Modelling in-sewer changes in wastewater quality under aerobic conditions

In the present paper a model for the simulation of in-sewer transformation processes occurring during the transport of wastewater under aerobic conditions is presented. The basic approach is to use a detailed description of the transport processes to separate the effect of advection-dispersion in the variations of concentration (of a certain pollutant in a certain pipe length) from those due to transformation processes. The focus is on dry weather flow and free surface flows. Processes such as reaeration, heterotrophic growth and decay, hydrolysis, ammonification and biofilm consumption of substrate and oxygen are considered. Transformation processes are modelled using a similar approach to that of the IAWQ Activated sludge model n°1. Results of tracer experiments are presented showing the relevance of dispersion in pipe flow. The results of sensitivity analysis on the model parameters are presented. The model results are especially sensitive to variations in yield and maximum growth rate for heterotrophs. Thus, these parameters should be determined experimentally for a given system. Less precise estimates for the other parameters have less impact on the model results. This aspect is especially relevant due to the lack of established methods for the estimation of these parameters for sewer systems modelling.