Design and operational charts for complete mixing activated sludge systems

This paper provides charts enabling the derivation of design and operational procedures for an activated sludge waste water treatment system including the final settler. These charts are based on a mathematical model including Monod relationships describing the microbial growth and the substrate removal in the aerator, while sedimentation in the final settler is assumed to follow the limiting flux theory. Charts represent the minimum required settler area, insuring that given effluent standards will be met, as a function of either the hydraulic retention time or the recycle rate. Most design and operational characteristics of the activated sludge system can be read directly on the charts, while a simple equation is provided for the sludge wasting rate.Operational procedures under constant or variable inflow conditions are first derived from the charts. It is concluded that operational policies including a constant recycle sludge concentration are especially attractive in response to various types of influent variations. Such policies allow one to maintain either the effluent substrate concentration or the recycle flow rate at the same level in most situations. Furthermore, the required settler area remains reasonably small. Large aerators provide considerable operational flexibility and appear to be required when hydraulic overloads are expected.The charts allow one to design the activated sludge system in such a way that expected overload conditions can be overcome. Least-cost design is briefly studied through simplified cost functions. This allows derivation of the best compromise between sufficient operational flexibility and least-cost design, leading to the “optimal overdesign” of the system.Parameter variability has a significant influence on the charts. Hence, correct determination of the model parameters is required before calibrating and using the charts.