Simulation of respirometry-based detection and mitigation of activated sludge toxicity

This paper presents the results of an evaluation of a toxicity mitigation control strategy using the IWA simulation benchmark (Respirometry in control of the activated sludge process: benchmarking control strategies, IWA Scientific and Technical Report #11, IWA, London, England, 2002; The COST simulation benchmark: description and simulator manual, Office for Official Publications of the European Community, Luxembourg, 2001, 154pp). The aim of the proposed strategy is to minimise the impact of a toxic influent shock load on process performance. To do this, the strategy depends on a respiration rate measurement to detect toxic events and uses an equalisation tank to store toxic influent. A defined control algorithm specific for the benchmark plant was developed and controls the reintroduction of the stored influent back into the process stream once the influent is deemed non-toxic. Implemented into the benchmark-defined nitrifying configuration, the evaluation required several alterations to the simulation benchmark including three model changes, the addition of a soluble unbiodegradable toxic state variable and the development of two toxic influent files. The results of the simulations indicate that the impact of the toxicant can be significantly mitigated through the use of off-line storage and reintroduction of suspected influent.