Jar testing simulates the coagulation / ﬂocculation process in a water treatment plant. In a set of beakers, a deﬁned amount of raw plant water is mixed with varying amounts of coagulant. Each beaker receives an increasing incremental dose of coagulant and is then stirred for a set amount of time to mimic plant operation. The formation of ﬂoc during this test simulates plant operation. Observations from the floc formation indicate a preferred range of coagulant dosage. Thus, jar testing can help identify the parameters that promote an appropriate final water quality.
While this jar test is simple to perform, it does require time, typically about 2-3 hours, including preparation and wait for settlement. Since water quality can change seasonally (summer algae growth, flood events, forest fires) the jar test has to be repeated regularly.
A significantly faster method is the measurement of zeta potential expressed in milli Volt (mV). Here, the water is exposed to an electric and scientifically supported field, and the movement of colloidal particles indicates their net charge under the specific water conditions. We present details of the zeta potential method and compare results with jar testing. At the example of alum (Aluminium Sulfate, Al2(SO4)3) as coagulant, we show that the optimal dose is related to a favorable zeta potential range of -8mV to +3mV.
Not shown here, the use of ferric (Iron Chloride or Iron Sulfate) typically correlates with the same zeta potential range to optimize water treatment plant operation.
Each coagulant chemistry has an optimal pH range where it is most effective, so it is also important to monitor pH in the plant and maintain the similar pH during the jar testing.