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.
Principle
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Coagulants are used in water treatment plants
- to remove natural suspended and colloidal matter,
- to remove material which do not settle in plain sedimentation, and
- 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 apparatus
- Glass beakers
- Pipette
- Nephelometer
- pH meter
Reagents (click to check the preparation of reagents)
- Alum solution (1mL containing 10 mg of alum)
- Lime
- Acid/alkali
- Take 1-litre beakers and fill them with sample up to the mark.
- Keep each beaker below each paddle and lower the paddles, such that each one is about 1cm above the bottom.
- Find the pH of the sample and adjust it to 6 to 8.5.
- Pipette 1, 2, 3, 4, 5, 6 mL of the alum solution into the test samples.
- Immediately run the paddles at 100 rpm for 1 minute.
- Reduce the speed to 30-40 rpm and run at this rate for 30 minutes.
- Stop the machine, lift out the paddles and allow to settle for 30 minutes.
- Find the residual turbidity of the supernatant using nephelometer.
- Plot a graph with alum dosage along x-axis and turbidity along y-axis.
- The dosage of alum, which represents least turbidity, gives Optimum Coagulant Dosage (O.C.D.).
- Repeat steps 1-10 with higher dose of alum, if necessary.
Observation
Optimum coagulant dosage = ..........