Determination of Optimum Coagulant Content in Water by JAR TEST

Determination of Optimum Coagulant Content in water by JAR TEST-Environmental Engg Lab can find out by following this method

Coagulants are used in water treatment plant.

1. Remove the natural suspended and colloidal matters.

2. Remove materials which do not settle is plain sedimentation

3. Assist infiltration. Alum is the most widely used coagulant.

Nowadays coagulant aids are also being used in conjunction with alum to reduce alum dose required. When a coagulant is added to water containing turbidity, flocs will be formed.

Floc formation depends upon the quantity of coagulant added (coagulant dose), pH, temperature, flocculation time etc. The test is supposed to be conducted as a routine experimental in all the water treatment plants.

Determination of Optimum Coagulant Content in water by JAR TEST(Jar test experiment)- is a simple device which will help in determining the optimum coagulant dose required. The jar test device consists of a number of stirrers provided with paddles.

The paddles can be rotated with varying speed with the help of a motor and regulator. The sample will be taken in jars/ beakers and varying doses of coagulants will be added simultaneously to all the beakers.

The paddles will be rotated at 100 rpm for 1 minute at 40 rpm for 9 minutes corresponding to the flash mixing and slow mixing in the flocculator.

After 20-30 minutes settling, supernatant will be carefully decanted from all the jars to measure the turbidity. The dose which gives the least turbidity will be taken as the optimum coagulant dose.


Determination of Optimum Coagulant Content in water
Determination of Optimum coagulant content in water (Jar test)


Apparatus used in jar test experiment

  1. Jar testing apparatus
  2. Turbidity Meter
  3.  Beakers


  1. Alum solution (1 percent)

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Procedure for jar test experiment

The first procedure that we gonna do in jar test experiment is,

  1. Measure initial turbidity using turbidity meter
  2. Measure and adjust the pH of the sample between 6 and 8
  3. Measure 1000 mL of the sample in 6 beakers and place them in the jar testing apparatus.
  4. Switch on the instrument and adjust the speed of the paddles to 100 rpm
  5. Add varying doses of alum in increasing order corresponding to 2, 4, 6, 8, 10 and 12 mg per litre the beakers simultaneously and start a stopwatch.
  6. Allow the rapid mix at 100 rpm for one minute
  7. Bring down the speed to 40 rpm and allow the show mix for 9 minutes.
  8. Switch of the instrument and allow 10 minutes settling
  9. Take out supernatant (about 50 mL) without disturbing the settled flocs if possible simultaneously from all the beakers.
  10. Measures the turbidity of all the samples with the help of a turbidity meter.
  11. Repeat the steps 1 to 8 with a higher dosage of alum if necessary
  12. Draw graph of settled water turbidity with alum dose.
  13. Note down economical or optimal dose from the graph,
  14. The optimum alum dose for the given sample of water will get in mg/L.

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The below video shows the jar test experiment by nctel.

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