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Strategies to Combat Centralized Sewer Overflows

Dennis Hallahan of Infiltrator Water Technologies explains several ways to manage CSOs, including the use of decentralized systems to attenuate flow, reduce stormwater volume, prevent CSOs, and improve water quality. 

Strategies to Combat Centralized  Sewer Overflows 

Stormwater infrastructure  plays a critical role in cities and communities worldwide, conveying large volumes of water to protect local infrastructure from surface flooding. In the United States (US) and almost every other country, the stormwater infrastructure is required to serve double duty by handling both stormwater and sanitary sewage. These combined sewers convey the everyday sanitary sewer volumes and then, when it rains, the stormwater volumes as well, which can lead to CSOs. The US Environmental Protec­tion Agency (EPA) published a report in 2001, which estimated CSO volumes in the US of 1.3 trillion gallons {4.9 trillion liters) per year. Nearly 50 years after the Clean Water ACT (CWA) passage, US communities con­tinue to be in violation resulting in EPA enforcement actions.  Centralized treatment systems must contend with the problem of Infiltration and Inflow (I&I). This contributes to overflows -both with CSO and sanitary sewer overflows (SSO) - and direct dis­charge of untreated waste during rainfall events when the capacity of the sanitary sewer piping and/ or the capacity of the wastewater treatment plant may be exceeded.  According to the EPA's 2004 Report to Congress: Impacts and Control of CSOs and SSOs, the number of US communities with stormwater utilities or fees has grown from approximately 1,400 in 2013 to 1,600 in 2016. In ap­proximately 772 US communities, wastewater and stormwater drain into the same combined sewer sys­tem and in most communities this infrastructure piping is too costly to replace. In April 1994, the EPA issued the CSO Control Policy, the national framework for control of CSOs, through the National Pollut­ant Discharge Elimination System permitting program. It mandates the dramatic reduction or elimina­tion of CSOs, so the EPA began working with municipalities to im­prove antiquated sewage systems.

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