Jar Test Procedure: How to Use a Flocculation Jar Test Apparatus for Water Treatment

What Is the Jar Test?

The jar test is the standard laboratory procedure used by water treatment facilities to determine the optimal type and dose of coagulant and flocculant needed to clarify source water. By replicating the coagulation-flocculation-sedimentation process at bench scale, engineers and scientists can dial in chemical doses before scaling up to full plant operations — saving significant chemical costs and ensuring treated water meets turbidity, color, and regulatory standards.

SciMed's Flocculation Jar Test Apparatus is engineered for precision: variable-speed paddle mixing, simultaneous multi-jar operation, and robust construction for daily laboratory use.

Understanding Coagulation vs. Flocculation

Coagulation is the destabilization of colloidal particles in water by adding a chemical coagulant (alum, ferric sulfate, PAC). Rapid mixing disperses the coagulant and neutralizes the negative charges that keep particles suspended.

Flocculation follows: gentle, slow mixing allows destabilized particles to collide and aggregate into larger “floc” particles heavy enough to settle by gravity or be removed by filtration.

Jar Test Step-by-Step Procedure

  1. Collect representative raw water samples — Test water that reflects actual intake conditions. Measure initial turbidity (NTU), pH, temperature, and alkalinity.
  2. Fill jars — Fill all test beakers (typically 1–2 liters each) with equal volumes of raw water. Run a minimum of 4–6 jars simultaneously to test a dose-response curve.
  3. Set rapid mix speed — Set the paddle mixer to 100–150 RPM. Add increasing doses of coagulant to each jar simultaneously (e.g., 5, 10, 15, 20, 25, 30 mg/L alum).
  4. Rapid mix phase — Mix at high speed for 1–2 minutes to fully disperse the coagulant.
  5. Slow mix phase — Reduce speed to 20–40 RPM for 15–20 minutes to allow floc formation. Observe floc size and structure.
  6. Settling phase — Stop the mixers and allow 30–60 minutes of quiescent settling.
  7. Sample and analyze — Carefully withdraw samples from just below the water surface (without disturbing sediment). Measure residual turbidity, pH, and color.
  8. Identify optimal dose — The jar with the lowest residual turbidity at the lowest coagulant dose represents the optimal treatment condition.

Key Parameters to Record

  • Raw water turbidity (NTU) and final turbidity
  • Raw water pH and post-treatment pH
  • Coagulant dose (mg/L)
  • Floc formation time and floc size rating
  • Settled water color and clarity
  • Residual aluminum or iron (if required by regulation)

Applications Beyond Drinking Water

Jar testing is also used in:

  • Wastewater treatment — Phosphorus removal, suspended solids reduction
  • Industrial process water — Cooling tower makeup, paper mill effluent
  • Research and development — Testing novel coagulants or polymer aids

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