Combined Sewer Overflow

A combined sewer is a type of sewer system that collects sanitary sewage and stormwater runoff in a single-pipe system. Combined sewers can cause serious water pollution problems due to combined sewer overflows, which are caused by large variations in flow between dry and wet weather. This type of sewer design is no longer used in building new communities, but many older cities continue to operate combined sewers.

Many cities that installed sewage collection systems – in the early 20th century or before – used single-pipe systems that collect both sewage and urban runoff from streets and roofs. This type of collection system is referred to as a combined sewer system (CSS). The cities’ rationale when these systems were built was that it would be cheaper to build just a single system. Most cities at that time did not have sewage treatment plants, so there was no perceived public health advantage in constructing a separate storm sewer system.

Combined sewer systems are found throughout the United States, but are most heavily concentrated in the Northeast and Great Lakes regions. State and local authorities have generally not allowed the construction of new CSSs since the first half of the 20th century.

When constructed, combined sewer systems were typically sized to carry three to five times the average dry weather flows. As cities added sewage treatment plants, relief structures were installed in the collection system so that the flow could be discharged into a river or stream during large storm events when the capacity of the pipe exceeded the capacity of the wastewater treatment plant. By using these devices, called regulators, to discharge the excessive flow into a nearby water body, sewer backups in homes and streets are prevented.

A combined sewer overflow (CSO) is the discharge of wastewater and stormwater from a combined sewer system directly into a river, stream, lake or ocean. Overflow frequency and duration varies both from system to system, and from outfall to outfall, within a single combined sewer system. Some CSO outfalls discharge infrequently, while others activate every time it rains. During heavy rainfall events when the stormwater exceeds the sanitary flow, the CSO is diluted.

The stormwater component contributes a significant amount of pollutants to CSO. Each storm is different in the quantity and type of pollutants it contributes. For example, storms that occur in late summer when it has not rained for a while have the most pollutants. Pollutants like oil, grease, fecal coliform from pet and wildlife waste and pesticides get flushed into the sewer system. In cold weather areas, pollutants from cars, people and animals also accumulate on hard surfaces and grass during the winter and then are flushed into the sewer systems during heavy spring rains.

Mitigation of CSO is something that must be approached with an open mind. Some of the methods used are

  • Sewer separation, building a separate sanitary collection system.
  • To expand the treatment capacity and build a bigger plant.
  • Underground storage of combined flows for later treatment, construct storage tunnels or retention basins.
  • Install fine screening and disinfection to remove sanitary trash and reduce pathogens.

All of the above mentioned mitigation methods have one thing in common. They are expensive to design and build.

Some less expensive methods are available, such as

  • Installing permeable paving and pervious concrete to allow water to perk through to the soil below, slowing runoff.
  • Installing green roofs on building to again reduce the flow to the storm or combined sewer.
  • Installing rain gardens in green spaces provided in the city limits again to slow or stop runoff.

Combined sewers are something we have from earlier generations of planners that did the best they could with the knowledge they had. Hindsight is 20-20; we must make the best of what we have.

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