Stormwater Treatment Pop Quiz

QUESTION: Why do the floc particles in the picture appear different (note same water sample used, with untreated control on the left)?

ANSWER: It is due to basic chemistry. The jar labelled ACH has been dosed with a small amount (20ppm) of coagulant which works by neutralising or destabilising the negative charge on clay particles which is causing them to repel and hence stay suspended in solution. The result of using a coagulant is typically a very fine floc layer settles out with very clear low turbidity water above, assuming you have got your dose rates correct… The jar labelled Chitosan has been dosed with a flocculent (at approximately 200ppm). Flocculants work by physically forming larger particles through contact and adhesion to effectively increase particle size to a point allowing flocculation (i.e. settlement) to occur. As a result when using flocculants you will see larger floc particles, which are typically more stable than the small flocs formed when using a coagulant. A potential downside of flocculants is if the flocs form too rapidly they may settle before binding all suspended particles leaving a slightly cloudy water column. To address this and get the best performance from flocculants apply evenly throughout the water body and provide continuous slow mixing to encourage contact and adhesion of suspended clay particles, which may not be practical in all instances.

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If you want to know more about coagulants and flocculants or stormwater treatment on construction sites, check out our news page which contains a number of useful and informative articles and book yourself in to attend our sediment basin training course.