Jar Test Guide for Coagulants and Flocculants

What is a Jar Test?

A jar test is a laboratory-scale simulation of full-scale coagulation-flocculation processes used to determine the optimal type and dosage of coagulants and flocculants for water and wastewater treatment. By testing multiple chemical combinations under controlled conditions, operators can identify the most cost-effective treatment program before implementing changes at full scale. Jar testing is essential for both initial system design and ongoing optimization.

When to Run Jar Tests

• New system design: Before installing treatment equipment, to select chemicals and determine required dosages
• Process optimization: When effluent quality is poor or chemical costs are too high
• Seasonal changes: Water quality changes with temperature, rainfall, and production patterns
• New chemical suppliers: To verify performance before switching suppliers
• Effluent limit changes: When discharge standards become more stringent
• Regular QA/QC: Monthly or quarterly to ensure ongoing optimal performance

Equipment and Materials Needed

Item	Specification	Notes
Jar test apparatus	4-6 paddle stirrers, variable speed	Phipps & Bird or equivalent
Beakers / jars	1000 mL or 2000 mL	Clear glass or plastic
Sample water	1-2 liters per jar	Must be representative; test within 24 hours
Coagulants	PAC, PFS, alum, ferric chloride	Prepare 1% or 10% solutions
Flocculants	Anionic / cationic PAM	Prepare 0.1% solution, fresh
pH adjustment chemicals	NaOH (1M) and H₂SO₄ (1M)	For pH optimization
pH meter	Calibrated	Measure before and after
Turbidity meter	NTU scale	Measure supernatant quality
Pipettes / syringes	Various sizes	Accurate chemical dosing
Stopwatch / timer	—	Track mixing and settling times

Jar Test Mixing Speed (RPM): Rapid Mix vs Slow Mix

Mixing speed is the variable most often copied blindly from one jar test report to the next. The two mixing phases do different jobs, so they need different speeds:

• Rapid mix (coagulation): a short, intense mix that spreads the coagulant through the whole jar before it hydrolyses. The procedure below starts at 100-200 RPM (G-value ~200-300 s⁻¹).
• Slow mix (flocculation): gentle stirring that lets destabilised particles collide and grow without tearing the floc apart. The procedure below uses 20-50 RPM (G-value ~20-50 s⁻¹).

Treat these as starting points, not a standard. The same RPM gives a different G-value on paddles and jars of different shapes, so check your jar tester’s manual or G-value chart, and always record the RPM and mixing time next to each result. If the floc looks fine but breaks up during the slow mix, lower the speed; if it stays as pin floc, try a slightly longer or faster flocculation stage before changing the dose. For a deeper look at tuning the two stages, see how to optimize flocculation mixing speed and rapid mix vs slow mix in flocculation.

For RPM-to-G conversion and Gt alignment, see Velocity Gradient (G Value) in Jar Testing.

Step-by-Step Jar Test Procedure

Step 1: Sample Preparation

1. Collect a representative water sample. Ensure it is well-mixed and reflects actual process conditions.
2. Measure initial water quality parameters: pH, temperature, turbidity, color, COD, alkalinity.
3. Pour equal volumes (typically 1000 mL) of sample into each jar.
4. Label each jar with the test condition (dosage, chemical type, etc.).

Step 2: pH Adjustment (if needed)

1. If testing pH effect, adjust each jar to a different pH using dilute acid or base.
2. Stir gently and measure pH to confirm target value.
3. Common pH range to test: 5.5, 6.0, 6.5, 7.0, 7.5, 8.0

Step 3: Coagulant Addition (Rapid Mix)

1. Start rapid mixing at 100-200 RPM (G-value ~200-300 s⁻¹).
2. Add coagulant (PAC, PFS, alum, etc.) to each jar at different dosages.
3. Mix rapidly for 1-3 minutes to ensure uniform dispersion and charge neutralization.
4. Typical dosage series for initial screening: 10, 20, 50, 100, 150, 200 mg/L

Step 4: Flocculant Addition (Slow Mix)

1. Reduce mixing speed to 20-50 RPM (G-value ~20-50 s⁻¹) for flocculation.
2. Add flocculant (PAM) if testing polymer aid. Typical dosage: 0.5-5 mg/L.
3. Mix slowly for 10-20 minutes to allow floc formation and growth.
4. Observe floc size, strength, and settling characteristics during this period.

Step 5: Settling

1. Stop mixing and start timer.
2. Allow flocs to settle undisturbed for 10-30 minutes.
3. Record time for 50% of jar to clear (interface settling rate).
4. Observe final settled sludge volume and supernatant clarity.

Step 6: Measurement and Evaluation

1. Collect supernatant sample from 2-3 cm below the surface (avoid disturbing settled sludge).
2. Measure key parameters: turbidity, pH, color, COD, total phosphorus, etc.
3. Record sludge volume (SV30): settled sludge volume after 30 min, in mL/L.
4. Compare results across all test conditions.

Experimental Design Strategies

Screening Test (First Round)

When testing a new water source, start with a wide range to identify the effective dosage range:

Jar	1	2	3	4	5	6
PAC (mg/L)	10	25	50	100	150	200

Use this to find the approximate optimal range, then narrow down in the next round.

Optimization Test (Second Round)

Based on screening results, test a narrower range with smaller increments:

Jar	1	2	3	4	5	6
PAC (mg/L)	40	50	60	70	80	100

Flocculant Optimization

Once optimal coagulant dosage is found, test PAM dosage:

Jar	PAC (mg/L)	Anionic PAM (mg/L)
1 (control)	50	0
2	50	0.5
3	50	1.0
4	50	2.0
5	50	3.0
6	50	5.0

pH Optimization

Test different pH values at constant coagulant dosage to find the optimal pH range.

Product Comparison Test

When comparing products from different suppliers, test all at the same dosage and conditions:

Jar	Product	Dosage
1	Product A (current)	50 mg/L
2	Product B (supplier 1)	50 mg/L
3	Product C (supplier 2)	50 mg/L
4	Product A	60 mg/L
5	Product B	60 mg/L
6	Product C	60 mg/L

How to Interpret Results

Key Performance Indicators

• Supernatant turbidity: Primary indicator of treatment effectiveness
• Floc size: Larger flocs settle faster; very small flocs indicate under-dosing
• Settling rate: How quickly the water clears; faster = better
• Sludge volume (SV30): Lower volume = less sludge disposal cost
• Final pH: Must be within acceptable range for discharge or downstream processes
• Residual coagulant: Avoid over-dosing which can increase residual metal

Common Patterns

Observation	What It Means	Action
Small, scattered flocs	Under-dosing or wrong pH	Increase dosage; adjust pH
Large flocs, cloudy water	Restabilization (over-dose)	Decrease coagulant dosage
Flocs break on mixing	Weak flocs; need polymer	Add PAM as flocculant aid
Good settling but turbid top	Pin floc carryover	Add PAM or increase dosage slightly
Very large, fluffy flocs	PAM over-dose	Reduce polymer dosage
Different results each jar	Sample not homogeneous	Mix sample thoroughly before pouring

Common Mistakes to Avoid

• Not mixing sample thoroughly: Settled solids cause inconsistent results between jars.
• Adding chemicals all at once: Add coagulant while stirrers are already running for uniform mixing.
• Wrong mixing speed: Too fast breaks flocs; too slow prevents proper formation.
• Not measuring initial water quality: Changes in influent quality can invalidate comparisons.
• Testing too narrow a range: Always include a control and a wide range in the first round.
• Single round testing: At least 2-3 rounds needed for true optimization.
• Ignoring temperature: Cold water reduces coagulation efficiency; may need higher dosage.
• Not verifying with full-scale: Jar tests predict but don’t guarantee full-scale performance.
• Using old polymer solutions: PAM solutions degrade after 24-48 hours; always prepare fresh.

Scale-Up Considerations

Jar test results provide a starting point, but full-scale operation typically requires adjustments:

• Dosage adjustment: Full-scale dosage is usually 80-120% of jar test optimal value.
• Mixing intensity: Ensure full-scale mixers provide appropriate G-values (rapid mix: 300-500 s⁻¹, flocculation: 20-50 s⁻¹).
• Retention time: Verify full-scale tanks provide sufficient contact time.
• Sludge handling: Jar test sludge volume can help estimate full-scale sludge production.
• Pilot testing: For large systems, run a pilot before full-scale implementation.
• Regular re-testing: Water quality changes over time; re-test quarterly or after process changes.

Jar RPM vs Plant Mixer Speed

Jar test RPM does not translate 1:1 to a full-scale mixer. A plant flash mixer, a hydraulic flocculator and a 2-litre jar have completely different geometry, so the useful link between them is mixing intensity (G-value) and contact time, not shaft speed. Use the jar test to find which chemical and dose range works, then ask your equipment or chemical supplier to check that the plant mixers deliver comparable intensity and time.

Advanced Jar Testing Techniques

• Zeta potential measurement: Quantifies particle charge to optimize coagulant dosage.
• Streaming current detector: Real-time charge measurement for process control.
• Filterability test: Filter supernatant to predict downstream filtration performance.
• Floc strength test: Re-shear flocs and measure re-flocculation ability.
• Sequential dosing: Test split addition of coagulant for better performance.
• Dual coagulant testing: Test PAC + PFS blends for synergistic effects.

Frequently Asked Questions

What RPM should I use for a jar test?

Use your jar tester’s rapid-mix and flocculation settings as a starting point (this guide starts at 100-200 RPM for rapid mix and 20-50 RPM for slow mix), then adjust based on floc behaviour. Report RPM and time with every result instead of copying one number from the internet.

Is jar test mixing speed the same as plant mixer speed?

No. Equipment geometry differs, so compare mixing intensity (G-value) and contact time rather than RPM. See the scale-up and jar vs plant sections above.

How many jars or doses should I run?

Run enough jars to bracket the dose you expect to need, plus one blank with no chemical. If the best result sits at the edge of the range, run a second round around it. You can turn the result into a plant dose with the dosage calculator, and for organics-driven projects see the COD removal methods guide.

Conclusion

Jar testing is the foundation of effective coagulation-flocculation process design and optimization. A well-designed jar test program can significantly reduce chemical costs while improving effluent quality. The key is to follow a systematic approach: start with broad screening, narrow down to optimal ranges, test multiple parameters (dosage, pH, polymer), and verify results at full scale. Regular jar testing should be part of every water treatment plant’s quality assurance program.

Need help with jar testing or interpreting results? Contact our technical team for expert guidance on optimizing your coagulation program.