Experiment No

Experiment No. 1

JAR TEST

Objective:

To determine the optimum dose of given coagulant [Al₂(SO₄)₃.18H₂O]

Apparatus Required:

· Flock tester

· Magnetic stirrer

· Jar of capacity 500ml

· Pipettes of 1ml and 2ml

· Dash mixture

Chemical Required:

· Water sample

· 1% alum [Al₂(SO₄)₃.18H₂O] solution

Theory:

Water contains very fine suspended clay particles, which are not removed by plain sedimentation. In addition to fine suspended matter, water also contains electrically charged colloidal matter, which are continuously in motion and never settle down due to gravitational force. This type of fine clay particles and colloidal impurities are removed by sedimentation with coagulation.

In this process certain chemicals are added to water, which is called coagulants. When it is added to water an insoluble, gelatinous, precipitation is formed which absorb and entangle very fine suspended matter and colloidal impurities. Then the water is sent in sedimentation basins where sedimentations of fine and colloidal articles take place through the precipitate.

The most common coagulant, which is widely used, is aluminium sulphate [Al₂(SO₄)₃.18H₂O]. It is also called simply alum. Alum reacts with the water in the presence of alkalinity; if natural alkanity is not present sufficient lime is added. The following reactions take place with the various types of alkalinity.

Al₂(SO₄)₃.18H₂0 + 3Ca(HCO₃)₂ ---------2Al(OH)₃ + 3CaSO₄ + 18 H₂O + 6C0₂

Al₂(SO₄)₃.18H₂0 + 3Ca(OH)₂ -----------2Al(OH)₃ + 3CaSO₄ + 18 H₂O

Al₂(SO₄)₃.18H₂0 + 3Na₂CO₃ -----------2Al(OH)₃ + 3Na₂SO₄ + 18 H₂O + 3C0₂

The insoluble and colloidal aluminium hydroxide [Al(OH)₃] forms the flock, which remove the fine suspended and colloidal impurities. The pH value of water should be between 6.5 to 8.5 for best results. The amount of alum required for coagulation mainly depends on the turbidity, pH-value, temperature and colour of water. The quantity of optimum dose of alum is determined by practical test in the lab. The most commonly used test is jar test in the lab. Usually the dose of alum varies from 5 mg/lit for relatively clear water to about 85 gm/lit for very turbid waters.

Procedure:

1. The pH-value, temperature, colour and turbidity of sample water were determined.

2. The sample water was taken in each beaker up to 500ml

3. The beakers were put in the flock tester and magnetic stirrer was put in the beaker carefully.

4. A dash mixture was place on the bench infront of each beakers and the flock tester was switched on and set it for 10 minutes.

5. The different doses of 1% alum were put into each of the six dash mixing tray using 1ml, 2ml of pipettes.

6. The dash mixture was put in the beaker in same time plunge in such way that it nearly bottom of the beaker.

7. The pH of water in all beakers was measured at 10 minutes.

8. The stirring was stopped and removed when the time reached 30 minutes

9. It has to be allowed for 30 minutes to settle flock.

10. All the beakers were observed taking top water.

Observation:

Temperature of sample water = 16°C

Colour of the sample water = 40°H

Turbidity of sample water = 3 NTU

pH value of sample water = 6.47

Volume of sample water taken = 500ml

Sample	Dose (ml)	pH	Turbidity	Remarks	Concentration of alum in (mg/h)
1	0.5	7.8	8.5		10
2	1.0	7.44	3.7	Optimum dose	20
3	1.5	7.66	4.0		30
4	2.0	7.58	75.0		40
5	2.5	7.55	30.0		50
6	3.0	7.62	40.0		60

Conclusion:

From the observation, it is concluded that the optimum dose of the coagulant is 2.0 ml as it gives the best floc (i.e. maximum turbidity). This is the best result as the pH-value lies between 6.5 and 8.5. So the water with 2ml alum is better than others.

Hence the amount of optimum dose of alum can be determined in the laboratory. In water works this test is done frequently to determine optimum dose and economical use of coagulants. Similarly when the quality and characteristics of water change, the test should be done continuously.