A downpour can elicit many emotions, but only those in public works think of rain in terms of overloaded sewers and surcharged manholes. Sometimes even the forecast of rain can bring about visions of backed-up basements, overflowing inlets, and angry citizens.
For wastewater operators in a community with combined sewers—those carrying both sanitary and storm flows—protection of the wastewater plant is a primary concern when the weather is stormy. The operator must draw upon knowledge and experience to make operational decisions to maximize treatment while preventing damage to plant facilities from the extra flow.
In a combined sewer system, excess flow caused by wet weather is either diverted within the collection system or within the footprint of the treatment plant. The U.S. Environmental Protection Agency defines the discharge from a collection sewer system at a point prior to the publicly owned treatment works (POTW) as a combined sewer overflow (CSO). In a 2004 report to Congress, the EPA listed 9348 issued permits for CSO outfalls in 32 states with an estimated annual discharge of 850 billion gallons. The majority of these states are located in the Northeast and Great Lakes regions.
If during wet weather, high flows reach the POTW, biological units designed with limited capacities can become threatened, and a portion of the flow must be diverted within the plant to prevent these facilities from flooding. Typically the flows that exceed the capacity of the biological treatment units receive primary treatment and chemical disinfection and are then mixed with the plant's fully treated effluent prior to discharge into the receiving stream. This process of splitting flows and mixing of effluents during we weather events is referred to as blending.
Wet Weather Concerns
Rainfall or wet weather events have been a concern for engineers and operators ever since the first sewer pipe was designed and constructed. For communities installing sewers in the 1880s, the pipes were usually designed to collect both sanitary and storm flows, creating a combined sewer system. Up until the 1900s, most collection systems discharged directly to a receiving stream.
As the role of pathogens in causing disease became understood and concern for the environment grew, efforts to reduce pollution in streams focused on providing some form of primary treatment for sanitary flows. The cost of providing treatment made it prohibitive to handle all storm flows at the newly constructed plants. To limit flows reaching the plant, many of the old direct discharge points in the collection system were converted to CSOs, allowing diversion of combined sewage during wet weather.
After the Clean Water Act was passed in 1972 and the EPA was established, more stringent water quality standards for plant discharges drove the construction of facilities that provided secondary treatment for sanitary flows. Due to the variation in water quality and quantity, each community developed a customized approach to meet effluent standards using a combination of clarification, biological processes, and chemical disinfection. While the primary treatment process may have been designed to handle a portion of the storm flows, the high cost of constructing biological units and difficulties in operating biological treatment units during high flow conditions did not allow for treatment of all wet weather flows.
Once acceptable treatment of sanitary flows was achieved, attention was directed to the wet weather flows that were being discharged into streams. In 1994 the EPA issued a CSO Control Policy representing “a comprehensive national strategy to ensure that municipalities, permitting authorities, water quality standards authorities, and the public engage in a comprehensive and coordinated effort to achieve cost-effective CSO controls that ultimately meet appropriate health and environmental objectives.”
Under the CSO Policy, CSO communities were required to implement, by Jan. 1, 1997, minimum technology-based controls often referred to as the “nine minimum controls.” These controls, as defined by the EPA, are “measures that can reduce the prevalence and impacts of CSOs and that are not expected to require significant engineering studies or major construction.” The CSO Control Policy brought wet weather flows into the limelight and changed years of operational practices in which wet weather flows were routinely diverted in the collection system.
“One of the nine minimum controls is maximizing flows to the plant,” said Kevin Weiss with the U.S. EPA. “This practice reduces overflows by getting the water to the plant.” But because treatment units at the plants are not sized for this excess flow, operational tools such as blending are used to accommodate wet weather volumes.
“The permit program addresses this bypass or routing around biological units,” said Weiss. “A POTW can have a bypass approved under limited circumstances, if their long-term plan demonstrates there is no feasible alternative. However, this is considered an interim solution; we don't expect it to last forever.”
In administering the policy, the EPA has recognized that each plan is site-specific and implementation within each city is heavily dependent on finances. In order to achieve a determination of “no feasible alternatives” an analysis of a community's financial capabilities must be performed in accordance with EPA guidelines. Based on this financial study and other water quality considerations, a phased approach to CSO control is developed.
“Our goal is, at some point in the future, to have municipalities eliminate the bypass at the plant,” said Weiss. “However, we realize that this can take a long time to implement for some CSO cities.”