Modeling Risk of Failure in Nitrification: Simple Model Incorporating Abundance and Diversity

A risk based approach to engineering provides a rational way to balance cost against the need to avoid failure. Such an approach has not been systematically incorporated into the design and operation of wastewater treatment plants. This is perhaps because engineers do not have the means to make risk based predictions of performance. We have adapted a classical technique for incorporating risk in engineering predictions to the oxidation of ammonia, the first step in nitrification. The approach incorporated random changes in load, aeration, and kinetic parameters. Two and three species models were used to evaluate the effect of increasing aeration on the risk of failure. Surprisingly, increased aeration did not lead to a monotonically decreasing risk of failure. Intermediate aeration rates typically increased the standard deviation of the effluent ammonia and thus the risk of failure. Reliable performance was predicted when there was a high abundance of one species or a similar abundance of both. These preliminary but encouraging results suggest risk based approaches offer new and important insights into the operation of biological treatment systems.