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What is I&I?
Infiltration and Inflow (I&I) is clean storm and/or groundwater that enters the sanitary sewer system through holes, breaks, joint failures, connection failures, illegal connections (sump pumps, down spouts/gutters, and footing drains), and from cross-connections with storm sewers. Most inflow comes from stormwater and most infiltration comes from groundwater. High groundwater levels and storm events can contribute to excessive sewer flows.
I&I affects the volume of flow conveyed through the system and pumping stations and the cost to treat the flows. Ultimately, this affects the taxes businesses and residents pay to operate and maintain the sewers. Private property I&I can contribute substantially to the problem. An 8-inch diameter sewer can adequately convey flow from roughly 200 homes. However, only 8 sump pumps operating at full capacity can overload the same 8-inch diameter sewer.
The Town of Amherst Engineering Department maintains a proactive approach to sanitary sewer maintenance which includes sewerline flushing, root treatment, manhole inspection and GPS. To minimize and eliminate extraneous stormwater from entering the sanitary sewer system a townwide program is currently being utilized. Some of the investigative techniques are used to identify sources of I&I include the following:
- Flow Monitoring
- Determines the amount of I&I entering a sanitary sewer system. Sensors are placed into the flow and the data is analyzed to determine if more investigation is required in the study area.
- Smoke Testing
- Identifies connected downspouts and other inflow sources, such as cross connections with the storm sewer system and deficiencies in and around manholes and sewers. The testing involves placing a blower over a manhole to push a non-toxic, odorless “smoke” into the sewerline and laterals.
- Televising and Dye Testing
- a non-toxic dye is added to a possible I&I source like storm drains and ditches. The sewerline is then televised, using a camera to look for cracks, root intrusions, and dye colored infiltration. The video from the camera also serves as a condition assessment for the sewers to determine locations for repairs and rehabilitation projects.
- Time of Sale Inspections
- Includes interior and exterior plumbing inspections to help property owners eliminate illegal or improper stormwater connections like sump pumps, roof drains and more. For more information see the Time of Sale Law webpage.
- I&I Reduction Program
- Private developers are required to offset peak flows from their proposed projects on a 4:1 ratio. For an explanation of the program see the Sewer Remediation Trust Fund. This program requires that developers calculate the peak sewage flow for the their project and then multiply the peak sewage flow by the 4:1 ratio of required mitigation flow to peak sewage flow to determine the flow that must be mitigated by the proposed project.Forinformationregarding the acceptable mitigation measures and costs of mitigation credits, please see the Town of Amherst I&I Table.
Once I&I sources are identified, several repair and rehabilitation methods are used, including replacement, spot repair, and manhole repair. Trenchless Technologies are also being used, including PipePatch and Cured-In-Place Pipe (CIPP) Lining to help the town rehabilitate the sewer infrastructure.