Observed and simulated effect of rain events on the behaviour of an activated sludge plant removing nitrogen

This study deals with wastewater treatment plant (WWTP) loading and performance when receiving combined sewage. A yearly data set from an activated sludge plant operating at very low food-to-mass ratio (F/M) and being intermittently aerated was analyzed statistically to compare loading and discharge during dry and wet weather. The results show that despite some loading increase, the nitrogen removal performance is only slightly affected by rain, whereas higher soluble COD load discharge is observed. This approach was supplemented with dynamic simulations with the Activated Sludge Model No 1 (ASM1). It was first calibrated on data from an intensive 48-h dry weather sampling campaign at 20degreesC. A good fit between simulated and measured variables was obtained after adjusting five parameters of the ASM1. This parameter set was then validated on a wet weather sampling campaign. Once the confidence in the simulations' results was established, several wet-weather scenarios were simulated, with maximum flow and variable pollutant loading. They show that the-higher COD load discharge is due to a hydraulic flush of soluble inert compounds, and how the biological potential can be used to treat almost any realistically possible carbon and nitrogen overload.