Rehabilitation of the Alcântara Sewage Treatment Plant - Effect of the design capacity on CSO discharges

Abstract

Within the scope of the Alcântara STP rehabilitation project, enhanced primary treatment is being installed for the treatment of wet-weather flows. A simplified model using rainfall historical series was used to assess the CSO discharges for different treatment capacities. The Alcântara catchment, the largest of Lisbon, was represented by only two sub-catchments in parallel. One sub-catchment represents the fast component of the hydrographs, and the other simulates the slower response from the catchment causing a tail in the hydrographs. The monitoring period was not long enough to allow modelling the base flow variation. Despite its simplicity, the model reproduces with significant approximation measured hydrographs. However, its results must be analysed cautiously, mainly due to the uncertainty on the measured data and on the base flow. This model allowed the simulation of a 19 years rainfall time series for six different scenarios. Results showed significant reductions of the volume, frequency and duration of CSO discharges associated with increasing the STP capacity. Higher reductions are achieved for the bathing season compared with the rest of the year, contributing to improved conditions for the recreation uses in the Tagus estuary.