Understanding Inflow & Infiltration: An Engineering Perspective

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Inflow and infiltration (I&I) is surface runoff and groundwater that has entered the sanitary sewer system. There are a number of ways these waters can enter the sewer system and each pathway has particular characteristics that are described with a flow hydrograph.

The first step in reducing I&I is to understand its source.

This table is a great resource as it outlines the various components that comprise sanitary and I&I flows:

Ground Water Infiltration (GWI)

These rates in the Pacific Northwest typically occur in the 3,000 - 5,000 L/ha/d range. High GWI can be found in:

  • Catchments with poorly draining soils
  • Deep sewers
  • Structural decay
  • Ungasketed joints

Achieving complete I&I reduction in these catchments will likely require an intense focus on rehabilitation of the lower portions of the system.

Storm Water Inflow (SWI)

SWI usually results from:

  • Directly connected roof leaders
  • Foundation drains
  • Sump pumps
  • Catch basins
  • Defective sewer access points, including manhole frames and covers, cleanouts and inspection chambers

SWI is indicated in flow monitoring records as sharp responses to rainfall events during summer months, but is nearly impossible to separate from Rainfall-induced Infiltration (RDII) during winter months. This can be quantified at a return period using the I&I Envelope Method.

Rainfall-induced Infiltration (RII)

RII enters the sewer system through:

  • Leaking points
  • Structural defects in pipes and manholes

Service connections are typically buried at shallow depths and are easily damaged. Additionally, brick risers and other component interfaces in manholes are not typically sealed. This allows for rapid infiltration of groundwater to occur.

Infiltration in pipes and lower portions of manholes occurs more slowly, but can be significant during longer wet weather events.

It’s difficult to visually detect RII Because CCTV inspections are usually conducted during dry weather to prevent submergence of the camera. RII becomes fully developed when soils surrounding the sanitary sewer become saturated. This is also determined using the I&I Envelope Method, indicated by flow events with the highest ratio of Rainfall-Dependent Inflow & Infiltration (RDII) to rainfall.

Why care about I & I?

Before we get too much further into this, we should probably discuss why we should even care about inflow and infiltration into our sanitary sewers. Whether you’re an engineer or an urban planner, inflow and infiltration (I&I) surface runoff and groundwater that has entered the sanitary sewer system should be on your radar.

Sanitary sewer systems are designed to carry a specific volume of wastewater, usually from systems including toilets and sinks in homes and businesses. Meanwhile, stormwater sewers are designed to carry rainwater and groundwater away, when this water flows into sanitary sewers it taxes the load on the overall system, including wastewater treatment plants. The result is public health risks and extra costs to your community.

Extra load and Dangerous Consequences

Wet weather and storm events can quickly fill sanitary sewer systems beyond capacity. At which point, wastewater flows at higher-than-usual water levels and can flood homes and spill onto streets. These wastewaters and potential pathogens are a significant public health risk. As wasterwater flow into water bodies like streams and lakes, contamination becomes a major issue for interconnected waterways, harming the flora and fauna that reside there and can prompt officials to issue beach advisories and closures.

EPA Regulations

The Environmental Protection Agency (EPA) requires regulated agencies possessing a National Pollutant Discharge Elimination System (NPDES) permit to stop all wastewater overflows from reaching United States waters. It’s a daunting and nearly impossible task because I&I can’t be stopped altogether and will continue to increase over time as the infrastructure ages. Since the late 1980's public awareness and interest has grown in upgrading sanitary sewer infrastructure and municipalities are continuing to respond.

The best thing municipalities can do is to understand the I&I within their systems, and determine if larger responses could result in a problem. I&I events are based on climatic events and as such their return periods can be calculated. A return period I&I response that causes a sewer backup or overflow can be calculated for every pipe in a city. If the return period is less than the expected service level promised to the rate payers or results in an overflow that harms the aquatic environment, action is required. Since I&I increases with sewer age, it’s important for utilities to understand the current response and calculate the corresponding return period. Information such as this helps to avoid overflows and resulting fines—and starts with effective monitoring tools.

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