Pollution loads discharged from combined sewer overflows: theoretical approach and long term numerical assessment

The paper describes a theoretical analysis and a numerical assessment of pollutant loads discharged from Combined Sewer Overflows (CSOs) - with or without stormwater tanks - into the environment. The theoretical approach was based on certain simple assumptions, reasonably valid if the time scale of the problem involved is longh enough (month/ year), in that single-event simulation is not interesting at all. The analysis led to a general solution, for a fixed rainfall regime, of the problem of pollution load evaluation. Two main parameters related to the rainfall regime were found to be significant: the total volume of water discharged from the structure and the effective mixing factor between dry and storm waters. A numerical assessment of these two parameters was then made, on an annual basis, by means of long-term rainfall series recorded in Milan, Italy. Both the "simple" CSO structure and the CSO coupled with stormwater tanks (on-line and off-line) were considered. The resulting graphs make it possible to evaluate the total annual load discharged from CSOs into the environmente and the potential reduction obtained by adding a storage capacity to the overflow. This estimation could be of interest for persistent pollutants (phosphorus, heavy metals) discharged into low-recirculation bodies (lakes estuaries, lagoons, closed seas).