Analysis of rising sludge risk in Activated Sludge Systems: from operational strategies to clarifier design

The main objective of this paper is to evaluate the risk of suffering rising sludge problems under different modes of operation and clarifier designs using the IWA Benchmark Simulation Model no 1 (BSM1). First, as a reference case, waste flow, dissolved oxygen, external recirculation and the settling area were modified and the risk of rising sludge quantified. In a second series of simulations, the bio-kinetic parameters and the influent fractions of the Activated Sludge Model No 1 (ASM1) were considered uncertain and both Monte-Carlo and sensitivity analyses of rising sludge risk were carried out. The results of this study showed that rising sludge problems could be avoided either decreasing nitrification efficiency or reducing the biomass residence time in the bottom of the secondary clarifier. Also, the Monte Carlo simulations revealed that the uncertainty of the model predictions differed for the different evaluated scenarios. This difference was strongly related to the effect of the input uncertainty on the nitrogen removal bacteria. Finally, it was found that ηg (anoxic growth rate correction factor) was responsible for causing the majority of the rising sludge risk output uncertainty for all the evaluated scenarios. Nevertheless, when nitrification efficiency was low, the autotrophic maximum specific growth rate (μA) become more important having an important role on the overall model predictions. Summarizing, the study allowed a better understanding of rising sludge in wastewater treatment plants, the evaluation of different ways to overcome these problems (in both values and uncertainty), the identification of the model parameters with the strongest influence on its possible uncertainty and finally some practical implications from an operational and design perspective.