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Management of Wet Weather Flows by Municipal Utilities

(PDF Version)

Adopted by WEF Board of Trustees: April 30, 2010  

The Water Environment Federation (WEF) supports environmentally sound and cost effective management of wet weather flows. The debate over the appropriate management and control of wet weather discharges has been going on for more than two decades. While there have been some successes during this time, most notably the negotiation of the Combined Sewer Overflow (CSO) Control Policy adopted in 1994, the complexity of issues, both in terms of technology to address the diversity of flow and water quality scenarios and the use of the existing regulatory structure to address permitting all of the different types of discharges, have made further progress difficult. In addition, EPA has not provided national guidance for design and operation of sanitary sewer systems to promote the reduction of sanitary sewer overflows (SSO). In some instances, it is reported that EPA Regions have their own interpretation leading to confusion and potential conflict among discharging communities and within EPA.

This statement provides background on the issues and WEF’s position, which includes a proposal to work with EPA in the development of a definitive national strategy and related guidance on planning and permitting for wet weather flows.


The Clean Water Act (CWA) was developed almost 40 years ago. The issues of primary concern at that time are very different from the issues now facing municipalities. The sources most contributing to nonattainment of water quality now are diffuse and wet weather sources, not the traditional point sources of pollution, which were the concerns when the CWA was adopted. WEF has recently adopted a position statement encouraging modernization of the CWA to incorporate updated tools and policies necessary to assure continued water quality improvements.1  

Discharges that result from rainfall and snowmelt (wet weather events) include not just storm water runoff, but system capacity overloads resulting in CSOs, SSOs, and peak wet weather flows that threaten the ability of wastewater treatment plants (WWTPs) to function properly. Wet weather discharges are by nature intermittent, somewhat unpredictable, and not easily characterized in terms of treatment options. They are extremely variable from one wet weather event to the next with respect to frequency, duration, volume, season and sub-surface conditions. This unpredictability and variability presents a significant challenge to WWTPs in managing wet weather discharges; climate change may impact such unpredictability and variability even further. It also makes it difficult for EPA and authorized states to apply standard conditions for managing wet weather operations through the National Pollutant Discharge Elimination System (NPDES) permit process.

While the flows and conditions are highly variable, there are important commonalities among the types of wet weather discharges described in this statement. They are all triggered by rainfall or snowmelt. They involve discharges of various combinations of pathogens, floatable material, sediment and suspended solids, oxygen-demanding substances, and a host of conventional and toxic pollutants. In many cases the impacts on the environment may be more a result of flow rates and volume than the pollutants carried in the flow. Compared to industrial and dry-weather municipal discharges, they are difficult to predict, measure, and control.

The commonalities among wet weather discharges can be physical as well, in that pollution sources can be physically or hydraulically connected such that controlling one source can have impacts elsewhere in the collection system or stormwater drainage system, such as:

  • Sanitary and combined sewers are often interconnected; meaning that controls implemented in combined system can impact discharges in the separate system and vice versa.
  • Storm water runoff may be contaminated with sanitary sewage if there are illicit sanitary connections to storm sewers.
  • SSOs caused by infiltration and inflow into sanitary sewers are a major source of pollutants in localized areas.
  • Separation of combined sewers creates “new” sanitary sewers and storm sewers, changing, rather than eliminating pollutant contributions and adding to regulatory issues that must be addressed.
  • High groundwater can cause exceedence of storm, sanitary and combined sewer system capacities.
  • CSO Long Term Control Programs and SSO controls affect the flow and volume of peak wet weather flow sent to the treatment plant and the necessity, in many cases, to evaluate and utilize peak flow management at the treatment plant.
  • Wet weather programs involve similar activities during the planning stages—focused analysis of the conveyance system and its response to wet weather events, and evaluation of costs and benefits of different long term control measures—and during implementation activities such as permit issuance, compliance monitoring, recordkeeping, and reporting.
  • In addition to traditional controls, wet weather programs can benefit from appropriate green infrastructure that focuses on infiltration, evapotranspiration, capture and reuse of stormwater to maintain or restore natural hydrologies. At the largest scale, the preservation and restoration of natural landscape features (such as forests, floodplains and wetlands) are critical components of green stormwater infrastructure. On a smaller scale, green infrastructure practices include rain gardens, porous pavements, green roofs, infiltration planters, trees and tree boxes, and rainwater harvesting for non-potable uses such as toilet flushing and landscape irrigation.

Recognizing that these commonalities could be addressed in a way that provides for greater efficiency, more comprehensive planning, and less redundancy among permitting requirements, over the past ten years EPA and other stakeholders have continued a dialogue about how municipal wet weather issues could be better integrated on a watershed basis.

WEF Position In order to assist municipal utilities and NPDES permitting authorities accomplish their missions in an economical and environmentally responsible manner, WEF believes that the EPA must develop a comprehensive strategy and provide practical guidance that allow municipalities to manage wet weather flows in a holistic manner. EPA’s strategy and guidance should be based upon the wet weather management principles and guidance practices described in the following management tools:

  • WEF’s “Guide to Managing Peak Wet Weather Flows in Municipal Wastewater Collection and Treatment Systems”. Developed in 2006, the Guide was made possible by a Water Quality Cooperative Agreement between EPA and WEF. The Guide outlines an approach for analysis of wastewater flow collection and treatment during wet weather conditions, development of sound and effective practices for municipal facility planning, and design and operation for optimum management of wet weather flows. The Guide provides a risk-based method for WWTPs to be more proactive in planning for wet weather flows and describes a process that can be used to build support for real-world solutions that effectively use resources to improve water quality.
  • WEF/NACWA’s “Core Attributes of Effectively Managed Collection Systems”. In the absence of clear federal guidance, the Partner Organizations WEF and the National Association of Clean Water Agencies (NACWA) with technical reviews by the American Public Works Association (APWA) and the American Society of Civil Engineers (ASCE) have developed these baseline attributes as fundamental elements in the effective management of sanitary sewer collection systems. These core attributes are intended to provide guidance for wastewater agency collection system managers to evaluate their existing programs and confirm they are performing in both dry and wet weather conditions according to industry-established best management practices, or have practices that are lacking and need enhancement.
  • Addressing wet weather discharges in a holistic manner following the Guide can provide for greater efficiency, more comprehensive planning, and less redundancy among permitting requirements. Non-traditional approaches may be required to address the challenges posed by wet weather discharges. Such challenges must be addressed on several fronts:
    • Ensuring that the existing framework of regulation and policy is fully implemented for those discharges covered by existing programs.
    • Adjusting the existing regulation and policy framework to reflect the difficult decisions municipalities face in controlling episodic, variable, and largely unpredictable wet weather sources.
    • For example, in many WWTPs, the plants are designed with additional capacity in the primary clarification units, or more recently with a peak flow treatment facility, to accommodate peak flows to the plant. When the plant flow to the secondary treatment facilities (which have almost always been biological units) is increased to the maximum capacity that retains biological stability, additional flow is routed to auxiliary treatment systems and then recombined with flows that have gone through the other treatment trains. This practice is referred to as “blending.” This long-accepted sound engineering practice has been followed at WWTPs around the country, but treated differently in the NPDES permitting process by EPA and the states over the years. Recently, the practice has been questioned, and the NPDES permitting practices have been changing in the Regions and states, with no clear guidance on how to permit or the standard for review.
    • Supporting flexible decision-making at the state and regional board level to achieve watershed objectives in the most cost-effective manner;
    • Establishing Water Quality as a leading parameter when requiring agencies to establish compliance measures; and
    • Providing flexibility and time to apply tools that focus on sustainable practices, green infrastructure and pollution prevention.

Given the many similarities and requirements among wet weather control programs, and the fact that these programs are all impacting WWTPs, recognition of these similarities should be addressed in a positive manner using the proposed framework described above within the existing NPDES program.

WEF would like to partner with EPA to develop a strategy and guidance for integration of municipal wet weather programs that will assist permitting authorities with design and implementation of permits to address the unique nature of wet weather discharges and the controls necessary to achieve water quality goals. The strategy and guidance will enable permitting authorities to utilize flexibility available in the NPDES permit program to focus on water quality goals. The strategy and guidance will also encourage urban areas to develop their wet weather programs in a comprehensive manner following the WEF Guide that organizes NPDES information and planning to help decision makers prioritize wet weather control programs.

Founded in 1928, the Water Environment Federation (WEF) is a not-for-profit technical and educational organization of 36,000 individual members and 75 affiliated Member Associations representing water quality professionals around the world. WEF members, Member Associations and staff proudly work to achieve our mission to provide bold leadership, champion innovation, connect water professionals, and leverage knowledge to support clean and safe water worldwide. To learn more, visit