Modeling an activated sludge process followed by fixed-film reactors : a case study

Throughout this study, Peschiera Borromeo wastewater treatment plant (WWTP) has been examined. Its biological processes include activated sludge and fixed-film reactors for pre-denitrification and nitrification. The thesis includes detailed information about biofiltration technology and how it has been implemented in one of the two lines that are operating in Peschiera Borromeo WWTP. A software tool (WEST for OPTIMIZATION) allowed to build the conceptual model of the sequence of the two biological processes, and simulate how they work under different operating conditions. The input data about the flowrates or concentrations of influent or effluent water have been obtained from the plant management, and by periodical sampling and laboratory analysis. A parameter estimation analysis function of the software allowed to spot the most sensitive parameters which govern the biochemical processes, such as growth rates of different populations (e.g.: autotrophic and heterotrophic biomass). The model has been calibrated by implementing the corrected parameters of the growth rate and checked against effluent concentrations and sludge production. Different scenario analyses have been compared in terms of effluent quality index, including COD (chemical oxygen demand), BOD (biochemical oxygen demand), total nitrogen, ammoniacal nitrogen, nitrate nitrogen and total suspended solids. This thesis, on the other hand, has allowed to check the performance of the software, in terms of its applicability to field conditions and in terms of its accuracy of in obtaining realistic outputs even with a limited set of input data. It has been possible to study possible modifications of operation conditions, which do not require considerable make-over or large investments. The results have been compared and will give some guidelines to engineers and managers of the WWTP to evaluate possible improvements in operation, such as lower energy consumption, while achieving an effluent quality which complies with the legal constraints for discharge into the Lambro river.