Determination of Jar Test (Alum Dose selection) for turbid water

Determination of Jar Test (Alum Dose selection) for turbid water

Discussion:

Coagulant and Coagulation

Coagulants are used in all water treatment plants and in some sewage u

1hey are used:

1. To remove natural suspended and colloidal material

2. To remove materials which do not settle readily in chemical treatment proc

3. To assist in filtration by forming mats on sand filters and

4. To assist in the vacuum filtration of sewage sludge.

A coagulant reacts with the natural alkalinity in solution in the liquid treated, or with other chemicals. to 1orm an insoluble, flocculent precipitate. The precipitate clarifies the e coagulating, absorbing and entraining suspended and colloidal material. It may also remove color gases in solution. A portion of the precipitate formed in water filtration plants is carried through sedimentation basins to be deposited on the filter beds where it further assists in the clarification of the water and removal of color, odor, and taste producing compounds.

The chemicals most commonly used as coagulants in water treatment are aluminum sulphatc (alum iron suphate (copperas), ferric sulfate, iron chloride and sodium aluminates. Their use results in the formation of the hydroxides or aluminates, which are the effective coagulating agents.

Alum is most widely used coagulant. Commercial alum contains combinations of Al2. (SO4)2. I8H2O and Al2 (SO4)3.1 9H20. Alum reacts with the natural alkalinity of the water or, if the alkalinity is insufficient, with the added alkalinity in the form of lime [Ca(OH)2] or soda ash (Na2CO3). Producing precipitate, which is usually, considered to be aluminum hydroxide. Ordinarily there is sufficient natural alkalinity present to react with the coagulant.

Reactions of Coagulants:

Presence of Alkalinity

Al2 (SO4)3 + 14H2O +6HC03 ­­­→ 2Al (OH) 3 +6CO2 + 14H2O+3SO4

FeCl3 + 3HCO → 3Fe (OH)3 + 3CO2 + 3C

In absence (Without) of Alkalinity

Al2 (SO4) 3+ 14H20 → 2AI (OH) 3 + 3H2SO4 + 8H20

FeCl3 + 3H2O → Fe (OH) 3 + 3HCI

CLEANING THE WATER

Untreated water supplies contain suspended material that does not settle quickly. Such water is said to be turbid, which means cloudy. When turbid water is treated with certain chemicals, suspended materials settles more quickly.

The chemicals used to increase the settling rate are called coagulants. They form a sticky substance called floc. Suspended particles stick to floc. Eventually enough particles stick to the floc to form large clumps. The clumps settle to t e bottom

The amount of coagulant depends upon the amount and kind of suspended matter. I low much coagulant to use must be determined for e ach sample of turbid water. In this experiment, lind which amount of coagulant produces good floc formation and cause the fastest settling.

The coagulant you will use is a solution containing 1 gram of aluminum sulfate in 1 liter of solution. Good floc formation may not mean the fastest settling rate. If you watch closely and follow directions carefully, you will get an accurate answer.

ALUM-DOSE SELECTION

Equipment and Supplies:

• Beakers 1-liter or 400-ml, 7 Nos.
  
• Graduated cylinder 100-ml, 7 Nos.
  
• Pipettes and cylinder or bulbs or burettes 25-ml, 6Nos.
  
• Stirrer
  
• Turbid water sample
  
• Aluminum Sulfate solution, 1000 mg/L
• Procedure:
  

1. Set up seven 1-liter or 400-ml beakers numbered 1 through 7, respectively, for each group.
   
2. Pour 400-ml of turbid after into 1-liter beakers. Or 200-ml of turbid water into a 400-ml beaker.
   
3. Use a pipette to add the assigned volume of the aluminum sulfate, for both quantities are given in the chart below.
   
4. Stir the liquid in the beaker vigorously for 2 minutes. Than stir the liquid very slowly (10 to 15 revolutions per minutes).
   
5. Set the beakers aside to settle for 20 minutes. Do not disturb the samples of water as you observe them during the 20-minutes.
   
6. Inspect all seven beakers with the members of your team during and at the end of the 20-minutes settling time. Describe the samples as follows:
   
7. Which sample formed the flock the faster?
   
8. Which sample seems to have the most flock?
   
9. Which sample seems to form the largest particles?
   
10. Which sample seems to have least flock?
    
11. Which sample seems to settle best?
    

CHIART: