Article of Interest

Infiltration / Inflow

Turning a problem into an opportunity to regain capacity
and lower operating costs

At many municipal wastewater treatment facilities, over half the flow is clean groundwater (infiltration) and rain water or storm water (inflow). During wet weather, it is not uncommon for some facilities to receive 90% of the flow from these non-sewage sources.

Infiltration and inflow (I/I) creates significant adverse impacts

• I/I ties up system capacity and can cause the need to expand capacity of the collection system and/or the treatment plant.
• I/I can cause sanitary sewer overflows (SSOs) and combined sewer overflows (CSOs) creating regulatory issues and environmental impacts.
• I/I can adversely impact treatment plant operations and cause an increase in pollutants discharged.
• I/I increases the cost of transporting and treating the wastewater and increases the user rates.
• I/I depletes groundwater resources making a significant volume of water unavailable for water supply or for base flow to stressed rivers and streams.

While eliminating all I/I in the sewer system is not practical, often a significant portion of it can be cost effectively removed. The first step is to size up the magnitude of the I/I problem by reviewing your plant flow records and developing some metrics (e.g. gallons of I/I per inch diameter-mile of pipe, gallons per capita, etc). Another simple way to get a sense of the magnitude of I/I is to look at your influent BOD and TSS concentrations. In the absence of I/I, there is about 400mg/l of BOD and TSS in typical municipal sewage. So, for example, if your treatment plant averages 200 mg/l, likely about one half of the flow is from I/I.

System flow monitoring to assess the I/I distribution
If it appears that your facility may have excessive I/I based on these simple metrics, it is usually worthwhile to do some system flow monitoring during the spring to understand how the I/I problem is distributed throughout the sewer system. Based on the flow metering results, sewer system evaluation survey (SSES) tools are employed to more specifically pinpoint the source of the I/I. These tools include smoke testing, TV inspections, dye testing, manhole inspections and building inspections. Once the sources are identified, an analysis can be performed to determine which sources can be cost-effectively removed. The results of such an analysis can then be used to generate an asset management capital improvement plan.

Strategies to control private I/I
Often much of the I/I  problem is from private sources such as roof leaders, foundation drains and sump pumps. Accordingly, it is important to have a strategy to deal with private I/I sources. There are a number of sewer use ordinances and incentive based systems to encourage the users to remove these sources.

Wright-Pierce is helping communities throughout New England address I/I problems. If you would like to discuss strategies to abate your I/I issues, please contact us.