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'''Infiltration/Inflow'''
==Background==
Early [[combined sewer]]s used [[surface runoff]] to dilute waste from [[toilet]]s and carry it away from urban areas into natural waterways. [[Sewage treatment]] can remove some [[pollutant]]s from toilet waste, but treatment
==Infiltration==
[[Groundwater]] entering sanitary sewers through defective pipe joints and broken pipes is called ''infiltration''.<ref name="king">King, James J. ''The Environmental Dictionary'' (1995) John Wiley & Sons ISBN 0-471-11995-4 p.335</ref> Pipes may leak because of careless installation; or they may 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.
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.<ref name="eddy">Metcalf & Eddy ''Wastewater Engineering'' (1972) McGraw-Hill pp.39-44</ref> ==Inflow==
Water entering sanitary sewers from inappropriate connections is called ''inflow''.<ref name="king"/> Typical sources include roof drains, cellar drains, and yard drains where urban features prevent surface runoff, and storm drains are not conveniently accessible or identifiable. Inflow tends to peak during precipitation events, and causes greater flow variation than infiltration. 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.<ref name="hammer">Hammer, Mark J. ''Water and Waste-Water Technology'' (1975) John Wiley & Sons ISBN 0-471-34726-4 pp.303-304&441-442</ref>
==Significance==
Dilution of sewage directly increases costs of pumping and [[chlorination]], ozonation, or [[ultraviolet disinfection]]. Physical treatment structures including screens and pumps must be enlarged to handle the peak flow. Primary clarifiers must also be enlarged to treat average flows, although [[primary treatment]] of peak flows may be accomplished in [[retention basin]]s. Biological [[secondary treatment]] is effective only while the concentration of soluble pollutants (typically measured as [[biochemical oxygen demand]] or BOD) remains high enough to sustain a population of microorganisms digesting those pollutants. Secondary treatment is expected to remove 85 percent of soluble organic pollutants from sewage containing 200 mg/L BOD,<ref>U.S. Environmental Protection Agency (EPA). Washington, DC. [http://www.access.gpo.gov/nara/cfr/waisidx_07/40cfr133_07.html "Secondary Treatment Regulation."] ''Code of Federal Regulations,'' 40 CFR Part 133.</ref> but BOD removal by conventional biological secondary treatment ceases as BOD concentrations entering the treatment facility are diluted below about 20 mg/L.
Dilution of sewage increases costs of pumping, requires increased volumes of treatment chemicals, and generally decreases the efficiency of treatment (increases the mass of pollutants discharged) by producing larger volumes of treated sewage with similar pollutant concentrations. 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.<ref name="hammer"/>▼
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==Correction==
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