Introduction To Domestic Wastewater Treatment, and Physical Treatment Processes
Introduction To Domestic Wastewater Treatment, and Physical Treatment Processes
Introduction To Domestic Wastewater Treatment, and Physical Treatment Processes
1
Overview of the Module
Module 6:
Introduction to domestic wastewater treatment,
and Physical treatment processes :
Waste-water management, schematic of waste-water treatment
plant, primary, secondary and tertiary treatments, system of
sanitation, sewage appurtenances, sewage and storm water
pumping stations, quantity estimation of sewage and storm
water, hydraulic design of sewers and storm water. Flow
equalization, screening, design of fine screens, aeration, grit,
effect of grit, grit removal facility, skimming tank, water quality
and estimation of organic content. 2
Schematic of process flow of a wastewater
(sewage) treatment system (STP)
3
4
Background information
Urbanization has encouraged the migration of people from villages
to the urban areas.
This has given rise to a number of environmental problems such as,
water supply with desirable quality and quantity, wastewater
generation and its collection, treatment and disposal.
In urban areas for domestic and industrial uses the source of water
is generally reservoir, river, lake, and wells.
Out of this total water supplied, generally 60 to 80% contributes as a
wastewater.
In most of the cities, wastewater is let out partially treated or
untreated and it either percolates into the ground and in turn
contaminates the ground water or it is discharged into the natural
drainage system causing pollution in downstream water bodies.
The importance of water quality as a factor constraining water use
has often gone unacknowledged in the analyses of water scarcity.
Water scarcity is a function not only of volumetric supply, but also of
quality sufficient to meet the demand.
The drinking water demand is perhaps the largest demand for high
quality water apart from many industrial uses which also require 5
high quality water.
Background information
Agriculture, by far the largest consumer of water, also suffers when
water supplies become saline.
In India, water pollution comes from the main sources such as
domestic sewage, industrial effluents, leachets from landfills, and
run-off from solid waste dumps and agriculture land.
Domestic sewage (black water) and sullage (grey water) is the main
source of water pollution in India, especially in and around large
urban centers.
The regular monitoring of the water quality in the rivers and wells in
the country revealed that the total coliform counts far exceeds the
desired level in water to be fit for human consumption.
In the past disposal of waste from water closets was carried out
manually and wastewater generated from kitchen and bathrooms was
allowed to flow along the open drains.
This primitive method was modified and replace by a water carriage
system, in which these wastes are mixed with sufficient quantity of
water.
This waste is carried through closed conduits under the conditions of
gravity flow.
This mixture of water and waste products is known as sewage.
6
Advantages of a water carriage system
The carriage of wastes on head or carts is not required.
Bad smell, which was unavoidable during open transport of sewage,
is not occurring due to transport of this polluted water in closed
conduits.
The old system was posing the health hazards to sweepers and to
the nearby residents, because of the possibilities of flies and insects
transmitting disease germs from the accessible carts to the residents
food eatables.
This is avoided in water carriage system because of transport of
night soil in close conduits.
The human excreta is washed away as soon as it is produced in
water carriage system, thus storing is not required as required in the
old system of manual disposal.
Thus, no bad smells are produced in closed conduit transport.
In the old system, the wastewater generated from the kitchen and
bathrooms was required to be carried through open roadside drains
for disposal. This is avoided in sewerage system as the open drains
could generate bad odours when used for disposal of organic
wastes. 7
Advantages of a water carriage system
The water carriage system does not occupy floor area, as the sewers
are laid underground.
In addition, the construction of toilets one above the other is
possible in water carriage system and combining latrine and
bathrooms together as water closets is possible.
This is one of the important advantages of water carriage system.
8
Definitions of relevant technical terms
Industrial wastewater:
It is the wastewater generated from the industrial and commercial
areas.
This wastewater contains objectionable organic and inorganic
compounds that may not be amenable to conventional treatment
processes.
Night Soil:
It is a term used to indicate the human and animal excreta.
Sanitary sewage:
Sewage originated from the residential buildings comes under this
category.
This is very foul in nature.
It is the wastewater generated from the lavatory basins, urinals and
water closets of residential buildings, office building, theatre and
other institutions.
It is also referred as domestic wastewater. 9
Definitions of relevant technical terms
Sewage:
It indicates the liquid waste originating from the domestic uses of
water.
It includes sullage, discharge from toilets, urinals, wastewater
generated from commercial establishments, institutions, industrial
establishments and also the groundwater and stormwater that may
enter into the sewers.
Its decomposition produces large quantities of malodorous gases,
and it contains numerous pathogenic or disease producing bacteria,
along with high concentration of organic matter and suspended
solids.
Sewage Treatment Plant (STP):
It is a facility designed to receive the waste from domestic,
commercial and industrial sources and to remove materials that
damage water quality and compromise public health and safety when
discharged into water receiving systems or land.
It is combination of unit operations and unit processes developed to
treat the sewage to desirable standards to suit effluent norms defined
by regulating authority.
10
Definitions of relevant technical terms
Sewer:
It is an underground conduit or drain through which sewage is
carried to a point of discharge or disposal. There are three
types of sewer systems that are commonly used for sewage
collection.
Separate sewers are those which carry the house hold and
industrial wastes only.
Storm water drains are those which carry rain water from the
roofs and street surfaces.
Combine sewers are those which carry both sewage and storm
water together in the same conduit.
House sewer (or drain) is used to discharge the sewage from a
building to a street sewer.
Lateral sewer is a sewer which collects sewage directly from
the household buildings.
Branch sewer or submain sewer is a sewer which receives
sewage from a relatively small area.
11
Definitions of relevant technical terms
Sewer:
Main sewer or trunk sewer is a sewer that receives sewage from
many tributary branches and sewers, serving as an outlet for a
large territory.
Depressed sewer is a section of sewer constructed lower than
adjacent sections to pass beneath an obstacle or obstruction. It
runs full under the force of gravity and at greater than
atmospheric pressure. The sewage enters and leaves the
depressed sewer at atmospheric pressure.
Intercepting sewer is a sewer laid transversely to main sewer
system to intercept the dry weather flow of sewage and
additional surface and storm water as may be desirable. An
intercepting sewer is usually a large sewer, flowing parallel to a
natural drainage channel, into which a number of main or outfall
sewers discharge.
Outfall sewer receives entire sewage from the collection system
and finally it is discharged to a common point.
Relief sewer or overflow sewer is used to carry the flow 12
in
excess of the capacity of an existing sewer.
Definitions of relevant technical terms
Sewerage:
The term sewerage refers the infrastructure which includes
device, equipment and appurtenances for the collection,
transportation and pumping of sewage, but excluding works
for the treatment of sewage.
Basically it is a water carriage system designed and
constructed for collecting and carrying of sewage through
sewers.
Storm water:
It indicates the rain water of the locality.
Subsoil water:
Groundwater that enters into the sewers through leakages is
called subsoil water.
13
Definitions of relevant technical terms
Sullage:
This refers to the wastewater generated from bathrooms,
kitchens, washing place and wash basins, etc.
Composition of this waste does not involve higher
concentration of organic matter and it is less polluted water as
compared to sewage.
Wastewater:
The term wastewater includes both organic and inorganic
constituents, in soluble or suspended form, and mineral
content of liquid waste carried through liquid media.
Generally the organic portion of the wastewater undergoes
biological decompositions and the mineral matter may
combine with water to form dissolved solids.
14
Sources of sewage
The wastewater generated from the household activities
contributes to the major part of the sewage.
The wastewater generated from recreational activities, public
utilities, commercial complexes, and institutions is also
discharged in to sewers.
The wastewater discharged from small and medium scale
industries situated within the municipal limits and
discharging partially treated or untreated wastewater in to the
sewers also contributes for municipal wastewater.
15
Sewage discharge
The quality of sewage and its characteristics show a marked
range of hourly variation and hence peak, average and minimum
discharge are important considerations.
The process loadings in the sewage treatment are based on the
daily average characteristics as determined from a 24 hours
weighted composite samples.
In the absence of any data an average quantity of 150 LPCD may
be adopted for design.
The hydraulic design load varies from component to component
of the treatment plant with all appurtenances, conduits, channels
etc., being designed for the maximum discharge, which may vary
from 2.0 to 3.5 times the average discharge.
Sedimentation tanks are designed on the basis of average
discharge, while consideration of both maximum and minimum
discharge is important in the design of screens and grit
chambers.
Secondary treatment is generally designed for average discharge,
with sufficient safety margin to accommodate the peak discharge.
16
Effects of untreated wastewater disposal
The daily activities of human beings produce both liquid and solid
wastes.
The liquid portion of the wastewater is necessarily the water supplied
by the authority or through private water sources, after it has fouled by
variety of uses.
The sources of wastewater generation can be defined as a
combination of the liquid or water-carried wastes removed from
residences, institutions, and commercial and industrial
establishments, together with groundwater, surface water, and storm
water as may be present.
If the untreated wastewater is allowed to accumulate, it will lead to
highly unhygienic conditions.
The organic matter present in the wastewater will undergo
decomposition with production of large quantities of malodorous
gases.
If the wastewater is discharged without treatment in the water body,
this will result in the depletion of Dissolved Oxygen (DO) from the
water bodies.
Due to depletion of DO, the survival of aquatic life will become difficult,
finally leading to anaerobic conditions in the receiving waters. 17
Effects of untreated wastewater disposal
The nutrients present in the wastewater can stimulate the growth of
aquatic plants, leading to problems like eutrophication.
In addition, the untreated domestic wastewater usually contains
numerous pathogenic or disease causing microorganisms, that dwell
in the human intestinal tract or it may be present in certain industrial
wastewaters.
Apart from this, the wastewater contains inorganic gritty materials.
The continuous deposition of this inorganic material may reduce the
capacity of water body considerably over a period.
Generally domestic sewage does not contain any inorganic matter or
organic compounds in highly toxic concentration.
However, depending upon the type of industries discharging into the
public sewers and the dilution that is offered by sewage; the
municipal wastewater may have these inorganic substances or toxic
organic compounds with the concentration more than the discharge
limits stipulated by the authorities.
Certain compounds, such as sulphates, metals such as chromium,
etc., if presents in higher concentration, may disturb the secondary
treatment of the sewage. 18
Objectives of sewage collection and disposal
The objective of sewage collection and disposal is to ensure that
sewage discharged from communities is properly collected,
transported, treated to the required degree so as not to cause danger to
human health or unacceptable damage to the natural environment and
finally disposed off without causing any health or environmental
problems. Thus, efficient sewerage scheme can achieve the following:
21
Schematic of process flow of a wastewater
(sewage) treatment system (STP)
22
Schematic of process flow of a wastewater
(sewage) treatment system (STP)
23
Schematic of process flow of a wastewater
(sewage) treatment system (STP)
24
History of wastewater treatment system
The treatment and safe disposal of wastewater is necessary.
This will facilitate protection of environment and environmental
conservation, because the wastewater collected from cities and
towns must ultimately be returned to receiving water body or to the
land or reused to fulfill certain needs.
The sewage treatment plants constructed near the end of nineteenth
century were designed to remove suspended matter alone by the
principal of simple gravity settling.
It soon became apparent that primary treatment alone was
insufficient to protect the water quality of the receiving water body.
This was mainly due to the presence of organic material, in colloidal
and dissolved form, in the sewage after settling.
Thus, in the beginning of twentieth century several treatment
systems, called secondary treatment, were developed with the
objective of organic matter removal.
For this secondary treatment, biological methods are generally used.
The aerobic biological treatment processes were popularly used as a
secondary treatment, and these processes are still at the first
choice. 25
History of wastewater treatment system
In the second half of twentieth century, it became clear that the
discharge of effluents from even the most efficient secondary treatment
plant could lead to the deterioration of the quality of receiving water
body.
This could be attributed partly to the discharge of ammonia in the
effluent.
In the receiving water body this discharge exerts an oxygen demand for
the biological oxidation of ammonia to nitrate, a process called
nitrification.
However, even when nitrification is carried out at the treatment plant
itself, the discharge of effluent can still be detrimental to the water
quality due to introduction of nitrogen in the form of nitrate and
phosphorus as phosphate.
The tolerance limits of nitrates for the water when used as raw water for
public water supplies and bathing ghats is 50 mg/L as NO3.
The availability of nitrogen and phosphorous tends to cause an
excessive growth of aquatic life notably, autotrophic organisms such as
algae, that can use carbon dioxide rather than organic material as a
sources for cell synthesis.
Almost invariably biological methods are used in the treatment systems
to effect secondary treatment for removal of organic material. 26
History of wastewater treatment system
In biological treatment systems, the organic material is metabolized by
bacteria.
Depending upon the requirement for the final effluent quality, tertiary treatment
methods and/or pathogen removal may be included.
Today majority of wastewater treatment plants uses aerobic metabolism for the
removal of organic matter.
The popularly used aerobic processes are the activated sludge process,
oxidation ditch, trickling filter, and aerated lagoons.
Stabilization ponds use both the aerobic and anaerobic mechanisms.
In the recent years, due to increase in power cost and subsequent increase in
operation cost of aerobic processes, more attention is being paid for the use of
anaerobic treatment systems for the treatment of wastewater including sewage.
Recently the high anaerobic process such as Upflow Anaerobic Sludge Blanket
(UASB) reactor is used for sewage treatment at many places.
Depending on the mode of disposal the tertiary treatment may be given for
killing pathogens, nutrient removal, suspended solids removal, etc.
Generally secondary treatment followed by disinfection will meet the effluent
standards for disposal into water bodies.
When the treated sewage is disposed off on land for irrigation, the level of
disinfection needs will depend on the type of secondary treatment and type of
crops with restricted or unrestricted public access. 27
(A) Pre- and primary treatment
28
(B) Secondary treatment
29
(C) Tertiary treatment
30
(D) Solid treatment
31
Summary of waste-water treatment processes
32
33
Background information
For safe disposal of the sewage generated from a locality efficient
collection, conveyance, adequate treatment and proper disposal of
treated sewage is necessary. To achieve this, following conditions
should be satisfied:
34
Types of sewerage system
35
Sketches of sewer systems
36
Sketches of sewer systems
37
Advantages and disadvantages of combined system
38
Advantages and disadvantages of separate system
39
Advantages and disadvantages of partially separate system
40
Considerations for the type of system
41
Sketches of sewer lines
42
Patterns of collection system
43
Perpendicular pattern
44
Interceptor pattern
45
Radial pattern
46
Fan pattern
47
Fan pattern
48
Zone pattern
49
50
Types of sewer appurtenances
51
Manholes
Man-holes: Man holes are the openings of either circular or
rectangular in shape constructed on the alignment of a sewer line to
enable a person to enter the sewer for inspection, cleaning and
flushing. They serve as ventilators for sewers, by the provisions of
perforated man-hole covers. Also they facilitate the laying of sewer
lines in convenient length.
Man-holes are provided at all junctions of two or more sewers,
whenever diameter of sewer changes, whenever direction of sewer
line changes and when sewers of different elevations join together.
52
Manholes
Special Man-holes:
Junction chambers: Man-hole constructed at the intersection of two
large sewers.
Drop man-hole: When the difference in elevation of the invert levels
of the incoming and outgoing sewers of the man-hole is more than
60 cm, the interception is made by dropping the incoming sewer
vertically outside and then it is jointed to the man-hole chamber.
Flushing man-holes: They are located at the head of a sewer to flush
out the deposits in the sewer with water.
53
Classification of manholes
54
Classification of manholes
55
Classification of manholes
56
Inverted syphons
57
Storm-water inlets
58
Storm-water inlets
59
Catch basins
60
Clean-outs
61
Lamp hole
62
Other technical terms
63
64
Background knowledge
65
Types of pumps
66
Head requirement
67
Power calculation
68
Numerical example
69
Numerical example
70
Numerical example
71
Numerical example
72
73
Quantity estimation of sewage
Dry Weather Flow:
Dry weather flow is the flow that occurs in sewers in separate
sewerage system or the flow that occurs during dry seasons in
combined system.
This flow indicates the flow of sanitary sewage.
This depends upon the rate of water supply, type of area served,
economic conditions of the people, weather conditions and
infiltration of groundwater in the sewers, if sewers are laid below
groundwater table.
Evaluation of Sewage Discharge:
Correct estimation of sewage discharge is necessary; otherwise
sewers may prove inadequate resulting in overflow or may prove
too large in diameter, which may make the system uneconomical
and hydraulically inefficient.
Hence, before designing the sewerage system it is important to
know the discharge / quantity of the sewage, which will flow in it
after completion of the project and at the end of design period.
74
Quantity estimation of sewage
Addition due to unaccounted private water supplies:
People using water supply from private wells, tube wells, etc. contribute
to the wastewater generation more than the water supplied by municipal
authority.
Similarly, certain industries utilize their own source of water.
Part of this water, after desired uses, is converted into wastewater and
ultimately discharged into sewers.
This quantity can be estimated by actual field observations.
Addition due to infiltration:
This is additional quantity due to groundwater seepage in to sewers
through faulty joints or cracks formed in the pipes.
The quantity of the water depends upon the height of the water table
above the sewer invert level.
If water table is well below the sewer invert level, the infiltration can
occur only after rain when water is moving down through soil.
Quantity of the water entering in sewers depends upon the permeability
of the ground soil and it is very difficult to estimate. While estimating
the design discharge, following suggested discharge can be considered
(Table on next slide). 75
Quantity estimation of sewage
Storm water drainage may also infiltrate into sewers. This inflow
is difficult to calculate.
Generally, no extra provision is made for this quantity.
This extra quantity can be taken care of by extra empty space left
at the top in the sewers, which are designed for running ¾ full at
maximum design discharge.
77
Variations in sewage flow
Variation occurs in the flow of sewage over annual average daily flow.
Fluctuation in flow occurs from hour to hour and from season to
season.
The typical hourly variation in the sewage flow is shown in the Figure on
next slide.
If the flow is gauged near its origin, the peak flow will be quite
pronounced.
The peak will defer if the sewage has to travel long distance.
This is because of the time required in collecting sufficient quantity of
sewage required to fill the sewers and time required in travelling.
As sewage flow in sewer lines, more and more sewage is mixed in it due
to continuous increase in the area being served by the sewer line.
This leads to reduction in the fluctuations in the sewage flow and the lag
period goes on increasing.
The magnitude of variation in the sewage quantity varies from place to
place and it is very difficult to predict.
For smaller township this variation will be more pronounced due to
lower length and travel time before sewage reach to the main sewer and
for large cities this variation will be less. 78
Variations in sewage flow
Figure: Typical hourly variations in sewage flow
79
Variations in sewage flow
80
81
Design period
The future period for which the provision is made in designing the
capacities of the various components of the sewerage scheme is known
as the design period. The design period depends upon the following:
Ease and difficulty in expansion.
Amount and availability of investment.
Anticipated rate of population growth, including shifts in
communities, industries and commercial investments.
Hydraulic constraints of the systems designed.
Life of the material and equipment.
82
83
84
85
Quantity estimation of Storm-water
86
Quantity estimation of Storm-water
87
Quantity estimation of Storm-water
88
Quantity estimation of Storm-water
89
Quantity estimation of Storm-water
90
Quantity estimation of Storm-water
91
Quantity estimation of Storm-water
92
1-Numerical example: Quantity of Storm-water
93
2-Numerical example: Quantity of Storm-water
94
2-Numerical example: Quantity of Storm-water
95
2-Numerical example: Quantity of Storm-water
96
97
Requirements of design and planning of sewerage system
98
Difference between water supply pipes and sewer pipes
99
Provision of freeboard in sewers
100
Hydraulic formulae for determining flow velocities
101
Hydraulic formulae for determining flow velocities
102
Minimum velocity: Self cleansing velocity
103
Minimum velocity: Self cleansing velocity
104
Maximum velocity or Non-scouring velocity
105
Effect of flow variations on velocities in a sewer
106
Hydraulic characteristics of circular sewer running full
or partially full
107
Hydraulic characteristics of circular sewer running full
or partially full
108
Hydraulic characteristics of circular sewer running full
or partially full
109
Hydraulic characteristics of circular sewer running full
or partially full
110
Hydraulic characteristics of circular sewer running full
or partially full
111
1-Numerical example: Hydraulics of sewers
112
1-Numerical example: Hydraulics of sewers
113
2-Numerical example: Hydraulics of sewers
114
2-Numerical example: Hydraulics of sewers
115
3-Numerical example: Hydraulics of sewers
116
3-Numerical example: Hydraulics of sewers
117
4-Numerical example: Hydraulics of sewers
118
4-Numerical example: Hydraulics of sewers
119
4-Numerical example: Hydraulics of sewers
120
Design of storm water drains for separate system
121
Laying of sewer pipes
122
Laying of sewer pipes
123
Laying of sewer pipes
124
Important factors considered for selecting material
for sewer
125
Materials for sewers: Asbestos cement
126
Materials for sewers: PCC or RCC
127
Materials for sewers: Vitrified clay or stoneware
128
Materials for sewers: Bricks and cast iron
129
Materials for sewers: Steel pipes
130
Materials for sewers: Ductile pipes
131
Materials for sewers: Plastic and glass fiber
132
Materials for sewers: Highly density polyethylene
133
Shapes of sewer pipes
134
Shapes of sewer pipes
135
Shapes of sewer pipes
136
137
Background information
The wastewater to be treated in the wastewater treatment plant has a
lot of variations in flow rate, concentration of pollutants and
characteristics.
A wastewater treatment plant already designed for some flow rate and
loading rate can’t sustain such large seasonal or other variations in
flow rate.
Flow equalization is a method to overcome problems related to
fluctuations in flow rate & pollution load.
Flow equalisation basin is located after most of the primary treatment
units such as screening and grit removal but before primary
sedimentation.
Flow equalisation method controls the short term, high volumes of
incoming flow, called surges, through the use of basin.
It helps in equalizing the flow rate and optimizing the time required for
treatment in secondary and tertiary processes.
It also helps in lowering the strength wastewater by diluting it with
wastewater already present in the equalization basin.
Basin volume and dimensions, mixing and air requirements, etc. are
the basic things that are considered in designing an equalisation
basin. 138
Advantages & disadvantages of flow equalization
Advantages:
Helps in improving the performance of down- stream
operations and reduces the operating & capital cost of down-
stream process.
Biological treatment is enhanced because of elimination of
shock load due to flow rate & pollution load.
Thickener/ settler and filter performance gets enhanced and
their required surface area gets reduced.
Disadvantages:
Large land area may be required.
Additional capital and operating cost may be required.
May cause odor problem for nearby residential colonies.
139
Types of flow equalization
140
Types of flow equalization
(2) Off-line equalization:
In this case, only over-flow above a predetermined value is
diverted into the basin.
It helps in reducing the pumping requirements.
In this method of equalization, variations in loading rate can be
reduced considerable.
Off-line equalisation is commonly used for the capture of the
“first flush” from combined collections systems.
141
Determination of volume of flow equalization basin
142
Determination of volume of flow equalization basin
143
1-Numerical example: Equalization tank
144
1-Numerical example: Equalization tank
145
146
Background information
Aeration removes odour and tastes due to volatile gases like
hydrogen sulphide and due to algae and related organisms.
Aeration also oxidize iron and manganese, increases dissolved
oxygen content in water, removes CO2 and reduces corrosion and
removes methane and other flammable gases.
Principle of treatment underlines on the fact that volatile gases in
water escape into atmosphere from the air-water interface and
atmospheric oxygen takes their place in water, provided the water
body can expose itself over a vast surface to the atmosphere. This
process continues until an equilibrium is reached depending on the
partial pressure of each specific gas in the atmosphere.
148
Calculation of solubility of gases
149
Calculation of solubility of gases
150
1-Numerical example: Aeration
151
2-Numerical example: Aeration
152
3-Numerical example: Aeration
153
Types of Aeration
(1) Diffused or Submerged aeration:
Submerged aeration systems are used in lakes, reservoirs, and
wastewater treatment facilities to increase dissolved oxygen (DO) levels
and promote water circulation.
Submerged diffusers release air or pure oxygen bubbles at depth,
producing a free, turbulent bubble-plume that rises to the water surface
through buoyant forces.
The ascending bubble plume entrains water, causing vertical circulation
and lateral surface spreading.
Oxygen transfers to the water across the bubble interfaces as the
bubbles rise from the diffuser to the water surface.
(2) Spray aeration:
Spray aeration removes low levels of volatile contaminants.
In a spray aeration system, water enters through the top of the unit and
emerges through spray heads in a fine mist.
Treated water collects in a vented tank below the spray heads.
Volatile contaminants are released and vented to the outside.
(3) Water fall type of aeration:
It involves flow of water over media forming droplets or thin film of water
so as to contact with air. 154
Types of aerators
1) Gravity Aerators (Cascades): In gravity aerators, water is allowed to
fall by gravity such that a large area of water is exposed to atmosphere,
sometimes aided by turbulence.
158
Screens: Types of screens
Screens can be broadly classified depending upon the opening size provided
as coarse screen (bar screens) and fine screens.
Based on the cleaning operation they are classified as manually cleaned
screens or mechanically cleaned screens.
Due to need of more and more compact treatment facilities many
advancement in the screen design are coming up.
Screens: Types of screens
Screen Chamber
It consists of rectangular channel. Floor of the channel is
normally 7 to 15 cm lower than the invert of the incoming
sewer. Bed of the channel may be flat or made with desired
slope. This channel is design to avoid deposition of grit and
other materials in to it.
Sufficient straight approach length should be provided to
assure uniform distribution of screenings over the entire
screen area.
At least two bar racks, each designed to carry peak flow, must
be provided. Arrangement of stopping the flow and draining
the channel should be made for routine maintenance.
The entrance structure should have a smooth transition or
divergence to avoid excessive head loss and deposition of
solids (Figure on next slide).
Effluent structure should be having uniform convergence. The
effluent from the individual rack may be combined or kept
separate as necessary.
161
Screen Chamber
162
Requirements and specifications for design of bar
screen
Requirements and specifications for design of bar
screen
Requirements and specifications for design of bar screen
Requirements and specifications for design of bar
screen
1-Numerical example of bar screen
1-Numerical example of bar screen
1-Numerical example of bar screen
1-Numerical example of bar screen
1-Numerical example of bar screen
172
Grit chambers: Background knowledge
Grit chamber is the second unit operation used in primary treatment
of wastewater and it is intended to remove suspended inorganic
particles such as sandy and gritty matter from the wastewater.
This is usually limited to municipal wastewater and generally not
required for industrial effluent treatment plant, except some industrial
wastewaters which may have grit.
The grit chamber is used to remove grit, consisting of sand, gravel,
cinder, or other heavy solids materials that have specific gravity
much higher than those of the organic solids in wastewater.
Grit chambers are provided to protect moving mechanical equipment
from abrasion and abnormal wear; avoid deposition in pipelines,
channels, and conduits; and to reduce frequency of digester
cleaning.
Separate removal of suspended inorganic solids in grit chamber and
suspended organic solids in primary sedimentation tank is necessary
due to different nature and mode of disposal of these solids.
Grit can be disposed off after washing, to remove higher size organic
matter settled along with grit particles; whereas, the suspended
solids settled in primary sedimentation tank, being organic matter,
173
requires further treatment before disposal.
Types of grit chambers
Grit chambers are of two types:
Mechanically cleaned.
Manually cleaned.
In mechanically cleaned grit chamber, scraper blades collect the grit
settled on the floor of the grit chamber. The grit so collected is elevated
to the ground level by several mechanisms such as bucket elevators, jet
pump and air lift. The grit washing mechanisms are also of several
designs most of which are agitation devices using either water or air to
produce washing action.
Manually cleaned grit chambers should be cleaned atleast once a week.
The simplest method of cleaning is by means of shovel.
Aerated grit chamber
An aerated grit chamber consists of a standard spiral flow aeration tank
provided with air diffusion tubes placed on one side of the tank.
The grit particles tend to settle down to the bottom of the tank at rates
dependent upon the particle size and the bottom velocity of roll of the spiral
flow, which in turn depends on the rate of air diffusion through diffuser tubes
and shape of aeration tank.
The heavier particles settle down whereas the lighter organic particles are
carried with roll of the spiral motion.
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2-Numerical example: Grit chamber
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2-Numerical example: Grit chamber
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Skimming tank: Background knowledge
The floating solid materials such as soap, vegetables, debris, fruit skins,
pieces of corks, etc. and oil and grease are removed from the wastewater in
skimming tanks.
A skimming tank is a chamber designed so that floating matter rises and
remains on surface of the wastewater until removed, while the liquid flows
continuously through outlet or partition below the water lines.
The detention time in skimming tank is 5 minutes. To prevent heavy solids
from settling at the bed, compressed air is blown through the diffusers
placed in the floor of the tank.
Skimming tanks are used to remove lighter, floating substances, including
oil, grease, soap, pieces of cork and wood, vegetable debris, and fruit skins.
Tank can be rectangular or circular, designed for detention period of 1 to 15
minutes. Typical detention time of about 5 min is adopted in design.
The submerged outlet is located opposite the inlet and at lower elevation to
assist in flotation and remove any solids that may settle.
Due to compress air supply, the oily matters rise upward and are collected in
the side trough, from where they are removed.
In conventional sewage treatment plant separate skimming tank is not used
and these materials are removed by providing baffle ahead of the effluent
end of the primary sedimentation tank.
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Skimming tank
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Oxygen demand
Biochemical oxygen demand
Biochemical oxygen demand
Biochemical oxygen demand
Biochemical oxygen demand
Biochemical oxygen demand
Biochemical oxygen demand
Biochemical oxygen demand
Biochemical oxygen demand
Biochemical oxygen demand
Numerical example
Numerical example
BOD model
BOD model
BOD model
BOD reaction rate
BOD reaction rate
Numerical example
Numerical example
Numerical example
Numerical example
Numerical example
Chemical oxygen demand (COD)
Chemical oxygen demand (COD)
Chemical oxygen demand (COD)
Theoretical oxygen demand (ThOD)