Is Private Lateral Inflow and Infiltration Mitigation Effective?

LEARNING OBJECTIVE
This paper presents an overview of the process to mitigate private property inflow and infiltration (I/I) impacts on a coastal community's wastewater collection system through inspection and rehabilitation of private laterals. This paper will present the pilot study results and provide a case-study of localized private laterals rehabilitation methodologies and a cost-benefit analysis of potential private laterals I/I reduction. Flow reduction to the City's public wastewater collection system and to the WRFs resulting from private-side I/I mitigation efforts will be summarized and presented. The process implemented, lessons learned, and results of the study will guide the next steps for the implementation of a city-wide Private Lateral I/I Program.
ABSTRACT BACKGROUND In 2015 and 2016 the City of St. Petersburg, Florida (City) experienced extreme wet weather resulting with unauthorized discharges due to high infiltration/inflow rates and capacity limitations in the sewer collection system, water reclamation facility (WRF) effluent filters, and injection wells. The City engaged a team of professional engineers to evaluate the wastewater collection system and WRFs to identify the most cost-effective solution to mitigate potential similar overflows during future storm events. The City advanced development of a Wet Weather Overflow Mitigation Program (WWOMP). The WWOMP Phase I Study recommended improvements to the WRFs and collection system to reduce the likelihood of future overflows. Collection system improvements were focused on public and private side I/I removals to reduce flows to the WRFs and increase capacity in the sanitary sewer system. The City initiated an I/I pilot study in the Maximo Moorings neighborhood within the Southwest WRF (SWWRF) basin. The initial phases focused on inspection and rehabilitation of the City's public infrastructure. Pre-rehabilitation and post-rehabilitation flow monitoring was performed to estimate the potential I/I reduction associated with rehabilitation of the public infrastructure. Results generally concluded that rehabilitation of public infrastructure alone did not achieve the desired I/I reductions. Private sources of I/I are often also significant contributors to wet weather flow. A Water Environment Research Foundation (WERF) Study reported a range of private sector I/I contributions of 7% to 80%, with a mean of 40% based on a survey of 26 utilities (WERF, 2006). The City subsequently implemented a Private Lateral I/I Pilot Study expanding inspection and rehabilitation to the private laterals in the Maximo Moorings neighborhood. Because I/I is highly dependent on the characteristics of the targeted collection system, rehabilitation programs focused on removing private sources of I/I often have wide-ranging results. This same variability in I/I from system to system and from basin to basin is expected in St. Petersburg and will be incorporated as the City plans for the future implementation of a City-Wide Private Lateral I/I Program.
OBJECTIVES The objectives of the Private Lateral I/I Pilot Study is twofold: (1) to collect data to better understand the I/I reduction associated with rehabilitation/replacement of private laterals in the study areas and (2) to guide the next steps and methods of reducing private sources of I/I throughout the City. METHODOLOGY Two similar neighborhoods were selected for the study in the event of anomalies or deficiencies in the data before and after rehabilitation. A study area and control area were identified and monitored in each neighborhood. In the study areas, defects resulting in I/I were identified and rehabilitated. The control area provided a baseline to compare flows for the same rainfall and groundwater conditions. The City selected the Maximo Moorings and the Greater Pinellas Point study areas due to the following characteristics: - Built between the 1950s and 1970s when the installation of Orangeburg pipe, a bituminized fiber pipe that has a tendency to break, was prevalent. - Located within the SWWRF basin and sit at a similar elevation with most of the gravity sewers below groundwater levels. Both study area's characteristics are representative of a majority of the City's residential neighborhoods. - Significant public infrastructure rehabilitation was performed previously in both neighbourhoods, with roughly 80% of the public gravity sewers recently lined (2016-2018). The project team performed an 8-step process for implementing the study as follows: 1. Pre-rehabilitation flow monitoring 2. Pre-qualification for plumbers 3. Data management procedures 4. Public outreach, pre- and post-Covid 5. Private lateral inspections 6. Rehabilitation 7. Post-rehabilitation monitoring 8. Post Pilot Study survey and lessons learned for citywide private lateral program implementation
RESULTS AND CONCLUSIONS Pre-rehabilitation and post-rehabilitation flow monitoring was performed to collect data on dry weather and wet weather flows generated in the Study Areas. For the pre-rehabilitation condition, the I/I was characterized as a baseline for comparison against post-construction flow data to estimate the effectiveness of private lateral rehabilitation. The control areas account for variation in rain and antecedent moisture conditions between the two monitoring periods and help identify wet weather events during the post-construction flow monitoring period. Rainfall impacts both the antecedent moisture conditions and the sanitary sewer responses (refer to Figure 2). Therefore, rainfall experienced during the pilot study was compared to historical rainfall to show how the pre-rehabilitation and post-rehabilitation rainfall data compared to historic norms and also to each other. In the pre-construction flow monitoring period, excessive levels of rainfall-derived inflow and infiltration (RDII) and groundwater infiltration (GWI) were observed in Greater Pinellas Point. Excessive levels of RDII and high levels of GWI were observed in Maximo Moorings. Figure 3 shows the total volume of rain compared to the portion that enters the sanitary sewer system for each event and incremental metershed for the pre-construction period. The antecedent moisture conditions are indicated by the size of the bubbles based on amount of rainfall that fell during the two weeks prior to the event. Linear regressions, assuming there is no RDII when there is no rain, are fitted to understand the trend for each metershed. Events with less than 1.5 inches of rain show less correlation to the trendlines than events with more than 1.5 inches of rain. These trends will be compared to the post-construction RDII results to understand the effectiveness of private lateral rehabilitation in this area. Post-construction flow monitoring began July 2021 and will continue through December 2021. The following will be summarized and presented: - pre- and post-construction study flows and reductions achieved, - inspection and rehabilitation costs, - cost-effectiveness evaluation, - Pilot Study findings and recommendations, - and lastly, the City's next steps in pursuing system-wide private lateral I/I mitigation.