With recent flooding in cities across the country,there has been increasing concerns about the state of our water treatment infrastructure. How can cities keep growing when many sewer systems are already at the breaking point?

First, we have to understand what’s already in place.


Simple diagram of typical water treatment plant from the EPA.

Today, most cities have a combined sewer system – waste water from inside buildings (toilets, sinks, etc.) and runoff from the streets go into the same sewer pipes under the streets. When heavy rains occur, the pipes essentially overflow, and the waste water treatment plants have to release the combined dirty and rain water somewhere. In cities like Chicago and New York, that dirty water goes into nearby body of water. This keeps the streets from becoming sewers themselves.

Another approach, which is employed in areas of Philadelphia, for example is a separate sewer system. Separate sewers carry waste water separately from rainwater. If an entire city were on this system, the excess loads on the system could be alleviated by allowing only the rainwater pipes to flow, un-treated, into the nearby body of water.
The other part of the issue of water treatment in urban areas is that cities have very little open ground for rainwater to soak into. Streets, buildings, sidewalks fill up the land and when it rains, there is nowhere for the water to go besided to the sewer. Parks and yards help, but are typically not spread evenly in an urban area.

Chicago City Hall Green Roof

There are many ways like green roofs and swales that are now being employed to give the water a place to go, besides the sewer. New development offers a chance to put these solutions into play, project by project, on a larger scale. Some of these solutions (green roofs) can be employed by property owners of existing buildings. All efforts in this vein will reduce the loads on the combined sewer systems.
With something as critical as our water, changing the system over time is the only feasbile way to go. It’s not reasonable to shut a city’s water and sewer system down to do a full system overhaul. This is not to say larger plans are not being considered. In Chicago, for instance, UrbanLab has made a proposal for chaning the way we treat water.