Sewer systems that convey both sanitary sewage and storm water through a single pipe are referred to as combined sewer systems (CSS). In dry weather and during light to moderate rainfall, the CSS is able to convey all flows to the wastewater treatment facility. During periods of heavy rainfall, however, the capacity of the CSS may be exceeded, often causing untreated combined sewage and storm water to back up into basements and to overflow from manholes onto surface streets. Traditionally, CSS outfalls were designed to discharge directly into receiving waters during combined sewer overflows (CSOs), increasing pollution to those waters.
Many cities have separated their sewers, but other regions are currently working to separate their combined sewers to reduce pollution, and the costs are high. King County's plan to reduce CSOs would cost a billion dollars.
An incremental and low cost step to reduce stormwater flows is to prevent the water from reaching the sewer in the first place. Green Infrastructure is an approach to using natural systems to reduce effects of urban development. The general principle is water dispersion rather than collection, and infiltration rather than retention. One great example is in Indianapolis. Stormwater planters and bioswales with native grasses run almost the entire length of the city’s Cultural Trail, a state-of-the-art bicycle and pedestrian route built over the past six years that wends its way for eight miles through the downtown streets of Indiana’s largest city.
The footage below offers a great look at how they work in action, with water filling up the trenches and then slowly draining off without ever entering the sewer system. The plantings also serve to separate bicycles from both car traffic and from pedestrians along the path. The project as a whole has added 500 trees and eight acres of green space to Indianapolis, and at the same time it's saving the city money in water treatment costs.