Combined Sewers Sometimes Too Much of a Good Thing

Overflows Cause Water Quality Issues

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Posted: Wednesday, February 2, 2011 2:54 pm

The best thing about combined sewer systems - that they collect stormwater, sanitary sewage and industrial wastewater all in the same pipe - is also the worst thing about combined sewer systems.

When flow levels are low or normal, such systems efficiently transport all of the wastewater to a treatment plant, where it is treated and discharged safely to a nearby waterway. However, during periods of heavy rainfall or snow melt, the wastewater volume in a combined system can exceed the capacity of the treatment facility and overflow directly into nearby waterways. As a result, combined sewer overflows (CSO) are among the major sources of water quality problems.

Retention/treatment basins have been installed in many combined sewer systems. These basins are designed to capture the sewage and stormwater and provide initial treatment and disinfection.

The Detroit Water and Sewerage Department (DWSD) provides water service to approximately 4 million people in southeast Michigan. It is one of the largest water utilities in the United States. The department has undertaken several projects to correct CSO problems.

The Detroit wastewater service area has a total of 15 CSO retention treatment basins (RTBs) that function when heavy rainstorms overload the sewer system. During 2010, these RTBs prevented 6.4 billion gallons of CSO from reaching waterways, according to a 2010 Annual Report issued by the Public Education Work Group composed of DWSD and wholesale customers.

DWSD has reassessed previous CSO control projects for the Rouge River, due to difficult economic times and a changing economic landscape.

"The declining population has actually resulted in more green space from vacant parcels, many without structures. This demographic change impacts the amount of stormwater generated by the area and ultimately the size of required facilities," the report said.

The re-evaluation resulted in a decision to reduce capital costs and increase green infrastructure within the expanded green space by constructing first flush capture basins instead of a previously planned tunnel.

"DWSD is proceeding with a combination of downsized conventional CSO control facilities plus a new ‘green infrastructure' component that reduces the amount of stormwater that enters the combined sewer," the report said.

"The new CSO control program, which was approved by the Michigan Department of Natural Resources and Environment (MDNRE) in May 2010, carries an estimated capital cost of $850 million. To minimize the financial impact on Detroit residents who will bear 83 percent of the cost pursuant to a Federal Court Order, the program will be phased in over more than 30 years. The specific timetable for completing the projects will depend on the city's financial capability as determined by an evaluation of economic conditions to be conducted every five years.

"The conventional CSO control facilities that have been proposed include storage tanks to capture the most heavily polluted ‘first flush' that occurs at the start of a rain event, in conjunction with an innovative, low cost screening and disinfection system. The screening is to be accomplished using disposable mesh nets which are to be installed at the CSO outfalls, and disinfection will be achieved by injecting sodium hypochlorite into the combined sewers upstream of the outfall."

The Southeast Michigan Council of Governments (SEMCOG), a regional planning partnership, received a $308,000 grant through MDNRE to work with DWSD to identify, prioritize and select green infrastructure projects across the Upper Rouge Tributary Combined Sewer area.

"Targeted green infrastructure opportunities include demolition of abandoned buildings and greening of those respective properties, disconnection of downspouts, implementation of green infrastructure techniques on municipal properties, targeted techniques along roadways and parking areas, and tree planting along roadways and open spaces. DWSD has committed to spending $50 million to implement the green infrastructure program over the next 20 years, in addition to the $850 million conventional CSO control facility expenditure," the report said.

The green infrastructure program is expected to reduce inflow to combined sewers by at least 10 to 20 percent upon completion. The program also:

• Provides quick water quality benefits while conventional CSO facilities are designed, sited, financed and built;

• Acknowledges the ongoing demographic and population changes that are profoundly affecting the tributary area;

• Is consistent with Detroit Mayor Dave Bing's objective to remove abandoned homes and deal with urban blight;

• Is consistent with the approach being used in many other urban areas such as Philadelphia, PA; Portland OR; Cincinnati, OH; and Washington, DC.

MDNRE reports that the green infrastructure program will:

• Reduce discharges, which will reduce E. coli and other water quality pollutants;

• Improve stream hydrology;

• Reduce in-stream temperature, thereby improving fish and stream quality;

• Improve habitat, reduce the urban heat island effect, improve aesthetics and property values, and reduce the peak rate, which might reduce flooding.

A report issued by DWSD in 2009 recommended the use of green infrastructure technologies as a component of the long-term CSO control program. The report recommended several initiatives.

"Roads represent 6,700 acres of the total tributary area, which includes both the pavement surface and the total right-of-way for local, county, and state roads. They are also 38 percent of the impervious surface in the tributary area. As such, there are numerous opportunities to incorporate green infrastructure practices within the right-of-way and reduce impervious surfaces within this land use," the 2009 DWSD report said.

"There are three major highways running through the tributary area (the Lodge Freeway, Interstate 96, and the Southfield Freeway) that represent a significant portion of the road impervious surfaces. The state of Michigan should be partners in this effort by integrating greening techniques along these corridors, such as native vegetation and tree planting in the right-of-way."

The report recommends analyzing traffic information of certain roadways in order to ensure that the current capacity level is necessary.

"In areas where road lanes can be reduced, a boulevard or bump-outs should be added to manage stormwater runoff by incorporating green infrastructure techniques. The benefits, in addition to stormwater runoff volume reduction, include traffic and pedestrian safety, as well as aesthetics," the 2009 DWSD report said.

"Where boulevards and other techniques are not possible, street trees should be added in the right-of-way. In highly impervious areas, tree planter boxes, which can absorb additional stormwater runoff, should be utilized. The priority tree planting should be installation of street trees along road rights-of-way."

The report calls for existing boulevards to be retrofitted to include green infrastructure techniques, including trees and bioswales. Bioswales are landscape elements designed to remove silt and pollution from surface runoff water. The report also recommends the greening of municipal facilities.

"Municipal facility parking areas' stormwater runoff should be managed using bioswales. Downspouts should be disconnected from all municipally-owned buildings and direct runoff to pervious areas. This will reduce volume to the system, as well as being a positive example to other stakeholders," the 2009 DWSD report explained. The report added that there should be a goal of converting 50 percent of grass and bare ground on municipal property to trees.

The report also recommended disconnecting downspouts on private property and offering an incentive program to do so.

The goal of the program should be 100 percent disconnection of residential and school downspouts and direct runoff to pervious areas on school property, including rain gardens, and 25 percent of the impervious surfaces on commercial, industrial, and institutional properties that are managed through disconnection or bioswales in their parking lots, the 2009 DWSD report said.

City-owned parks also provide good opportunities to implement green infrastructure, according to the report.

The goal in public parks should be to convert 75 percent of grass to trees with forest understory, 80 percent of bare ground to trees, and 50 percent of trees with grass understory to trees with forest understory.

"Parks, especially those slated by the city for repositioning, should be analyzed for advanced green infrastructure techniques, such as managing road/residential runoff in park areas through rain gardens, stormwater wetlands, etc. This practice is being successfully implemented in fully utilized parks in the city of Philadelphia," the DWSD reported.

The report said that tree canopy on bare ground within residential areas should be increased to 100 percent. A total of 50 percent of grass and bare ground on commercial, industrial, and institutional land should be converted to trees.

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