What does it indicate when about 80% of floc has settled in the jar test within one or two minutes after mixing is stopped?

When approximately 80% of floc has settled in the jar test within one or two minutes after mixing is stopped, it indicates that the coagulant dose is likely too high and should be decreased. A rapid settling of floc suggests that the particles have formed larger aggregates, which can indicate an excess of coagulant. While this may initially seem beneficial, excessive coagulant can lead to instability in the floc structure, making it more susceptible to re-suspension during further processing.

Additionally, if the settling occurs too quickly, there may not be enough time for optimal interactions to occur between the floc and other particles in the water, which can diminish the overall effectiveness of the treatment process. Proper floc formation is crucial for efficient removal of impurities, and a balanced coagulant dosage is key to achieving this. Therefore, adjusting the dosage to a more appropriate level can enhance the treatment process by promoting better floc stability and performance.