Evaluation and modification of the simpletreat chemical fate model for activated sludge sewage treatment plants

The SimpleTreat model of Struijs et al. (1991a,b) for chemical fate in a generic activated sludge type wastewater treatment plant (WWTP) has been examined together with an improved model version (vers. 2.1jm) which gives more flexibility in the specification of the WWTP design and operating conditions and which includes a more accurate description of the process of volatilization. Model predictions for 15 different organic chemicals have tentatively been compared with observed removals in three specific Danish WWTP's, all low loaded plants with nitrogen removal. Biodegradation rate constants were either default values for readily or inherently degradable chemicals proposed in the EU Technical Guidance Document on Environmental Risk Assessment of New and Existing Substances (EUTG), or were judged from literature data. The model predictions were generally in fair agreement with the observed removals although exceptions were noted. For 10 chemicals the data allowed estimations of “full scale” biodegradation rate constants by model calibration. Rate constants (extrapolated to 20°C reference temperature) ranged from 0.07 L/(g SS ·d) for pentachlorophenol to 12 L/(g SS ·d) for the every readily degradable compound, p-cresol. Finally, example calculations with parameter values representing a generic Danish WWTP have been compared with example results stated in the EUTG. The predicted emissions in the generic EU WWTP were up to 3-4 times less to water and air and the emission with excess sludge up to 10 times higher than the corresponding figures for an average Danish WWTP.