Impact of rainwater incursions in urban sewage collection network upon wastewater collection and quality in tributary to sewage treatment plant during storms

Abstract

In spite of separate wastewater collection systems being the most popular type of sewers among Brazilian designers, considerable wastewater treatment plant (WWTP) inflow increases and corresponding variations in the influent and effluent quality have been noticed during and after urban storms. However, the impacts of storms over the contributing urban wastershed upon the wastewater collection and treatment system go far beyond the disturbances cited above. Many times, apart from producing an effluent of insufficient quality, the system may also face an excess of wastewater that cannot be treated, so being discharged in natura into the environment - the so called “overflow”. This may commit even more the quality of receiving waters. Such a phenomenon happens, supposedly, due to the presence of illicit connections of rain water pipes to the sewage collection network, as well as infiltrations from the former into the latter - particularly through leakage points along both systems. This research intends to investigate the impact of the urban storm water incursions into the wastewater collection network through a quasi-continuous monitoring of the influent flow to the WWTP. Such consequences are demonstrated through the analysis of hydrographs and comparative concentration diagrams (for several water-quality determinants along the time), considering dry days versus rainy days - which can show, respectively, the variations of inflow rate and quality in the influent sewage. The greatest reason for revealing this problem is to answer the urgent need for increasing the conscience of professionals in this area, governors, decision makers and the population using the sewers network – who frequently connects rainwater pipes to sewers in an illicit way. It has been noticed that even a great part of the technical and scientific community related to water-resource engineering and to sanitation is not aware of the problem in its full extension.