Time Scale Behaviour of a Biological Wastewater Treatment Facility

The effects of recycle and purge ratios (as responsible for the two of the main time scales of the wastewater treatment facility: the residence time of the biological reactors - anoxic and aerobic, and the doubling time of the microorganisms accountable for the biodegradation of substrates), on the performance of a biological treatment process for wastewater are investigated in this study. A modified form of the ASM3 model with two-step nitrification-denitrification (ASM3-2N) [1] was used for the biological process. Three operating time windows with data collected for two weeks: "dry weather", "stormy weather" and "rainy weather" were used to test the system at its largest time scale (http://www.benchmarkwwtp.org). The system under study has two bioreactors in series, followed by a separator, a recycle (through which values the time scale of the bioreactors are modified) and a purge, responsible for the time scale of the activated sludge. Recycle ratios from 0.1 to 0.9 were used and purge ratios from 0.001 to 0.01. The time scale of the bioreactors overcomes the time scale of the biological process, since the latter is, to some extent, determined by the former. This means that the recycle and purge ratios are not independent so, when they are used as command variables to control the process, they should be treated accordingly.