Evaluation of Process Uncertainty in Activated Sludge Treatment by Probabilistic Modeling

Abstract

Simulation-based design and analysis of biotreatment systems depends to a great extent on the accuracy of the model parameters used. In many cases, site-specific parameter estimates are not available and parameter values recommended in the literature are adopted along with a measure of judgment to account for bioprocess uncertainty. To better quantify process certitude associated with this approach and thus provide guidance to process designers lacking site-specific parameter values, stochastic simulations were conducted and the empirical frequency distributions derived from the simulation results were interpreted in terms of process certitude. In general, the simulation results confirmed field experience and current practice in biotreatment from a probabilistic standpoint and provided context in terms of process certitude. The certitude of achieving specific treatment levels increased with sludge age, validating the application of a design safety factor on the minimum solids retention time. The increase in process certitude was not substantial, however, beyond a sludge age of about 4 days for the removal of organic material or about 10 days for the removal of ammonia. Process certitude was also declined when the hydraulic behavior of the treatment system approached complete mixing, which could result from such operational strategies as increasing the recycle ratio or the utilization of step feed.