Water Treatment Using Sludge Blanket Clarifiers

Abstract

A sludge blanket clarifier is a treatment unit combining flocculation and upward flow sedimentation, which is more efficient compared to the conventional sedimentation tank. It has the ability to treat water at a faster rate, resulting in less space requirement. The removal of suspended particles takes place by a combination of flocculation, sedimentation and straining as the water passes upwards through the blanket of sludge formed within the clarifier. The performance of the blanket appears to directly depend on the raw water quality, coagulant concentration, temperature and up flow velocity. However, there are no direct relationships available to help the operators to adjust these variables to obtain satisfactory performance of the clarifier. Hence it is essential to identify a characteristic that can be used as an indicator to monitor the sludge blanket. A study of literature showed that a parameter called “sludge cohesion coefficient (SCC)” of sludge has been used in France (Degremont, 1991) to characterize the sludge blanket. However, the procedure given was not clear, and parameters to characterize performance of sludge blanket clarifiers in the tropical environments could not be found in the literature studied. Therefore this research was conducted to study the behaviour of a sludge blanket using the parameter SCC, which was done in two stages. During first stage development of a standard test procedure and introduction of cohesion coefficient ranges which can be expected under tropical climatic condition were done. Under this stage several laboratory tests were conducted and a test procedure was finalized. In the second stage, using the developed test procedure, SCC of synthetic raw water samples was measured to develop relations between the SCC and other variables. Results showed that there is an optimum range of alum dose that produced the sludge blanket with the highest SCC under controlled raw water conditions.