Water Treatment and Plant Design PDF

BDU, IOT (2015/16) Wree-3152 Water Treatment and Plant Design
1. WATER QUALITY
Water quality refers to the chemical, physical, biological, and radiological characteristics of
water. It is a measure of the condition of water relative to the requirements of one or more
biotic species and or to any human need or purpose. Water quality is measured by several
factors, such as the concentration of dissolved oxygen, bacteria levels, the amount of salt (or
salinity), or the amount of material suspended in the water (turbidity). In some bodies of water,
the concentration of microscopic algae and quantities of pesticides, herbicides, heavy metals,
and other contaminants may also be measured to determine water quality.
Although scientific measurements are used to define water quality, it is not a simple thing to
say “that water is good” or “that water is bad.” So, the determination is typically made relative
to the purpose of the water – is it for drinking or to wash a car with or for some other purpose?
Poor water quality can pose a health risk for people. Poor water quality can also pose a health
risk for ecosystems.
Absolutely pure water is never found in nature and contains number of impurities in varying
amounts. The rainwater which is originally pure also absorbs various gases, dust and other
impurities while falling. This water when moves on the ground further carries salt, organic and
inorganic impurities. So this water before supplying to the public should be treated and purified
for the safety of public health, economy and protection of various industrial processes. It is the
most essential for the water work engineer to thoroughly check, analyze and do the treatment of
the raw water obtained from the sources, before its distribution. The water supplied to the
public should be strictly according to the standards laid down from time to time.
The following are important requirements of water for domestic use:

1. It should be colorless and sparkling clear
2. It should be of good taste and free from odor
3. It should be reasonably soft
4. It should be free from disease producing bacteria of organisms
5. It should be free from objectionable dissolved gases such as sulphureted hydrogen but
should have sufficient quantity of dissolved oxygen
6. It should be free from harmful salts
7. It should be free from objectionable minerals like Iron, Manganese, Lead, Arsenic and
other poisonous minerals
8. It should be free from radioactive minerals like radium, strontium …
9. It should be reasonably free from phenolic compounds, chlorides, fluorides, iodine
10. It should not lead to scale formation and should non-corrosive
Lecture Note Page 1

1.1 Common Impurities in Water

Impurities in water are classified into three heads:
i. Suspended impurities
Suspended impurities are those impurities which normally remain in suspension. They are
microscopic and make water turbid.
Sources of suspended impurities:
- Living organisms: bacteria, algae, protozoa
- Inorganic: clay silt, sand
- Organic: plant and animal particles, vegetable, industrial and domestic by-products
ii. Colloidal impurities
Are electrically charged impurities. Usually very small in size and remains in constant motion
and don’t settle.
Sources colloidal impurities:
Clay, amino acids, silica, organic waste products, Iron oxides, manganese oxides
iii. Dissolved impurities
Dissolved impurities are not visible but they are large in quantity since water is a good solvent.
They cause bad taste, hardness and alkalinity. Sometimes they are harmful.
Sources of dissolved impurities:
a. Salt
- Ca and Mg: bicarbonates, carbonates, sulphates and chlorides
- Na: bicarbonates, carbonates, sulphates, chlorides, and fluorides
b. Metals and compounds: Iron oxide, Mn, Pb, As, Ba, Cd, Cn, Br, Se, Nitrates
c. Vegetable dyes
d. Gases: O2, CO2, H2S
1.2 Quality of Water Source

Natural water is available from the following sources:
i. Pure water
Chemically pure water is a combination of two elements: hydrogen and oxygen with a chemical
symbol of H2O. It is not possible to find absolutely pure water in nature. It is entirely free from
all the three types of impurities.
ii. Rain water
When precipitation takes place, the rain water, falling through atmosphere absorbs various
gases and vapors which are normally present in the atmosphere. Thus, rain water absorbs
Lecture Note Page 2

oxygen, nitrogen, carbon dioxide, rare gases, sweep particulates, salt nuclei (principally
chlorides), radio-active fallout etc.
iii. Surface runoff water
When the rain water falling on the ground surface takes the form of surface runoff, it picks up
particulates (silt and clay), organic matter, nitrates, phosphorous etc. the characteristics of the
surface runoff water thus depends upon the topography and vegetation of the catchment, along
with land use and management. Thus the surface runoff water contains
- Mineral and organic particulates picked up by erosion
- Soil bacteria and other organism
- Salt and soluble substances
- Natural and synthetic fertilizers
iv. River water
Since the surface runoff water normally joins the river and streams, the characteristics of river
water are particularly the same as those of surface runoff water. However, if the river receives
the municipal and industrial wastewaters, additional pollution parameters are added to it.
v. Lake and pond water
When water is ponded in lakes and reservoirs, algae and similar other organisms grow on the
surface, giving rise to odors, tastes and color. Decaying vegetation further intensify these
elements. In addition to this, thermal stratification takes place in the reservoirs resulting in
- Low dissolved oxygen
- Dissolution of iron and manganese
- Production of H2S
- Increase in CO2
- Reduction in pH of water
vi. Ground water
When rain water infiltrates in to the ground to join the water table, it becomes ground water.
Groundwater has the following characteristics:
- It absorbs gases of decomposition and degradable organic matter (such as H2S, CH4)
within the pores of soil mantle through which it percolates
- In earth strata which in rich is organic matter, oxygen is removed from percolating
water and CO2 is added
- Groundwater has lower pH
- Soil minerals are dissolved in groundwater. Carbonates, sulphates and chlorides are
added, resulting in hardness
Lecture Note Page 3

- Groundwater may contain iron and manganese in soluble form

vii. Effects of impoundment on water quality
When water is impounded in the reservoirs, especially for storage purpose, its characteristics
are changed. The impoundment may produce both beneficial as well as detrimental effects on
water quality.
Beneficial effects:
- Reduction in turbidity at upper levels, because of reduced velocities and long detention
time
- Reduction in hardness caused by algae carbon dioxide and subsequent precipitation of
calcium carbonate
- Organic oxidation
- Reduction in BOD caused by bio-degradation during storage
- Color reduction due to setting silts
- Reduction in coliform density due to natural die off during storage
Detrimental effects:
- Lower atmospheric reaeration caused by reduced velocities and increased depth
- Increased algal blooms with resultant taste and odour problems, in addition to the
aesthetic aspect
- Back up of pollutants present in the receiving water
- Thermal stratification resulting in low DO, dissolution of iron and manganese,
production of H2S, increase in CO2 and reduction in pH.
1.3 Water Quality Characteristics

Water quality refers to the chemical, physical and biological characteristics of water. It is a
measure of the condition of water relative to the requirements of one or more biotic species and
or to any human need or purpose.
For the purpose of classification, the impurities present in water may be divided into the
following three categories.
A. Physical Characteristics
Physical characteristics are parameters that can be detected with our sense organ which include:
 Turbidity
 Color
 Taste and odor
 Temperature
 Foam
Lecture Note Page 4

1. Turbidity
Turbidity is caused due to presence of suspended and colloidal solids. The suspended solids
may be dead algae or other organisms. It is generally silt, clay rock fragments and metal oxides
from soil.
The amount and character of turbidity will depend upon:
The type of soil over which the water has run and
The velocity of the water
When the water becomes quite, the heavier and larger suspended particles settle quickly, while
the lighter and more finely divided ones settle very slowly. Very finely divided clay may
require months of complete quiescence for settlement. Groundwater is normally clear because,
slow movement through the soil has filtered out the turbidity. Lake waters are clearer than
stream waters, and streams in dry weather are clearer than streams in flood because of the
smaller velocity and because dry-weather flow is mainly ground water seepage. Low inorganic
turbidity (silt and clay) may result in a relatively high organic turbidity (color). The explanation
of this is that low inorganic turbidity permits sunlight to penetrate freely into the water and
stimulates a heavier growth of algae, and further, that organics tend to be absorbed upon soil
fractions suspended in water.
Turbidity is a measure of resistance of water to the passage of light through it. Turbidity is
expressed as NTU (Nephelometric Turbidity Units) or PPM (parts per million) or Milligrams
per liter (mg/l).
Turbidity is measured by:

1) Turbidity rod or Tape 2) Jacksons Turbidimeter 3) Bali’s Turbidimeter
The sample to be tested is poured into a test tube and placed in the meter and a unit of turbidity
is read directly on the scale by a needle or by digital display.
Water
sample
Figure 1: Turbidity meter
Drinking water should not have turbidity more than 10 NTU. This test is useful in determining
the detention time in settling for raw water and the dosage of coagulants required to remove
Lecture Note Page 5

turbidity. Sedimentation with or without chemical coagulation and filtration are used to remove
it.
2. Color
Color is caused by materials in solution or colloidal conditions and should be distinguished
from turbidity, which may cause an apparent (not true) color.
True color is caused by dyes derived from decomposing vegetation. Colored water is not only
undesirable because of consumer objections to its appearance but also it may discolor clothing
and adversely affect industrial processes.
Before testing the color of water, total suspended solids should be removed by centrifugal force
in a special apparatus. The color produced by one milligram of platinum in a liter of water has
been fixed as the unit of color. The permissible color for domestic water is 20ppm on platinum
cobalt scale.
3. Temperature
Temperature increase may affect the portability of water, and temperature above 15 0c is
objectionable to drinking water. The temperature of surface waters governs to a large extent the
biological species present and thereof activity. Temperature has an effect on most chemical
reactions that occur in natural water systems. It also has pronounced effect on the solubility of
gases in water.
4. Foam
Foam, from various industrial waste contributions and detergents, is primarily objectionable
from the aesthetic standpoint.
5. Tastes and Odor
The terms taste and odor are themselves definitive of this parameter. Because the sensations of
taste and smell are closely related and often confused, a wide variety of tastes and odors may be
attributed to water by consumers. Substances that produce an odor in water will almost in
variably impart a taste as well. The converse is not true, as there are many mineral substances
that produce taste but no odor.
Many substances with which water comes into contact in nature or during human use may
import perceptible taste and odor. These include minerals, metals, and salts from the soil, and
products from biological reactions, and constituents of wastewater. Inorganic substances are
more likely to produce tastes unaccompanied by odor. Alkaline material imports a bitter taste to
water, while metallic salts may give salty or bitter taste.
Organic material, on the other hand, is likely to produce both taste and odor. A multitude of
organic chemicals may cause taste & odor problems in water with petroleum-based products
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being prime offenders. Biological decomposition of organics may also result in taste-and odor-
producing liquids and gases in water. Principal among these are the reduced products of sulfur
that impart a rotten egg taste and odor. Also certain species of algae secrete an oily substance
that may result in both taste and odor.
Consumers find taste and odor aesthetically displeasing for obvious reasons. Because water is
thought of as tasteless and odorless, the consumer associates taste and odor with contamination
and may prefer to use a tasteless, odorless water that might actually pose more of a health
threat.
B. Chemical Characteristics
1. Total Solids
Total solids include the solids in suspension, colloidal and in dissolved form. Suspended solids
are important as pollutants and pathogens are carried on the surface of particles. The smaller
the particle size, the greater the total surface area per unit mass of particle, and so the higher the
pollutant load that is likely to be carried. The quantity of suspended solids is determined by
filtering the sample of water through fine filter, drying and weighing. The quantity of dissolved
and colloidal solids is determined by evaporating the filtered water obtained from the
suspended solid test and weighing the residue. The total solids in a water sample can be directly
determined by evaporating the water and weighing the residue, the residue of total solids is
fused in a muffle furnace the organic solids will decompose whereas only inorganic solids will
remain. By weighing we can determine the inorganic solids and deducting it from the total
solids, we can calculate organic solids.
Figure 2: Filter paper Figure 3: Using filter paper to remove dirt from water
2. Alkalinity
It is defined as the quantity of ions in water that will react to neutralize hydrogen ions.
Alkalinity is thus the measure of the ability of water to neutralize acids. By far the most
Lecture Note Page 7

constituents of alkalinity in natural waters are carbonate (CO32-), bicarbonate (HCO3-) and
hydroxide (OH-). These compounds result from the dissolution of mineral substances in the soil
atmosphere.
Effects:
i) Non pleasant taste
ii) Reaction between alkaline constituent and cation produces precipitation in pipe
3. pH
pH is a measure of the concentration of free hydrogen ion in water. It expresses the molar
concentration of the hydrogen ion as its negative logarithm. Water and other chemicals in
solution therein, will ionize to a greater or lesser degree. Pure water is only weakly ionized.
The ionization reaction of water may be written:
HOH  H+ +OH-
The reaction has an equilibrium defined by the equation:
[H][OH]/ [HOH] = Kw
In which HOH, H, OH is the chemical activities of the water hydrogen and hydroxyl ion
respectively. Since water is solvent, its activity is defined as being unity. In dilute solution,
molar concentrations are frequently substituted for activities yielding
[H][OH) = Kw (10-14 at 20oC)
Taking negative logs of both sides, Log [H] + Log [OH] = -14
- Log [H] - Log [OH] = 14
Defining –Log = p; pH + pOH = 14
In neutral solutions at equilibrium (OH) = (H), hence pH = pOH = 7.
1
Mathematically it is expressed as; pH = -log [H+] = log   7
[H ]
Increasing acidity leads to higher values of [H+], thus to lower values of pH. Low pH is
associated with high acidity, high pH with caustic alkalinity. It may be readily measured
potentially by use of a pH meter.
pH is important in the control of a number of water treatment and waste treatment processes
and in control of corrosion.
Lecture Note Page 8

Figure 4: pH meter Figure 5: Effect of pH on steel or Iron

4. Dissolved Oxygen (DO)
Dissolved oxygen is present in variable quantities in water. Its content in surface waters is
dependent upon the amount and character of the unstable organic matter in the water. Clean
surface waters are normally saturated with DO.
The amount of oxygen that water can hold is small and affected by the temperature. The higher
the temperature, the smaller will be the DO. Gases are less soluble in warmer water.
Temperature (0C) 0 10 20 30
DO (mg/1) 14.6 11.3 9.1 7.6
Oxygen saturated waters have pleasant taste and waters lacking in DO have an insipid tastes.
Drinking water is thus aerated if necessary to ensure maximum DO. The presence of oxygen in
the water in dissolved form keeps it fresh and sparkling. But more quantity of oxygen causes
corrosion to the pipes material.
Observing a heated pot of water, one can observe that bubbles form on the walls of the pot
prior to reaching the boiling point. These cannot be filled with only water vapor because liquid
water will not begin to vaporize until it has reached its boiling point. One can surmise that this
gas is oxygen, or at least a mixture of gases from the air, because bubbles of this sort form in
water from virtually every source: what other gas mixture besides air is in constant contact with
water? When these bubbles form, they eventually grow to a sufficient size to leave the surface
of the pot and escape to the air: the dissolved gas in the liquid has decreased. This seems to
support the hypothesis that dissolved oxygen will decrease when temperature is increased.
5. Oxygen Demand
Organic compounds are generally unstable to be oxidized biologically or chemically to stable,

relatively inert end produce such as CO2, H2O & NO3. Indicators used for estimation of the
oxygen demanding substance in water are Biological Oxygen Demand (BOD), Chemical
Oxygen Demand (COD), Total Oxygen Demand (TOD) and Total Organic Carbon (TOC).
Lecture Note Page 9
An indication of the organic content of water can be detected by measuring the amount of
oxygen required for stabilization.
BOD is the quantity of oxygen required for the biochemical oxidation of the decomposable
matter in a sample at specified temperature within specified time. Unpolluted waters should
have less than 5 mg/l of BOD found from an incubation period of five days and a temperature
of 20°c. Higher values of BOD lead to the suspicion regarding the quality of water.
6. Nitrogen
The forms most important to water quality engineering include:
a) Organic – nitrogen: in the form of protein, amino acids and urea
b) Ammonia – nitrogen: nitrogen as ammonium salts. eg. (NH4).CO3
c) Nitrite- nitrogen: an intermediate oxidation stage. Not normally present in large quantity.
d) Nitrate- nitrogen: final oxidation product of nitrogen.
e) Gaseous nitrogen (N2)
The presence of nitrogen compounds in surface waters usually indicates pollution. Excessive
amount of ammonia and organic nitrogen may result from recent sewage discharges or runoff
contamination by relatively fresh pollution. Therefore, water containing high org-N &
ammonia –N levels are considered to be potentially dangerous. While waters in which most of
nitrogen is in nitrate form are considered to somewhat stabilized to constitute prior pollution.
Effect (s):
- NO3 – poisoning in human and animal babies (human below 6 month old)
– will substitute O2 in blood vessel (methaemoglobinaemenia)
- Excessive algae breeding and aquatic plants
7. Hardness
Hard water is formed when water percolates through deposits of calcium and magnesium-
containing minerals such as limestone, chalk and dolomite. Hardness is caused by the sum of
the alkali earth elements present in water although the major constituents are usually calcium
and magnesium. These materials in water react with soap, causing precipitation which as scum
or curd on the water surface. Until enough soap has been dissolved to react with all these
materials, no lather can be formed. Water that behaves like this is said to be ‘hard ‘. The
hardness compounds are temporary and permanent:
1. Temporary hardness (carbonate hardness)
 Calcium bicarbonate (Ca(HCO3) 2)
 Magnesium bicarbonate (Mg(HCO3) 2)
Lecture Note Page 10

2. Permanent hardness’ (non- carbonate hardness)

 Calcium sulfate (CaSO4)
 Magnesium chloride (MgSO4)
 Calcium chloride (CaCl2)
 Magnesium chloride (MgCl2)
The most usual compounds causing alkalinity are calcium and magnesium bicarbonate, happen
also to cause the temporary hardness. Hence, when the alkalinity and hardness are equal, all the
hardness is temporary. If the total hardness is greater than the alkalinity, then the excess
hardness represents permanent hardness. On the other hand, if the total hardness is less than the
alkalinity, the difference indicates the presence of sodium bicarbonate, which adds to the
alkalinity but doesn’t increase the hardness.
Total hardness = Ca-hardness + Mg-hardness
Total hardness = Carbonate hardness + non-carbonate hardness
A generally accepted classification of hardness is as follows:

Soft < 50 mg/1 as CaCO3
Moderately hard 50 – 150 mg/1 as CaCO3
Hard 150 – 300mg/1 as CaCO3
Very hard > 300 mg/1 as CaCO3
Generally a hardness of 100 to 150 mg/liter is desirable. Excess of hardness leads to the
following effects:
1. Large soap consumption in washing and bathing
2. Fabrics when washed become rough and strained with precipitates.
3. Hard water is not fit for industrial use like textiles, paper making, dye and ice-cream
manufactures.
4. The precipitates clog the pores on the skin and makes the skin rough
5. Precipitates can choke pipe lines and values
6. It forms scales in the boiler tubes and reduces their efficiency
7. Very hard water is not palatable
Softening is practiced when hardness exceeds 300mg/lit. Water hardness more than 600mg/lit
have to rejected for drinking purpose.
8. Chloride
The natural waters near the mines and sea dissolve sodium chloride and also presence of
chlorides may be due to mixing of saline water and sewage in the water. Excess of chlorides is
dangerous and unfit for use. The chlorides can be reduced by diluting the water. Chloride may

demonstrate an adverse physiological effect when present in concentration greater than

250mg/l and with people who are acclimated. However, a local population that is acclimated to
the chloride content may not exhibit adverse effect from excessive chloride concentration.
Because of high chloride content of urine, chlorides have sometimes been used as an indication
of pollution.
9. Fluoride
It is generally associated with a few types of sedimentary or igneous rocks; fluoride is seldom
found in surface waters and appears in groundwater in only few geographical regions. Fluoride
is toxic to humans and other animals in large quantities, while small concentrations can
beneficial.
Concentrations of approximately 1.0mg/1 in drinking water help to prevent dental cavities in
children. During formation of permanent teeth, fluoride combines chemically with tooth
enamel, resulting in harder, stronger teeth that are more resistant to decay. Fluoride is often
added to drinking water supplies if quantities for good dental formation are not naturally
present.
Effect (s):
Excessive intakes of fluoride can result in discoloration of teeth. Noticeable discoloration,
called mottling, is relatively common when fluoride concentrations in drinking water exceed
2.0mg/1, but is rare when concentration is less than 1.5mg/1. Adult tooth are not affected by
fluoride, although both the benefits and liabilities of fluoride during teeth formation years carry
over into adulthood. Excessive concentrations of greater than 5mg/1 in drinking water can also
result in bone fluorosis and other skeletal abnormalities.
10. Metals and other chemical substances
Water contains various minerals or metal substances such as iron, manganese, copper, lead,
barium, cadmium, selenium, arsenic etc.
The concentration of iron and manganese should not allow more than 0.3ppm. Excess will
cause discoloration of clothes during washing and incrustation in water mains due to deposition
of ferric hydroxide and manganese oxide. Lead and barium are very toxic, low ppm of these is
allowed. Arsenic, Selenium are poisonous, therefore they must be removed totally. Human
beings are affected by presence of high quantity of copper in the water.

C. Biological Characteristics
A feature of most natural water is that they contain a wide variety of micro – organisms
forming a balance ecological system. The types and numbers of the various groups of micro –
organisms present are related to water quality and other environmental factors.
Microorganisms that bring diseases are called “pathogen”. Their quantities are very small
compared to other microorganisms. The experiments to determine the presence of all pathogens
takes a long time and very expensive. Water polluted by pathogenic microorganisms may
penetrate into private and or public water supplies either before or after treatment. The presence
of pathogenic microorganism is shown by indicator microorganism.
Properties of indicator microorganism:
- Can be used for all types of water
- Always present when pathogen is present
- Always absent when pathogen is absent
- Easily experimented and give reliable results
Typical indicators used are coliform group which includes:
- Fecal coliform e.g. E.Coli
- Total coliform e.g. fecal coliform, soil coliform and others
Microbiological indicators of water quality or pollution are therefore of particular concern

because of their relationships to human and animal health.
1. Bacterium
Many are found in water. Some bacteria are indicator of pollution but are harmless; other few
in number are pathogenic. Bacterial-born diseases include: typhoid fever, cholera, and bacterial
dysentery
2. Viruses
These are group of infectious which are smaller than ordinary bacteria and that require
susceptible host cells for multiplication and activity. Viral-born diseases include infectious
hepatitis and poliomyelitis.
3. Algae
These are small, chlorophyll bearing generally one–celled plants of varying shapes and sizes
which live in water. When present in large numbers they may cause turbidity in water and an
apparent color. They cause trouble in water works by undue clogging of filters, but their most
troublesome characteristics in the taste and odor that they may cause.

4. Protozoa
They are the lowest and simplest forms of animal life. Protozoa–born diseases include
giardiasis and amebic dysentery.
1.4 Examination of Water Quality

Various types of impurities present in water can be determined by water by water analysis. This
analysis is done both for raw water which will enable us to determine the outline or processes
of water purification. Water analysis of purified water is done to know whether the degree of
purification has reached the required standard or not. Following are the purposes of water
analysis of raw water and purified water:
1. To classify the water with respect to general level of mineral constituents
2. To determine the degree of clarity and ascertain the nature of matter in suspension
3. To determine the chemical and bacteriological pollution of water
4. To determine the presence/absence of an excess of any particular constituent affecting
potable quality and general use
5. To determine the level of organic impurities
6. To set the outlines of purification process and specify various stages in it
7. To ascertain whether purification of water has reached to the required standards or not
Thus, examination of water is used to classify, prescribe treatment, control treatment and
purification processes and maintain public supplies of an appropriate standard of organic
quality, clarity and palatability.
1.Sampling
Samples must be taken from locations that are representative of the water source, treatment
plant, storage facilities, distribution network, points at which water is delivered to the
consumer, and points of use.
Location of sampling points

One objective of surveillance is to assess the quality of the water supplied by the supply agency
and of that at the point of use, so that samples of both should be taken. Any signiﬁcant
difference between the two has important implications for remedial strategies.
In selecting sampling points, each locality should be considered individually; however, the
following general criteria are usually applicable:
- Sampling points should be selected such that the samples taken are representative of the
different sources from which water is obtained by the public or enters the system.

- These points should include those that yield samples representative of the conditions at
the most unfavourable sources or places in the supply system, particularly points of
possible contamination such as unprotected sources, loops, reservoirs, low-pressure
zones, ends of the system, etc.
- Sampling points should be uniformly distributed throughout a piped distribution system,
taking population distribution into account; the number of sampling points should be
proportional to the number of links or branches.
- The points chosen should generally yield samples that are representative of the system
as a whole and of its main components.
- Sampling points should be located in such a way that water can be sampled from
reserve tanks and reservoirs, etc.
- In systems with more than one water source, the locations of the sampling points should
take account of the number of inhabitants served by each source.
- There should be at least one sampling point directly after the clean-water outlet from
each treatment plant.
Sampling frequency
The most important tests used in water-quality surveillance or quality control are those for
microbiological quality (by the measurement of indicator bacteria) and turbidity and for free
chlorine residual and pH where chlorination is used. These tests should be carried out whenever
a sample is taken, regardless of how many other physical or chemical variables are to be
measured.
Storage of samples for microbiological analysis

Although recommendations vary, the time between sample collection and analysis should, in
general, not exceed 6 hours, and 24 hours is considered the absolute maximum. It is assumed
that the samples are immediately placed in a lightproof insulated box containing melting ice or
ice-packs with water to ensure rapid cooling. If ice is not available, the transportation time must
not exceed 2 hours.
It is imperative that samples are kept in the dark and that cooling is rapid. If these conditions
are not met, the samples should be discarded. When water that contains or may contain even
traces of chlorine is sampled, the chlorine must be inactivated. If it is not, microbes may be
killed during transit and an erroneous result will be obtained. The bottles in which the samples
are placed should therefore contain sodium thiosulfate to neutralize any chlorine present. The
box used to carry samples should be cleaned and disinfected after each use to avoid

contaminating the surfaces of the bottles and the sampler’s hands. Normally plastics are used
for chemical analysis (except for oil & grease) and glass for bacteriological analysis.
As each sample is collected, it must be clearly identified so there is no chance of confusing it

with any other sample. Samples should be tagged or labeled with the following information:
- Sample location
- Date and time of collection
- Type of sample (either grab or composite)
- Name of collector
- Preservatives added
- Any unusual conditions at the time of sampling
- Other information deemed pertinent (an identifying sample number, a list of analyses to
be performed, type of container, etc.)
2. Standard Tests
i. Titration (volumetric) method
Volumetric or titrimetric analyses are quantitative analytical techniques which employ a
titration in comparing an unknown with a standard. In a titration, a volume of a standardized
solution containing a known concentration of reactant "A" is added incrementally to a sample
containing an unknown concentration of reactant "B". The titration proceeds until reactant "B"
is just consumed (stoichiometric completion). This is known as the equivalence point. At this
point the number of equivalents of "A" added to the unknown equals the number of equivalents
of "B" originally present in the unknown.
The basic requirements or components of a volumetric method are:
i. A standard solution (i.e., titrant) of known concentration which reacts with the analyte
with a known and repeatable stoichiometry (i.e., acid/base, precipitation, redox,
complexation)
ii. A device to measure the mass or volume of sample (e.g., pipet, graduated cylinder,
volumetric flask, analytical balance)
iii. A device to measure the volume of the titrant added (i.e., buret)
iv. If the titrant-analyte reaction is not sufficiently specific, a pretreatment to remove
interferents
v. A means by which the endpoint can be determined. This may be an internal indicator
(e.g., phenolphthalein) or an external indicator (e.g., pH meter).

Table 1: Volumetric methods for environmental analysis

Analyte Titrant Indicator Pretreatment Method
Acid/Base
Alkalinity HCl methyl orange none
Acidity NaOH phenolphthalein none
N(-III) H2SO4 methyl red digest/distill Macro-Kjeldahl
(NNH4OH) & Acidimetric
Volatile Acids NaOH phenolphthalein distillation Distillation
Precipitation
Chloride Ag potassium
chromate
Chloride Hg diphenylcarbazone Mercuric
Nitrate
Complexation or Chelation
Ca EDTA Eriochrome Blue
Black R
Hardness EDTA Eriochrome Black T
CN Ag p-dimethylamino
benzalrhodanine
Volumetric methods may be based on acid/base reactions, precipitation reactions, complexation

reactions and redox reactions. Table 1 presents a summary of the volumetric methods
commonly used for environmental analysis. The acid/base methods generally use a strong acid
or base as a titrant with methyl orange/red (acid titration) or phenolphthalein (base titration) as
the indicator. For all but acidity and alkalinity determinations, the analyte must be separated
from the major cations and anions prior to titration. Precipitative volumetric analysis relies on
the formation of solid phase with a very low solubility product constant. In environmental
analysis, it may be used for chloride determination. Specific indicators are used to detect excess
silver or mercury. Complexometric titrations often employ ethylenediaminetetraacetic acid or
EDTA. This is a hexadentate ligand which binds very strongly to many metals. For calcium and
total hardness determination, a couple of specific dyes are used to determine the presence of
excess cation.
The recommended determinations to be made by titration method are: Chloride (Cl-),

carbonates (CO32-), bicarbonates (HCO3), DO, BOD, COD, calcium (Ca++), magnesium
(Mg++), bromide (Br), hydroxide (OH-), sulfide(S-), sulfite(SO32), acidity, alkalinity etc.
ii. Colorimetric method (using color as the basis)

Measuring amount of color produced by mixing with reagents at fixed wavelength (using
spectrophotometer) or comparison with colored standards or discs (comparator)

The recommended determinations made by colorimetric method are: color, turbidity, iron
(Fe++), manganese (Mn++), chlorine (Cl2), fluoride (F-), nitrate (NO3-), nitrite (NO2), phosphate
(PO4---), ammonia (NH4+), arsenic, phenols, etc.
iii. Gravimetric method (using weight as the basis)

Using weight of insoluble precipitates or evaporated residues in glassware or metal and
accurate analytical balance
The recommended determinations made by gravimetric methods are: sulfate (SO4), Oil and
grease, TDS, TSS, TS, etc.
iv. Electrical method

In this method, probes are used to measure electrical potential in millivolts against standard cell
voltage.
The recommended determinations made by electrical methods are: pH, Fluoride (F-), DO,
nitrate (NO3), etc.
v. Flame spectra (emission & absorption) method

At fixed wave length characteristics to ions being determined measuring intensity of emission
or absorption of light produced by ions exited in flame or heated sources.
The recommended determinations made by flame spectra methods are: sodium (Na+),
potassium (K+), lithium (Li+), etc.
3. Bacteriological water analysis

Bacteriological water analysis is a method of analyzing water to estimate the numbers of
bacteria present and, if needed, to find out what sort of bacteria they are. It represents one
aspect of water quality. It is a microbiological analytical procedure which uses samples of
water and from these samples determines the concentration of bacteria. It is then possible to
draw inferences about the suitability of the water for use from these concentrations. This
process is used, for example, to routinely confirm that water is safe for human consumption or
that bathing and recreational waters are safe to use.
The common feature of all these routine screening procedures is that the primary analysis is for
indicator organisms rather than the pathogens that might cause concern. Indicator organisms
are bacteria such as non-specific coliforms, Escherichia coli (E-coli) and Pseudomonas
aeruginosa that are very commonly found in the human or animal gut and which, if detected,
may suggest the presence of sewage. Indicator organisms are used because even when a person
is infected with a more pathogenic bacteria, they will still be excreting many millions times

more indicator organisms than pathogens. It is therefore reasonable to surmise that if indicator
organism levels are low, then pathogen levels will be very much lower or absent. Judgements
as to suitability of water for use are based on very extensive precedents and relate to the
probability of any sample population of bacteria being able to be infective at a reasonable
statistical level of confidence.
Analysis is usually performed using culture, biochemical and sometimes optical methods.
When indicator organisms levels exceed pre-set triggers, specific analysis for pathogens may
then be undertaken and these can be quickly detected (where suspected) using specific culture
methods or molecular biology.
Methods
i. Plate count
The plate count method relies on bacteria growing a colony on a nutrient medium so that the
colony becomes visible to the naked eye and the number of colonies on a plate can be counted.
To be effective, the dilution of the original sample must be arranged so that on average between
30 and 300 colonies of the target bacterium is grown. Fewer than 30 colonies makes the
interpretation statistically unsound whilst greater than 300 colonies often results in overlapping
colonies and imprecision in the count. To ensure that an appropriate number of colonies will be
generated several dilutions are normally cultured.
The laboratory procedure involves making serial dilutions of the sample (1:10, 1:100, 1:1000,
etc.) in sterile water and cultivating these on nutrient agar in a dish that is sealed and incubated.
Typical media include plate count agar for a general count or MacConkey agar to count Gram-
negative bacteria such as E. coli. Typically one set of plates is incubated at 22°C and for 24
hours and a second set at 37°C for 24 hours. The composition of the nutrient usually includes
reagents that resist the growth of non-target organisms and make the target organism easily
identified, often by a colour change in the medium. Some recent methods include a fluorescent
agent so that counting of the colonies can be automated. At the end of the incubation period the
colonies are counted by eye, a procedure that takes a few moments and does not require a
microscope as the colonies are typically a few millimeters across.
ii. Membrane filtration
Most modern laboratories use a refinement of total plate count in which serial dilutions of the
sample are vacuum filtered through purpose made membrane filters and these filters are
themselves laid on nutrient medium within sealed plates. The methodology is otherwise similar
to conventional total plate counts. Membranes have a printed millimeter grid printed on and can
be reliably used to count the number of colonies under a binocular microscope.

1.5 Water Quality Standards

Public water supplies are obliged to provide a supply of wholesome water which is suitable and
safe for drinking purposes. The quality of water for various uses should satisfy the required
standards.
Water quality standards may be set by regional, national, or international bodies. Guidelines for
drinking water quality have established by the World Health Organization (WHO) (Table 2).
Table 2: WHO guideline for drinking water quality

Parameter Unit Guideline value
Microbial quality
Fecal coli forms Number/ 100 ml Zero*
Coli form organisms Number /100 ml Zero*
Arsenic mg/1 0.05

Cadmium mg/1 0.005
Chromium mg/1 0.05
Cyanide mg/1 0.1
Fluoride mg/1 0.5 - 1.5(3)
Lead mg/1 0.05
Mercury mg/1 0.001
Nitrate mg/1 10
Selenium mg/1 0.01
Aluminum mg/1 0.2

Chloride mg/1 250
Color True color unit (TCU) 5(15)
Copper mg/1 1.0
Hardness mg/1(as CaCO3) 300
Iron mg/1 0.3(3)
Manganese mg/1 0.3
pH 6.5 - 8.5
Sodium mg/1 200
Total dissolved solids mg/1 500
Sulfate mg/1 250
Taste and odor Non objectionable
Turbidity NTU 5(10)
Zinc mg/1 5.0

4.3. Sources of Water Pollution

Following are the main sources of water pollution.
1. Domestic Sewage
If domestic sewage is not properly after it is produced or if the effluent received at the end of
sewage treatment is not of adequate standard, there are chances of water pollution.
The indiscriminate way of handling domestic sewage may lead to the pollution of underground
sources of water supply such, as wells. Similarly if sewage or partly treated sewage is directly
discharged into surface waters such as rivers, the waters of such rivers get contained.
2. Industrial Wastes
If industrial wastes are thrown into water bodies without proper treatments, they are likely to
pollute the watercourses. The industrial wastes may carry harmful substances such as grease,
oil, explosives, highly odorous substances, etc.
3. Catchment Area
Depending upon the characteristics of catchment area, water passing such area will be
accordingly contained. The advances made in agricultural activities and extensive use of
fertilizers and insecticides are main factors, which may cause serious pollution of surface
waters.
4. Distribution System
The water is delivered to the consumers through a distribution of pipes which are laid
underground. If there are cracks in pipes or if joints are leaky, the flowing water gets
contaminated by the surrounding substances around the pipes.
5. Oily Wastes
The discharge of oily wastes from ships and tankers using oil as fuel may lead to pollution.
6. Radioactive Wastes
The discharge of radioactive wastes from industries dealing with radioactive substance may
seriously pollute the waters. It may be noted that radioactive substances may not have color,
odour, turbidity or taste. They can only be detected by and measured by the use of special
precise instruments.
7. Travel of Water
Depending upon the properties of ground through which water travels to reach the source of
water supply; it is charged with the impurities.
2. WATER TREATMENT
2.1 Introduction
All surface water and some groundwater require treatment prior to consumption to ensure that
they do not represent a health risk to the user. Health risks to consumers from poor quality
water can be due to microbiological, chemical, physical or radioactive contamination.
However, microbiological contamination is generally the most important to human health as

this leads to infectious diseases which affect all populations groups, many of which may cause
epidemics and can be fatal. Chemical contamination, with the exception of a few substances
such as cyanide and nitrate, tends to represent a more long-term health risk. An example of this
is nitrate which can cause methaemoglobinaemenia in babies. Substances in water which affect
the clarity, colour or taste of water may make water objectionable to consumers and hence
ability to recover costs. As many microorganisms are found associated with particles in water,
physical contamination may also represent a health risk as it extends microbial survival.
Most treatment systems are designed to remove microbiological contamination and those
physical constituents which affect the acceptability or promote microorganism survival largely
related to the suspended solids in the water. A disinfectant is nearly always included in
treatment plants of any size. This is done for two main reasons: firstly it is added to inactivate
any remaining bacteria as the final unit of treatment; and, more importantly, to provide a
residual disinfectant which will kill any bacteria introduced during storage and/or distribution.
The degree of treatment depends on the quality of the water source.
2.2 Objective of Water Treatment
The main objective of the treatment process is to remove the impurities of raw water and bring
the quality of water to the required standard. The objective may be summarized as follow:
i. Preventing disease transmission
Organisms that cause disease must be removed or inactivated to make the water safe.
Such organisms are small animals (invertebrates) and their eggs (ova), protozoa and
their cysts, bacteria which may form spores, and viruses.
Chlorine is most commonly used to inactivate such pathogens, but the effectiveness of
chlorine on some forms e.g. cysts and ova are much less than others, and suspended
material in the water may shelter the pathogens from the chlorine.

ii. Making the water acceptable

If the consumers regard the water as unsatisfactory they may use an alternative source
which is hazardous. The taste, appearance and suitability for washing clothes shall all
be considered.
iii. Protecting the distribution system
- Corrosion reduces the life of the pipes, reduces their carrying capacity, and forms
deposits which may colour the water.
- Depositions in pipes may result from unsatisfactory addition of chemicals, reactions
within the system or poor turbidity removal
2.3 Methods of Water Treatment
The common methods/processes of water treatment (water purification) are:

1. Screening and grit removal
2. Aeration
3. Plain sedimentation
4. Coagulation and flocculation
5. Secondary sedimentation
6. Filtration
7. Adsorption
8. Softening
9. Disinfections

3. PRELIMINARY WATER TREATMENT PROCESS
3.1 Screening
Screening of water which is one form of pre-treatment is done by passing the water through
closely spaced bars, gratings or perforated plates. Screening usually involves a simple
screening or straining operation to remove large solids and floating matter as leaves, dead
animals, fish etc. Screening does not change the chemical or bacteriological quality of the
water. It serves to retain the coarse material and suspended matter that are larger than the screen
openings.
Purposes:
(i) Removal of floating and suspended matter which clogs pipes, damages pumps, etc.
(ii) Clarification by removal of suspended matter to lighten the load on subsequent treatment
processes.
Types of screen
a. Coarse/Bar screens
b. Fine/Mesh screens - with opening of 0.05 – 2cm
Coarse screen spacing is typically between 7.5 and 10cm. Mostly bars are kept inclined so that
they can be cleaned easily with a rake. Angle of inclination of bars is 60 - 75° if screening is
very small and 30 - 45° if larger amount is retained over the screen bar.
Velocity of flow should be low towards the screen bar (0.1 - 0.2m/sec). It may be increased to
0.3 - 0.5 after the screen to prevent settling there. Between the openings the velocity should be
restricted to up to 0.7m/sec to avoid forcing through the suspended solids. If regular cleaning is
done an allowance for loss of heads of up to 0.1 to 0.2m is made. However to allow for delay
and mechanical failures a loss of head allowance between 0.5 to 1.0m is made.
hl
V1
V2
Figure 1: Profile for water flowing through a bar screen

Headloss through coarse screen
Where:
c = empirical discharge coefficient to account for turbulence and eddy motion. (c = 0.7 for
clean bar and 0.6 for clogged bar screen)
V2 = velocity of flow through openings
V1 = approaching velocity of upstream channel
g = gravitational acceleration (9.81m/s2)
Headloss through fine screen
Where
c = empirical discharge coefficient to account for turbulence and eddy motion (c = 0.6)
g = gravitational acceleration (9.81m/s2)
Q = discharge (m3/s)
A = effective opening area of the screen
Example 1
Determine the building up of headloss through a bar screen when 50% of the flow area is
blocked off by the accumulation of coarse solids assume the following conditions are applied.
Approach velocity = 0.6m/s
Velocity through a clean bar screen = 0.9m/s
Open area for flow through clean bar screen = 0.19m2
Solution:
i. Compute the headloss through a clean bar screen
ii. Compute the headloss through a clogged bar screen

Q Q 
Vi    And Vf   
 Ai   Af 
Af = 50%Ai

 50 Ai  Ai
Af   
 100  2
From this the velocity through a clogged bar screen is doubled
Example 2
Design a coarse screen and calculate the headloss through the rack, using the following
information:
Peak design wet weather flow = 0.631 m3/s
Velocity through rack at peak wet weather flow = 0.90 m/s
Velocity through rack at maximum design dry weather flow = 0.6 m/s
 = 60°, with a mechanical cleaning device
Upstream depth of wastewater = 1.12m
Solution:
Step 1: Calculate bar spacing and dimensions
(a) Determine total clear area (A) through the rack
(b) Calculate total width of the opening at the rack, w
(c) Choose a 25 mm clear opening

(d) Calculate number of opening, n
Note: Use 24 bars with 10 mm (0.01 m) width and 50 mm thick.

(e) Calculate the width (W) of the chamber
(f) Calculate the height of the rack
Allowing at least 0.6 m of freeboard, a 2m height is selected.

(g) Determine the efficiency coefficient, EC

Note: The efficiency coefficient is available from the manufacturer.
3.2 Aeration
Aeration is the process of bringing water in intimate contact with air, while doing so water
absorbs oxygen from the air.
Objective:
i. Removes taste and odour caused by gases due to organic decomposition

ii. Increases the dissolved oxygen content of the water
iii. Removes hydrogen sulphide H2S, and hence odor due to this is also removed
iv. Decreases the CO2 content of water and thereby reduces its corrosiveness and raises its
pH value
v. Converts iron and manganese from their soluble states to their insoluble states, so that
these precipitated and removed
vi. Due to agitation of water during aeration, bacteria may be killed to some extent
vii. Effective in removing volatile substances from water
Aeration removes or modifies the constituents of water using two methods - scrubbing action
and oxidation. Scrubbing action is caused by turbulence which results when the water and air
mix together. The scrubbing action physically removes gases from solution in the water,
allowing them to escape into the surrounding air. Carbon dioxide and hydrogen sulfide are
being removed by scrubbing action. Scrubbing action will remove tastes and odors from water
if the problem is caused by relatively volatile gases and organic compounds.
Oxidation is the other process through which aeration purifies water. Oxidation is the addition
of oxygen, the removal of hydrogen, or the removal of electrons from an element or compound.
When air is mixed with water, some impurities in the water, such as iron and manganese,
become oxidized. Once oxidized, these chemicals fall out of solution and become suspended in
the water. The suspended material can then be removed later in the treatment process through
filtration.
Carbon dioxide gas dissolves easily in water, resulting in carbonic acid:
H2O + CO2 <===> H2CO3

The carbonic acid increases the acidity of the water (lowers the pH) which can cause corrosion
of pipes in the distribution system. A large concentration of carbon dioxide in water can also
keep iron in solution, making it difficult to remove from the water.
Carbon dioxide in the water can be dealt with either through aeration of the water or through
addition of lime. Aeration is usually used in water with a high concentration of carbon dioxide.
The Iron and Manganese may be removed as a precipitate after aeration. Chemically, these
reactions may be written as follows:
4Fe2+ + O2 + 10H2O  4Fe(OH) 3 + 8H+

2Mn2+ + O2 + 2 H2O  2MnO2  + 4H+
Efficiency
The efficiency of the aeration process depends almost entirely on the amount of surface contact
between the air and water. This contact is controlled primarily by the size of the water droplet
or air bubble.
Problems with Aeration

Aeration typically raises the dissolved oxygen content of the raw water. In most cases, this is
beneficial since a greater concentration of dissolved oxygen in the water can remove a flat
taste. However, too much oxygen in the water can cause a variety of problems resulting from
the water becoming supersaturated. Supersaturated water can cause corrosion (the gradual
decomposition of metal surfaces) and sedimentation problems. In addition, air binding occurs
when excess oxygen comes out of solution in the filter, resulting in air bubbles which harm
both the filtration and backwash process.
Aeration can also cause other problems unrelated to the supersaturated water. Aeration can be a
very energy-intensive treatment method which can result in overuse of energy. In addition,
aeration of water can promote algal growth in the water and can clog filters.
Different types of aerators are available

 Gravity aerator
 Spray aerator
 Air diffuser
 Mechanical aerator

i. Gravity aerators
a) Cascade towers
Inlet
chamber
Collection
Chamber
b) Inclined apron possibly shaded with plates
Inlet
chamber
Collection
Chamber
Figure 1: Gravity aerators
c) Tray aerator
In tray aerator water falls through a series of trays perforated with small holes, 5 - 12mm
diameter and 25 - 75mm spacing center to center. They are often built in stacks of 4 - 6 trays
giving a total height of 1.2 - 3m. The trays may be filled with layers of activated
charcoal/carbon or gravel of 50mm size to insure purification.
Figure 2: Tray aerator

ii. Spray aerators: - spray droplets of water into the air from stationary or moving orifices or
nozzles. Water is pumped through pressure nozzles to spray in the open air as in fountain to
a height of about 2.5m.
Figure 3: Spray aerator
iii. Air diffuser

In diffused aeration systems, water is contained in basins. Compressed air is forced into this
system through the diffusers. This air bubbles up through the water, mixing water and air and
introducing oxygen into the water.
Figure 4: Air diffusion aerator

iv. Mechanical Aerator
Mechanical aeration systems are fairly simple, but they are not among the most common
purification techniques. These aerators work by vigorously agitating source water with
mechanical mixers. As the waters churn, they become infused with purifying air.
Mechanical aeration systems are able to remove most volatile contaminants, but they are
limited to removals of 50 to 80 percent, depending on conditions.
Figure 5: Mechanical aerator

4. PLAIN SEDIMENTATION
Sedimentation is a solid-liquid separation utilizing gravitational settling to remove suspended
solids. Most of the suspended particles present in water have specific gravity > 1. When water
has little or no movement, suspended solids sink to the bottom under the force of gravity and
form sediment.
Plain sedimentation - when impurities are separated from water by the action of gravity alone
Coagulant aided sedimentation - when the particles are too small to be removed by gravity
and aided with coagulants to increase size and agglomeration.
Particle-fluid separation processes are difficult to describe by theoretical analysis, mainly
because the particles involved are not regular in shape, density, or size.
The various regimes in settling of particles are commonly referred to as
Type – I: Discrete particle settling
Type – II: Flocculant settling
Type – III: Hindered (zone) settling
Type – IV: Compression settling
Type – I – Discrete particle settling

It is the settling of discrete particles in low concentration, with flocculation and other inter-
particle effects being negligible.
- Particles that the size, shape and specific gravity do not change over time.
- These particles settle at constant settling velocity.
- They settle as individual particles and do not flocculate during settling.
Examples: Settling of sand, grit
Applications:
- Plain sedimentation for sand removal prior to coagulation
- Settling of waters treated for iron and manganese content
Plain sedimentation is the removal of particles (silt, sand, clay, etc.) through gravity settling in
basins. No chemicals are added to enhance the sedimentation process.
Type – II – Flocculant settling

It is the settling of flocculent particles in a dilute suspension. As coalescence occurs, particle
masses increase and particles settle more rapidly.
- Particles that change the size, shape and perhaps specific gravity over time.
- Particles flocculate during sedimentation.
- These types of particles occur in alum or iron coagulation.

Type – III – Hindered (zone) settling

Settling in which particle concentration causes inter-particle effects.
- Flocculation and rate of settling is a function of particle concentration.
- Particles remain in a fixed position relative to each other, and all settle at a constant
velocity
- Mass of particles settle as a zone
- Zones of different particle concentrations (different layers) may develop as a result of
particles with different settling velocities
- State of compression is reached at the bottom.
Type – IV – Compression settling
Settling of particles that are of such a high concentration that the particles touch each other and
settling can occur only by compression of the compacting mass.
- Compression settling occurs at lower depths of the sedimentation tanks
- Rate of compression is dependent on time and the force caused by the weight of solids
above the compression layer.
- Both discrete and flocculant particles may settle by zone compression settling
- However, flocculent particles are the most common type encountered.
Figure 6: Settling regimes depend upon closeness of particles to each other

Principle of plain sedimentation - Discrete particles
The terminal settling velocity of a single discrete particle is derived from the forces
(gravitational force, buoyant force, and drag force) that act on the particle. The sedimentation
of discrete particles may be described by Newton’s Law, from which the terminal settling
velocity is found by:
Forces acting on a free falling particle in a fluid are:

FD: Drag Force
FB: Buoyancy Force
FG: Gravitational Force
FD = FG - FB
Drag Force on a particle traveling in a resistant fluid:
CD: Drag coefficient

v: settling velocity
: density of fluid
A: projected area of particle in the direction of flow
Gravitational Force:
p: density of particle

g: gravitational acceleration
: volume of particle
Buoyancy Force:
: density of fluid
g: gravitational acceleration
: volume of particle
From Newton’s Law

Terminal settling velocity of a particle of any shape

Terminal settling velocity of a particle of a solid spherical particle (d: diameter of a sphere):
This is known as the Stokes equation.

Terminal velocity (vt) is independent of horizontal and vertical movement of the liquid. Nature
of the flow can be described by the Reynolds number (Re). Drag coefficient depends on the
nature of the flow around the particle. The values of drag coefficient depend on the density of
water (), relative velocity (u), particle diameter (d), and viscosity of water (µ), which gives the
Reynolds number Re.
The value of Cd decreases as the Reynolds number increases.

i. For Re less than 2 or 1, Cd is related to R by the linear expression as follows:
For laminar flow conditions
ii. 2 < Re < 500 - 1000, Transition zone
The value of vt is solved by iteration. First, assume the flow is laminar and calculate Cd,
compute vt and Re and with the computed Re, compute Cd until the values of vt converges.
iii. 500 - 1000 < Re < 200,000, Turbulent flow zone
Terminal velocity becomes

Figure 7: Variation of drag coefficient Cd with Reynolds number Re for single particle
sedimentation
Example 1
Find the terminal settling velocity of a spherical particle with diameter of 0.5mm and a specific
gravity (sg) of 2.65 settling through water at 200 C ( =1.002*10-3Ns/m2, w = 1000kg/m3).
Solution
Given
D = 0.5mm
Sg = 2.65
 =1.002*10-3Ns/m2, w = 1000kg/m3
Assume the flow is Laminar
Check the flow is laminar or not

The flow is transitional flow
The terminal settling velocity is calculated as:
Check Re
The revised terminal settling velocity is
Check Re
The revised terminal settling velocity is
Therefore, vt = 0.09m/sec
Example 2: Estimate the terminal settling velocity in water at a temperature of 15ºc (µ =

0.00113Ns/m2) of spherical silicon particles with specific gravity 2.40 and average diameter of
(a) 0.05mm and (b) 1.0mm

Design Aspects of Sedimentation Tanks (also called clarifiers)

In practice, settling of the particles is governed by the resultant of horizontal velocity of water
and the vertical downward velocity of the particle. The path of the settling particle is as shown
in Figure 8.
The critical particle in the settling zone of an ideal rectangular sedimentation tank, for design
purposes, will be one that enters at the top of the settling zone and settles with a velocity just
sufficient to reach the sludge zone at the outlet end of the tank.
In an ideal sedimentation tank with horizontal or radial flow pattern, particles with settling
velocity less than vs (vt) can still be removed partially.
Figure 8: Horizontal and vertical velocity of settling of particles
The design aspects of sedimentary tanks are:

1. Velocity of flow
2. Capacity of tank
3. Inlet and outlet arrangements
4. Settling and sludge zones
5. Shapes of tanks
6. Miscellaneous considerations
(1) Velocity of flow: The velocity of flow of water in sedimentation tanks should be sufficient
enough to cause the hydraulic subsidence of suspended impurities. It should remain
uniform throughout the tank and it is generally not allowed to exceed 150mm to 300mm
per minute.
(2) Capacity of tank: Capacity of tank is calculated by
i) Detention period
ii) Overflow rate

i) Detention period: The theoretical time taken by a particle of water to pass between entry
and exit of a settling tank is known as the known as the detention period. The capacity of tank
is calculated by:
C = Q * T where C → Capacity of tank
Q → Discharge or rate of flow
T → Detention period in hours
The detention period depends on the quality of suspended impurities present in water. For plain
sedimentation tanks, the detention period is found to vary from 3 to 8 hours.
ii) Overflow Rate (Vo): In this method it is assumed that the settlement of a particle at the
bottom of the tank does not depend on the depth of tank and depends upon the surface area of
the tank.
Settling time =
Detention time = where,
To get the desired settling with most efficient tank size, tR = ts which occurs when Vo = Vs.
Where, L → Length of tank

W → Width of tank
Ap → Plan area of tank
Vs → Velocity of descend of a particle to the bottom of tank
Vo → Overflow rate or surface loading rate
V → Horizontal velocity of particle
It shows that all those particle having settling velocity greater than or equal to will be
removed. In fact, it was the case when the particle entering at full height H of the tank was
considered. Truly speaking, even the smaller particles having settling velocities ( ) lower than
will also settle down, if they happen to enter at some other height h of the tank. In that case,
when the particles are entering at some other height h of the tank, all those particles having
their settling velocities, Vs  will settle down.
The ratio of removal of this size particle to that of settling value is given by:
Where, Xr – the ratio of removal of the given size particle

Vs - the settling velocity of the given size particle
Vs - the settling velocity of the design/stated particle

For example:
Suppose the tank has 400m2 surface area and rate of inflow is 1.6m3/s, then
Hence,
- All particles having will be removed
- Only 50% of the particles having will be removed
- Only 25% of the particles having will be removed and so on.
It, therefore, follows that the quantity i.e. the discharge per unit of plan area is a very
important term for the design of settling tanks.
The efficiency of a sedimentation tank indicates the overall percentage removal of suspended
matter at a given overflow rate .
Prediction of efficiency of basin requires a settling column analysis from which the cumulative
frequency distribution curve may be obtained Figure 9.
Figure 9: Cumulative particles removal versus settling velocity curve
According to the equation, , the weight fraction removal of particles having

velocity < will be:
Hence, the overall fraction of particles removed, F, would be:
Approximation:
In which is the fraction with  .

(3) Inlet and Outlet Arrangements
Inlet zone
The inlet is a device, which is provided to distribute the water inside a tank. The two primary
purposes of the inlet zone of a sedimentation basin are to distribute the water and to control the
water's velocity as it enters the basin. In addition, inlet devices act to prevent turbulence of the
water. The incoming flow in a sedimentation basin must be evenly distributed across the width
of the basin to prevent short-circuiting. Short-circuiting is a problematic circumstance in which
water bypasses the normal flow path through the basin and reaches the outlet in less than the
normal detention time. It is undesirable since it may result in shorter contact, reaction or
settling times in comparison with the presumed detention time.
Outlet Zone
The outlet zone controls the water flowing out of the sedimentation basin - both the amount of
water leaving the basin and the location in the basin from which the out flowing water is
drawn. Like the inlet zone, the outlet zone is designed to prevent short-circuiting of water in the
basin. In addition, a good outlet will ensure that only well-settled water leaves the basin and
enters the filter. The outlet can also be used to control the water level in the basin.
Outlets are designed to ensure that the water flowing out of the sedimentation basin has the
minimum amount of particle or floc suspended in it. The best quality water is usually found at
the very top of the sedimentation basin, so outlets are usually designed to skim this water off
the sedimentation basin.
Outlet arrangement consists of
(i) weir, notches or orifices
(ii) effluent trough or launder
(iii) outlet pipe
Weir loading rates are limited to prevent high approach velocities near the outlet. Weirs
frequently consist of V-notches approximately 50mm in depth, placed 150 – 300mm on
centers, with a baffle in front of the weir to prevent floating material from escaping the
sedimentation basin and clogging the filters.
Figure 10: Zones of sedimentation tank

(4) Settling and Sludge Zones

Settling Zone
After passing through the inlet zone, water enters the settling zone where water velocity is
greatly reduced. This is where the bulk of particle or floc settling occurs and this zone will
make up the largest volume of the sedimentation basin. For optimal performance, the settling
zone requires a slow, even flow of water.
The settling zone may be simply a large expanse of open water. But in some cases, tube settlers
and lamella plates are included in the settling zone. Tube settlers and lamella plates increase the
settling efficiency and speed in sedimentation basins. Each tube or plate functions as a
miniature sedimentation basin, greatly increasing the settling area. Tube settlers and lamella
plates are very useful in plants where site area is limited, in packaged plants, or to increase the
capacity of shallow basins.
Sludge Zone
The sludge zone is found across the bottom of the sedimentation basin where the sludge
collects temporarily. Velocity in this zone should be very slow to prevent re-suspension of
sludge.
A drain at the bottom of the basin allows the sludge to be easily removed from the tank. The
tank bottom should slope toward the drains to further facilitate sludge removal. In some plants,
sludge removal is achieved continuously using automated equipment. In other plants, sludge
must be removed manually. If removed manually, the basin should be cleaned at least twice per
year or more often if excessive sludge buildup occurs. Many plants have at least two
sedimentation basins so that water can continue to be treated while one basin is being cleaned,
maintained, and inspected.
(5) Shapes of Tanks

Following are the three shapes of settling tank.
a) Rectangular tanks with horizontal flow
b) Circular tanks with radial or spiral flow
The detention time for a circular tank
c) Hopper bottom tanks with vertical flow

Figure 11: Typical sedimentation tanks: (a) rectangular horizontal flow tank; (b) circular,
radial-flow tank; (c) hopper-bottomed, upward flow tank

The following are the parameters for satisfactory performance of sedimentation tank.
1. Detention period ….. 3 to 4 hours for plain settling
2 to 2.5 hours for floc settling
1 to 1.5 hours for vertical flow type
2. Overflow rate ……… 15 - 30 m3/m2/day for plain settling
30 - 40m3/m2/day for horizontal flow
40 - 50m3/m2/day for vertical flow
3. Velocity of flow…….. 0.5 to 1.0 cm/sec
4. Weir loading………... 300m3/m/day
5. L:W …………………. 3:1 to 5:1
Breadth of tank…….. (10 to 12m) to 30 to 50m
6. Depth of tank………. 2.5 to 5m (with a preferred value of 3m)
7. Diameter of circular tank…. up to 60m
8. Solids removal efficiency….. 50%
9. Turbidity of water after sedimentation ….. 15 to 20 NTU
10. Inlet and outlet zones………. 0.75 to 1.0m
11. Free board…………………… 0.5m
12. Sludge Zone…………………. 0.5m
Overview of design calculations

Determine the surface area, dimensions, and volume of the sedimentation tank as well as the
weir length. Design of a sedimentation basin involves the following steps:
1. Divide flow into at least two tanks.
2. Calculate the required surface area.
3. Calculate the required volume.
4. Calculate the tank depth.
5. Calculate the tank width and length.
6. Check flow through velocity.
7. If velocity is too high, repeat calculations with more tanks.
8. Calculate the weir length.
Example 2:
A water treatment plant has four clarifiers treating 0.175 m3/s of water. Each clarifier is 4.88m
wide, 24.4m long and 4.57m deep. Determine: (a) the detention time, (b) overflow rate, (c)
horizontal velocity, and (d) weir loading rate assuming the weir length is 2.5 times the basin
width.

Exercise 1:
Find the settling velocity (vs) for sand particles with a diameter of 0.02mm. ρp= 2650 kg/m3, µ=
1.002*10‐3Ns/m2 at 20°C.
Exercise 2:
Design plain sedimentation tanks to be used to remove grit and sand from river water that is
used to produce 20,000m3/d drinking water. Use the overflow rate and horizontal velocities
calculated in exercise 1. Use two tanks.
Exercise 3:
Repeat Exercise 2 using circular tank. Note: the maximum tank diameter is 40
Example 3:
A clarifier is designed to have a surface overflow rate of 28.53 m3/m2/d. Estimate the overall
removal with the settling analysis data and particle size distribution in cols. 1 and 2 of Table 1.
The wastewater temperature is 15c and the specific gravity of the particles is 1.20.
Table 1: Results of settling analysis test and estimation of overall solid removal
Particle size Weight fraction Settling velocity Weight fraction
in mm > size, % in mm/sec < size, percent
0.1 12 0.968 88
(88% of the particles pass the 0.10 sieve)
0.08 18 0.620 82
0.07 35 0.475 65
0.06 72 0.349 28
0.05 86 0.242 14
0.04 94 0.155 6
(6% of the particles pass the 0.04 sieve)
0.02 99 0.039 1
0.01 100 0.010 0
Answer: F = 92%

Example 4:
Given: A settling basin is designed to have a surface overflow rate of 32.6 m/day = 0.37mm/s
Find: The overall removal obtained for a suspension with the size distribution given below. The
specific gravity of the particles is 1.2 and T = 20C,  = 1.027*10-3Ns/m2,  = 1000kg/m3.
Table 2: Results of settling analysis test and estimation of overall solid removal
Particle size, mm 0.10 0.08 0.07 0.06 0.04 0.02 0.01
Weight fraction 10 15 40 70 93 99 100
greater than size, 40% of the 100% of the
percent particles > 0.07 particles > 0.01
Weight fraction 90 85 60 30 7 1 0
less than size, 90% of the 7% of the
percent particles pass particles pass
the 0.10 sieve the 0.04 sieve
Answer: F = 90%

5. COAGULATION AND FLOCCULATION

5.1 Coagulation
The hydraulic settling values of small size particles in water are very small and therefore, they
require longer time to settle in plain sedimentation tanks. For example, a slit particle of size
0.05mm will require about 11hrs to settle down through a depth of 3m and clay particle of size
0.002mm will require about 4 days’ time to settle the same height of 3m at normal temperature
of about 25ºc. Moreover, water may be containing colloidal impurities which are even finer
than 0.0001mm and which also carry electrical charge on them. Due to electrical charge they
remain continuously in motion and never settle down by gravity in water. Therefore, when
water is turbid due to presence of such fine size and colloidal impurities, plain sedimentation is
of no use.
For dealing water with such impurities a chemical process was evolved. This process removes
all these impurities within reasonable period of 2 – 3hrs. This chemical process is called
coagulation and the chemical used in the process is called coagulant. The objective of
coagulation is to unit several colloidal particles together to form bigger sized settable flocs
which may settle down in the tank.
The principle of coagulation can be explained from the following two conditions:
1. Floc formation
When coagulants (chemicals) are dissolved in water and thoroughly mixed with it, they
produce a thick gelatinous precipitate. This precipitate is known as floc and this floc has got the
property of arresting suspended impurities in water during downward travel towards the bottom
of tank. The gelatinous precipitate has therefore, the property of removing fine and colloidal
particles quickly.
2. Electric charges
Most particles dissolved in water have a negative charge, so they tend to repel each other. As a
result, they stay dispersed and dissolved or colloidal in the water.
The purpose of most coagulant chemicals is to neutralize the negative charges on the
turbidity particles to prevent those particles from repelling each other. The amount of coagulant
which should be added to the water will depend on the zeta potential, a measurement of the
magnitude of electrical charge surrounding the colloidal particles. You can think of the zeta
potential as the amount of repulsive force which keeps the particles in the water. If the zeta
potential is large, then more coagulants will be needed.

Coagulants tend to be positively charged. Due to their positive charge, they are attracted to the
negative particles in the water, as shown below.
Figure 1: Negatively charged particles Figure 2: Positively charged coagulants attract to

repel each other due to electricity negatively charged particles due to electricity
The combination of positive and negative charge results in a neutral. As a result, the particles
no longer repel each other. The next force which will affect the particles is known as van der
Waal's forces. Van der Waal's forces refer to the tendency of particles in nature to attract each
other weakly if they have no charge.
Figure 3: Neutrally charged particles attract due to van der Waal's forces
Once the particles in water are not repelling each other, van der Waal's forces make the
particles drift toward each other and join together into a group. When enough particles have
joined together, they become floc and will settle out of the water.
Figure 4: Particles and coagulants join together into floc
Destabilization of Colloidal Dispersion

Colloids persist as small particles because they carry negative surface charge and therefore
repel each other. Colloids, by definition, do not settle and colloid removal requires that they
should be agglomerated in to large particles. This requires surface charge to be destabilized.
Particle destabilization can be achieved by four mechanisms:
1. Double layer compression
Addition of electrolyte to water shrinks the layer of charged ions around the particle. If reduced
enough, the attractive Van der Waals force (which acts close to particle) can overcome
repulsive electrical force.

Diffused double layer created by cations attaching to negatively charged particles (fixed layer)
and cations and anions loosely attaching in outer diffused layer.
Figure 5: A negatively colloidal particle with its electrostatic field

2. Adsorption and charge neutralization
Adding positively charged ions that adsorb to particle surface can reduce surface charge
and repulsion
3. Entrapment in precipitate
Al and Fe salts added at right pH will precipitate as flocs with colloids as nuclei.
4. Particle bridging
Large organic molecules (both anionic and caionic) attach to multiple particles ‘bridging’
them (often used in addition to metal salts)
Once particles are coagulated, they can be flocculated.

Factors affecting coagulation:

1. Type of coagulant
2. Dose of coagulant
3. Characteristic of water
a) Type and quantity of suspended matter
b) Temperature of water
c) pH of water
4. Time and method of mixing
Common Coagulant Types

Coagulant chemicals come in two main types - primary coagulants and coagulant
aids. Primary coagulants neutralize the electrical charges of particles in the water which
causes the particles to clump together. Coagulant aids add density to slow-settling flocs and
add toughness to the flocs so that they will not break up during the mixing and settling
processes and are generally used to reduce flocculation time. Lime, calcium carbonate and
bentonite are examples of coagulant aids.
In water treatment plants, the following are the coagulants most commonly used:
i. Aluminum sulfate [Al 2(SO4) 3.18H2O]
It is also called alum. It is the most widely used chemical coagulant in water purification work.
Alum reacts with water only in the presence of alkalinity. If natural alkalinity is not present,
lime may be added to develop alkalinity. It reacts with alkaline water to form aluminum
hydroxide (floc), calcium sulphate and carbon dioxide. Due to the formation of calcium
sulphate, hardness and corrosiveness of water is slightly increased.
Chemical reaction taking place

i) Al 2(SO4)3.18H2O + 3Ca (HCO3) 2  2Al(OH) 3 + 3CaSO4 + 6CO2 +18H2O
ii) Al2(SO4)3.18H2O + 3Ca(OH)2  2Al(OH)3 + 3CaSO4 + 18H2O
iii) Al2 (SO4)3.18H2O + 3Na2CO3  2Al(OH)3 + 3Na2SO4 + 3CO2 + 18H2O
The chemical is found to be most effective between pH ranges of 6.5 to 8.5. Aluminum
hydroxide is relatively insoluble within this range. Its dose may vary from 5 to 30mg/lit, for
normal water usually dose being 14mg/l. Actually, dose of coagulant depends on various
factors such as turbidity, colour, taste, pH value, temperature etc.
Due to the following reason, alum is the most widely used chemical coagulant.
1. It is very cheap
2. It removes taste and color in addition to turbidity
3. It is very efficient

4. Flocs formed are more stable and heavy

5. It is not harmful to health
6. It is simple in working, doesn’t require skilled supervision for dosing
ii. Sodium aluminates (Na2Al2O4)

In the process of coagulation, it can remove carbonate and non-carbonate hardness. It reacts
with calcium and magnesium salts to form flocculent aluminates of these elements.
Chemical reactions:
i) Na2Al2O4 + Ca (HCO3) 2  CaAl2O4 + Na2CO3 + CO2 + H2O
ii) Na2Al2O4 + CaSO4  CaAL2O4 + Na2SO4
iii) Na2Al2O4 + CaCl2  CaAl2O4 + 2NaCl
The pH should be within the range of 6 and 8.5.
iii. Chlorinated Copperas
Combination of Ferric sulphate and Ferric chloride
When solution of Ferrous sulphate is mixed with chlorine, both Ferric sulphate and Ferric
chloride are produced.
6FeSO4.7H2O + 3Cl2 2Fe3(SO4)2 + 2FeCl3 + 42H2O
Ferric sulphate and Ferric chloride each is an effective floc and so also their combination. Both
Ferric sulphate and Ferric chloride can be used independently with lime as a coagulant. If
alkalinity is insufficient, lime is added.
Chemical reaction taking place
2FeCl3 + 3Ca(OH)2  2Fe(OH)3 + CaCl2
Fe2(SO4)3 + 3Ca(OH)2  2Fe(OH)3 + 3CaSO4
Ferric chloride effective pH range 3.5 – 6.5 or above 8.5 and Ferric sulphate is effective with
pH range of 4 – 7 or above 9.
iv. Polyelectrolytes
They are special types of polymers. They may be anionic, cationic, and non-ionic depending
upon the charge they carry. Out of these only cationic polyelectrolytes can be used
independently as effective coagulants while others are used as coagulant-aids along with alum.
Uses:
- Broaden the pH range over which satisfactory flocculation could occur
- Reduce the quantity of primary coagulant required
Dosage: usually 1mg/l

Example 3:
Find out the quantity of alum required to treat 18 million liters of water per day. The dosage of
alum is 14mg/lit. Also work out the amount of CO2 released per liter of treated water.
Solution:
The chemical reaction of alum is given by:

Al 2(SO4)3.18H2O + 3Ca(HCO3) 2  2Al(OH) 3 + 3CaSO4 + 6CO2 +18H2O
Calculate the molecular weight of alum and carbon dioxide:
Molecular weight of alum (Al 2(SO4)3.18H2O) = 2*27 + 3*32 + 16* (4*3 + 18) + 36*1 = 666
Molecular weight of carbon dioxide: (6CO2) = 6*44 = 264
666 mg of alum releases 264 mg of carbon dioxide.
Therefore, 252 kg of alum will release
Dose of coagulant
The water is tested to determine what sort and amount of treatment will be required. Then
chemicals are added to the water in a set amount per day.
The dosage is the required concentration of the chemical within the water (mg/l). Tests of
water characteristics determine the required dosage of each chemical. A higher dose means that
more chemical is getting into the water.
The chemical feed is the amount of chemical you add to the water (mg/day). The chemical
feed rate is typically set on a chemical feeder of some sort so that the chemical is automatically
added to the water throughout the day. A higher chemical feed rate means that chemicals are
being added to the water more quickly.
Feeding of coagulant
In order to feed chemicals to the water regularly and accurately, some type of feeding
equipment must be used.
Coagulants may be put in raw water either in powder form or in solution form.
1. Dry-Feed Type
Dry powder of coagulant is filled in the conical hopper. The hoppers are fitted with agitating
plates which prevent the chemical from being stabilized. Agitating plates are used to prevent

arching of chemicals. Feeding is regulated by the speed of toothed wheel or helical spring
(Figure 6). Activated carbon and lime are added to raw water in powder form.
Figure 6: Dry feeding devices

Chemical feed rate of a dry chemical is given by:
2. Wet feeding type

First, solution of required strength of coagulant is prepared. The solution is filled in the tank
and allowed to mix in the mixing channel in required proportion to the quantity of water. It can
be easily controlled with automatic devices.
Mixing devices
The process of floc formation greatly depends upon the effective mixing (rapid mixing) of
coagulant with the raw water.
Rapid mixing of the mixture of coagulant and raw water is used to:
- Disperse chemicals uniformly throughout the mixing basin
- Allow adequate contact between the coagulant and particles
- Formation of microflocs
The mixing is done by mixing device.
1. Hydraulic jump - flume with considerable slope is developed
2. Pump method - centrifugal pump is used to raise raw water
3. Compressed air method – compressed air is diffused from bottom of the mixing tank
4. Mixing channels
Mixing of raw water and coagulant is made to pass through the channel in which flume
has been done. Vertical baffles are also fixed at the end of the flumed part on both sides
of the channel (Figure 7).

Figure 7: Mixing channel
5. Mixing basin with baffle wall

6. Mechanical mixing basins
Mechanical means are used to agitate the mixture to achieve the objective of thorough
mixing. Flash mixers and deflector plate mixers are used.
A. Flash mixer
The mixing of coagulant in water is achieved by rotating vigorously fans fixed in the mixing
basin. The deflecting wall avoids short-circuiting and deflects the water flow towards the fan
blades. Chemical pipe discharges the coagulant just near the rotating fan (Figure 8).
Figure 8: Flash mixer
Design criteria of flash mixer:

1. Detention period – 30 to 60 sec
2. Velocity of flow – 0.9 m/sec
3. Depth – 1 to 3m
4. Rotation per minute of blade – 100
5. Power required – 0.041kW/1000m3/day

B. Deflector plate mixer

Mixing is achieved by diffusing water through a deflection plate. Water enters from inlet pipe
and comes out through a hole provided below the deflector plate where it gets agitated.
Chemical pipe discharges the coagulant just near the deflector plate, where it gets thoroughly
mixed with water (Figure 9).
Figure 9: Deflector plate mixer

The formula used to determine the chemical feed rate of liquid chemicals is shown below:
Example 1: If you added 230 mg of polymer to 100 ml of water, you would have a liquid
polymer solution with a concentration of 2.3 mg/ml.
Example 2: Consider a situation in which the flow of water into the plant is 5 MGD and the
alum dosage is 10 mg/l. The liquid alum has a concentration of 520 mg/ml. The chemical
feeder setting would be determined as follows:
The setting on the liquid alum feeder should be 252 ml/min.
5.2 Flocculation
After adding the coagulant to the raw water, rapid agitation is developed in the mixture to
obtain a thorough mixing. Next to rapid mixing, mixture is kept slowly agitated for about 30 to
60 minute. Slow mixing process in which particles are brought into contact in order to promote

their agglomeration is called flocculation. The tank or basin in which flocculation process is
carried out is called flocculation chamber.
Flocculation occurs by:
1. Brownian motion (the random motion of particles suspended in a fluid resulting from
their collision) - important for small particles (< 0.5m) --- Perikinetic flocculation
2. Stirring– mechanical stirring strong enough to cause particle collisions but not so
strong as to break up particles. --- Orthokinetic flocculation
3. Differential settlement – larger, faster particles catch up with smaller, slower particles
The velocity of flow in the flocculation chamber is kept between 12 – 18cm/sec. Activated
carbon in powder form can be used to speed up the flocculation.
The rate of agglomeration or flocculation is dependent upon:
- Type and concentration of turbidity
- Type of coagulant and its dose
- Temporal mean velocity gradient caused by mixing – G in the basin
Velocity Gradient
- The mean velocity gradient is the rate of change of velocity per unit distance normal to
the section - (meter per second per meter) (T-1).
- The velocity of water flowing through the flocculation basin must be within a very
specific range, designed to gently mix the water without breaking apart the floc.
- Measurement of the intensity of mixing in the chamber.
- Determines how much the water is agitated in the tank.
- Determines how much energy is used to operate the flocculator.
The value of G can be computed in terms of power input by the following equation:
Where P – power dissipated (watt)

µ - absolute viscosity (Ns/m2)
V - the volume to which P is applied (m3)
G - temporal mean velocity gradient caused by mixing (s-1)
Power can also be calculated by:
Where: P = power input, Watt or Nm/s

FD = drag force on paddles, N
vp = velocity of paddles (velocity relative to the water), m/s

Where:
CD = coefficient of drag, 1.8 for flat blades
Ap = area of paddle blades, m2
= density of water, kg/m3
Therefore,
The flocculation technique most commonly used involves mechanical agitation with rotating
paddle wheels or vertical mounted turbines (Figure 10).
Figure 10: Flocculator

The design criteria of a horizontal continuous flow rectangular basin flocculator:
Depth of tank : 3 – 4.5m
Detention time : 30 – 45 minute; normal: 30 minute
Velocity of flow : 0.2 – 0.8m/s; normal: 0.4m/s
Total area of paddles : 10 – 25% of cross-sectional area of the tank
Peripheral velocity of blades : 0.2 – 0.6m/s; normal: 0.3 – 0.4m/s
Velocity gradient (G) : 10 – 75 s-1
Factor G*t : 104 - 105
Power consumption : 49 – 196 kW/Mm3/d
Outlet flow velocity : 0.15 – 0.25 m/s
5.2.1 Clarifier (Secondary Sedimentation tank)

After flocculation, water enters the settling tank which is normally called a clarifier. Water is
retained in the sedimentation tank for a sufficient period to permit the settlement of the floc to
the bottom. The principle of design of clarifier is the same as for plain sedimentation basin

except that its detention period is lower. The detention period commonly adopted is 2 to 2.5hrs
with an overflow rate of 1 to 1.2 m/hr.
Section of sedimentation unit consisting of flash mixer, flocculator and settling basin (Fig. 11)
Figure 11: Section of secondary sedimentation unit
Flocculent settling (Type II settling)

Flocculent particles resulting from coagulation will agglomerate while settling with a resultant
increase in particle size. The density of the composite particle will decrease due inclusion of
water, however, the settling velocity will increase. (0.1 to 3mm best floc size)
The clarification of dilute suspensions of flocculating particle is a function of:
- Settling property of the particles
- Flocculating characteristic of the suspension
Example 4:
Design a settling tank (coagulation–sedimentation) with continuous flow for treating water for
a population of 48,000 persons with an average daily consumption of 135lit/head. Take
detention period of 2.5hrs and maximum day factor of 1.8.
Example5:
Design a conventional vertical-shaft rapid mix tank unit for uniformly dispersing coagulant in
10 MLD of settled raw water as per design parameters given below:
Detention time (t): 20 – 60 s
Ratio of tank height (H) to diameter (D): (1:1 to 1:3)
Ratio of impeller diameter (DI) to tank diameter (D): (0.2:1 to 0.4:1)
Velocity gradient (G): > 300s-1
Gt: 10000 – 20000
Tank diameter (D): <3m
Paddle tip speed (vp): 1.75 – 2.0 m/s
Velocity of paddle relative to water (v): 0.75 * paddle tip speed

Paddle area (Ap)/Tank section area (AT): 10:100 – 20:100

Coefficient of drag on impeller blade (CD): 1.8
Maximum length of each impeller blade (L): 0.25 * impeller diameter
Maximum width of impeller blade (B): 0.20 * impeller diameter
Impeller height from bottom (HB): 1.0 * impeller diameter
Kinematic viscosity: 1.003*10-6 m2/s
Dynamic viscosity of water: 1.002 * 10-3 Ns/m2
Determine tank dimensions (provide a freeboard of 0.5 m), impeller diameter, paddle
dimensions, number of paddles, clearance of the impeller from tank bottom, paddle rotation
speed and power input requirement.
Solution:
Let the detention time (t) be 40s
Therefore, volume of tank (V)
Let the tank diameter (D) be 2 m

Tank cross-sectional area (Acs)
Tank height (H) = 1.5m, provide freeboard of 0.5m

Total height (HT) = 2 m
Tank height (H) to tank diameter (D) ratio
(within the range of 0.33 – 1.0, hence ok!)

Let velocity gradient, G = 400 s-1
Therefore,
[10000, 20000], hence ok!
Let the paddle tip speed (vp) be 1.8 m/s [1.75, 2.0 m/s]
Hence, velocity of paddle relative to water (v)
To calculate the paddle area the following empirical formula can be used:

Tank sectional area = (D)*(H) = (1.5)*(2) = 3.0m2
[0.1, 0.2], hence ok!

Let the impeller diameter be 0.8 m, i.e.,
Choose length of each impeller blade (L) as 0.20m, i.e.,
Choose breadth of each impeller blade (B) as 0.15m, i.e.,
Area of each blade (Ab)
Therefore, number of blades (nb) to be provided
Clearance of the paddles from the tank bottom = 0.8m

Paddle rotation speed (w, radians/s)
Power requirement is given by:
P = 74.2 kW/Mm3/d within the range of 49 – 196 kW/Mm3/d

Exercise:
1. Design a conventional rectangular horizontal-shaft flocculation tank unit for 10 MLD of
settled raw water after coagulant addition and rapid mixing as per design parameters given
below:
Detention time (t): 10 – 30 minutes
Velocity gradient (G): 20 – 75 s-1
Gt: 2*104 – 6*104
Tank Depth (D): <5m
Paddle tip speed (vp): 0.25 – 0.75 m/s
Velocity of paddle relative to water (v): 0.75 * paddle tip speed
Paddle area (Ap)/Tank section area (AT): 0.1 – 0.2
Coefficient of drag on paddle blade (CD): 1.8
Maximum length of each paddle (L): 5.0 m
Maximum width of each paddle (b): 0.50 m
Kinematic viscosity: 1.003*10-6 m2/s
Dynamic viscosity of water: 1.002*10-3 Ns/m2
Freeboard: 0.50 m
Draw a net sketch of the designed tank (top and front view) clearly showing tank dimensions,
paddle shaft position, paddle blade dimensions, water level, etc. Also mention paddle rotation
speed and power requirement.
Exercise 2:
A mechanical flocculator is used to treat 38,000m3/day water with detention time 20 minutes.
a) Design the dimension of the tank if L : W : d = 1 : 4 : 2
b) Find the power required when velocity gradient is 55s-1 and dynamic viscosity 1.002*
10-3 Ns/m2
c) If the tank have 3 paddles and every paddle have 4 plate with relative velocity of
paddles is 0.38 m/s and coefficient drag is 1.8, find the area of 1 plate.

Jar Test
Purpose
To determine the optimum concentration of coagulant to be added to the source water
In many water treatment plants, changing water characteristics require the operator to adjust
coagulant dosages at intervals to achieve optimal coagulation. Different dosages of coagulants
are tested using a jar test, which mimics the conditions found in the treatment plant. The first
step of the jar test involves adding coagulant to the source water and mixing the water rapidly
(as it would be mixed in the flash mix chamber) to completely dissolves the coagulant in the
water. Then the water is mixed more slowly for a longer time period, mimicking the
flocculation basin conditions and allowing the forming floc particles to cluster together.
Finally, the mixer is stopped and the floc is allowed to settle out, as it would in the
sedimentation basin.
The type of source water will have a large impact on how often jar tests are performed. Plants
which treat groundwater may have very little turbidity to remove are unlikely to be affected by
weather-related changes in water conditions. As a result, groundwater plants may perform jar
tests seldom, if at all, although they can have problems with removing the more difficult small
suspended particles typically found in groundwater. Surface water plants, in contrast, tend to
treat water with a high turbidity which is susceptible to sudden changes in water quality.
Operators at these plants will perform jar tests frequently, especially after rains, to adjust the
coagulant dosage and deal with the changing source water turbidity.
Figure 11: Jar test apparatus
Procedure
1. Decide on six dosages of the chemical(s).
Use the chemicals in use at the treatment plant. These chemicals may include
coagulants, coagulant aids, and lime.

The dosages should be in a series with the lowest dosage being lower than the dosage
currently used in the plant and the highest dosage being higher than the dosage currently
used in the plant.
If pre-lime has to be fed, it is usually best to hold the amount of lime constant and vary
the coagulant dosage.
2. Prepare a stock solution of the chemical(s).
You will need to prepare a stock solution for each type of chemical used. The strength
of the stock solution will depend on the chemical dosages which you decided to use in
step 1. The table below shows what strength stock solution you should prepare in each
circumstance.
Approximate dosage Stock solution 1ml added to
required, mg/l concentration, mg/l 1L sample equals
1-10 mg/l 1,000 mg/l 1 mg/l
10-50 mg/l 10,000 mg/l 10 mg/l
50-500 mg/l 100,000 mg/l 100 mg/l
For example, if all of your dosages are between 1 and 10 mg/L, then you should prepare
a stock solution with a concentration of 1,000 mg/L. This means that you could prepare
the stock solution by dissolving 1,000mg of the chemical in 1L of distilled
water. However, this would produce a much larger quantity of stock solution than you
need and would waste chemicals. You will probably choose instead to dissolve 250 mg
of the chemical in 250 mL of distilled water.
3. Collect eight liter sample of the water to be tested. This should be the raw water.
4. Measure 1,000 ml of raw water and place in a beaker. Repeat for the remaining beakers.
5. Place beakers in the stirring machine.
6. With a measuring pipet, add the correct dosage of lime and then of coagulant solution to
each beaker as rapidly as possible.
7. With the stirring paddles lowered into the beakers, start the stirring machine and operate
it for one to three minute at a speed of 80 RPM. While the stirrer operates, record the
appearance of the water in each beaker. Note: the presence or absence of floc, the
cloudy or clear appearance of water, and the color of the water and floc.
The stirring equipment should be operated as closely as possible to the conditions in the
flash mix and/or flocculation facilities of the plant. Mixing speed and time may vary at
your plant from the times and speeds listed in this and the following step.

8. Reduce the stirring speed to 20 RPM and continue stirring for 30 minutes. Record a
description of the floc in each beaker 5, 10, 15, 20, 25, and 30 minutes after addition of
the chemicals.
9. Stop the stirring apparatus and allow the samples in the beakers to settle for 30 minutes.
Record a description of the floc in each beaker after 15 minutes of settling and again
after 30 minutes of settling.
10. Determine which coagulant dosage has the best flocculation time and the most floc
settled out. This is the optimal coagulant dosage.
A hazy sample indicates poor coagulation. Properly coagulated water contains floc
particles that are well-formed and dense, with the liquid between the particles clear.
11. Test the turbidity of the water in each beaker using a turbidometer.
12. If lime or a coagulant aid is fed at your plant in addition to the primary coagulant, you
should repeat the jar test to determine the optimum dosage of lime or coagulant aid. Use
the concentration of coagulant chosen in steps 10 and 11 and alter the dosage of lime or
coagulant aid.
13. Using the procedure outlined in step 11, measure the turbidity of water at three
locations in the treatment plant - influent, top of filter, and filter effluent.

6. FILTRATION
In this chapter we will answer the following questions:
How does filtration fit into the water treatment process?
How does filtration clean water?
What types of filters are used for water treatment?
How are filters cleaned?
What media are used in filters?
What factors affect filter efficiency?
The effluent obtained after coagulation does not satisfy the drinking water standard and is not
safe. So it requires further treatments. Filtration is one of the water purification process in
which water is allowed to pass through a porous medium to remove remaining flocs or
suspended solids from the previous treatment processes. In the filter, up to 99.5% of the
suspended solids in the water can be removed, including minerals, floc, and microorganisms.
Filtration process assist significantly by reducing the load on the disinfection process,
increasing disinfection efficiency.
Theory of Filtration
Filtration consists of passing water through a thick layer of sand. During the passage of water
through sand, the following effects take place.
i) Suspended matter and colloidal matter are removed
ii) Chemical characteristic of water get changed
iii) Number of bacteria considerably reduced
These phenomena can be explained on the basis of the following mechanisms of filtration.
1. Mechanical straining
Mechanical straining of suspended particles in the sand pores. Passing the water through a
filter in which the pores are smaller than the particles to be removed.
Figure 1: Straining in a filter media

2. Sedimentation and Adsorption

- The interstices between the sand grains act as sedimentation basins in which the
suspended particles smaller than the voids in the filter-bed settle upon the sides of the
sand grains.
- The particles stick on the grains because of the physical attraction between the two
particles of matter and the presence of the gelatinous coating formed on the sand grains
by the previously deposited bacteria and colloidal matter. Adsorption is the gathering of
dissolved solids onto the surface of sand.
3. Electrolytic action
Due to the friction between medium and suspended solids, certain amount of dissolved and
suspended matter is ionized. Suspended matter in water is ionized, carries charge of one
polarity and the particles of sand in filter which are also ionized, possess electrical charges
of opposite polarity. These neutralize each other; change the chemical character of water.
4. Biological Action
The growth and life process of the living cells, biological metabolism. The surface layer
gets coated with a film in which the bacterial activities are the highest and which feed on
the organic impurities. The bacteria convert organic impurities by a complex biochemical
action into simple, harmless compounds – purification of water.
Rate of Filtration
Rate of filtration (loading rate) is the flow rate of water applied per unit area of the filter. It is
the velocity of the water approaching the face of the filter:
Where va = face velocity, m/d = loading rate, m3/d/m2

Q = flow rate onto filter surface, m3/d
As = surface area of filter, m2

Types of filters
Two types of filter:
1. Gravity filter system
i. Slow Sand Filter (SSF)
ii. Rapid Sand Filter (RSF)
2. Pressure filter system
i. Slow Sand Filters

The slow sand filter removes particles from the water through adsorption and straining. It also
removes a great deal of turbidity from water using biological action. A layer of dirt, debris, and
microorganisms builds up on the top of the sand. This layer is known as schmutzdecke, which
is German for "dirty skin." The schmutzdecke breaks down organic particles in the water
biologically, and is also very effective in straining out even very small inorganic particles from
water.
Slow sand filters are best suited for the filtration of water for small towns. The sand used for
the filtration is specified by the effective size and uniformity coefficient. The effective size,
D10, which is the sieve in mm that permits 10% sand by weight to pass. The uniformity
coefficient is calculated by the ratio of D60 and D10.
Construction
Slow sand filter is made up of a top layer of fine sand of effective size 0.2 to 0.3mm and
uniformity coefficient 2 to 3. The thickness of the layer may be 75 to 90 cm. Below the fine
sand layer, a layer of coarse sand of such size whose voids do not permit the fine sand to pass
through it. The thickness of this layer may be 30cm. The lowermost layer is a graded gravel of
size 2 to 45mm and thickness is about 20 to 30cm. The gravel is laid in layers such that the
smallest sizes are at the top. The gravel layer is used to retain the coarse sand layer and is laid
over the network of open jointed clay pipe or concrete pipes called under drainage. Water
collected by the under drainage is passed into the out chamber (Figure 11).

Figure 11: Section of slow sand filter
Operation
The water from sedimentation tanks enters the slow sand filter through a submersible inlet as
shown in figure 11. This water is uniformly spread over a sand bed without causing any
disturbances. The water passes through the filter media at an average rate of 2.4 to
3.6m3/m2/day. This rate of filtration is continued until the difference between the water level on
the filter and in the inlet chamber is slightly less than the depth of water above the sand. The
difference of water above the sand bed and in the outlet chamber is called the loss of head.
During filtration as the filter media gets clogged due to the impurities, which stay in the pores,
the resistance to the passage of water and loss of head also increases. When the loss of head
reaches 60cm, filtration is stopped and about 2 to 3cm from the top of bed is scrapped and
replaced with clean sand before putting back into service to the filter.
The scrapped sand is washed with the water, dried and stored for return to the filter at the time
of the next washing. The filter can run for 6 to 8 weeks before it becomes necessary to replace
the sand layer.
Uses
The slow sand filters are effective in removal of 98 to 99% of bacteria of raw water and
completely all suspended impurities and turbidity is reduced to 1 NTU. Slow sand filters also
removes odours, tastes and colours from the water but not pathogenic bacteria which requires
disinfection to safeguard against water-borne diseases. The slow sand filter requires large area
for their construction and high initial cost for establishment. The rate of filtration is also very
slow.

Maintenance
The algae growth on the overflow weir should be stopped. Rate of filtration should be
maintained constant and free from fluctuation. Filter head indicator should be in good working
condition. Trees around the plant should be controlled to avoid bird droppings on the filter bed,
No coagulant should be used before slow sand filtration since the floc will clog the bed quickly.
ii. Rapid Sand Filter

The rapid sand filter differs from the slow sand filter in a variety of ways, the most important of
which are the much greater filtration rate ranging from 100 to 150m3/m2/day, the ability to
clean automatically using backwashing and require small filter area. The mechanism of particle
removal also differs in the two types of filters - rapid sand filters do not use biological filtration
and depend primarily on adsorption and some straining.
The main features of rapid sand filter are as follows

Effective size of sand - 0.45 to 0.70mm
Uniformity coefficient of sand - 1.2 to 1.7
Depth of sand - 60 to 75cm
Filter gravel - 2 to 50mm size (Increase size towards bottom)
Depth of gravel - 45cm
Depth of water over sand during filtration - 1 to 2m
Overall depth of filter including 0.5m free board - 2.5m
Area of single filter unit - 100m2 in two parts of each 50m2
Loss of head - max 1.8 to 3.0m
Turbidity of filtered water - 1 NTU
Operation
The water from coagulation sedimentation tank enters the filter unit through inlet pipe and
uniformly distributed on the whole sand bed. Water after passing through the sand bed is
collected through the under drainage system in the filtered water well. The outlet chamber in
this filter is also equipped with filter rate controller.
Three factors can be used to assess when a filter needs backwashing. Some plants use the
length of the, filter run, arbitrarily scheduling backwashing after 24 - 48 hours or some other
length of filter operation. Other plants monitor turbidity of the effluent water and head loss
within the filter to determine when the filter is clogged enough to need cleaning.

Headloss is a loss of pressure (also known as head) by water flowing through the filter. When
water flows through a clogged filter, friction causes the water to lose energy, so that the water
leaving the filter is under less pressure than the water entering the filter. Head loss is displayed
on a headloss gauge. Once the headloss within the filter has reached between six and ten hours,
a filter should be backwashed.
The Rose equation is used to determine the headloss resulting from the water passing through
the filter medium. It is applicable to rapid sand filters with a uniform near spherical or spherical
medium. It is given by:
Where, hl - frictional head loss in filter depth L, m

v - approach velocity, m/s
CD - drag force coefficient
d - diameter of sand grains, m
 - shape factor (round sand 0.82, angular sand 0.73, pulverized coal 0.73)
 - porosity, dimensionless
Washing of Filter
Washing of filter is done by the back flow of water through the sand bed as shown in Figure 12.
First the valve ‘V1’ is closed and the water is drained out from the filter leaving a few
centimeter depth of water on the top of sand bed. Keeping all values closed the compressed air
is passed through the separate pipe system for 2 - 3 minutes, which agitates the sand bed and
stirrer it well causing the loosening of dirt, clay etc. inside the sand bed.
Now valve ‘V4’ and ‘V5’ are opened gradually, the wash water tank, rises through the laterals,
the strainers gravel and sand bed. Due to back flow of water the sand expands and all the
impurities are carried away with the wash water to the drains through the channels, which are
kept for this purpose.
Washing process is continued till the sand bed appears clearly. The washing of filter is done
generally after 24 - 48hours and it takes 10 minutes and during backwashing the sand bed
expands by about 50%.

Figure 12: Rapid sand filter
Construction
Rapid sand filter consists of the following five parts:
1. Enclosure tank – A water tight tank is constructed either masonry or concrete
2. Under drainage system – may be perforated pipe system or pipe and stracher system
3. Base material – gravel should free from clay, dust, silt and vegetable matter. Should be
durable, hard, round and strong and depth 40cm.
4. Filter media of sand – The depth of sand 60 to 75cm
5. Appurtenances – Air compressors useful for washing of filter and wash water troughs for
collection of dirty water after washing of filter.
Rapid sand filter bring down the turbidity of water to 1 NTU. This filter needs constant and
skilled supervision to maintain the filter gauge, expansion gauge and rate of flow controller and
periodical backwash.

Comparison of slow sand filter and rapid sand filter

Table 4.1 Comparison of slow sand filter and rapid sand filter
No Parameter SSF RSF
Not more than 10 NTU hence
1 Raw water turbidity Not more than 30 NTU
coagulation is required
2 Rate of filtration 2.4 – 3.6 m3/m2/d 100 – 150 m3/m2/d
3 Loss of head 0.6 – 0.7m 1.8 – 3.0m
4 Filter material d10 = 0.15 - 0.35mm d10 = 0.45 – 0.7mm
Uc <= 2 - 3 Uc <= 1.2 – 1.7
Only relatively fine media Sand, anthracite, coal,
used, usually sand magnetite, plastics,
5 Filter depth 0.6 - 1.5m 0.5 - 2.0m
6 Construction cost High Low
7 Land requirement Large Small (20% of SSF)
8 Operational cost Very low high
9 Equipment and accessories Few, simple Many and sophisticated
10 Level of sophistication Low Very high
11 Filtration Mechanism Biological action, straining, Primarily adsorption. Also some
and adsorption. straining.
12 Cleaning of filter Scraping of 2.5 cm thick layer Backwash with clean water
washing and replacing. under pressure to detach the dirt
Cleaning interval that is on the sand.
replacement of sand at 1 to 2 Backwashing daily or on
months alternate days.
13 Efficiency Bacterial removal There is no removal of bacteria.
Taste, odour, colour and Removal of colour, taste, odour
turbidity removal and turbidity is good
14 Common Applications Small groundwater systems. Most commonly used type of
filter for surface water treatment.
iii. Pressure Filter
Pressure filter is type of rapid sand filter in a closed water tight cylinder through which the
water passes through the sand bed under pressure. All the operation of the filter is similar to
rapid gravity filter; expect that the coagulated water is directly applied to the filter without
mixing and flocculation. These filters are used for industrial plants but these are not economical
on large scale.
Pressure filters may be vertical pressure filter and horizontal pressure filter. Backwash is
carried by reversing the flow with values. The rate of flow is 120 to 300m3/m2/day.

Figure 13: Vertical pressure filters
Factors Influencing Efficiency of Filter

The efficiency of a filter is influenced by a variety of factors. To a large extent, the efficiency is
determined by the characteristics of the water being treated and by the efficiency of previous
stages in the treatment process.
The chemical characteristics of the water being treated can influence both the preceding
coagulation/flocculation and the filtration process. In addition, the characteristics of the
particles in the water are especially important to the filtration process. Size, shape, and
chemical characteristics of the particles will all influence filtration. For example, floc which is
too large will clog the filter rapidly, requiring frequent backwashing, or can break up and pass
through the filter, decreasing water quality.
The types and degree of previous treatment processes greatly influence filtration as well.
Conventional, direct, and in-line filtration will all have different levels of efficiency. Finally,
the type of filter used and the operation of the filter will influence filter efficiency.
Example 1:
11.75 MLD of water after secondary sedimentation (Turbidity: < 10 NTU) is to be filtered
through a battery of rapid sand filters to reduce water turbidity to < 1.0 NTU. Based on pilot
plant studies, it was determined that 60 cm deep filter beds of sand (0.5 mm average sand
diameter) were suitable for this purpose. It was further determined that such beds could be
safely operated up to 7.5 hours at a filtration rate of 10 m3/m2/hr, without the terminal head-loss
reaching 3m. Filter backwashing rate was 1 m3/m2/min and the backwash time was 5 minutes.
A filter unit will be off-line for 30 minutes during each backwash operation. Based on this
information, determine the numbers of filter units to be provided and dimensions of each unit.
The maximum surface per filter is limited to 50m2. Determine how much filtered water is
required for backwashing each day. Also determine the schedule of operation of the filters in a
typical day.
Solution:
Note that the total water to be filtered is more than 11.75 MLD, since filter backwash
(say, B m3/d) is cycled back to the head for the flocculation tank and filtered again.
So, total water to be filtered = (11750 + B) m3/d
Considering a filter is off-line 1.5 hours/day for backwashing
(11750 + B) m3 of water must be filtered in 22.5 hours at a filtration rate of 10 m3/m2/hr
Therefore,
3 2
Hence total water produced = 12590 m in 22.5 hrs using 55.95m filter area
2
Provide 2 filters (in parallel), each with cross-sectional area of 55.95/2 = 27.98m
Take L/B = 1.4 [1.3:1 – 1.5:1]
L = 6.25m; B = 4.47m
3
12590 m filtered in 22.5 hours
3
840 m filtered in 1.5 hours = 90 minutes
Water (for consumption) actually produced by a filter for 22.5 – 1.5 = 21 hours
3
In 21 hours of useful operation per day, the filter produced 11750 m of water
3
The filter produces further 840 m of water in 1.5 hours of operation to be used for
backwashing
The filter is off-line (producing no water) for 1.5 hours a day for backwashing
Example 2:
Design a slow sand filter for a community of 40,000 population. The per capita water demand
of the water supply is 180lit/capita per day and the rate of filtration is 150lit/m2/hr. MDF = 1.8.

Example 3:
Design a rapid sand filter to treat water for 240,000 population, the per capital water
consumption of the town is 200lit/head/day. The filter works all the 24hrs. Assume rate of
filtration as 6000lit/m2/hr and maximum day factor is 1.8.
Example 4:
Determine the percentage of filtered water required for wash-water based on the following
criteria:
Flow, Qf = 300 l/s
Rate of filtration, Vof = 170 m3/m2/day
Time of washing = 10 min
Rate of washing, VoBW = 15 mm/s (0.9 m3/m2/min)
Ans: 5.3%

7. DISINFECTION
The process of killing harmful bacteria from water and making it safe to the consumers is said
to be disinfection. The materials which are used for disinfection of water are called the
disinfectants.
Requirements of Good Disinfectant
1) Destroy bacteria/pathogens within a practicable period of time, over an expected range
of water temperature.
2) Effective at variable compositions, concentration and conditions of water treated.
3) Neither toxic to humans and domestic animals nor unpalatable or otherwise
objectionable in required concentration.
4) Don’t change water properties
5) Have residual in a sufficient concentration to provide protection against recontamination
6) Can be determined easily, quickly, and preferably automatically.
7) Dispensable at reasonable cost
8) Safe and easy to store, transport, handle and supply
9) Don’t form toxic by-products due to their reactions with any naturally occurring
materials in water.
Methods of Disinfection
The disinfection of water can be done by one of the following methods:
a) Boiling of water e) Excess lime
b) Ultra–Violate rays f) Potassium permanganate [KMnO4]
c) Iodine and bromine g) Chlorine
d) Ozone O3
The most common method of disinfection is the use of chlorine i.e. chlorination. The various
chlorine compounds which are available in the market and used as disinfectants are:
1. Calcium hypochlorite [Ca(OCl)2] – power from
2. Sodium hypochlorite [NaOCl] – liquid from
3. Free chlorine Cl2 – Gaseous form
Chlorination Chemistry
When chlorine is added to water, a variety of chemical processes take place. The chlorine reacts
with compounds in the water and with the water itself. Some of the results of these reactions
(known as the chlorine residual) are able to kill microorganisms in the water.

BDU, IOT (2015/16) Wree-3152 Water Treatment and Plant
Design
Chlorine Demand
When chlorine enters water, it immediately begins to react with compounds found in the
water. The chlorine will react with organic compounds and form trihalomethanes. It will also
react with reducing agents such as hydrogen sulfide, ferrous ions, manganous ions, and nitrite
ions.
Let's consider one example, in which chlorine reacts with hydrogen sulfide in water. Two
different reactions can occur:
Hydrogen Sulfide + Chlorine + Oxygen Ion Elemental Sulfur + Water + Chloride Ions
H2S + Cl2 + O2- S + H2O + 2Cl-

Hydrogen Sulfide + Chlorine + Water Sulfuric Acid + Hydrochloric Acid
H2S + 4Cl2 + 4H2O H2SO4 + 8HCl

In the first reaction, hydrogen sulfide reacts with chlorine and oxygen to create elemental sulfur,
water, and chloride ions. The elemental sulfur precipitates out of the water and can cause odor
problems. In the second reaction, hydrogen sulfide reacts with chlorine and water to create
sulfuric acid and hydrochloric acid.
Each of these reactions uses up the chlorine in the water, producing chloride ions or hydrochloric
acid which have no disinfecting properties. The total amount of chlorine which is used up in
reactions with compounds in the water is known as the chlorine demand. A sufficient quantity
of chlorine must be added to the water so that, after the chlorine demand is met, there is still
some chlorine left to kill microorganisms in the water.
Reactions of Chlorine Gas with Water
At the same time that chlorine is being used up by compounds in the water, some of the chlorine
reacts with the water itself. The reaction depends on the type of chlorine added to the water as
well as on the pH of the water itself.
Chlorine gas is compressed into a liquid and stored in metal cylinders. The gas is difficult to
handle since it is toxic, heavy, corrosive, and an irritant. At high concentrations, chlorine gas can
even be fatal.
When chlorine gas enters the water, the following reaction occurs:
Chlorine + Water Hypochlorous Acid + Hydrochloric Acid
Cl2 + H2O HOCl + HCl
The chlorine reacts with water and breaks down into hypochlorous acid and hydrochloric
acid. Hypochlorous acid may further break down, depending on pH:

Design
Hypochlorous Acid ↔ Hydrogen Ion + Hypochlorite Ion
HOCl ↔ H+ + OCl-
The concentration of hypochlorous acid and hypochlorite ions in chlorinated water will depend
on the water's pH. A higher pH facilitates the formation of more hypochlorite ions and results in
less hypochlorous acid in the water. This is an important reaction to understand because
hypochlorous acid is the most effective form of free chlorine residual, meaning that it is
chlorine available to kill microorganisms in the water. Hypochlorite ions are much less efficient
disinfectants. So disinfection is more efficient at a low pH (with large quantities of hypochlorous
acid in the water) than at a high pH (with large quantities of hypochlorite ions in the water.)
Hypochlorites
Instead of using chlorine gas, some plants apply chlorine to water as a hypochlorite, also known
as a bleach. Hypochlorites are less pure than chlorine gas, which means that they are also less
dangerous. However, they have the major disadvantage that they decompose in strength over
time while in storage. Temperature, light, and physical energy can all break down hypochlorites
before they are able to react with pathogens in water.
There are three types of hypochlorites - sodium hypochlorite, calcium hypochlorite, and
commercial bleach:
Hypochlorites and bleaches work in the same general manner as chlorine gas. They react with
water and form the disinfectant hypochlorous acid. The reactions of sodium hypochlorite and
calcium hypochlorite with water are shown below:
Calcium hypochlorite + Water Hypochlorous Acid + Calcium Hydroxide
Ca(OCl)2 + 2H2O 2HOCl + Ca(OH)2
Sodium hypochlorite + Water Hypochlorous Acid + Sodium Hydroxide
NaOCl + H2O HOCl + NaOH
Chloramines
Some plants use chloramines rather than hypochlorous acid to disinfect the water. To produce
chloramines, first chlorine gas or hypochlorite is added to the water to produce hypochlorous
acid. Then ammonia is added to the water to react with the hypochlorous acid and produce a
chloramine.
Three types of chloramines can be formed in water - monochloramine, dichloramine, and

trichloramine. Monochloramine is formed from the reaction of hypochlorous acid with ammonia:
Ammonia + Hypochlorous Acid Monochloramine + Water
NH3 + HOCl NH2Cl + H2O

Design
Monochloramine may then react with more hypochlorous acid to form a dichloramine:
Monochloramine + Hypochlorous Acid Dichloramine + Water
NH2Cl + HOCl NHCl2 + H2O
Finally, the dichloramine may react with hypochlorous acid to form a trichloramine:
Dichloramine + Hypochlorous Acid Trichloramine + Water
NHCl2 + HOCl NCl3 + H2O
The number of these reactions which will take place in any given situation depends on the pH of
the water. In most cases, both monochloramines and dichloramines are formed.
Monochloramines and dichloramines can both be used as a disinfecting agent, called combined
chlorine residual because the chlorine is combined with nitrogen. This is in contrast to the free
chlorine residual of hypochlorous acid which is used in other types of chlorination.
Chloramines are weaker than chlorine, but are more stable, so they are often used as the
disinfectant in the distribution lines of water treatment systems. Despite their stability,
chloramines can be broken down by bacteria, heat, and light. Chloramines are effective at killing
bacteria and will also kill some protozoans, but they are very ineffective at killing viruses.
Dosage of Chlorine
The chlorine dose required depends on two considerations: the chlorine demand and the desired
chlorine residual.
Example 1:
A water treatment plant has used 150.6 kg of chlorine during the past 24 hours. The flow for the
same period was 19.95 MLD. What is the dose of chlorine applied to the water in mg/l?
Solution:
(A) Plain Chlorination

Plain chlorination is the process of addition of chlorine only when the surface water with no
other treatment is required. The water of lakes and springs is pure and can be used after plain
chlorination. A rate of 0.8mg/lit/hour at 15N/cm2 pressure is the normal dosage so as to maintain
in residual chlorine of 0.2 mg/lit.
Design
(B) Super Chlorination
Super chlorination is defined as administration of a dose considerably in excess of that necessary
for the adequate bacterial purification of water. About 10 to 15 mg/lit is applied with a contact
time of 10 to 30 minutes under the circumstances such as during epidemic breakout water is to
be dechlorinated before supply to the distribution system.
(C) Brake Point Chlorination

The graph below shows what happens when chlorine (either chlorine gas or a hypochlorite) is
added to water. First (between points O and A), chlorine reacts with reducing compounds in the
water, such as hydrogen sulfide. These compounds use up the chlorine, producing no chlorine
residual.
Figure 13: Break point chlorination
Next, between points A and B, the chlorine reacts with organics and ammonia which is naturally
found in the water. Some combined chlorine residual is formed - chloramines. Note that if
chloramines were to be used as the disinfecting agent, more ammonia would be added to the
water to react with the chlorine. The process would be stopped at point B.
In contrast, if hypochlorous acid is to be used as the chlorine residual, then chlorine will be
added past point B. Between points B and D, the chlorine will break down most of the
chloramines in the water, actually lowering the chlorine residual.
Finally, the water reaches the breakpoint, shown at point D. The breakpoint is the point at
which the chlorine demand has been totally satisfied - the chlorine has reacted with all reducing
agents, organics, and ammonia in the water. When more chlorine is added past the breakpoint,

Design
the chlorine reacts with water and forms hypochlorous acid in direct proportion to the amount of
chlorine added. This process, known as breakpoint chlorination, is the most common form of
chlorination, in which enough chlorine is added to the water to bring it past the breakpoint and to
create some free chlorine residual. The presence of free residual chlorine in drinking water is
correlated with the absence of disease-causing organisms, and thus is a measure of the potability
of water. “A residual concentration of free chlorine of greater than or equal to 0.5 mg/litre after
at least 30 minutes contact time at pH less than 8.0.” WHO (1993)
(D) De-chlorination
Removal of excess chlorine resulting from super chlorination in part or completely is called ‘De-
chlorination’. Excess chlorine in water gives pungent smell and corrodes the pipe lines. Hence
excess chlorine is to be removed before supply.
Physical methods like aeration, heating and absorption on charcoal may be adopted. Chemical
methods like sulphur dioxide (SO2), Sodium Bi-sulphate (NaHSO3), Sodium Thiosulphate
(Na2S2O8) are used.
Points of Chlorination
Chlorine applied at various stages of treatment and distribution accordingly they are known as
pre, post and re-chlorination.
a) Pre-Chlorination
Chlorine applied prior to the sedimentation and filtration process is known as pre-chlorination.
This is practiced when the water is heavily polluted and to remove taste, odour, colour and
growth of algae on treatment units. Pre-chlorination improves coagulation and post chlorination
dosage may be reduced.
b) Post Chlorination
When the chlorine is added in the water after all the treatment is known as post-chlorination.
After chlorination water is sent for distribution to the consumers.
c) Re-Chlorination
In long distribution systems, chlorine residual may fall tendering the water unsafe. Application
of excess chlorine to compensate for this may lead to unpleasant smell to consumers at the points
nearer to treatment point in such cases chlorine is applied again that is re-chlorinated at
intermediate points generally at service reservoirs and booster pumping stations.
Example1:
Consider water from a polluted river having BOD5 = 5 mg/L, TKN = 1 mg/l (as N), and MPN:
106 organisms / ml. This water will be treated in a conventional water treatment plant and

Design
supplied for potable purposes. Compute the chlorine dose (in mg/l as Cl2) required per liter of
this water (consider both pre and post-chlorination) such that after treatment BOD5, TKN, NH3-
N are negligible and MPN < 1organism/ml.
Assumptions:
Assume that 1 mg/l (as Cl2) chlorine is required to destroy 1 mg/l of BOD5.
Assume TKN is completely converted to NH3-N during pre-chlorination.
The average time between post-chlorination and water consumption by the end users is 1 hr
The product of disinfectant dose (C in mg/l) and the contact time (t in minutes) for 5 and 6
log-kills using free residual chlorine as disinfectant is 96 and 120 respectively.
Assume 2 log-kill of microorganism during water treatment up to just before the post-
chlorination step.
Solution:
Chlorine dose required during pre-chlorination for destruction of BOD5 = 5 mg/l as Cl2
All TKN in water is converted to NH3-N during this process.
Hence ammonia concentration in water before post-chlorination = 1 mg/l (as N)
Breakpoint chlorination has to be performed to destroy ammonia in water.
Relevant equation:
2NH3 + 3HOCl N2 + 3HCl + 3H2O
Ammonia concentration in water
Chlorine required for destruction of ammonia
Therefore, breakpoint chlorination dose
Initial microorganism concentration = 106 /ml

Removal during water treatment up to post-chlorination = 2 Log
Hence microorganism concentration just before post chlorination = 104 /ml
To get this concentration below 1 /ml, 5 log kills are required
“C.t” for 5 log kills = 96
Contact time = 1 hour = 60 minutes
Therefore, the required free chlorine residual dose is
Therefore, total chlorine dose required

Design
8. SOFTENING
Hardness in water is due to cations such as calcium and magnesium (divalent cations) and to a
lesser extent to aluminum, iron and other divalent and trivalent cations. Calcium and magnesium
are the two major cations responsible for hardness in natural waters. Therefore, for most waters:
Calcium and magnesium ions are released into water as it dissolves rocks and minerals. These
mineral ions in the water can cause scale buildup in plumbing, fixtures and appliances and affect
performance. These, while not undesirable from a health standpoint, may make the water less
suitable for some non-potable uses.
The cations are usually associated with the following anions: HCO3-, CO32- (These are alkalinity
anions), and SO42-, NO3-, and Cl- (These are acidity anions). Carbonate hardness is the amount of
hardness associated with alkalinity ions. The difference between the equivalents of the hardness
ions, calcium and magnesium, and the equivalents of bicarbonate and carbonate anions is
noncarbonated hardness.
The compounds producing temporary and permanent hardness in water are shown in Table 1.
The former precipitated as scale when water is heated; the latter not. A large proportion of waters
from the underground sources are hard, particularly waters from chalk and limestone which often
have a carbonate hardness of 200 - 300mg/l. A major source of non-carbonate hardness in
surface waters is the calcium sulfate present in clays and other deposits.
Table 1: Types of hardness
Temporary hardness Permanent hardness
Calcium Bicarbonate Ca(HCO3)2 Calcium Sulphate CaSO4
Magnesium Bicarbonate Mg(HCO3)2 Magnesium Sulphate MgSO4
Calcium Chloride CaCL2
Magnesium Chloride MgCL2
Softening is the process used for the reduction of hardness. Softened, or conditioned, water is
water that has the detrimental hardness minerals removed. It is not essential to soften the water in
order to make the water safe for public uses. Hard water contributes to wearing out our clothing
and other fabrics, our appliances and the plumbing in our homes and businesses.
Thus, advantage of softening lies chiefly in:
- Reduction in soap use.
- Longer life for water heaters.
- Less incrustation of pipes.

Design
Hence whether or not the hardness of water should be reduced, depends on the relation between
the cost of treatment and the obtained result in saving and satisfaction to the consumers.
Industrial users of municipal water benefit through the lower cost of producing process and
boiler waters from softened water. In the hot water heater, heat causes some removal of calcium
carbonate and magnesium carbonate from the water resulting in scale buildup. Scale buildup in
the water heater can slow the heating process and increase the energy usage.
Methods of removal of hardness
1. Boiling
2. Lime addition
3. Lime-soda process
4. Caustic soda process
5. Zeolite process
Methods 1 and 2 are suitable for removal of temporary hardness and 3 to 5 for both temporary
and permanent hardness.
1. Boiling
2. Addition of lime
3. Lime-soda process
Lime softening involves a relatively complicated series of chemical reactions which will be
discussed in depth below. The goal of all of these reactions is to change the calcium and
magnesium compounds in water into calcium carbonate and magnesium hydroxide. These are
the least soluble calcium and magnesium compounds and thus will settle out of the water at the
lowest concentrations. Carbon dioxide is the primary compound which creates the initial demand
for lime. Once the carbon dioxide demand has been met, the lime is free to react with and
remove carbonate hardness from the water.
In many cases, only the carbonate hardness needs to be removed, requiring only the addition of
lime. However, if non-carbonate hardness also needs to be removed from water, then soda ash
(Na2CO3) must be added to the water along with lime.

Design
The precipitation of CaCO3 and Mg(OH)2 is pH dependent. The optimum pH for CaCO3
precipitation by lime addition is from 9 to 9.5. While the effective precipitation of Mg(OH)2
under water-treatment plant conditions requires a pH of about 11.0. Since most natural waters
have a pH considerably below these values, it is often necessary to artificially raise the pH. This
can be accomplished by the addition of an excess amount of lime.
CaO + H2O Ca2+ + 2OH-
The addition of about 1.25mequiv/l of lime is sufficient to raise the pH to 11.0.
Recarbonation
The reactions which remove carbonate and non-carbonate hardness from water require a high pH
and produce water with a high concentration of dissolved lime and calcium carbonate. If allowed
to enter the distribution system in this state, the high pH would cause corrosion of pipes and the
excess calcium carbonate would precipitate out, causing scale. So the water must be
recarbonated, which is the process of stabilizing the water by lowering the pH and precipitating
out excess lime and calcium carbonate.
The goal of recarbonation is to produce stable water, which is water in chemical balance,
containing the concentration of calcium carbonate in which it will neither tend to precipitate out
of the water (causing scale) nor dissolve into the water (causing corrosion). This goal is usually
achieved by pumping carbon dioxide into the water. Excess lime reacts with carbon dioxide in
the reaction shown below, producing calcium carbonate:
Lime + Carbon dioxide → Calcium carbonate + Water
Ca(OH)2 + CO2 → CaCO3 + H2O
Recarbonation also lowers the pH, which encourages the precipitation of calcium carbonate and
magnesium hydroxide.

Design
4. Caustic Soda
Caustic soda (NaOH), also known as sodium hydroxide, can replace soda ash and some of the
lime in the treatment process. The treatment process using caustic soda follows the same steps as
that of lime-soda ash softening.
First, carbon dioxide reacts with the caustic soda to make sodium carbonate and water.
Carbon dioxide + Caustic soda → Sodium Carbonate + Water
CO2 + 2NaOH → Na2CO3 + H2O
Then the remaining caustic soda can react with calcium bicarbonate and magnesium bicarbonate.
Calcium bicarbonate + Caustic soda → Calcium carbonate + Soda ash + Water
Ca(HCO3)2 + 2NaOH → CaCO3 + Na2CO3 + 2H2O
Magnesium bicarbonate + Caustic soda → Magnesium hydroxide + Soda ash + Water
Mg(HCO3)2 + 4NaOH → Mg(OH)2 + 2Na2CO3 + 2H2O
The caustic soda can also react with magnesium non-carbonate hardness, as shown below. Also
note that the reactions between caustic soda and carbonate hardness produced soda ash, which can
react with non-carbonate hardness as well.
Magnesium sulfate + Caustic soda → Magnesium hydroxide + Sodium sulfate
MgSO4 + 2NaOH → Mg(OH)2 + Na2SO4
5. Zeolite process
This is also known as the base-exchange or Ion exchange process. The hardness may be
completely removed by this process.
Zeolites are compounds (silicates of aluminum and sodium) which replace sodium Ions with
calcium and magnesium Ions when hard water is passes through a bed of zeolites. The zeolite
can be regenerated by passing a concentrated solution of sodium chloride through the bed. The
chemical reactions involved are:

Design
9. MISCELLANEOUS TREATMENT OF WATER
a. Removal of silica
b. Fluoridation
c. Removal of oil
Remarks in Water Treatment

1) The processes selected for the treatment of potable water depend on the quality of raw
water supply.
2) Most ground waters are clear and pathogen-free and not contain significant amounts of
organic materials. Such waters may often be used directly with a minimal does of
chlorine to prevent contamination in the distribution system.
3) Other ground waters may contain large quantities of dissolved solids or gases. When
these include excessive amounts of iron, manganese, or hardness, some treatment
methods described in section 2.3 may be required.
4) Surface waters often contain a wide variety of contaminants than groundwater, and
treatment processes may be more complex.
5) Ground waters like springs can directly be used after chlorination.

Jar Test Data Sheet
Water Source:
Date: Time:
Coagulant Type: Current Dosage:

Coagulant Aid Type: Current Dosage:
Lime Type: Current Dosage:
Fast Stir Time: Fast Stir Speed:

Slow Stir Time: Slow Stir Speed:
Dosages
Jar 1 Jar 2 Jar 3 Jar 4 Jar 5 Jar 6
Volume of Raw Water (mL)
Coagulant Dose (mg/L)
Coagulant Aid Dose (mL)
Lime Dose (mL)
Appearance of Floc in Jars during Fast Stir

Time After Chemical Addition Jar 1 Jar 2 Jar 3 Jar 4 Jar 5 Jar 6
1st Appearance
Appearance of Floc in Jars during Slow Stir

Time After Chemical Addition Jar 1 Jar 2 Jar 3 Jar 4 Jar 5 Jar 6
5 Minutes
10 Minutes
15 Minutes
20 Minutes
25 Minutes
30 Minutes
Appearance of Floc in Jars during Settling

Time After Stirring Ends Jar 1 Jar 2 Jar 3 Jar 4 Jar 5 Jar 6
15 Minutes
30 Minutes
Turbidity after Settling

Jar 1 Jar 2 Jar 3 Jar 4 Jar 5 Jar 6
Turbidity (NTU)
Turbidity in the Treatment Plant

Influent Top of Filter Filter Effluent
Turbidity (NTU)

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What are the common types of impurities found in water?

What are the common types of impurities found in water?

What factors determine the quality of water sources?

What factors determine the quality of water sources?

BDU, IOT (2015/16) Wree-3152 Water Treatment and Plant Design

The following are important requirements of water for domestic use:

Lecture Note Page 1

1.1 Common Impurities in Water

1.2 Quality of Water Source

Lecture Note Page 2

Lecture Note Page 3

- Groundwater may contain iron and manganese in soluble form

1.3 Water Quality Characteristics

Lecture Note Page 4

Turbidity is measured by:

Figure 1: Turbidity meter

Lecture Note Page 5

5. Tastes and Odor

Lecture Note Page 6

Lecture Note Page 7

Lecture Note Page 8

Figure 4: pH meter Figure 5: Effect of pH on steel or Iron

Organic compounds are generally unstable to be oxidized biologically or chemically to stable,

Lecture Note Page 10

2. Permanent hardness’ (non- carbonate hardness)

A generally accepted classification of hardness is as follows:

Lecture Note Page 11

demonstrate an adverse physiological effect when present in concentration greater than

10. Metals and other chemical substances

Lecture Note Page 12

Microbiological indicators of water quality or pollution are therefore of particular concern

Lecture Note Page 13

1.4 Examination of Water Quality

Location of sampling points

Lecture Note Page 14

Storage of samples for microbiological analysis

Lecture Note Page 15

As each sample is collected, it must be clearly identified so there is no chance of confusing it

Lecture Note Page 16

Table 1: Volumetric methods for environmental analysis

Volumetric methods may be based on acid/base reactions, precipitation reactions, complexation

The recommended determinations to be made by titration method are: Chloride (Cl-),

ii. Colorimetric method (using color as the basis)

Lecture Note Page 17

iii. Gravimetric method (using weight as the basis)

iv. Electrical method

v. Flame spectra (emission & absorption) method

3. Bacteriological water analysis

Lecture Note Page 18

Lecture Note Page 19

1.5 Water Quality Standards

Table 2: WHO guideline for drinking water quality

Arsenic mg/1 0.05

Aluminum mg/1 0.2

Lecture Note Page 20

4.3. Sources of Water Pollution

However, microbiological contamination is generally the most important to human health as

2.2 Objective of Water Treatment