Water Treatment Lab Experiments — Jar Test, Filtration and Chlorination Procedures

Introduction

Water treatment lab experiments are performed in environmental engineering and civil engineering departments to understand the physical, chemical, and biological processes used to treat drinking water and wastewater. This guide covers the three most important experiments: jar test (coagulation-flocculation), rapid sand filtration, and chlorination.

Experiment 1: Jar Test (Coagulation-Flocculation)

Aim

To determine the optimum dose of coagulant (alum) for effective removal of turbidity from turbid water samples.

Apparatus

• Jar test apparatus (6-paddle stirrer with 1-litre beakers)
• Turbidity meter (nephelometer)
• Alum (aluminium sulphate) solution
• pH meter
• Stopwatch

Procedure

1. Fill 6 jars with 1 litre of turbid water sample each. Measure initial turbidity (NTU) and pH.
2. Add varying doses of alum solution to each jar (e.g., 5, 10, 15, 20, 25, 30 mg/L).
3. Apply rapid mixing at 100 rpm for 1 minute (flash mix), then slow mixing at 30 rpm for 20 minutes (flocculation).
4. Allow 30 minutes for sedimentation without stirring.
5. Collect the supernatant carefully without disturbing the sludge.
6. Measure residual turbidity (NTU) and pH of each sample.
7. Plot residual turbidity vs. alum dose — the dose giving minimum turbidity is the optimum.

Observation Table — Jar Test

Experiment 2: Rapid Sand Filtration

Aim

To study the operation of a rapid sand filter and to determine filtration rate and filter efficiency for turbidity removal.

Procedure

1. Fill the filter column with layers: gravel (bottom), coarse sand, fine sand (top).
2. Pass pre-settled turbid water through the filter at a controlled rate.
3. Collect filtered water samples at intervals (0, 5, 10, 20, 30 minutes).
4. Measure turbidity of filtered water at each interval using a turbidimeter.
5. Calculate filtration efficiency: % removal = [(initial turbidity − final turbidity) / initial turbidity] × 100.
6. Measure the hydraulic loading rate: Q/A (m³/m²/hour).

Experiment 3: Chlorination (Breakpoint Chlorination)

Aim

To determine the breakpoint chlorine demand of a water sample and to establish the chlorine dose required for effective disinfection.

Procedure

1. Take several samples of water in 250 mL Erlenmeyer flasks.
2. Add increasing doses of chlorine (as sodium hypochlorite) to each flask (0, 1, 2, 4, 6, 8, 10 mg/L as Cl₂).
3. Allow contact time of 30 minutes at room temperature.
4. Measure residual chlorine in each sample using an amperometric titrator or DPD colorimetric method.
5. Plot chlorine dose vs. residual chlorine — the breakpoint is where residual chlorine reaches minimum.
6. The dose beyond the breakpoint provides a free chlorine residual for disinfection.

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Chlorination and Disinfection — Detailed Procedure

Break-Point Chlorination

Break-point chlorination is the process of adding chlorine to water until the chlorine demand is fully satisfied and a free chlorine residual begins to appear. The “break point” is the dose at which combined chlorine (chloramines) is destroyed and free available chlorine starts to accumulate.

The chlorine demand curve shows four zones:

1. Zone 1: Chlorine reacts with reducing agents (Fe, Mn, H2S) — no residual
2. Zone 2: Chlorine reacts with ammonia to form chloramines — combined residual rises
3. Zone 3: Chloramines are oxidised and destroyed — residual falls to the break point
4. Zone 4: Free chlorine residual rises linearly with added dose

Determination of Chlorine Demand — Procedure

1. Prepare a series of water samples (200 mL each) in identical jars.
2. Add increasing measured doses of chlorine solution (e.g., 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 mg/L) to each jar.
3. Mix and allow a contact time of 30 minutes in the dark.
4. Measure residual chlorine in each jar using the DPD colorimetric method or iodometric titration.
5. Plot residual chlorine (y-axis) vs. applied chlorine dose (x-axis) — identify the break point.
6. Chlorine demand = applied dose at break point – free residual at that dose.

WHO and IS 10500 Residual Chlorine Standards

• Free residual chlorine at consumer tap: 0.2 mg/L (minimum, IS 10500:2012)
• Free residual chlorine at treatment plant outlet: 0.5 mg/L
• Contact time (CT value): minimum 30 minutes at pH < 8.0 for effective disinfection
• Maximum permissible residual: 1.0 mg/L (taste threshold consideration)

DPD Method for Residual Chlorine Measurement

The N,N-diethyl-p-phenylenediamine (DPD) method is the standard colorimetric procedure. DPD reacts with free chlorine to produce a pink/magenta colour, the intensity of which is proportional to chlorine concentration, measured at 515 nm in a spectrophotometer or by visual comparison with a colour disc.

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