THE OPTIMAL DESIGN OF ACTIVATED SLUDGE PROCESS

Abstract

Activated sludge process (ASP) is a widely used aerobic biological wastewater treatment process. However, expenditures involved, both in terms of capital and operation and maintenance costs, are quite considerable. Thus even a few percent saving at the design stage would be of significant value. In the present study an attempt is made to develop a rational approach to minimum cost design of component units of ASP viz, the reactor (aeration tank) secondary clarifier and the sludge recycle. Cost of anaerobic digestor of excess sludge produced is also considered. Procedure for optimal design of activated sludge process, considering construction and operational costs, is developed. Biological reactor design is based on microbial kinetic concepts and continuous culture of microorganism theory. Biomass concentration in the aeration tank and return sludge are utilized as the primary independent design variables to which system performance is related for activated sludge process. Liquid-biomass separation is based on the batch solids flux theory-a rational approach to design secondary clarifier. Optimization problem is framed as minimization of capital and operating costs subject to the constraints determined by effluent quality criteria and process variables affecting the performance. Interior penalty function method is used for cost optimization. Results suggest that the optimal cost increases linearly . with increase in influent BOD and optimal unit cost decreases exponentially with increase in wastewater flow. Major factor which appears to be contributing in reduction_ in cost is biological solids retention time and hence aeration tank volume.