Cost optimization of activated sludge systems

Abstract

AbstractActivated sludge is a widely used aerobic biological waste‐water treatment process. A rational approach to least cost design of an integrated system is described which includes the following processes: activated sludge reactor, final settling tanks, gravity thickening, and aerobic sludge digestion. Both capital and operation and maintenance costs are considered. Biological reactor design is based on microbial kinetic concepts and continuous culture of microorganisms theory. Biological solids retention time (θc) is utilized as the primary independent design variable to which system performance is related, e.g., effluent quality, ammonia oxidation, and excess sludge production. Liquid‐biomass separation is based on the batch flux technique, a rational approach to design of gravity separators (final settling tanks). Trade‐offs among reactor volume, clarifier size, recycle pumping capacity, thickener capacity, digester volume, air requirements, and sludge production are discussed. The optimum design is taken as the combination of these parameters within the acceptable design domain, determined by effluent quality criteria, that results in minimum cost. While the method described is general, design of a given treatment system depends on availability, from lab or pilot studies, of system specific numerical values for biological growth coefficients and biomass setting characteristics. A design example illustrates the approach.