Using Green Infrastructure to Reduce Combined Sewer Overflow
One of the principal functions of the Detroit Water & Sewerage Department (DWSD) under its NPDES permit is to control combined sewer overflow (CSO) following rainstorms.
Combined sewers carry both human waste and storm runoff. Heavy rains cause flow in these sewers to overwhelm wastewater treatment plants. The polluted effluent then contaminates lakes and streams, threatening public health and damaging the environment.
In some large metropolitan areas, huge underground reservoirs or tunnels have been built to hold polluted storm runoff until the treatment plant catches up.
Several years ago, such a reservoir was contemplated for Detroit, but the city couldn’t afford it. The plan was scaled back, but not enough to be within the city’s ability to pay for it.
State regulators worked with local officials to develop plans for a number of smaller, surface containment facilities. They would hold the initial, more contaminated runoff (so-called “first flush”) until the treatment plant could provide both stages of treatment, disinfection and solids (sludge) removal.
Under this last plan, the remaining storm surge (after the first flush) would be disinfected and released to public waters without removing most of the solids. Nutrients in the solids generate algae growth which in turn depletes oxygen downstream.
In addition, the plan to control overflow called for the application of enhanced natural processes to absorb or at least slow down storm runoff. These processes are commonly known as “green infrastructure.”
State officials included short, vague references to green infrastructure in DWSD’s proposed NPDES permit renewal. Green infrastructure was intended to reduce CSO by 10 to 15 percent. (In my opinion, green infrastructure has the potential for much greater CSO reduction than that.)
The Southeast Michigan Council of Governments (SEMCOG) was enlisted to assist in fleshing out more precise detail concerning green infrastructure. The effort got off to a slow start, but environmental organizations, notably the Sierra Club, helped to keep the project on track.
Today, as the result of city-wide publicity and neighborhood organizing, some of this green methodology is being implemented. Trees are being planted, downspouts disconnected from sewers and rain gardens established in backyards and on roofs. Soon, we can expect to see roadside swales created and pervious concrete used for roads and parking lots.
Some suburban communities have been reluctant to adopt measures like these, but public interest in clean waterways and stable water services rates will drive improved storm runoff control.