Proposed EPA Policy on Permit Requirements for
Peak Wet Weather Discharges from
Wastewater Treatment Plants Serving
Sanitary Sewer Collection Systems
December 2005
EPA seeks comments on a proposed policy regarding implementing requirements for wet weather
blending at municipal publicly owned wastewater treatment plants serving sanitary sewer collection
systems. The proposed policy seeks to ensure that all feasible solutions are used by local
governments when peak wet weather flows to the wastewater treatment plant exceed the treatment
capacity of secondary treatment units. The proposed policy applies only to publicly owned
wastewater treatment plants that serve sanitary sewers and, during wet weather, divert a portion of
the flow around secondary treatment units andrecombine the flow with flow from the secondary
treatment units. It is EPA 's goal to ensure that all feasible solutions are used by local governments
when addressing problems related to heavy wet weather flows and to improve treatment of
wastewater to protect human health and the environment.
Background
Many municipal sewage treatment systems experience problems during heavy downpours when
flows to the wastewater treatment plant exceed the treatment capacity of existing secondary
treatment units. Many municipalities manage peak wet weather flows by routing some peak flow
around traditional biological secondary treatment units, blending the rerouted flow with the flow
receiving secondary treatment; and disinfecting and discharging. In an attempt to address this issue,
EPA proposed a policy addressing National Pollutant Discharge Elimination System (NPDES)
permit requirements for municipal wastewater treatment plants (serving sanitary sewers) during wet
weather conditions in November 2003. The 2003 proposed policy is intended to provide clarity
about managing peak wastewater flows that are sometimes diverted from secondary treatment unit
processes during significant wet weather events. EPA received more than 98,000 public comments.
EPA stopped working on the proposal in May 2005 in order to review different approaches and new
information.
In October 2005, the Natural Resources Defense Council (NRDC) and the National Association of
Clean Water Agencies (NACWA) developed joint recommendations to address peak wet weather
flow diversions at wastewater treatment plants that are serving sanitary sewer collection systems.
Their approach describes limited circumstances when NPDES permits can approve anticipated wet
weather blending as a "bypass" at publicly owned treatment work (POTW) treatment plants serving
sanitary sewers. Their recommended policy would apply only to blended discharge from sewage
treatment plants serving sanitary sewer collection systems. It would not apply to overflows in
collection systems; dry weather diversions; diversions around primary or tertiary treatment units; or
diverted flows that are not recombined with flow from the secondary treatment units prior to
discharge.
EPA's proposed policy is informed by and reflects those joint recommendations.
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Additional Background
A bypass is an "intentional diversion of waste streams from any portion of a treatment facility."
Bypasses are prohibited unless they are 1) unavoidable to prevent severe property damage or
personal injury; 2) there were no feasible alternatives; and 3) the NPDES authority was notified.
EPA's 1994 CSO Policy provides guidance on approval of anticipated bypasses in a permit for
POTW plants serving combined sewers. That policy describes approvals of "CSO-related bypass"
in permits under the bypass regulation that recognizes "approved anticipated bypass". Under the
CSO Policy, permittees need to demonstrate that there are 'no feasible alternatives' for approved
bypass at the POTW plant in order for such CSO-related bypasses to be approved in a permit. The
CSO Policy's provision are intended to encourage delivery of maximum flows to the POTW
treatment plant, but also to ensure that bypasses would not cause water quality exceedences. The
approach in today's proposal for approval of anticipated bypasses at POTWs serving separate
sanitary sewers is based on application of the same bypass regulation, but analysis differs in scope
and focus due to the different types of sewers considered.
About this Guidance
EPA seeks comments on its proposed policy on peak wet weather flow diversions for publicly
owned wastewater treatment plants serving sanitary sewers. This policy seeks to ensure that all
feasible solutions are used by local governments when addressing problems related to heavy wet
weather flows.
During peak wet weather flows, limited diversions around biological treatment units at municipal
sewage treatment systems can maximize flows to treatment plants which can help prevent raw
sewage from being discharged into our nation's waters, backing up into homes and other buildings,
or damaging biological treatment units. Although increased flows may cause short-term
management challenges, the ultimate goal of this policy is to ensure, to the extent feasible, all wet
weather flows receive secondary treatment and other treatment necessary to meet the requirements
of the Clean Water Act.
The proposed policy provides that in limited situations, an NPDES permitting agency can approve
anticipated diversions around biological treatment units as a "bypass" in a permit, provided:
• the permittee demonstrates (and the NPDES authority agrees) there are no feasible
alternatives to the diversion;
• the diversion from the secondary treatment units receives a minimum of primary treatment
and any feasible supplemental treatment; and
• effluent limitations based on secondary treatment and water quality-based effluent limits will
be met.
Key provisions:
• This policy applies only to publicly owned wastewater treatment plants serving sanitary
sewer systems.
• All flows diverted from the secondary treatment units in peak wet weather events will
receive a minimum of primary treatment and any supplemental treatment or technology
shown feasible using the factors outlined in the proposed policy.
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• Discharges must meet effluent limitations, including the 85 percent removal requirement and
other secondary treatment requirements and any other more stringent limitations necessary to
meet water quality standards. Existing requirements that discharges meet limits would not
change.
• Diversions will not be approved when peak flows are largely due to poor collection system
maintenance or the lack of investment in or upgrades to treatment capacity.
• Permits that approve diversion should include an implementation schedule in the permit for
implementing feasible technologies.
• The availability of feasible alternatives would be determined on a POTW-by-POTW basis.
• Diversions are reported to the permitting authority and the public will be notified.
• This policy encourages public participation in the permitting process.
How to Get Additional Information
For additional information, including a copy of the proposed guidance, visit the NPDES website at
http://www.epa.gov/npdes/wetweather/ or contact Kevin Weiss at 202-564-0742 or
weiss.kevin@epa.gov.
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