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Do Combined Sewer Overflows Increase or Decrease Wastewater-Related Chemicals in Receiving Waters?

USGS scientists processing samples of combined sewer overflow water
USGS scientists processing samples of combined sewer overflow water collected after a storm. The samples were analyzed for selected wastewater-related chemicals. Photo credit: Patrick J. Phillips, USGS.

Recent research by U.S. Geological Survey (USGS) scientists indicates that combined sewer overflows (CSOs – see text box) and related municipal wastewater infrastructure can increase the amount of some wastewater-related chemicals delivered to receiving waters. CSOs occur during times of storm runoff when storm and sanitary waters are combined and treatment is bypassed. For chemicals that have high removal during treatment, bypassing treatment increases the mass of those chemicals delivered (loadings) to receiving waters. At the same time, concentrations of other wastewater chemicals that are not usually removed by treatment can decrease during times of CSOs because of dilution by the large runoff volumes.

USGS scientists conducted a study of CSOs in the Lake Champlain basin. They collected samples of wastewater-treatment-plant (WWTP) effluent and CSO effluent, and water samples from urban streams, large rivers, a reference (undeveloped) stream, and Lake Champlain. The samples were analyzed for a range of organic chemicals that are often associated with municipal wastewaters.  Although the highest concentrations of many chemicals were in WWTP-effluent samples, high concentrations were also found in samples of CSO effluent and storm runoff from urban streams subject to leaky sewer pipes or CSOs. For the reasons stated above, the mixture of chemicals detected differed substantially among sampling sites. For example, concentrations of caffeine, tris(2-butoxyethyl)phosphate (an industrial chemical with a range of uses including as a flame retardant), and cholesterol, which have high removal rates during normal wastewater treatment, generally had similar or higher concentrations in CSO effluent than in WWTP effluent. These chemicals also were higher in samples of urban stream stormflow than in samples of baseflow as a result of CSO discharge and leakage from near-surface sources during storms.

Combined Sewer Overflows (CSOs)

Combined sewer systems collect runoff and domestic and industrial discharge in the same sewer system and route it to a wastewater treatment plant for treatment. During high flows typically associated with runoff events, the combined volumetric flow rate of runoff and wastewaters can exceed the capacity of the sewer system or the treatment plant and flow is diverted directly to a stream, river, or other receiving water body without treatment.

The second phase of this study will focus on a more quantitative assessment of organic wastewater compounds in treated effluent and flow during CSOs at a single WWTP discharging to Lake Champlain. These results will better quantify the amounts of organic wastewater compounds originating from untreated flows resulting from CSOs.


Phillips, P.J., and Chalmers, A.T., 2009, Wastewater effluent, combined sewer overflows, and other sources of organic compounds to Lake Champlain: Journal of the American Water Works Association, v. 45, no. 1, p. 45-57, JAWRA-07-0175-P, doi:10.1111/j.1752-1688.2008.00288.x.

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