Determination of Optimum Dose of Alum using Jar Test

Apparatus

SUBMITTED BY
SHUVA CHANDRA BOSE
ID: 161116

SUBMITTED TO
DR. MD. AKRAMUL ALAM
PROFESSOR, CE, DUET
SHAHAB UDDIN
ASSISTANT PROFESSOR, CE, DUET

DEPARTMENT OF CIVIL ENGINEERING

DHAKA UNIVERSITY OF ENGINEERING & TECHNOLOGY, GAZIPUR

1.0 INTRODUCTION

Raw water or wastewater must be treated to remove turbidity, color and bacteria.
Colloidal particles are in the size range between dissolved substance and suspended
particles. The particles are too small to be removed by sedimentation or by normal
filtration processes. Colloidal particles exhibit the Tyndall effect; that is, when light
passes through liquid containing colloidal particles, the light is reflected by the particles.
The degree to which colloidal suspension reflects light at 90º angle to the entrance beam
is measured by turbidity. The unit of measure is a Turbidity Unit (TU) or Nephlometric
Turbidity Unit (NTU). It is determined by reference to a chemical mixture that produces a
reproducible refraction of light. Turbidities in excess of 5 TU are easily detectable in a
glass of water and are usually objectionable for aesthetic reasons. For a given particle
size, the higher the turbidity, the higher the concentration of colloidal particles.

Color is a useful term that is used to describe a solution state. But it is difficult to
distinguish ‘dissolved color’ and ‘colloidal color’. Some color is caused by colloidal iron
or manganese complexes. Although, the most common cause of color is from complex
organic compounds that originate from the decomposition of organic matter. Most color
seems to be between 3.5 and 10μm, which is colloidal. Color is measured by the ability of
the solution to absorb light. Color particles can be removed by the methods discussed for
dissolved or colloidal, depending upon the state of the color.

Finely dispersed solid (colloids) suspended in wastewater are stabilized by

negative electric charges on their surfaces, causing them to repel each other. Since this
prevents these charged particles from colliding to form larger masses, called flocs, they
do not settle. To assists in the removal of colloidal particles form suspension, chemical
coagulations and flocculation are required. These processes, usually done in sequence,
are a combination of physical and chemical procedures. Chemicals are mixed with
wastewater to promote the aggregation of the suspended solids into particles large enough
to settle or be removed. Coagulation is the destabilization of colloids by neutralizing the
forces that keep them apart. Cationic coagulants provide positive electric charges to
reduce the negative charge of the colloids. As a result, the particles collide to form larger
particles (floc). Rapid mixing is required to disperse the coagulant throughout the liquid.
The coagulants overdose can cause a complete charge reversal and destabilize the colloid
complex.

A coagulant is the substance (chemical) that is added to the water to accomplish

coagulation. There are three key properties of a coagulant;

1. Trivalent cation: As indicated in the last section, the colloids most commonly
found in natural waters are negatively charged; hence a cation is required to neutralize the
charge. A trivalent cation is the most efficient cation.
2. Nontoxic: This requirement is obvious for the production of safe water.
3. Insoluble in the neutral pH range. The coagulant that is added must precipitate
out of solution so that high concentrations of the ion are not left in the water. Such
precipitation greatly assists the colloid removal process.

The two most commonly used coagulants are aluminum (Al3+) and ferric iron
(Fe3+). Both meet above three requirements. Aluminum can be purchased as either dry or
liquid alum [Al2(SO4)3·14H2O]. Commercial alum has an average molecular weight of
594. When alum added to a water containing alkalinity, the following reaction occurs;
Al2(SO4)3·14H2O + 6HCO-3 ↔ 2Al(OH)3(s) + 6CO2 + 14H2O + 3SO42-

The above reaction shifts the carbonate equilibrium and decreases the pH. When
sufficient alkalinity is not present to neutralize the sulfuric acid production, the pH may
be greatly reduced;

Al2(SO4)3·14H2O ↔ 2Al(OH)3(s) + 3H2SO4 + 8H2O

If the second reaction occurs, lime or sodium carbonate may be added to neutralize the
acid. The optimal pH range for alum is approximately 5.5 to 6.5 with coagulation
possible between pH 5to pH 8 under some conditions.
 In flocculation process, the flocculating agent is added by slow and gentle mixing
to allow for contact between the small flocs and to agglomerate them into larger particles.
The newly formed agglomerated particles are quite fragile and can be broken apart by
shear forces during mixing. Increasing the dosage will increase the tendency of the floc to
float and not settle. Once suspended particles are flocculated into larger particles, they
can usually be removed from the liquid by sedimentation, provided that a sufficient
density difference exists between the suspended matter and liquid. When a filtering
process is used, the addition of a flocculants may not be required since the particles
formed by the coagulation reaction may be of sufficient size to allow removal. The
flocculation reaction not only increases the size of the floc particles to settle them faster,
but also affects the physicals nature of the floc, making these particles less gelatinous and
thereby easier to dewater.

1.1 OBJECTIVE:

1. To conduct jar test.

2. To show the effectiveness of chemical treatment in a water treatment facility.
3. To evaluate coagulation efficiency.
4. To determine the most effective dosage of the recommended coagulants and
flocculants.
5. To selects the best chemical or best dosage to feed on the basis of clarifies of effluent
and minimum cost of chemicals.
6. To gain a ’hands on’ understanding of the treatment process for removing suspended
solids from water.
2.0 EQUIPMENT AND MATERIAL

1. Jar test apparatus with six rotating paddles

2. Six (6) beaker

3. Thermometer

4. Time / Stopwatch
 5. pH meter

6. Turbidity meter

7. pipette

2.1 REAGENT

1. Coagulant e.g. aluminum sulfate (alum), polyaluminum chloride (PAC), ferrous

sulfate, ferric chloride, etc.
2. Coagulant aid e.g. pH adjusters (lime or sulfuric acid), activated silica, polyelectrlye
(e.g. synthetic polymer such as acrylamide), clays (e.g. bentonite, montmorillonite,
etc.)
3. Liquid sample.
3.0 PROCEDURES

1. The waste water from the treatment plant was prepared. The sodium was use to
stability the PH of the waste water to the neutral.
2. The temperature, pH, color, alkalinity and turbidity of the synthetic water sample
were measured.
3. 600ml was filled each of the prepared synthetic water suspension into six different
beakers (Plexiglas beakers)
4. The prescribed dose of coagulant was added to each jar by using a pipette. One jar has
no coagulant since a control sample was required.
5. If a coagulant aid is required, it is added to each jar (except for control sample) during
the last 15 seconds of the rapid mix stage.
6. Start stirring rapidly (60 to 80 rpm) for 3 minute (Rapid mix stage).
7. After the rapid mix stage, reduce the speed to 30 rpm for 20 minutes.
8. Floc formation were record ed by referring to the chart of particle sizes in final 10
minutes.
9. After the stirring period was over, stop the stirrer and the flocs was allowed to settle
for about 5 minutes as in scheme (iv)
10. 500mL of settle water was separate out into another beaker.
11. The temperature, pH, color, alkalinity and turbidity of the clarified water were
determined.
12. A graph of turbidity versus coagulant dose (mg/L) was plotted. The most effective
dose of coagulant (or with the present of coagulant aid) that gives the least turbid
results also determined.
13. The qualitative characteristics of floc as bad, moderate, good and very good were
recorded. Cloudy samples indicate bad coagulation while good coagulation refers to
rapid floc formation resulting in clear water formation on the upper portion of the
beaker.
14. The following graph: color versus coagulant dose, pH versus coagulant dose,
temperature versus coagulant dose, etc. were plotted. These graphs will assist students
in the interpretation of the coagulation-flocculation process.
 4.0 RESULT AND DATA ANALYSIS

JAR NO. 1 2 3 4 5 6
Initial pH 6.42 6.42 6.42 6.42 6.42 6.42
Initial Temperature (oC) 26.5 26.5 26.5 26.5 26.5 26.5
Coagulant dose (mg/L) 10 20 30 40 50 control
Agigate (minutes) 23 23 23 23 23 23
Fast (rpm) 70 70 70 70 70 70
Slow (rpm) 30 30 30 30 30 30
Settling Depth (mm) 10 8 4 3 2 2
Final pH 5.35 4.06 3.60 3.25 3.22 -
Final Temperature (oC) 23.1 23.1 22.9 22.9 22.9 -
Final Turbidity (NTU) 7 5 4 17 32 108
Very Very Moderately
Floc Formation fine fine moderate Coarse coarse fine

Time of floc formation = 5 minutes

Floc sizes for:

Very fine is 0.30mm to 0.75mm
Fine is 0.50mm to 0.75mm
Moderate fine is 1.00mm to 1.50mm.
From the graphs it could conclude;
The most effective coagulant dose is 10 mg/L, and 20 mg/L
The most effective pH is 5.35
The expected temperature was 22.90 at optimum coagulant dose.

5.0 DISCUSSION

We had successfully done this experiment because the objective of this

experiment, to conduct various experiments on chemical coagulation and flocculation and
to determine the optimum dose combination of coagulant aid (when used) which will
produce the highest removal of turbid water sample has achieved.

Jar tests have been used to evaluate the effectiveness of various coagulants and
flocculants under a variety of operating conditions for water treatment. . This procedure
allows individual polymers to be compared on such criteria as floc formation, settling
characteristics, and clarity. Generally, the best performing products provide fast floc
formation, rapid settling rate, and clear supernatant. This test should be performed on-
site, since large amounts of water may be required for testing.

Turbidity is essentially a measure of the cloudiness of the water which indicates

the presence of colloidal particles. The particles should be making sure removed from
the water before for the publics use. However these colloids are suspended in solution
and can be removed by sedimentation or filtration. Very simply, the particles in the
colloid range are too small to settle in a reasonable time period, and too small to be
trapped in the pores of a filter. For colloids to remain stable they must remain small. Most
colloids are stable because they posses a negative charge that repel other colloidal
particles before they collide with one another. The colloids are continually involved in
Brownian movement, which is merely random movement. Charges on colloids are
measured by placing Dc electrodes in a colloidal dispersion. The particles migrate to the
pole of opposite charge at a rate proportional to the potential gradient. Generally, the
larger the surface charge, the more stable the suspension.
 Based on this experiment, the first jar is serving as a control and no coagulant was
added. The coagulant doses increased in the containers from no 1to no 6. For this water,
as the dose of coagulant increased the residual turbidity improved. It is important to note
that the optimum coagulant dose is the dose which meets the specified turbidity required
on the regulatory permit. The addition of excess coagulant may reduce turbidity beyond
what is required but also could lead to the production of more sludge which would
require disposal.

The most effective dose of coagulant we get from the Graph turbidity versus
coagulant after the experiment is 20 mg/L. The most effective pH is 5.36.

Jar tests are used in these procedures to provide information on the most effective
flocculants, optimum dosage, optimum feed concentration, effects of dosage on removal
efficiencies, effects of concentration of influent suspension on removal efficiencies,
effects of mixing conditions, and effects of settling time.

The general approach used in these procedures is as follows:

a) Prepare stock suspension of sediment.
b) Test a small number (six) of polymers that have performed well on similar
dredged material which has 2-grams-per-litre suspensions and is a typical concentration
for effluent from a well-designed containment area for freshwater sediments containing
clays. If good removals are obtained at low dosages (10 milligrams per liter or less), then
select the most cost-effective polymer. If good removals are not obtained, examine the
polymer under improved mixing and settling conditions and test the performance of other
flocculants
c) The effects of settling time on the removal of suspended solids and turbidity
from a suspension of average concentration should be exanimate using the selected
dosage and likely mixing conditions.
d) The effects of the range of possible mixing conditions on the required dosage of
flocculants for a typical suspension should be exanimate.

6.0 CONCLUSION
 As conclusion, this experiment is successfully been done and it is because the
objective of this experiment which to conduct various experiments on chemical
coagulation and flocculation and to determine the optimum dose combination of
coagulant aid (when used) which will produce the highest removal of turbid water sample
has achieved.

Jar testing is an experimental method where optimal conditions are determined

empirically rather than theoretically. Jar test are meant to mimic the conditions and
processes that take place in the clarification portion of water and wastewater treatment
plants. The values that are obtained through the experiment are correlated and adjusted in
order to account for the actual treatment system. After the experiment, Graph turbidity
versus coagulant dose are plot, from the graph we get the most effective dose of
coagulant is 60mg/L

Base on the data, we conclude that although the turbidity is generally declines as
the amount of the alum which added into the water but there is a point where more alum
should not be added. This is because alum will make the water more acidic. Therefore, to
overcome these problems, buffer should be added with same amount of alum at the same
time the alum is added.

After this experiment, we realize that a successful Jar Test is very reliant upon the
proper preparation of the polymers being tested. Dilution technique ("make down") is
especially critical, since it involves compactly coiled large molecules in emulsions, prior
to activation. The polymer must be uncoiled to provide maximum contact with the
colloidal particles to be flocculated. If the following procedures are not followed, the Jar
Test results will be very unreliable. As conclusion, after we analyzed the data, we have
decided that the optimum dosage of alum for this experiment is 10 mg/L. we reach this
conclusion base on the fact that the turbidity minimum at 7 NTU.
7.0 REFERENCES:

G. J. Schroepfer, M. L. Robins, and R. H. Susag, (1964)“Research Program on the

Mississippi River in the Vicinty of Minneapolis and St. Paul,” Advances in Water
Pollution Research, vol. 1

L.Davis , I.Cornwell. Introduction to Environmental Engineering. Third Edition.

Lab sheet: Enviromental Engineering, Test: JAR Test

Website: 4th Feb 2006 (date retrieved)

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