---
title: "Short note: Jar test"
subject: "Water Supply Engineering"
chapter: 6
key: "semester_5/water_supply_engineering:jar-test-note"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/jar-test-note
site: BCT Engineering
---

# Short note: Jar test

Water Supply Engineering · Chapter 6: Water Treatment

- Asked in: 2070 Chaitra (old course)
- Video: [Water Treatment | Coagulation Process Operation and Jar Testing](https://www.youtube.com/watch?v=72uFoBjSqaI) — American Water College
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/jar-test-note
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-6

## Question

Write a short note on 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.

## Answer

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
1. Fill the six jars with equal volumes of raw water and note the temperature, pH, turbidity and alkalinity.
2. Add increasing doses of coagulant solution to the jars (e.g. 5, 10, 15, 20, 25, 30 mg/l), at the same time.
3. Rapid mix at 100 rpm for 1 minute (simulating the rapid mixing basin).
4. 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.
5. Switch off and allow the flocs to settle for 30 minutes (sedimentation).
6. 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does a jar test determine?
   A. The bacterial content of the filtered water
   B. The chlorine demand of the water
   C. The sand size for filters
   D. The optimum coagulant dose (and best pH) for a given raw water
   Answer: D. It simulates coagulation, flocculation and settling in beakers to find the best dose and pH. (Key point: Purpose of jar test)

2. Which sequence matches the jar test procedure?
   A. Rapid mix about 100 rpm for 1 min, slow mix 20 to 30 rpm for 15 to 20 min, then settle 30 min
   B. Slow mix 100 rpm for 1 min, rapid mix 30 rpm for 20 min, then settle 30 min
   C. Rapid mix 20 rpm for 30 min, then settle for 1 min
   D. Settle 30 min first, then rapid mix at 100 rpm
   Answer: A. Rapid mix simulates the mixing basin, slow mix the flocculator and settling the sedimentation tank. (Key point: Procedure speeds)

3. Which jar is taken as giving the optimum dose?
   A. The one with the highest dose, as it clears the most
   B. The one with the clearest water at the lowest dose, with a quick, big, firm floc
   C. The one in which the first floc appears last
   D. The one with the lowest pH regardless of clarity
   Answer: B. Optimum means best clarity for the least chemical, with good floc formation. (Key point: Choosing optimum dose)