In the activated sludge process, the characteristics of qualitative response observed in the aerator effluent depends largely on the induced microbiological reaction and the pattern of mixing. The latter controls the “age” distribution of mixing liquor. Authors introduced first two kinds of mixing model, one of which is modified complete mixed type and another is to apply for any intermediate mixing condition between complete mixing and plug flow, in order to explain the pattern of flow in the aeration tank. This serves especially for the investigation of microbiological characteristics of conventional and step aeration process. Next, the kinetics of microbial reaction was studied, referring to the previous formulation treating the metabolism of single substrate-organism combination, considering the significance of BOD5 while using it as index of amount of compounded substrates in the sewage ; consequently the over-all effect of miscellaneous sludge organisms was attributed to the term of activity, by which the modified mathematical representation of reaction for such mixed-culture system was proposed. Finally, the authors showed the possibility of calculating exact change in substrate concentration of the effluent by means of computations combining any proposed mixing model with reaction formula. For example, in case of zero- and first-order microbial reaction, the response in effluent substrate concentration was computed and summarized
