Development of chemical exposure prediction model for aerobic sewage treatment plant for biochemical wastewaters

Sewage treatment plant (STP) is a key transfer station for chemicals distributed into different environment compartment, and hence models of exposure prediction play a crucial role in the environmental risk assessment and pollution prevention of chemicals. A mass balance model namely Chinese Sewage treatment plant (C-STP(O)) was developed to predict the fate and exposure of chemicals in a conventional sewage treatment plant. The model was expressed as 9 mixed boxes by compartment of air, water, suspended solids, and settled solids. It was based on the minimum input data required on the notification in new chemicals, such as molecular weight, absorption coefficient, vapor pressure, water solubility, ready or inherent biodegradability. The environment conditions (Temperature=283 K, wind speed=2 m·s^-1) and the classic STP scenario parameters of China, especially the scenario parameters of water quality and sludge properties were adopted in C-STP(O) model to reflect Chinese characteristics, these parameters were sewage flow of 35000 m³·d^-1, influent BOD₅ of 0.15 g·L^-1, influent SS of 0.2 kg·m^-3, effluent SS of 0.02 kg·m^-3, BOD₅ removal in aerator of 90%, sludge density of 1.6 kg·L^-3 and organic carbon content of 0.18-0.19. It adopted the fugacity express for mechanism of linear absorption, first-order degradation, Whitman two resistances. An overall interphase transfer constant which was the sum of surface volatilization and stripping was used to assess the volatilization in aerator. The most important and uncertain input value was the biodegradation rate constant, and determination of which required a tier test strategy from ready or inherent biodegradability data to simulate test in STP. An extrapolated criterion of US EPA to derive biodegradation rate constant using the results of ready and inherent biodegradability was compared with that of EU and was recommended. C-STP(O) was valid to predict the relative emission of volatilization, absorption, degradation and effluent, on ground of both successful simulation of the removal of 26 chemicals from references with an accuracy rate of 81% and consistency of prediction and test with absolute difference of 2.5%-6.3% for 5 phenols. In cases of prediction of three chemicals with different properties, 1, 1, 2-trichloroethane, Naphthalene and DEHP, the predicted fate well satisfied the monitor data. The model is therefore believed to be a simple, robust and useful tool in fate study and exposure assessment of chemicals.