DWSD and Polluted Storm Runoff

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.

DWSD's Wastewater Process, History and Regulation

Perhaps the best summary (16 pages, including charts and maps) of wastewater treatment at DWSD, the city department's record the past few decades and how the State of Michigan enforces state and federal clean water laws can be found on the website of the Michigan Department of Environmental Quality (MDEQ):

http://www.michigan.gov/documents/deq/deq-wrd-npdes-DetroitWWTP_FS_415425_7.pdf

Excerpts:

"The sustained peak primary treatment capacity for wet-weather flows is 1,700 [million gallons per day (MGD)] and the sustained peak secondary treatment capacity for wet-weather flows is currently 930 MGD... All dry-weather flows and a significant amount of wet-weather flows receive full secondary treatment at the WWTP."  (p.3)

"There is currently no additional space at the plant where additional secondary capacity could be constructed. The DEQ has determined there are no feasible alternatives for providing secondary treatment to flows greater than 930 MGD."  (p.4)

[The National Pollution Discharge Elimination System (NPDES) permit] "... focuses on addressing three major issues. The first is providing effluent limits, solids handling requirements, and improved operations and maintenance through a robust Asset Management Program... The second is a reduction in the permitted total phosphorus loads to help reduce phosphorus loadings to Lake Erie. There are numerous sources of phosphorus loadings to Lake Erie, including the Detroit [Wastewater Treatment Plant (WWTP).]  Loadings reductions from point and nonpoint sources are likely needed to help reduce harmful algal blooms in Lake Erie.  The third is a new adaptive management [Combined Sewer Overflow (CSO)] Control Program that recognizes the significant achievements of the Detroit CSO Control Program over the last 20 years, sets forth schedules for completion of the core CSO Control Program, expands the use of Green Infrastructure, and moves forward with the remainder of the control program..."  (pp.10-11)

"The Detroit WWTP has had periods of violations of its NPDES Permit limits and conditions over the past 35 years...After one period of violations from 1997 through 1999, a second amended consent judgment was issued in 2000. These violations largely resulted from high solids inventories accumulating at the WWTP.  The high solids inventories resulted from ineffective operations to dewater solids and remove them from the WWTP...Violations again started in September 2009 and continued until November 2011...As was the case in the late 1990s, the immediate cause was high solids inventories in the WWTP."  (p.11)

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It may be as complete a picture of DWSD as you can get, as long as you overlook a page full of high crimes and misdemeanors.