- 2070 Chaitra (old course)
Short note: Jar test
Short answer
A jar test uses six beakers with variable‑speed paddles to simulate coagulation, flocculation, and sedimentation. Increasing coagulant doses are added, mixed, settled, and the supernatant turbidity measured; the lowest dose that yields clear water and firm floc is the optimum.
Write a short note on jar test.
A jar test is a laboratory test that finds the optimum coagulant dose (and the best pH) for a given raw water by simulating coagulation, flocculation and sedimentation in a set of beakers.
Apparatus
A jar-test (flocculator) apparatus has six beakers or jars (1 or 2 litres each) with variable-speed paddles driven by a common motor, a pH meter, a turbidimeter and a stop-watch.
Procedure
- Fill the six jars with equal volumes of raw water and note the temperature, pH, turbidity and alkalinity.
- Add increasing doses of coagulant solution to the jars (e.g. 5, 10, 15, 20, 25, 30 mg/l), at the same time.
- Rapid mix at 100 rpm for 1 minute (simulating the rapid mixing basin).
- Slow mix at 20 to 30 rpm for 15 to 20 minutes (simulating flocculation); note the time when the floc first appears and its size.
- Switch off and allow the flocs to settle for 30 minutes (sedimentation).
- Test the supernatant for turbidity, colour and residual coagulant.
Interpretation
The lowest dose that gives the clearest water, with a quick-forming, large and firm floc, is the optimum dose. The test is repeated at different pH (by adding lime or acid) to find the best pH and for different coagulants or coagulant aids.
Uses
- To find the optimum coagulant dose and pH, which changes with the raw water quality.
- To compare coagulants and coagulant aids and to save chemicals.
- To select the mixing and settling time needed for design.
✓ Answer checked by Claude Opus 5.5 (high effort)