HOW TO ESTABLISH PEAK FLOWS IN A COLLECTION SYSTEM USING A WATERSHED PLANNING AND PERMITTING APPROACH
Abstract:Municipalities share a liability under the Clean Water Act (CWA) for setting capacity peak flows for their wastewater collection systems. Almost universally, the fact remains that municipalities will experience wet weather conditions that will cause their collection system flows to exceed their systems' peak flows. When this situation develops, a sanitary sewer overflow (SSO) will occur, thus potentially creating a permit violation. In light of this eventuality and the CWA's strict prohibition of SSOs that reach the waters of the United States, municipalities must ultimately decide on how best to balance establishing peak flow standards and determining specific peak flows throughout the collection system with these other issues:
Dynamic sewer flows resulting in a large measure from infiltration and inflow (I/I) originating on private property;
Capital funding with accurate prioritization and scheduling;
Risks of public health and environmental effects from SSOs; and
Threats of third-party citizen lawsuits.
The U.S. Environmental Protection Agency (EPA) attempted to help the decision process by forming an SSO Subcommittee in 1994 to develop approaches to manage SSOs and to suggest more reasonable enforcement action priorities. In October 1999, EPA drafted its interpretation of the subcommittee's concepts in the form of an SSO Rule. The conclusion made from EPA's draft SSO Rule and other EPA supporting documents is that EPA strongly encourages a watershed approach to planning and permitting. Watershed approaches more directly link collection system peak flow standards to the basin's receiving stream's water quality, compared to more traditional forms of collection system planning. Watershed approaches support locally developed priorities for achieving water quality standards; this concept is echoed in the proposed Capacity, Management, and Operations and Maintenance (CMOM) provisions, which allow a municipality to establish its own peak flows.
Traditional planning that involves hydraulic modeling and evaluating multiple combinations of SSO control alternatives usually ends up selecting a collection system's peak flows based on criteria that do not study the local receiving stream's assimilative capacity for wet weather SSO pollutant loadings. Therefore, traditional planning may result in collection system capital improvement programs that are oversized and are not the most cost-effective solution to provide an essential public service and to achieve environmental improvements, pollution prevention, and other community goals.
The Water Works and Sanitary Sewer Board of the City of Montgomery (Board) has been a long-time advocate for the watershed approach to manage collection systems. The Board is using various stages of the watershed approach in two of its three sewer basins—the Towassa and the Catoma Basins. The Board's water quality assessment studies showed that, typically, the fecal coliform effect of the receiving streams from an SSO were insignificant compared to other sources in the watershed. The Board also recently incorporated the watershed approach process to set peak flows in its Towassa Basin. Interestingly, the Board set peak flows in the Towassa Basin in the early 1990s, somewhat arbitrarily, on a 2-year, 24-hour design storm. The resulting modeling showed that the cost of basin improvements would amount to more than 12 million. Recently, in the initial steps of the watershed approach process, the Board's hydraulic model and implementation of the recommendations indicated that an approximate equivalent to a 1-year, 24-hour storm would be appropriate to balance the issues originally presented above. As a result of the lesser storm being chosen, the peak flows were reduced and the basin improvements now are estimated to be approximately 3 million, a reduction of 9 million.
Document Type: Research Article
Publication date: January 1, 2001
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