What is a Combined Sewer?

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  • Written By: Jeremy Laukkonen
  • Edited By: Michelle Arevalo
  • Last Modified Date: 05 April 2020
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Most modern sewage systems utilize separate pipes for waste and storm drains, with sewage being directed to treatment plants while storm runoff goes directly to a nearby body of water. The system that this replaced, known as the combined sewer, only uses a single pipe for both waste and storm water. During normal dry conditions, the waste travels through the combined pipe to a treatment station. During heavy rain these pipes will fill. This can allow the combined sewage and rain water to breach an overflow dam and empty directly into a body of water, like a river or a lake.

Combined sewer systems were relatively inexpensive to install historically, which may have contributed to their widespread use. At the time they were popular, many cities didn't have waste treatment facilities, so the differentiation between types of waste may not have seemed as important. It was around the middle of the 20th century that environmental concerns and other pressures resulted in the banning of combined sewer systems in many locations.

While it is rare for new communities to be constructed with combined sewer systems, many cities around the world still have them in place. This may lead to health concerns during times of heavy rain, as local bodies of water can be contaminated with human waste. This may be mitigated through a number of different techniques, such as extra storage for the runoff or partial disinfection of the waste before it enters a body of water.


Some cities have tunnels or retention ponds to help mitigate the environmental impact of their combined sewer systems. These systems are designed to hold a large volume of waste and rainwater until the weather dries up and it can be treated. Such holding facilities will typically store the first runoff in a separate container. This is often known as the first flush, and will tend to have a higher concentration of harmful contaminants, like fertilizers and motor oil, that get carried off by the storm water. The contents of this compartment can then be sent to a treatment facility after the storm has passed.

Other cities may make use of what are known as flow through facilities, where the waste isn't actually treated. These typically strain out the solid waste and then introduce an antibacterial agent, like sodium hypochlorite, which is the active ingredient in most kinds of bleach. They will generally be designed with a large enough capacity that the sodium hypochlorite can eliminate much of the bacteria before the waste is able to flow into a natural body of water.



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