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Frequently Asked Questions
Combined Sewer Systems
What is a combined sewer?
A combined sewer is a sewer that receives sanitary sewage, footing drain water, and road drainage. In a community with combined sewers, the catch basins or grates in the street are connected to the combined sewer. In the first half of the twentieth century, these sewers were installed as an economical means of transporting both sanitary wastewater and storm drainage. During dry weather, the sanitary sewage flowed to the wastewater treatment plant. During a rainstorm, the flow rates increased dramatically. Excess water overflowed to rivers, streams, lakes, and the oceans. At the time combined sewers were constructed, the environmental impact of combined sewage discharges was viewed as relatively minor and temporary in duration. Since the flow consisted of 98% storm water and 2% sanitary sewage, the dilution factor was thought to sufficiently mitigate the health risks. In addition, river and stream flow was also very high during a storm; so another dilution factor came into play.
How large are combined sewers?
Combined sewers vary from 12 inch in diameter to more than 12 foot in diameter. In a residential sewer system, combined sewers were normally designed to handle a rainstorm that occurred every five or ten years. Excess flows from larger storms overflowed to rivers, streams, lakes, and the oceans.
How are combined sewers "separated"?
To correct the problems with combined sewers, some communities separated the sewers. In many cases, they installed a new storm sewer to handle the road drainage. They disconnected the catch basins and road grates from the former combined sewer and connected them to the storm sewer. The former combined sewer became a "separate sanitary sewer" and received only sanitary sewage and footing drain water.
In some cases, new sanitary sewers were installed and the former combined sewer was used to transport road drainage and footing drain water.
Why don't more communities separate their sewers?
The cost is high and the neighborhoods are disrupted during the construction.
Beside sewer separation, what is another way to correct the problems with a combined sewer system?
Some communities construct large retention basins that collect the flows that exceed the capacity of the sewer system. The flows are held in the basin until after the storm when they can be sent to the wastewater treatment plant. Sometimes, the combined wastewater is treated in the retention basin and then discharged.
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