Gainesville Regional Utilities (GRU) has historically invested significant resources to evaluate and reduce I&I sources but recognized the need for a more programmatic approach for prioritizing rehabilitation and replacement efforts. GRU provides wastewater service to approximately 200,000 people. The GRU wastewater collection system includes over 1,100 miles of piping, 15,500 manholes, 170 lift stations, and 65,000 customer connections.
Recognizing that GRU has excellent technical staff with great institutional knowledge of their system and many lessons learned about what works best for their system and staff, we developed the I&I Reduction Program in a highly collaborative process with them. A decision we made collectively at the beginning of the project was to use an early-out area to go through the tasks that GRU would perform on an annual basis after our team had developed the Program as a way to make refinements and fill data gaps prior to going into a full-scale production. By completing the field investigations, including flow monitoring, smoke testing, manhole inspections, and night-time flow isolation in the early-out area, the Program team was able to determine processes that can best be used moving forward in other parts of the system, with lessons learned being applied.
One of the next major elements of the project was converting lift stations runtimes to flow hydrographs for the most recent two years of data for approximately 170 lift stations. We then compared the hydrographs to NEXRAD rainfall and quantified dry- and wet-season groundwater infiltration, base wastewater flow, and rainfall-dependent I&I. This analysis allowed us to prioritize the lift station basins for field investigation based on where the biggest return would be for the rehabilitation effort at a cost that was significantly less that flow monitoring. Results were corroborated with 10 permanent meter locations, metered winter water use, and known problems at lift stations. We then used short-term flow monitoring in some of the larger basins to further identify specific locations that are most in need of rehabilitation for I&I flow reduction, using other data to also evaluate structural integrity and risk of failure. The Program is set up to have rolling priorities set 2-3 years in advance for field investigation – followed by targeted rehabilitation – so that each step can be properly planned and budgeted.
The Program team also developed level-of-service (LOS) criteria that consider the unique challenges that GRU faces, including being more influenced by groundwater infiltration than by rainfall-derived I&I. The LOS criteria are tied to key performance indicators (KPI), with assumptions, methods of measurement, frequency of measurement, regulatory and internal drivers, influencing factors, and potential solutions identified for each KPI. We also performed an economic analysis to determine at what point conveyance and treatment is more cost-effective than rehabilitation, understanding that the collection system degradation is an ongoing process. The economic analysis was also used to refine the LOS criteria to ensure that the criteria are affordable.