ENVIRONMENTAL ENGINEERING LABORATORY

EXPERIMENT 1 : COAGULATION AND FLOACCULATION

1.1 Objective

i. To determine the optimum alum dose

ii. pH for suspended solids removal from water using jar test.

iii. To determine the optimum concentration of coagulant to be added to the source water.

1.2 Coagulation and Flocculation

Suspended solids (particularly colloids) are often removed from water by chemical

coagulation and flocculation. Colloidal particles carry a small electrostatic charge that keeps

them in suspension. Coagulation is the addition of chemicals to neutralize the effect of colloidal

charges and allow aggregation of particles. Following coagulation the suspension is stirred

gently to promote particle collision and agglomeration in a process termed flocculation.

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

1.3 Procedure

Note :

Prior to the jar test procedure each 500ml sample must be corrected to the required pH using

H2SO4 or NaOH .In order to obtain the correct amount of acid or alkali for each sample a

separate titration must be carried out for each alum dose-pH combination. Use 100ml water

samples for your titration.

Determine the pH and turbidity of the raw water.

Test for optimum dose (pH constant

i) Fill six 1 liter beakers with 500ml the

given kaolin (clay) suspension (SS about

50 mg/l).

ii) Fill a burrete with 50ml alum solution of

concentration 2.0 g/L.

iii) Add to the beakers the amount of

H2SO4 or NaOH that would yield a final

pH of 7.0 (Fill the amount of H2SO4 or

NaOH used in Table 2.

iv) Add to the beakers alum solution

corresponding to doses of 0 (control),

20, 40, 60, 80 and 100 mg/l.

v) Mix the samples at high speed (80 rpm)

for 1 minute.

vi) Reduce mixing speed to 30 rpm and

continue mixing for 15 minutes.

vii) Stop the stirrer and let the flocs settle

for 20 minutes.

viii) Determine turbidity of supernatant.

ix) Plot turbidity vs. alum dose

RESULTS :

EXPERIMENT : Optimum dose (pH constant)

Sample volume = ml

Alum concentration = g/liter

Beaker Volume of Alum

(ml)

Alum dose (mg/L) Turbidity (NTU) (from

100ml supernatant)

1 0 0 0

2 5 20 124

3 10 40 76.7

4 15 60

96.6

5 20 80 89.6

6 25 100 140

DISCUSSION :

CONCLUSION :

QUESTIONS

1. Name some other chemicals that may be used as coagulants.

2. In a jar test experiment, why does the turbidity increase again after reaching a minimum

value?

3. Briefly discuss the effect of alkalinity on coagulation.