What is Jar Testing?
Jar testing is a method for determining the treatment method that will be used when treating wastewater. Specifically, it helps to determine which chemicals will be needed and the proper dose rates for those chemicals. Jar testing is essentially a miniature batch treatment tank with all the variables under control of the operator. It usually consists of a "jar" or beaker of a known volume and a variable speed mixer.
The mixer can be as simple as a glass rod stirred by hand, a laboratory stir plate with a magnetic stir bar, or a motor driven metal impeller (similar to the mixers found in many wastewater treatment reaction tanks). It is also a good idea to have a pH meter for jar tests, as many of the reactions that occur during treatment require specific pH ranges.
How do you Jar Test?
As with any experimentation, it is good practice to take notes, keeping track of the additions and observations you make during the testing. A water sample should be taken from the equalization tank (a holdingtank where all wastewaters are intermingled before being pumped to the wastewater treatment system). You should sample a specific volume, usually a liter. Next, agitate the water in the jar, and measure the pH. In waters that may contain cleaner as well as dissolved metals, it is common to lower the pH to 2.5 - 3.0 with dilute sulfuric acid if it isn't already at the required pH.
Once the pH is lowered, you should being adding chemicals. Most often, coagulant is added at this stage. Coagulant is measured in parts per million (ppm), which is one milligram of something in one liter of water. In this case, one ppm is a one hundredth of a milliliter in one liter of water, with 1 milliliter in 1 liter of water equaling one thousand ppm. Coagulant addition may range from 1 to several thousand ppm depending on what is being treated. Allow the coagulant to mix in the water.
Next, the pH should be raised using dilute sodium hydroxide, usually to a range of 9.0 - 10.0. If required, because of complexors that might be present in the water tying up the metals, a metal precipitant can be added at a dose of 50 - 200 ppm. When metal precipitant is mixed well, the polymer should be added. In water being treated for metals, an anionic polymer is commonly used. Polymer addition should be made with good mixing to evenly distribute the polymer throughout the water. Mix for 30 to 60 seconds, then turn off the mixer. A heavy precipitate or floc should form and begin to settle to the bottom of the jar.
At this point, the jar test is complete, and a sample should be filtered and then taken to a treatability laboratory for analysis.
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