Combined Sewer System Background Information

Page updated on Jan 17, 2018 at 10:14 AM

There are two types of storm sewer systems in the City of Alexandria, a separate sewer system, and a combined sewer system.  Separate sewer systems consist of two pipes: one pipe conveys stormwater runoff (rain water) from storm drains to local waterways with little or no treatment to remove pollutants.  The other pipe conveys sanitary sewage to a local wastewater treatment plant (operated by Alexandria Renew Enterprises) as shown in the figure below.

CSS Separate Sanitary and Stormwater Systems 
source: www.dcwater.com 

In contrast, Combined Sewer Systems (CSS) have only one pipe which conveys both sanitary sewage and stormwater to a local wastewater treatment plant. About 540 acres in the Old Town is served by a combined sewer system, which was designed to carry sewage from inside homes and businesses and stormwater runoff from streets, rooftops, and parking lots in a single pipe—a "combined sewer." During dry weather, all raw sewage flows to the treatment plant operated by Alexandria Renew Enterprises. When it rains, the pipes can become overloaded with polluted stormwater. This mixture of stormwater (about 90%) and raw sewage may overflow into local streams through one of the four permitted combined sewer outfalls.

CSS Combined Sewer System 
source: www.dcwater.com 

The City's Combined Sewer System (CSS), a legacy infrastructure built in 19th and early 20th century, is a combination of storm and sanitary sewer systems serving approximately 540 acres in Old Town. The map below shows where the City's CSS is located and the location of its combined sewer overflow (CSO) outfalls.

Overview Map of Combined & Sanitary Sewer System 

In 2013 the City of Alexandria received a 5-year renewal of its permit to operate its CSS from the Virginia Department of Environmental Quality (VDEQ) through the issuance of a Virginia Pollution Discharge Elimination Permit, which regulates discharges from the City's CSS through the CSO outfalls.   

As part of the City's Eco-City Alexandria initiative, and in coordination with VDEQ, this new permit ensures regulatory requirements are met and the environment is protected. Under the permit negotiated by the City with VDEQ, the City will be updating its existing 1999 Long Term Control Plan (LTCP) and implementing several additional projects to improve water quality in Hunting Creek, Potomac River, and Chesapeake Bay. The LTCP to be developed will identify the strategy, projects, schedule, additional infrastructure investments and long-term impacts on the community. In addition to developing the LTCP, the permit also requires the City to implement several important construction projects within the five year permit cycle. These projects are needed to improve the system and to continue to reduce the impacts.

The City plans to start planning and implementation of several new and expanded initiatives required under the new CSS permit , including the following:

  • Update the LTCP;
  • Construction projects to improve sewer system performance, and improve water quality;
  • Develop and implement green infrastructure pilot projects within the CSS; and 
  • Continue implementing the CSS Area Reduction Plan that requires developers to separate sewers, when feasible, in the CSS as redevelopment occurs.

 As part the permit requirements, the City submitted Work Plan to VDEQ which details the process and schedule for how the City will prepare the update to the LTCP, including public participation. The Work Plan was submitted in May 2014 and approved by VDEQ in July 2014. The City has been submitting to VDEQ a series of technical memoranda throughout the Long Term Control Plan Update process in accordance with the approved Work Plan. Technical memoranda submitted to VDEQ to date include the following:

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