Assessing the environmental impact of WWTP expansion: odour nuisance and its minimization

As part of the effort to provide proper wastewater disposal for the City of Milan, the expansion of the WWTP located in Peschiera Borromeo has been planned and is currently under completion. The plant, sized for a population of less than 300,000, will soon treat the wastes from the south-east areas of Milan for a total capacity of over 500,000 p.e. The paper describes the approach to find a satisfactory solution of an alleged odour problem that is slowing down the completion of the plant restructuring. This included a survey of the existing plant and analysis of its final design, identification of odour emission sources and their quantification, and the examination of different alternatives for odour abatement. These were carried out using air quality modelling techniques consisting of dynamic, continuous "puff" models capable of reproducing diffusion even at very low pollutant concentrations. Several intervention hypotheses were tested and compared with the few existing regulatory norms and guidelines for odour pollution. In all but the minimum and the no-action hypotheses, proposed actions would results in a drastic reduction of nuisance effects at the receptors. Under the strongest intervention hypothesis (the one adopted by the WWTP agency), odour impact would be virtually eliminated.