Inflow Quantification in Urban Sewer Networks

Abstract

The improper waterflow to wastewater treatment plants (WWTP) due to rainwater inflow, and infiltration is a growing concern due to the many problems it brings to the sector, ranging from infrastructure deterioration to environmental problems caused by untreated wastewater and to the eventual financial costs that these issues cause. The study was carried out at the Folhadela WWTP, Vila Real, Portugal, between May 2014 and May 2015, with the total effluent flows recorded every 2 min at the entrance of the WWTP. Rainfall data from the Vila Real Meteorological Station, corresponding to the same period, were used. The study allowed us to conclude that from the wastewater that flowed to the Folhadela WWTP, in the months of study, only 15% is domestic wastewater, and the remaining 85% were undesirable volumes. Of these, 47% were infiltration flows, and 38% were rainwater flows that are not taken into account when dimensioning networks and WWTPs. These flows also have the particularity of representing very high volumes in short periods of time, coinciding with heavy rains, representing a very high risk for drainage and treatment infrastructures. Regarding the infiltration flow rates, as a general rule, they are taken into account when dimensioning the networks as being a percentage of the total flow. However, it is necessary to take into account the magnitude and the evolution of these values according to the network age and state of conservation, as well as have straight regulations about the undue connections into the network.