Modelling of the Activated Sludge process with a stratified settling unit

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Proceedings of the International Congress on Modelling and Simulation, MODSIM


The activated sludge process is the most widely used process for the biological treatment of domestic and industrial wastewaters. Wastewater treatment plants using the activated sludge process are widely used in developed and developing countries. The activated sludge model number 1 (ASM #1) is an internationally accepted standard for activated sludge modelling. It describes nitrogen and chemical oxygen demand within suspended-growth treatment processes, including mechanisms for nitrification and denitrification. We analysed the biological treatment of wastewater when a single aerated reactor was investigated. The residence time was used as the bifurcation parameter and investigated how the autotrophic and heterotrophic biomass concentrations varied. The configuration analysed consisted of a continuously stirred tank reactor (CSTR) with an attached settling tank. The settling tank concentrated the particulate matter that flowed from the reactor, before being returned back into the reactor. We considered two different models of the settling tank and compared the behaviour. The first was a simple settling tank, where it was assumed that the particulate matter was concentrated by a calculated factor instantaneously. The second was a stratified settling tank, where the tank was divided into a number of layers. The settling occurred through gravity settling and this lead to a separating flow, where the feed is clarified towards the top of the tank and thickened towards the bottom. The result was that both models lead to three types of behaviour 1. For low residence times, the flow rate was too fast for the heterotrophic or autotrophic biomass to develop and therefore there was no reaction occurring in the reactor. We call this the Washout state, as the wastewater flows through the system without any change. 2. For slightly higher residence times, a transcritical bifurcation occurs where the heterotrophic biomass concentration first becomes non-zero. 3. Finally, there is a higher residence time when a second transcritical bifurcation occurs where both the heterotrophic and autotrophic biomass concentrations become non-zero. We found that for the stratified settling tank, both transcritical bifurcations occurred earlier than for the simple settling tank (0.029 days compared to 0.053 days for the first bifurcation and 0.117 days compared to 0.389 days for the second bifurcation). When the configuration with the stratified settling tank was in the Washout state, the settling tank was acting as a dilution tank, with a concentration factor below one.

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