A single model for mass transfer and growth for biodegradation rates in activated sludge

In the scope of environmental risk assessment for new and existing chemicals a generic model is developed to predict the possible concentration of pollutants after passage of sewage treatment plants. The model combines Monod kinetics with mass transfer rates through an imaginary boundary layer between mixed liquor and active biomass. For each specific substance a combined rate constant can be calculated. This combined rate constant is dependent on the influent concentration of the substance and the maximum growth rate of the substance as the main inherent substance property. All other parameters are inherent for the activated sludge process and can be quantified on the basis of generic default values. Monod kinetics for bacterial growth are modified for a concentration gradient between the mixed liquor and the suspended solids. This gradient causes a difference between an apparent half-saturation value (K(S app)) and the real half-saturation value (K(S real)). Real half-saturation values are estimated by linear extrapolation of a plot of the apparent values against mass transfer rates, whereas the slope of this plot gives the specific diffusion limited mass transfer rate constant (K(diff)). From the available data it is concluded that values for K(S real) for water-soluble substances are below 100 microg COD/L, whereas the value for K(diff) varies between 100 and 500/day. Compared to the currently applied default values, which are based on a classification between ready and inherent biodegradability, the proposed method is more sophisticated.