Sludge (de)flocculation Dynamics in Secondary Clarifiers

During the biological treatment of wastewater the waste is biodegraded by micro-organisms. This is one of the principal goals. A probably even more crucial unit process is the clarification. Indeed, clarifying the “purified” water is necessary to remove the particulate fraction in which the micro-organisms reside. According to Wanner (1994) the washout of sludge contributes to 50 to 80% of the effluent BOD. This stresses the importance of the clarifier. The efficiency of sludge sedimentation depends on several factors. Of course, the hydrodynamics are essential, but also the biology plays a considerable role. Settling of sludge flocs is dependent on the combination of porosity and size of the flocs. Depending on the operational circumstances in the clarifier the floc size distribution changes along the length of the unit because of flocculation and deflocculation processes. In that way, shear rate, pH, ion strength and zeta potential are very important variables, but one should not underestimate the relevance of biological factors. Hence, not considering effects of oxygen concentration, load and residence time in (de)flocculation studies would be a huge mistake.