Towards better practices in detection of wastewater pollution in stormwater sewers and volume estimation of SSO discharges

Separate sewerage systems are designed so that the wastewater and stormwater are carried separately. However, in practice, untreated wastewater discharges to receiving waters are not that rare, impairing the quality of the receiving waters and increasing the risks to public health and aquatic organisms. The two main causes for such discharges are wastewater that enters stormwater sewers as well as sanitary sewer overflows (SSOs) from the separate wastewater sewers. The overall aim of this licentiate thesis is to review and improve the strategies used for detection of wastewater inputs to stormwater sewers and their location, and to address the issue of quantification of untreated wastewater discharges into the receiving waters by developing two methods: one for SSO discharges and one for the estimation of the wastewater amounts in stormwater sewers.A review of methods for detecting the entry of wastewater into stormwater sewers and the location of the entry points has been carried out by a critical literature review of the effectiveness of the current methods, their advantages, weaknesses and limitations in use. Additionally, an evaluation of the factors affecting the performance of a selected number of methods has been made from specially designed field studies, focusing on detectability of certain indicative pollution parameters and the effects of travel distance. The parameters selected based on those identified as being potentially the most useful in the literature review were E. coli, total coliform, Enterococci, conductivity, turbidity, TSS, and ammonium.The estimation of the volumes of untreated wastewater discharged into receiving waters was addressed by developing two methods: (1) volume estimation of SSO discharges based on already available infrastructure—backflow preventing flap gates—by performing full-scale studies to establish flow rating curves as a function of water head; and (2) estimation of wastewater ingress volumes into a stormwater system by using data from field studies as input to a Monte Carlo simulation to generate the probability distribution of possible fractions of wastewater in stormwater sewers.The reviewed indicator parameters and methods all demonstrated potential for detecting stormwater contamination by wastewater. However, there was no single method or strategy that would work under all conditions investigated. Human waste specific indicators—microbiological (adopted in microbial source tracking methods) and chemical ma