Jar Testing

by Jenny Poland and Todd Pagano

What is jar testing?

The jar test is a common laboratory procedure used to determine the optimum operating conditions for water or wastewater treatment. This method allows adjustments in pH, variations in coagulant or polymer dose, alternating mixing speeds, or testing of different coagulant or polymer types, on a small scale in order to predict the functioning of a large scale treatment operation. A jar test simulates the coagulation and flocculation processes that encourage the removal of suspended colloids and organic matter which can lead to turbidity, odor and taste problems.

Jar Testing apparatus :

The jar testing apparatus (see Figure 1) contains six paddles which stir the contents of six 1 liter containers. One container acts as a control while the operating conditions can be varied among the remaining five containers. A rpm gage at the top-center of the device allows for the uniform control of the mixing speed in all of the containers.(see Figure 2.)

Jar Test Procedure:

The jar test procedures involves the following steps:

1. Fill the jar testing apparatus containers with sample water. One container will be used as a control while the other 5 containers can be adjusted depending on what conditions are being tested. For example, the pH of the jars can be adjusted or variations of coagulant dosages can be added to determine optimum operating conditions.
2. Add the coagulant to each container and stir at approximately 100 rpm for 1 minute. The rapid mix stage helps to disperse the coagulant throughout each container. Coagulants are chemical additions, such as metallic salts, which help cause smaller aggregates to form larger particles.
3. Reduce the stirring speed to 25 to 35 rpm and continue mixing for 15 to 20 minutes. This slower mixing speed helps promote floc formation by enhancing particle collisions which lead to larger flocs. These speeds are slow enough to prevent sheering of the floc due to turbulence caused by stirring to fast.
4. Turn off the mixers and allow the containers to settle for 30 to 45 minutes. Then measure the final turbidity in each container. The final turbidity can be evaluate roughly by sight or more accurately using a nephelometer.

The following animation shows a typical jar test. Varying amounts of coagulant doses were added to 5 of the 6 containers. The first jar on the left is serving as a control and no coagulant was added. The coagulant doses increased in the containers from left to right. For this water, as the dose of coagulant increased the residual turbidity improved. It is important to note that the optimum coagulant dose is the dose which meets the specified turbidity required on the regulatory permit. The addition of excess coagulant may reduce turbidity beyond what is required but also could lead to the production of more sludge which would require disposal.

Animation of jar test process

Jar Test Example:

The following results were achieved after a series of jar test on two sample waters, A and B, were treated with two different coagulants, alum and ferric chloride, at varying doses.

Water A had low alkalinity and required less coagulant to achieve good coagulation and flocculation than the higher alkalinity of Water B. This is seen by the curves of Water A reaching a minimum turbidity at ~20 and ~30 mg/L of aluminum sulfate and ferric chloride added, respectively. Plots of turbidity versus coagulant dose for Water A with alum, Water B with ferric chloride and Water B with alum all showed a continual decrease in turbidity with an increase in coagulant dose. This trend is a sign that sweep flocculation is the main coagulation mechanism occurring. Water A, with ferric chloride, showed a decrease followed by an increase (at ~40 mg/L) in turbidity with a corresponding increase in coagulant dose. This dictates that adsorption and charge neutralization is taking place due to the colloids restabilizing and not coagulating. Although not shown in the graphs, the addition of coagulant to the low alkalinity waters lead to a drop in the pH of Water A, which enhanced adsorption and charge neutralization. Therefore, higher coagulant doses are often needed with high alkalinity waters (like Water B) where pH remains fairly constant and sweep floc is the main coagulation mechanism. Although slightly less alum than ferric chloride was needed to reach an optimum level, the residual turbidity when using the alum coagulant did not fall below 1 NTU. This means that even though alum may require a slightly smaller dose, it still may not be able to meet the desired effluent regulations without the additional help of a filter or polymer.

Conclusion:

Jar testing is an experimental method where optimal conditions are determined emperically rather than theoretically. Jar test are meant to mimic the conditions and processes that take place in the clarification portion of water and wastewater treatment plants. The values that are obtained through the experiment are correlated and adjusted in order to account for the actual treatment system.

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