THE JAR TEST
Jar testing, to determine the proper coagulant dosage, continues to be one of the most effective tools available to surface water plant operators. Finished water quality, cost of production, length of filter runs and overall filter life, all depend on the proper application of chemicals to the raw water entering the treatment plant.
Before you start
The jar test, as with any coagulant test, will only provide accurate results when properly performed.
Because the jar test is intended to simulate conditions in your plant, developing the proper procedure is very important. Take time to observe what happens to the raw water in your plant after the chemicals have been added, then simulate this during the jar test. THE RPM OF THE STIRRER AND THE MINUTES TO COMPLETE THE TEST DEPEND ON CONDITIONS IN YOUR PLANT. If, for instance, your plant does not have a static or flash mixer, starting the test at high rpm would provide misleading results. This rule applies to flocculator speed, length of settling time and floc developement. Again, operate the jar test to simulate conditions in YOUR plant.
Equipment
1 ……….Phipps & Bird Six-Paddle Stirrer with Illuminated Base.
6 ……….Graduated beakers, 1000 mL or B-Ker2 2000 mL square jars (Phipps & Bird).
2 ……….10 mL graduated pipets.
1 ……….1000 mL graduated cylinders.
1 ……….Scale for weighing coagulants.
Lab apparatus to measure alkalinity, pH and turbidity.
Stock Solutions
Stock solutions are prepared by dissolving 10.0 grams of alum, soda ash, lime, etc. into 1000 mL distilled water. Each 1.0 mL of this stock solution will equal 10 mg\L when added to 1000 mL of water to be tested.
Because dry polymers tend to be fed at much lower concentrations, a stock solution should contain 1.0 gram polymer to 1000 mL distilled water. Each mL of this stock solution will equal 1.0 mg\L when added to 1000mL of water to be tested. You might want to use a 1.0 mL graduated pipet with 0.1 mL subdivisions when jar testing polymers.
Liquid polymers should be added in the concentration recommended by the manufacturers.
Information
First check the alkalinity and pH of the water to be tested. Water containing an alkalinity of at least 25 mg\L and pH of around 7.0 will coagulate without the addition of lime, soda ash, etc. (Adding chemicals for pH adjustments, in pre-treatment, can be a major waste if they are not needed). Keep in mind that this will lower pH.
Color, as opposed to turbidity, is almost always removed best at depressed pH values. The additon of acid or larger doses of alum may be needed to accomplish color removal. Some color is removed at pH 7.0 but higher raw water colors may require a full unit lower.
If, on the other hand, you are removing manganese in the floc process, the higher you can elevate the pH and still form floc, the better the manganese removal.