Sludge Quality: The Key to Secondary Clarifier Performance

Within the wastewater treatment profession there is considerable disagreement as to the best method for controlling the activated sludge process regardless of whether nutrient removal is an objective or not. There is little disagreement, however, as to the profound impact that the performance of the secondary clarifier has on the overall performance of the activated sludge process: total suspended solids (TSS) escaping the secondary clarifier are mostly organic and contain significant quantities of both nitrogen and phosphorus.There is no question that the aeration basin and secondary clarifier in any activated sludge system are inextricably connected to each other. This is the essence of Wanner's statement (2006): “The modern approach to the [design and operation of the] activated sludge process looks at both the biological and the separation stage as one system.” This explains the focus, pervasive throughout the wastewater industry, on “meeting permit.” In developing process control strategies, however, it is important for operations personnel to consider the process objectives of the aeration basin and the secondary clarifier separately to ensure they are both being met, because they are quite different.It is interesting to note that in the call for abstracts for this specialty conference on nutrient removal that “Process Control” shows up in both the categories Operational Issues and Design Issues, but not in Research Issues. It is the author's opinion, and the focus of this paper, that the profession needs to develop and standardize process control tools that allow operators to quantify sludge quality.A test apparatus and testing sequence is suggested for operators to measure activated sludge quality.