Reducing Infiltration and Inflow in Small Collection Systems: Environmental, Economic, and Additional Impacts

Wastewater treatment facilities and sewage collection systems provide essential services to society that can be negatively affected by increased infiltration and inflow (I&I). Ten case studies examined the construction and operation phases of wastewater infrastructure serving small communities (<4,000 people); the case studies were used to analyze the environmental, economic, and additional impacts and benefits from I&I mitigation using life-cycle assessment (LCA), cost data, and anecdotal information. Consistent with existing literature, I&I mitigation projects completed where initial flow was greater than 454 L (120 gal.) per capita per day were economically and environmentally advantageous. It was found that mitigating I&I is often one of the most significant practices existing facilities can implement to reduce environmental impacts. Overall environmental impacts for mechanical wastewater resource recovery facilities (WRRFs) were reduced in six of seven case studies. Conversely, overall environmental impacts from lagoons were reduced for one of three case studies. Environmental impact reductions were typically tied to reduced operational energy and/or water pollutant emissions. Generally, I&I mitigation costs are small compared to estimated original facility costs. Additional impacts and benefits include improved wet weather resilience and reduced maintenance, which were commonly cited by staff as highly important.

Consistent with existing literature, I&I mitigation projects completed where initial flow was greater than 454 L (120 gal.) per capita per day were economically and environmentally advantageous, especially for mechanical plants. Lagoon systems in this study exhibited more complex results after I&I mitigation projects, which were driven by characteristics such as limited electricity usage, minimum water depth requirements, and discharge elimination. Reduced operational energy usage and improved treatment were major benefits of I&I mitigation projects and realized by a majority of case studies surveyed. Avoided future material usage related to avoided construction or equipment was also shown to be beneficial. Some case studies completely offset material usage for mitigation projects by avoiding other improvements that would have otherwise been required. Avoided construction for plant expansion is an additional major economic and environmental impact benefit of I&I mitigation but is often difficult to quantify. Additional major benefits of I&I mitigation commonly cited by staff include improved wet weather resilience and reduced maintenance.