Water wuality in Portuguese pseudo-separate and combined systems: a conceptual modelling approach for data comparison

Abstract

Six sewer sections of different combined and pseudo-separate catchments were continuously monitored for flow and sampled at preset intervals, over dry and wet weather periods. First flushes with high TSS and COD levels were observed in the outlet sewers of a 10 ha combined catchment and of an 8 ha stormwater pseudo-separate system. Higher TSS and COD concentrations were also found during the rain events in a 22 ha combined catchment. However, in two upstream 5 ha combined catchments, only a slight increase in TSS levels and no changes in COD were observed during the rain events. These results support that most of the flushed matter is stored in the larger and flatter combined sewers. Even for small intensity rain events, important increases in TSS, VTSS and COD were observed at an interceptor sewer serving several small pseudo-separate systems (80 ha). Values of all parameters tended to decrease over successive rain events and, in a third sub event, were even smaller than the dry-weather ones. By using a simple conceptual-modelling approach, a suitable and useful relationship between precipitation and TSS loads could be established for each of the six monitored sections. However additional experimental data are needed to fully calibrate the models. If calibrated for a wide range of rain events, possibly these would allow for the evaluation of the TSS loads for longer periods.