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

Abstract: 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 literaturereview 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 MonteCarlo 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 markers (e.g., caffeine and carbamazepine)—as well as the Distributed Temperature Sensing method provided the best indication of wastewater pollution in the literature. However, these methods are currently beyond the capabilities of typical municipal expertise due to their costs and/or complexity and, therefore, combinations of methods that minimise the limitations of individual approaches have been identified in this thesis as the most effective provided these are tailored to specific cases.The travel distance from the point at which the wastewater entered the stormwater sewer is an important factor affecting the detectability of the investigated parameters, as the concentrations of the indicator parameters may change along the sewer length due to dilution, dispersion, physical, chemical, and microbiological processes. A Maximum Detection Distance (MDD) away from the origin of the input was determined which was in general shorter for the microbiological parameters than for the physicochemical parameters. Among the microorganisms analysed, Enterococci showed the shortest MDD of about 330–550 m, and E. coli — the longest of about 635–1245 m. Of the physicochemical parameters, the shortest MDD was for TSS (800–1130 m) and the longest for conductivity (1220–1560 m). MDD itself is not the main factor for selecting what parameters should be used, but rather how the sampling strategy for a particular parameter should be planned.The full-scale experiments on circular flap gates provided flow-rating curves for diameters of 200, 300, 400, 500 and 600 mm with high precision (R2>0.99). There are a large number of such gates installed in Sweden and the flow rating curves developed in this study could help practitioners to monitor SSO discharges with minimal onsite measurements (water head before and after the flap gate) in a reliable and inexpensive way.From the field study results, an equation was developed to estimate the volume of wastewater entering into the stormwater sewer. Monte Carlo simulation was selected as a means to increase the robustness of the outcome of the developed equation. The results of the simulation showed that the efficiency of the suggested method varied to a great extent depending on the selected combination of parameters: for example, for TSS and conductivity the percent error between the most probable and actual wastewater fractions in the sewer was less than 2%, while TSS combined with microbiological parameters failed as a combination to estimate the wastewater fraction in sewer.