ARCHIVE SITE - Last updated Jan. 19, 2017. Please visit for the latest NACWA information.


NACWA's Wet Weather Survey (Survey) provides critical information that will help to further understanding among state and federal regulators of the factors that publicly owned treatment works (POTWs) consider when sizing their sewer collection systems and treatment plants for peak wet weather flows. The Survey constitutes one step in an ongoing effort to highlight and examine the complex issues, including capacity-related issues, which comprise the broader picture of the current state of core wastewater infrastructure in the United States.

The United States Environmental Protection Agency (EPA), wastewater utilities, and other stakeholders continue to explore the development of appropriate regulatory programs and practices to reduce sanitary sewer overflows (SSOs), and to improve water quality. As this dialogue continues, NACWA believes it is essential to account for and understand how existing state and federal statutory and regulatory programs have shaped, and continue to affect, municipal planning, engineering decisions, and peak wet weather flow designs.

These existing design practices and planning considerations shed light not only on what is currently driving these sizing decisions, but also provide insight into how future policies and regulations governing wet weather flows should be crafted in order to preserve the tremendous achievements in water quality realized over the last thirty years.

The Wet Weather Survey Report (Report) has five main sections. Section 1 is an Executive Summary of the Report and the Survey’s findings. Section 2 provides an overview of the existing statutory and regulatory provisions and policies governing peak flows. Section 3 includes information on how NACWA designed and distributed the Survey and compiled the responses. Section 4 provides critical details on geographic distribution, government structure, and other Survey respondent profile information. Section 5 presents a summary of selected Survey question responses, grouped as follows:

  • Municipal Peak Flow Decision-Making Processes
  • Sizing Criteria Used for Collection Systems
  • Peak Flow Controls and Performance Measures Used
  • Factors Considered in Plant Sizing Decisions
  • Wet Weather Treatment Components and Processes
  • Typical Conveyance and Treatment Capacity Related Performance Challenges
  • Peak Flows and National Pollutant Discharge Elimination System (NPDES) Permits
  • Regulatory/Permitting Authority Role in the Capacity Setting Process
  • Regulatory Oversight or Enforcement Influence on the Capacity Setting Process.

Basic Findings

Survey responses highlighted several key characteristics of POTW design and planning processes:

  • Level of service to their communities and public health concerns are the major factors taken into consideration by POTWs when setting peak flow capacity (Questions 49 and 50).
  • POTWs use a variety of collection system sizing criteria protocols, the majority of which (approximately 71%) are dictated by permitting authority (PA) requirements or guidance (Question 22).
  • POTW operators are proactive in ensuring adequate treatment capacity. Nearly all of the respondents (99%) initiate action on their own to increase capacity if flows are approaching design capacity (Question 61).
  • Technology exists to improve peak flow treatment performance and capacity, but the lack of regulatory compliance clarity and outright objections of regulatory authorities have prevented POTWs from installing the new technology (Question 71).


Municipalities are using a wide array of tools and approaches to design systems and plants and to manage peak wet weather flows. These approaches are working and municipalities must continue to have sufficient flexibility and a suite of options available to them for dealing with wet weather flows.

A one-size-fits-all approach will not work for highly variable wet weather flows. Any type of national program or standard must be general in nature and performance-based. Prescriptive requirements that dictate what design practices and planning considerations to use will only hamper current efforts by the wastewater community to improve peak wet weather flow management capabilities.

Based on the Survey responses, NACWA recommends that:

  • EPA not seek to develop a single, prescriptive national collection system or POTW sizing requirement, which would be inappropriate given the diverse methods and approaches utilities follow to develop and make peak flow sizing decisions.
  • EPA develop, with input from NACWA, general collection system and POTW sizing guidance that incorporates performance-based and site-specific objectives for capacity determinations, which would be part of a SSO regulatory program.
  • EPA and the states clarify the role of the PA in POTW and collection system design, permitting, and subsequent enforcement, especially in those instances where enforcement actions cite planning decisions in which the PA was involved.
  • The water quality impacts of peak flows should be a critical consideration in the POTW and collection system design, permitting, and PA enforcement processes.