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Combined Sewer Overflow ( CSO ) Treatment With Constructed Wetlands

Whenever the affluent flow exceeds the treatment capacity of a WWTP, untreated effluent is directly discharged into the waterlines, endangering the environment and public health. In this context it is relevant to study natural and sustainable solutions of combined sewer overflow (CSO) treatment. The aim of this thesis is to analyse the treatment capacity of constructed wetlands (CW) with subsurface flow (planted and unplanted) when treating CSO, through an experimental facility located in Frielas WWTP, which had its start-up in 2011. As the facility was located inside the WWTP it was possible to analyse the treatment response when subjected to local conditions (environmental conditions as well as organic loading, since the CW were always fed with screened effluent). Regarding the experimental stage, two campaigns were carried out at different times, the first one was executed from April to June 2012 and a second one from December 2012 to January 2013. The analysed parameters were COD, pH, redox, dissolved oxygen, and also (only during the first experimental phase) TSS and Enterococci. It was further evaluated, in a qualitative way, the beds evapotranspiration and their porosity. Aiming to achieve a better knowledge on how a CW treats COD, the k-C* model for the hydraulic retention time was calculated. In summary, the COD removal efficiency was higher in planted beds, for the first experimental phase, while in the second phase greater efficiencies were obtained for the beds supplied with a lower organic load. In terms of residual concentrations (C*), in a adjustment of experimental results to a k-C* kinetic model, the second experimental phase registered higher values than the first phase, with C* up to 150 mg/L of COD.