Separate and combined sewer systems: a long-term modelling approach

摘要

Sewer systems convey mostly dry weather flow, coming from domestic and industrial sanitarynsewage as well as infiltration flow, and stormwater due to meteoric precipitations. Traditionally,nin urban drainage two types of sewer systems are adopted: separate and combined sewers.nThe former convey dry and wet weather flow separately into two different networks, while thenlatter convey dry and wet weather flow together. Which is the best solution in terms of costbenefitnanalysis still remains a controversial subject. The present study was aimed at comparingnthe pollution loads discharged to receiving bodies by Wastewater Treatment Plant (WWTP) andnCombined Sewer Overflow (CSO) for different kinds of sewer systems (combined and separate).nTo accomplish this objective, a comparison between the two systems was carried out usingnresults from simulations of catchments characterised by different dimensions, populationndensities and water supply rate. The analysis was based on a parsimonious mathematical modelnable to simulate the sewer system as well as the WWTP during both dry and wet weather. The rainnseries employed for the simulations was six years long. Several pollutants, both dissolved andnparticulate, were modelled. The results confirmed the uncertainties in the choice of one systemnversus the other, emphasising the concept that case-by-case solutions have to be undertaken.nFurther, the compared systems showed different responses in terms of effectiveness in reducingnthe discharged mass to the RWB in relation to the particular pollutant taken into account.