Optimal monitoring and risk limits for a wastewater biological treatment serial plant

The biological treatment is the most complex step in removing organic and inorganic pollutants from waste-waters, being very sensitive to input-flow oscillations, operating conditions and biomass evolution. Sudden increases in substrate concentration or some inhibitory substances, deterioration of the biomass, or few observed species, all of these lead to a repeated process identification for each waste and biomass type, and also to a difficult control because of the process complexity, low data reproducibility and flexibility of the aerator-settler unit. The numerical analysis, even under imperfect data, can lead to significant improvements in the wastewater treatment (WWT) by determining the optimum and safe plant operating conditions. When only one bioreactor-settler unit exists, a few number of optimisation criteria can be checked. Alternatively, the paper illustrates in a complex WWT plant case including several units, how the increased system flexibility allows using multiple plant operation policies under various optimisation criteria. The analysis is exemplified in a practical case of three serial WWT units by deriving various transient operating policies with considering compromises among process yield, economics and safety aspects.