Infiltration/Inflow

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Infiltration/Inflow (I/I or I&I) is the process of groundwater, or water from sources other than domestic wastewater, entering sanitary sewers. I/I causes dilution in sanitary sewers, which decreases the efficiency of treatment, and may cause sewage volumes to exceed design capacity. Although inflow is technically different from infiltration, it may be difficult to determine which is causing dilution problems in inaccessible sewers. The United States Environmental Protection Agency defines the term infiltration/inflow as combined contributions from both.[1][2]

Background

Early combined sewers used surface runoff to dilute waste from toilets and carry it away from urban areas into natural waterways. Sewage treatment can remove some pollutants from toilet waste, but treatment of diluted flow from combined sewers produces larger volumes of treated sewage with similar pollutant concentrations. Modern sanitary sewers are designed to transport domestic and industrial wastewater directly to treatment facilities without dilution.[3]

Infiltration

Groundwater entering sanitary sewers through defective pipe joints and broken pipes is called infiltration.[4] Pipes may leak because of careless installation; they may also be damaged after installation by differential ground movement, heavy vehicle traffic on roadways above the sewer, careless construction practices in nearby trenches, or degradation of the sewer pipe materials. In general, volume of leakage will increase over time. Damaged and broken sewer cleanouts are a major cause of infiltration into municipal sewer systems.[5]

Infiltration will occur where local groundwater elevation is higher than the sewer pipe. Gravel bedding materials in sewer pipe trenches act as a French drain. Groundwater flows parallel to the sewer until it reaches the area of damaged pipe. In areas of low groundwater, sewage may exfiltrate into groundwater from a leaking sewer.[6]

Inflow

Sanitary manhole inflow due to a flood[7]

Water entering sanitary sewers from inappropriate connections is called inflow.

foul flush of accumulated biofilm and sanitary solids scoured from the dry weather wetted perimeter of oversized sewers during peak flow turbulence.[8] Sources of inflow can sometimes be identified by smoke testing. Smoke is blown into the sewer during dry weather while observers watch for smoke emerging from yards, cellars, or roof gutters.[9]

Significance

Dilution of sewage directly increases costs of pumping and

disinfection by-products
by chemical disinfection prior to discharge.

Excessive infiltration/inflow may cause sanitary sewer overflows during wet weather.

High rates of infiltration/inflow may make the sanitary sewer incapable of carrying sewage from the design service area. Sewage may back up into the lowest homes during wet weather, or street manholes may overflow.[9]

Correction

Smoke test results may not correlate well with flow volumes; although they can identify potential problem locations. Where sewage flow is expected to be relatively uniform, significance of infiltration and inflow may be estimated by comparison of sewage flow at the same point during wet and dry weather or at two sequential points within the sewer system. Small areas with large flow differences can be identified if the sewer system provides adequate measuring locations. It may be necessary to replace a section of sewer line if flow differences cannot be corrected by removing identified connections.[9]

References

  1. ^ U.S. Environmental Protection Agency (EPA), Washington, D.C. Code of Federal Regulations. "Part 35—State and Local Assistance."
    • "Definitions; Excessive inflitation/inflow." 40 CFR 35.2005 (b)(16). 40 CFR 35.2005
    • "Grants for Construction of Treatment Works; Infiltration/Inflow." 40 CFR 35.2120
  2. ^ Guide for Estimating Infiltration and Inflow (PDF) (Technical report). U.S. EPA New England. June 2014. Retrieved 8 May 2023.
  3. ISBN 0-07-060929-2
    .
  4. ^
    ISBN 0-471-11995-4
    .
  5. ^ Private Sewer Laterals (PDF) (Report). Water Infrastructure Outreach. Boston, MA: EPA. June 2014.
  6. ^ Metcalf & Eddy Wastewater Engineering. New York: McGraw-Hill. 1972. pp. 39–44.
  7. ^ Hambledon Infiltration Reduction Plan (PDF). Southern Water. June 2016. pp. 6–7. Retrieved 25 February 2024.
  8. ^ Fan, Chi-Yuan; Field, Richard; Lai, Fu-hsiung. "Sewer-Sediment Control: Overview of an EPA Wet-Weather Flow Research Program" (PDF). EPA and University of California, Los Angeles. Archived from the original (PDF) on 13 March 2016. Retrieved 12 March 2016.
  9. ^
    ISBN 0-471-34726-4
    .
  10. ^ EPA. "Part 133—Secondary Treatment Regulation." 40 CFR 133
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