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):
"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.