CSO

What are CSOs?

CSO stands for Combined Sewer Overflow. Combined sewer systems are wastomestic sewage, and ewater collection systems that are designed to collect and convey rainwater runoff, d industrial wastewater in the same pipe¹. However, during heavy rainfall events, flows can exceed the capacity of the sewer system or treatment plant. For this reason, overflows are designed to convey excess flows to nearby water bodies.

The water coming from these overflows is untreated and may contain human and industrial waste, toxic materials, and debris¹. While it was originally thought that the water from CSOs would be dilute enough to have little effect on the receiving water body, recent scientific data suggests that CSOs do have a negative impact². In some cases, where wastewater treatment is at a high enough level, the intermittent CSOs have a more harmful effect on the water quality and ecosystem health than the continuous effluent from the treatment plant². CSOs are a major water pollution concern for approximately 772 cities in the US with combined sewer systems¹.

Definition of an overflow, from Water Environment Federation Manual of Practice FD-17: Prevention and Control of Sewer System Overflows²: “A CSO is the intentional or unintentional discharge of untreated sanitary wastewater mixed with stormwater runoff or snow melt and occurs when the carrying capacity of a single conveyance system is exceeded by the instantaneous rate of flow within the single conveyance system. To control where overflows occur, engineers design diversion structures at strategic locations within the single conveyance system that ultimately discharge extraneous commingled flow to receiving waters.”

Regulatory Perspective

The US Environmental Protection Agency (EPA) has developed a national framework for control of CSOs called the ‘Combined Sewer Overflow Control Policy’. The Policy was developed through the National Pollutant Discharge Elimination System (NPDES) permitting program, which requires that all point sources discharging pollutants to surface waters must have a permit³. The CSO Control Policy provides information on how to meet the pollution control goals of the Clean Water Act flexibly and cost-effectively. The Policy, published in 1994, can be found here here and contains four fundamental principles to ensure that CSO controls are cost-effective and meet local environmental objectives:

• Clear levels of control to meet health and environmental objectives
• Flexibility to consider the site-specific nature of CSOs and find the most cost-effective way to control them
• Phased implementation of CSO controls to accommodate a community’s financial capability
• Review and revision of water quality standards during the development of CSO control plans to reflect the site-specific wet weather impacts of CSOs³.

As part of the CSO Control Policy, all communities with combined sewer systems were required to implement ‘nine minimum controls’ by January 1, 1997. These controls are measures that can be implemented to reduce the effect of CSOs without large engineering studies³. The ‘nine minimum controls’ are summarized below and more details can be found here.

• Proper operation and regular maintenance programs for the sewer system and the CSOs
• Maximum use of the collection system for storage
• Review and modification of pre-treatment requirements to assure CSO impacts are minimized
• Maximization of flow to the publicly owned treatment works for treatment
• Prohibition of CSOs during dry weather
• Control of solid and floatable materials in CSOs
• Pollution prevention
• Public notification to ensure that the public receives adequate notification of CSO occurrences and CSO impacts
• Monitoring to effectively characterize CSO impacts and the efficacy of CSO controls⁴.

Communities with combined sewer systems should also develop long-term CSO control plans that will lead to compliance with the Clean Water Act. More information on the elements of the long-term CSO control plan can be found here.

EPA guidance documents providing information on the technical, financial, and permitting issues related to the CSO Control Policy  are available here.

Meeting Regulations

Since the CSO Control Policy is developed in accordance with the NPDES permitting program, permitting of CSOs is a part of meeting regulations. There are two permitting phases⁷:

• Phase I permit – Typically requires the permittee to implement the ‘nine minimum controls’ and to develop a long-term CSO control plan.
• Phase II permit – Typically issued after the Phase I permit, requires that the permittee implement controls identified in the long-term CSO control plan.

Where the Phase I permit is typically issued for one permit cycle, the Phase II permit can continue to be issued for several cycles, until all controls are implemented.

Due to the random nature and variable pollutant loads of CSO events, assessing the effects of CSOs is difficult. Planning for CSOs is difficult, but critical to a successful CSO abatement program⁵. While the ‘nine minimum controls’ can likely be implemented without a large engineering study, CSO abatement programs cannot. Considerations include:

• environmental setting – receiving water size and use, aquatic life, local meteorology, etc.
• regulatory climate – Federal regulations, local environmental requirements, etc.
• infrastructure – asset age, structural integrity, conveyance capacity, etc.
• socio-political setting – public awareness, political climate, etc.