Four different strategies including influent storage and reintroduction, step-feeding, rapid sludge recycle and waste sludge storage were evaluated using the denitrification layout of the IWA simulation benchmark. The control objective was to minimise deterioration in effluent quality caused by a certain toxic input event. In these strategies the maximum specific respiration rate (Rmax) was selected as a measured and controlled variable. To simplify the analysis, the toxicant was assumed to be a soluble and nonbiodegradable substance. Two toxic influent files were developed with square-wave input lasting 3 hours. To detect the influent toxicity, a pseudo-online flow-through respirometer was applied. A number of simulations were performed and the results suggested that the influent storage and reintroduction strategy provided the most optimistic results and other strategies could not mitigate the toxic effect. The influent storage and reintroduction strategy strongly depended on reintroduction flow rate from the storage tank. The simulation according to reintroduction flow could estimate the time required for completely treating toxic wastewater stored in the storage tank. Also the IWA simulation benchmark was enhanced to evaluate toxicity effect on the activated sludge process.

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