Jar Test Procedure and Calculation

The jar test is a crucial laboratory-scale simulation used in water and wastewater treatment to determine the optimal type and dosage of coagulants and flocculants. Mastering this procedure helps ensure efficient and cost-effective plant operation.

Follow this guide to understand the jar test methodology and how to interpret its results.

What is a Jar Test?

A jar test mimics the conditions of a full-scale water treatment plant’s coagulation and flocculation processes. It involves:

• Taking representative samples of raw water.
• Adding varying amounts of treatment chemicals (coagulants, polymers, pH adjusters) to different “jars” or beakers.
• Subjecting these samples to standardized mixing (rapid and slow) and settling periods.
• Observing and measuring parameters like turbidity, pH, color, and floc characteristics to identify the most effective treatment conditions.

Why Perform a Jar Test?

Regularly performing jar tests is vital for several reasons:

• Optimizing Chemical Dosage: Prevents under-dosing (ineffective treatment) or over-dosing (wasted chemicals and potential sludge issues).
• Adapting to Raw Water Quality Changes: Raw water characteristics (turbidity, alkalinity, temperature, organic content) can vary significantly due to weather, seasons, or upstream activities. Jar tests help adjust treatment accordingly.
• Evaluating New Chemicals: Allows for testing and comparison of different coagulants or flocculant aids.
• Cost Savings: Minimizing chemical usage directly impacts operational costs.
• Regulatory Compliance: Ensures treated water meets quality standards for turbidity, pathogens, and other contaminants.
• Process Troubleshooting: Helps identify issues within the coagulation/flocculation stages of treatment.

Equipment Needed

A standard jar testing setup includes:

• Jar Testing Apparatus: A multiple-stirrer unit (typically 4-6 positions) with variable speed control for rapid and slow mixing.
• Beakers/Jars: Usually 1-liter or 2-liter Griffin beakers or square acrylic jars.
• Pipettes/Syringes: For accurate addition of chemical solutions.
• Stopwatch or Timer.
• Turbidimeter: To measure the clarity of water samples.
• pH Meter: To measure and adjust pH if necessary.
• Graduated Cylinders: For preparing stock solutions.
• Raw Water Sample Container.
• Chemicals: Coagulants (e.g., alum, ferric chloride, PAC), flocculants/polymers, pH adjustment chemicals (e.g., lime, caustic soda, acid).
• Notebook/Data Sheet: For recording observations and results.

Jar Test Procedure: Step-by-Step

The following is a general procedure. Specific parameters (mixing speeds, times, chemical concentrations) may need adjustment based on plant design and raw water conditions.

1. Preparation

   Collect a representative sample of the raw water to be tested. Ensure it’s well-mixed. Prepare stock solutions of the coagulants and other chemicals to be tested. For example, a 1% (10 g/L or 10,000 mg/L) stock solution is common, where 1 mL added to 1 L of water equals a 10 mg/L dose.

2. Setting Up

   Fill each beaker/jar with an equal volume of raw water (e.g., 1000 mL). Place the beakers in the jar testing apparatus. Measure and record initial raw water parameters (turbidity, pH, temperature, alkalinity).

3. Coagulant Addition & Rapid Mix

   While the stirrers are off or at very low speed, add varying doses of coagulant to each jar. For example, Jar 1: 5 mg/L, Jar 2: 10 mg/L, Jar 3: 15 mg/L, etc. Include a control jar with no chemical addition if desired. Immediately after adding chemicals, start the rapid mix phase (e.g., 100-300 RPM for 1-3 minutes) to ensure quick and thorough dispersion of the coagulant.

4. Slow Mix (Flocculation)

   Reduce the stirring speed to a slow mix (e.g., 20-70 RPM for 15-30 minutes). This phase allows for the formation of flocs as particles collide and agglomerate.

5. Settling

   Turn off the stirrers and allow the flocs to settle for a specified period (e.g., 15-60 minutes). Observe the rate of settling and the clarity of the supernatant (the water above the settled solids).

6. Analysis & Observation

   Carefully decant or pipette supernatant from each jar (from a consistent depth, avoiding settled floc) and measure its turbidity. Also record pH. Observe and note the characteristics of the floc in each jar:

   • Size: Pinpoint, small, medium, large, feathery.
   • Settling characteristics: Fast, slow, does it break up easily?
   • Clarity of supernatant.

Key Considerations

• Temperature: Water temperature significantly affects reaction rates and flocculation. Try to conduct tests at a temperature close to the actual plant water.
• pH: Coagulants work best within specific pH ranges. pH adjustment might be necessary before or after coagulant addition.
• Alkalinity: Sufficient alkalinity is needed to buffer the pH drop caused by acidic coagulants like alum or ferric salts.
• Mixing Energy and Duration: The G-values (velocity gradient) for rapid and slow mix in the jar test should ideally reflect plant conditions.
• Chemical Stock Solution Age: Use freshly prepared stock solutions, especially for polymers, as they can degrade over time.
• Consistency: Maintain consistent procedures for sampling, mixing, and analysis for comparable results.

Safety Precautions

Always follow laboratory safety guidelines when performing jar tests:

• Wear appropriate Personal Protective Equipment (PPE), such as safety glasses, gloves, and a lab coat.
• Handle chemicals with care, referring to Safety Data Sheets (SDS) for specific hazards and handling instructions.
• Ensure good ventilation.
• Properly dispose of chemical waste according to local regulations.

Further Information & Resources

EPA Jar Test Guidance American Water Works Association (AWWA) Water Environment Federation (WEF)