The presence of excess water in centralized sewerage systems is known to have a multitude of unfavorable effects on the daily operation of the wastewater infrastructure. The additional volume of I/I-water decreases the hydraulic capacity of wastewater collection networks, reduces the efficiency of wastewater treatment processes, and increases the costs of transporting and treating the wastewater. Currently, most I/I studies in Latvia are conducted on the scale of the wastewater treatment plant service area and determine only the common performance indicators for a given year. However, data of such resolution are not sufficient to identify problem areas within the networks and to introduce cost-effective measures. The contribution of private sewer laterals to the overall I/I volume is an area of particular interest. Although it is possible to locate and quantify I/I from individual house connections, in practice, given the financial and time constraints, it is not feasible to apply a case-by-case approach. Thus, a simple method to predetermine the problematic parts of the system before conducting on-site inspections is needed. This study investigates the link between groundwater levels and observed night-time wastewater flows on a sub-catchment scale by performing a linear regression analysis (940 data points in total). The results show a direct correlation (R > 0.70 in all cases) between said parameters and highlight the impacts of poorly built and ill-maintained house connections. The presented approach can be widely adopted by system operators to help identify potential sources of diluted wastewater and to aid in the development of priority-based renovation plans.
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