Modelling the response of influent volumes of wastewater treatment plants under current and future conditions for effective wastewater management in combined sewerage systems
Saikia, Sukanya D.
Saikia, Sukanya D.
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Publication Date
2023-09-28
Type
Thesis
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Abstract
Wastewater treatment plants (WWTPs) are critical infrastructure globally and are essential to protect public health and the environment. With factors such as population growth, urbanisation, increase in water consumption etc., the amount of wastewater generated has increased significantly, which impact the operations of WWTPs. Of particular concern are WWTPs with combined sewerage systems (CSSs) which treat foul and storm wastewater collectively. Such WWTPs are also influenced by changes in the intensity and frequency of precipitation events. During instances of increased precipitation intensity and frequency of storm events, WWTPs with CSSs might encounter hydraulic overloading and release of untreated wastewater called combined sewer overflows (CSOs). On the other hand, lack of precipitation can lead to reduced flow and increased contaminant loading. In both the cases, with climate change and the associated changes in precipitation patterns, WWTPs with CSSs may become more susceptible to system failures that pose a threat to the receiving waters and the surrounding natural environment. As stricter environmental regulations are enforced to limit the occurrences of CSOs, it has become increasingly important to identify the variables that impact the functioning of WWTPs with CSSs. However, unavailability of real data is a key challenge in the research area of monitoring the performance of WWTPs and sewerage systems. Studies have conventionally used modelled data simulated from hydraulic models that do not cater to local characteristics of individual WWTPs and hence are often associated with large uncertainties. While considerable attention has been given to the impacts of climate variables (current and future) and urbanisation on effluent quantity and quality and the performance of the sewerage systems focusing on CSOs, the same cannot be said for wastewater influent volumes. Influent volume characteristics function differently as compared to CSOs and effluent volumes. Once CSOs leave the sewerage system, there are still variations in the flow that proceed towards the WWTPs during or after the spill. Hence influent volumes have a significant impact on subsequent WWTP processes and predicting how they might change can help prevent occurrences of overflows and aid in achieving resilience of WWTPs. Studies investigating the degree to which precipitation change (current and future), tidal level, river level, and urbanisation impacts influent volumes of WWTPs with CSSs remain unexplored. This research addresses these gaps by studying influent volume response characteristics of 14 WWTPs of varying sizes, connected with CSSs, that are spatially distributed over Ireland. The thesis uses real spatio-temporal datasets of precipitation, influent volumes and location-specific data of tidal and river level. The objective of this study was to develop methodologies under practical data constraints to build models that could define meaningful relationships among the different variables. Daily precipitation and daily mean river level were found to be statistically significant predictor variables of influent volumes at a daily scale. On a monthly basis, monthly average daily precipitation, number of wet days in a month (and thus zero rainfall days) were observed to be statistically significant. The daily and monthly variations in influent volumes for each of the WWTPs were assessed with the help of simple and multiple linear regression modelling analysis. These individual WWTP models helped to capture local characteristics specific to each WWTP. In addition, a novel pooled model was developed through spatio-temporal analysis across all the 14 WWTPs to derive generic trends in influent volumes across any WWTP. Probability of exceedance curves linking daily precipitation and influent volumes were also developed that could aid in identifying storm overflow events under various precipitation categories. These graphs could be potentially used for future climate scenarios using precipitation projections to estimate the projected frequency of storm overflow events. This research also analysed, for the first time, the evolution of influent volumes during mid-century period (2041 – 2060) as compared to current period. It predicted future influent volumes by leveraging high resolution multi-model regional climate model projections of precipitation intensity and extreme events for each WWTP and linking them to the developed data-driven models and probability of exceedance curves. This analysis offers valuable insights into how WWTPs might get impacted in future (e.g., exceedance of peak design capacity under extreme weather conditions) due to climate change. Finally, this research aims to investigate the degree to which urbanisation might potentially impact influent volumes of WWTPs with CSSs. Landsat 5 and Landsat 8 satellite images were used to perform landuse landcover classification of all the 14 agglomerations corresponding to each WWTP to estimate the change in built-up area. Percentage change in built-up area relative to agglomeration area was found to be statistically significant with moderate degree of correlation with influent volumes across all agglomerations. The findings of this research will help wastewater utilities (particularly the ones connected with CSSs) as end-users, take informed decision in their planning and adaptation strategies in order to establish resilient wastewater infrastructure at regional and local WWTP scales.
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NUI Galway