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  Section: Plant Protocol » Environmental Science Methodology
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Jar Test for Determining Optimum Coagulant Dosage

To determine the optimum coagulant dosage for clarifying the given sample of water by using alum as the coagulant and performing the jar test experiment.

    Coagulants are used in water treatment plants
  1. to remove natural suspended and colloidal matter,
  2. to remove material which do not settle in plain sedimentation, and
  3. to assist in filtration.
Alum [Al2S(SO4)3. 18H2O] is the most widely used coagulant. When alum solution is added to water, the molecules dissociate to yield SO42- and Al3+. The +ve species combine with negatively charged colloidal to neutralise part of the charge on the colloidal particle. Thus, agglomeration takes place. Coagulation is a quite complex phenomenon and the coagulant should be distributed uniformly throughout the solution. A flash mix accomplishes this.

Jar test is simple device used to determine this optimum coagulant dose required. The jar test, device consists of a number of stirrers (4 to 6) provided with paddles. The paddles can be rotated with varying speed with the help of a motor and regulator. Samples will be taken in jars or beakers and varying dose of coagulant will be added simultaneously to all the jars. The paddles will be rotated at 100 rpm for 1 minute and at 40 rpm for 20 to 30 minutes, corresponding to the flash mixing and slow mixing in the flocculator of the treatment plant. After 30 minutes settling, supernatant will be taken carefully from all the jars to measure turbidity. The dose, which gives the least turbidity, is taken as the optimum coagulant dose.

  1. Jar test apparatus
  2. Glass beakers
  3. Pipette
  4. Nephelometer
  5. pH meter

Reagents (» click to check the preparation of reagents)
  1. Alum solution (1mL containing 10 mg of alum)
  2. Lime
  3. Acid/alkali

  1. Take 1-litre beakers and fill them with sample up to the mark.
  2. Keep each beaker below each paddle and lower the paddles, such that each one is about 1cm above the bottom.
  3. Find the pH of the sample and adjust it to 6 to 8.5.
  4. Pipette 1, 2, 3, 4, 5, 6 mL of the alum solution into the test samples.
  5. Immediately run the paddles at 100 rpm for 1 minute.
  6. Reduce the speed to 30-40 rpm and run at this rate for 30 minutes.
  7. Stop the machine, lift out the paddles and allow to settle for 30 minutes.
  8. Find the residual turbidity of the supernatant using nephelometer.
  9. Plot a graph with alum dosage along x-axis and turbidity along y-axis.
  10. The dosage of alum, which represents least turbidity, gives Optimum Coagulant Dosage (O.C.D.).
  11. Repeat steps 1-10 with higher dose of alum, if necessary.

Optimum coagulant dosage = ..........


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