Jar Test Procedure for Coagulant and Flocculant Optimization — Step-by-Step Guide with Interpretation

Introduction

The jar test is the most important tool in a water treatment plant operator’s arsenal — yet it is frequently performed incorrectly, leading to over-dosing, under-dosing, and wasted chemicals. A properly conducted jar test can reduce chemical consumption by 20-40% while improving treated water quality. This guide provides a complete, step-by-step procedure with interpretation guidance and common mistakes to avoid.

Equipment Required

Equipment	Specification	Purpose
Jar test apparatus	6-place, variable speed (0-300 rpm), illuminated base preferred	Simultaneous testing of 6 dose levels under identical mixing conditions
Beakers	1000mL or 2000mL, Griffin (low-form), identical shape and size	Sample containers — must be identical for valid comparison
Graduated cylinders	10mL, 50mL, 100mL, 1000mL	Measuring samples and stock solutions
Pipettes / micropipettes	1-5mL and 0.1-1.0mL	Precise dosing of stock solutions
Turbidimeter	NTU, 0-1000 range	Supernatant clarity measurement
pH meter	Calibrated, ±0.1 pH	Pre- and post-treatment pH
Timer/stopwatch	—	Mixing and settling time
Thermometer	0-100°C	Sample temperature (affects coagulation kinetics)

Stock Solution Preparation

PAC Stock Solution (1% = 10,000 mg/L)

1. Weigh 10.0g PAC powder (spray dried or drum dried)
2. Add to ~800mL distilled water in a 1000mL volumetric flask or beaker
3. Stir at 200-300 rpm for 10-15 minutes until fully dissolved. Spray dried PAC dissolves in 3-8 min; drum dried may take 15-30 min
4. Make up to 1000mL with distilled water. Mix well
5. Use within 1 week. Store in a closed container, away from direct sunlight

Dosing calculation: 1mL of 1% PAC solution added to 1000mL sample = 10 mg/L PAC dose

PAM Stock Solution (0.1% = 1,000 mg/L)

1. Weigh 1.0g PAM powder
2. Add to ~800mL distilled water while stirring at 200-300 rpm. Critical: Sprinkle PAM slowly into the vortex. Dumping powder in creates undissolved “fisheye” gel particles
3. Stir at 200-300 rpm for 60 minutes minimum (90 minutes if water temperature <15°C)
4. Make up to 1000mL. Mix gently — avoid high shear
5. Use within 24 hours. PAM solution degrades over time

Dosing calculation: 1mL of 0.1% PAM solution added to 1000mL sample = 1 mg/L PAM dose

Jar Test Procedure

Phase 1: Preparation

1. Collect representative sample: 10-15L from the plant influent, after screening and equalization. Avoid sampling immediately after a process change or during CIP. Record time, date, source, and any visual observations (color, odor, turbidity)
2. Measure initial parameters: pH, temperature, turbidity, COD (if relevant). Record all values on the jar test data sheet
3. Warm/cool sample to process temperature: The jar test should be conducted at the same temperature as the full-scale plant. If plant water is 35°C but you test at 22°C, results will not translate
4. Fill 6 beakers: Exactly 1000mL each. Use a graduated cylinder for accuracy — beaker markings are often inaccurate
5. Label beakers: 1 through 6, with planned doses

Phase 2: PAC Dosing and Rapid Mix

1. Set stirrers to 200 rpm (rapid mix — G value ~300-600 s-1 for 1000mL beaker)
2. Dose PAC simultaneously into all 6 beakers: Use pipettes pre-loaded with the calculated volume. Suggested PAC doses (mg/L): Blank (0), 50, 100, 200, 300, 500. Adjust range based on prior experience — if your plant typically uses 200 mg/L, test 100, 150, 200, 250, 300, 400
3. Rapid mix for 1 minute (or 2 minutes for high-turbidity water). Goal: uniform, instantaneous coagulant distribution

Phase 3: PAM Dosing and Slow Mix

1. Reduce speed to 40-60 rpm (slow mix / flocculation — G value ~20-60 s-1)
2. Dose PAM: Suggested doses: 1, 2, 3, 5 mg/L on beakers 2-5 (skip blank). Or if optimizing both PAC and PAM simultaneously, use a matrix design
3. Slow mix for 10-15 minutes. Observe floc formation: initial pin flocs should appear within 30-60 seconds, growing to 2-5mm by 5-10 minutes. If no flocs form by 3 minutes, the PAC dose is likely too low

Phase 4: Settling

1. Stop stirrers, start timer. Observe for 15-30 minutes
2. Record settling velocity: Measure floc blanket descent every 2 minutes (or more frequently in first 5 minutes)
3. Note floc characteristics: Size (1-2mm = fine, 2-5mm = good, >5mm = excellent but may trap water), shape (spherical vs irregular), settling pattern (blanket vs individual flocs)

Phase 5: Supernatant Analysis

1. Sample from mid-depth: Use pipette or syringe, withdraw gently from ~5cm below the water surface. Do NOT decant — this disturbs settled flocs
2. Measure turbidity of each supernatant. Record NTU
3. Measure pH of each supernatant. Record alongside initial pH
4. Optionally measure COD, color, UV254 depending on treatment goals

Phase 6: Data Analysis and Dose Selection

1. Plot turbidity (y-axis) vs PAC dose (x-axis): The curve typically shows rapid improvement, then a plateau or minimum, then possible increase at overdose (re-stabilization)
2. Select the dose at the “knee” of the curve: The lowest dose that achieves target turbidity. Not the lowest turbidity — the marginal improvement from 1.5 NTU to 1.0 NTU may require double the PAC dose
3. Check pH: If pH at optimal turbidity dose is outside the desired range, adjust raw water pH and re-run the jar test
4. Verify with a confirmation jar test: Run triplicates at the selected dose to confirm reproducibility

Interpreting Jar Test Results

Observation	Interpretation	Action
No flocs at any dose	pH out of range for coagulation; interfering substance (chelating agent, surfactant); or PAC is degraded/hydrolyzed	Check pH; test raw water for complexing agents; try fresh PAC sample
Pin flocs only (<1mm)	PAC dose too low or insufficient PAM for bridging	Increase PAC dose; add or increase PAM dose
Large, fluffy, slow-settling flocs	PAM overdose — water trapped in floc structure	Reduce PAM dose. Large flocs that don’t settle are worse than medium flocs that settle fast
Cloudy/milky supernatant with no settleable flocs	PAC overdose — re-stabilization (particles re-charged positive, repel each other)	Reduce PAC dose. Re-stabilization is the clearest sign of overdose
Clear supernatant, fine surface scum	Oil/grease or floating solids not captured by flocs	Consider DAF instead of sedimentation. Increase PAM dose slightly
Good clarity but high residual Al (>0.2 mg/L)	Soluble Al species not precipitated; pH may be too low	Increase pH to 6.5-7.5; consider higher basicity PAC

Common Jar Test Mistakes

1. Using old or improperly prepared PAC/PAM solutions: Stale PAM solution loses 20-50% effectiveness. Always use fresh solutions
2. Testing at the wrong temperature: Cold water (<10°C) requires 20-40% higher PAC dose. If your jar test is at 22°C lab temp but plant water is 5°C, you will under-dose in the plant
3. Not mixing identically across beakers: Beaker shape, paddle position, and fill volume must be identical. A 10mm difference in paddle immersion changes mixing intensity ~25%
4. Testing on the wrong sample: Raw influent that has sat for hours has different characteristics than fresh, flowing influent. Sample just before testing
5. Choosing the clearest supernatant instead of the best value dose: The lowest turbidity dose may be 3x the cost of a slightly higher turbidity dose that still meets targets
6. Not running a blank (no chemical) control: You need to know how much settling occurs naturally. A 40% turbidity reduction without chemicals is not unusual for high-solids wastewater

HydroChemix provides free jar test protocol templates and technical support for chemical dosing optimization. Contact jingshuicc@gmail.com for a downloadable jar test data sheet and dosing recommendation for your specific wastewater.