WATER & WASTEWATER TREATMENT
Sedimentation as a water and wastewater treatment process A recently held webinar on sedimentation formed part of the WISA Process Controllers Division’s Back-to-Basics series. With most of the same principles applicable to both water and wastewater treatment, it was decided to discuss this process unit from both angles in one session.
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hen talking about sedimentation, it is impossible to leave out coagulation and flocculation – as without them, there would not be a proper sedimentation process,” says Mbali Sibiya, process and quality technician at Umgeni Water and member of WISA’s Process Controllers Division. What is coagulation and flocculation? Coagulation is the process of adding a coagulant (polymer, alum) to raw water to destabilise the colloidal matter in the water to allow for it to form settleable floc. This happens in the rapid mixing stage and is a key element in the removal of turbidity from water. Flocculation is the process of allowing the floc formed in the coagulation
process to grow in size by slow mixing. The process is followed by a sedimentation tank where the floc is allowed to settle out. The factors that affect these processes include: • Coagulant dose – the jar test is done to estimate the minimum coagulant dose required to achieve certain water quality goals. • pH – the efficiency of the coagulant depends on the pH of the water and coagulant itself. • Intensity of stirring or mixing (the G value). • Time – for chemical mixing and allowance of floc formation. “Jar testing is where we will add coagulants into the jars of the raw water that we are treating to determine the correct dosage of the coagulant
A rectangular settling tank in the water treatment works
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and coagulant aid. It is a procedure that mimics what takes place in the actual treatment plant. It will show the coagulation process and the formation of floc,” explains Sibiya. The process to choose the correct coagulants for the source water is a lengthy process, as only certain ones can work with certain types of raw water. Coagulants are affected by the minerals and type of suspended solids in the raw water. This is why water utilities conduct lengthy trials with different types of coagulants. The trial typically begins with the jar test. Sedimentation Sedimentation is the process in which the flocs that have been formed during coagulation and flocculation are allowed to settle from the water. The flocs collect
