A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Environment, Resources and Development

Thesis (M.Eng.) - Asian Institute of Technology, 1980

The study of an aerobic biological system under the treatment of rainfall, had been neglected till BANKS made a review on the subject of the effect of rainfall on surface reaeration in 1977 and WICKRAMANAYAKE (1979) made a rigorous research for setting up an empirical relationship to evaluate the reaeration coefficient due to rainfall, with the receiving body under completely-mixed regime. Based on the results of WICKRA MANAYAKE (1979), it was possible in the study to develop the process kinetics in an aerobic biological system in which the completely-mixed regime was attained. The developed kinetic model for predicting the effluent substrate concentration was found to be in the form: S = {u1D5F}i Si - {u1D5F} + kd {K a(C* - C) + {u1D5F}j cj} {u1D5F} {a ( {u1D5F} + kd) + b} from which the assessment of process efficiency could be undertaken by attributing the substrate removal to three contributions of rainfall treatment, namely E1 = 100 x {u1D5F}j {u1D5F} E2 = 100 x ({u1D5F} + kd) KL a(CL* - CL) {u1D5F}si {a ( {u1D5F} + kd) + b} and E3 = 100 x ({u1D5F} + kd) {u1D5F}j Cj) {u1D5F}si {a ( {u1D5F} + kd) + b} These relationships were then substantiated by experimental investi gations and, under the process conditions used in the study, all process parameters and variable were found. 11_,a was determined according to WICK RAMANAYAKE (1979). It was found that E1, that is, the dilution effect of rainfall afforded the major contribution and increased as rainfall intensity increased, while E2, the mass transfer effect and E3, the effect due to direct addition of oxygen, were minor factors and even insignificant compared to the dilution effect of rainfall.