NPDES Sewer Overflow and Bypass Event Download Summary

EPA collects data on sewer and bypass events and makes the data available through ECHO.

• NPDES Sewer Overflow and Bypass Overview
• Dataset Description
• Data Elements by Table
• Data Element Definitions and Reference Codes

NPDES Sewer Overflow and Bypasses Overview

Background

Sewage collection systems collect and transport domestic, commercial, and industrial wastewater and varying amounts of stormwater and infiltrated ground water to treatment facilities for appropriate treatment. The first sewage collection systems in the United States were built in the 19th century as combined sewers (carrying both storm water and sewerage) and were designed to overflow during periods of high rainfall. Releases from these systems are called combined sewer overflows (CSOs).

Starting in the 20th century most cities designed and built separate sewerage systems to only collect domestic, commercial, and industrial wastewater. Occasionally, these separate collection systems release raw sewage. These types of releases are called sanitary sewer overflows (SSOs). A number of factors can cause or contribute to an SSO, including high levels of inflow and infiltration, blockages caused by roots, grease, sediment, or other materials, and structural, mechanical, or electrical failures.

EPA’s NPDES regulations define “bypass” as the “intentional diversion of waste streams from any portion of a treatment facility.” See 40 CFR 122.41(m)(1)(i). EPA regulations require permittees to submit a report detailing these bypass events within 5-days of an unanticipated bypass events and to submit a notice 10-days before any anticipated bypass events.

CSOs and SSOs (“sewer overflows”) and bypasses can release raw sewage into drinking water supplies, basements, parks, recreational streams, beaches, on city streets and backyards, and other areas where people are in close contact with the overflow. Sewer overflows and bypasses are of special concern to public health because they may expose citizens to bacteria, viruses, intestinal parasites, and other microorganisms that can cause serious illness such as gastroenteritis, hepatitis, cryptosporidiosis, and giardiasis. Sensitive populations, children, the elderly, and those with weakened immune systems, can be at a higher risk of illness from exposure to sewage from sewer overflows and bypasses. The threat to public health and the environment posed by sewer overflows and bypasses is not necessarily limited to large volume or extended-duration overflows.

Regulatory Requirements on Sewer Overflows and Bypasses

EPA’s NPDES regulations require the permittee to “properly operate and maintain all facilities and systems of treatment and control (and related appurtenances) which are installed or used by the permittee to achieve compliance with the conditions of this permit.” See 40 CFR 122.41(e). The proper operation and maintenance of collection systems and treatment plants is critical to minimizing the frequency and volume of sewer overflows and bypass events. Another standard permit condition regarding the duty to mitigate states that “the permittee shall take all reasonable steps to minimize or prevent any discharge... in violation of [the] permit which has a reasonable likelihood of adversely affecting human health or the environment” [see 40 CFR 122.41(d)]. Noncompliant sewer overflows include:

• wet-weather CSO discharges that are not compliant with permit requirements.
• dry-weather discharges from combined sewer systems (prohibited under the CSO Control Policy - see 19 April 1994; 59 FR 18689),
• SSOs that discharge to waters of the United States, and
• SSOs that result from noncompliance with the standard NPDES permit requirements in 40 CFR 122.41.¹

EPA’s NPDES regulations prohibit bypasses unless all of the following three conditions are met [see 40 CFR 122.41(m)(4)]:

• The bypass was unavoidable to prevent loss of life, personal injury, or severe property damage.
• There were no feasible alternatives to the bypass, such as the use of auxiliary treatment facilities, retention of untreated wastes, or maintenance during normal periods of equipment downtime. This condition is not satisfied if adequate back-up equipment should have been installed in the exercise of reasonable engineering judgment to prevent a bypass which occurred during normal periods of equipment downtime or preventive maintenance.
• The permittee submitted a notice providing information on the bypass to the NPDES permitting authority.

EPA regulations require reporting on sewer overflows and bypass events. In particular, reporting requirements for sewer overflows and bypass events are specified in the “Conditions applicable to all permits” section in EPA’s NPDES regulations (see 40 CFR 122.41). The requirements provided in §122.41 apply to all types and categories of NPDES permits and must be included in all permits (see 40 CFR 123.25 for applicability to state NPDES permits).² Historically, sewer overflow and bypass information were most often reported on paper or in non-standard formats. This non-standard reporting limited the availability and utility of these data. 

To address this concern, EPA developed the NPDES Electronic Reporting rule (40 CFR part 127) ("NPDES eRule"), which requires permittees to electronically submit their sewer overflow and bypass event reports. These reports cover the following events:

• anticipated and unanticipated bypasses,
• wet-weather CSOs that “endanger health or the environment,”
• dry-weather discharges from combined sewer systems,
• SSOs that discharge to waters of the United States, and
• SSOs that are not in compliance with the standard NPDES permit requirements in 40 CFR 122.41.

The NPDES eRule Phase 2 Implementation Dashboard identifies how states are collecting these sewer overflow and bypass event reports (i.e., by using EPA's electronic reporting tool, called "NeT-SewerOverflow", or by state-built tools).

¹ SSOs that result in discharges to waters of the United States or result from noncompliance with standard conditions including failure to properly operate and maintain the collection system constitute noncompliance (see 1 June 2010; 75 FR 30395, 30398, and NPDES Compliance Inspection Manual, EPA305-K-17-001, Page 299, Interim Revised Version, January 2017).↩ 
² U.S. EPA, 2010. NPDES Permit Writers’ Manual, EPA-833-K-10-001, Page 1-1, September.↩ 

Data Caveat

This national data set is currently in development and does not represent all sewer overflow and bypass events covered by the NPDES eRule. EPA is working collaboratively with states to implement Phase 2 of the NPDES eRule (see compliance deadlines at 40 CFR 127.16 and alternative Phase 2 compliance deadlines). More sewer overflow and bypass event data will be available for download on this page as more states start collecting and sharing these data with EPA. Please see the alternative Phase 2 compliance webpage for how states that elect to build their own electronic reporting tools can share these data with EPA.

Dataset Description

This Zipped collection of CSV files represents the Sewer Overflow and Bypass data that has been submitted to EPA in this standardized electronic format.  It’s comprised of 7 of the 8 data files (the center table and the 6 to the left of the center) depicted in the Entity Relationship File shown in the Entity Relationship Diagram, below. These data represents the latest version of each “Report” submitted of a Sewer Overflow or Bypass Event.

EPA has organized the data into the following tables:

• collection_system_permits: Contains relevant information regarding the EPA or State NPDES Permitted facility that submitted the "Sewer Overflow or Bypass Event Report".
• sewer_overflow_bypass_report_events: The central "parent" table for all of the following "sewer_overflow_bypass..." tables.
• sewer_overflow_bypass_causes: Identifies the causes of the Sewer Overflow or Sewer Bypass Event.
• sewer_overflow_bypass_corrective_actions: Identifies short term and long term corrective action taken for the Sewer Overflow or Bypass Event.
• sewer_overflow_bypass_impacts: Details the human health and environmental impacts of the Sewer Overflow or Bypass Event.
• sewer_overflow_bypass_receiving_waters: Identifies waterbodies (streams, creeks, rivers, lakes, etc.) affected by the event.
• sewer_overflow_bypass_types: Identifies the type(s) of the event - Combined Sewer Overflow (CSO), Sanitary Sewer Overflow (SSO), or Bypass (BYP).
• sewer_overflow_treatment_codes: Identifies the treatment plant equipment bypassed for Bypass Events or Identifies the treatments that the facility will take to address the Sewer Overflow Events.

Unique Primary Keys for Each Table

Key field columns are present in all files and may be used to join and relate data between files. The permit_identifier column, contained in all tables, represents the unique Federal "NPDES_ID" assigned to the regulated facility entity which is used by EPA and the States in their reporting to EPA. Each table has column(s) which comprise a unique primary key for the table, shown below:

• collection_system_permits: permit_identifier, collection_system_identifier
• sewer_overflow_bypass_report_events: sewer_overflow_bypass_event_key
• sewer_overflow_bypass_causes: sewer_overflow_bypass_event_key, sewer_overflow_bypass_cause_id
• sewer_overflow_bypass_corrective_actions: sewer_overflow_bypass_event_key, sewer_overflow_bypass_corrective_action_id
• sewer_overflow_bypass_impacts: sewer_overflow_bypass_event_key, sewer_overflow_bypass_impact_id
• sewer_overflow_bypass_receiving_waters: sewer_overflow_bypass_event_key, sewer_overflow_bypass_receiving_water_id
• sewer_overflow_bypass_types: sewer_overflow_bypass_event_key, sewer_overflow_bypass_type_code_id
• sewer_overflow_treatment_codes: sewer_overflow_bypass_event_key, sewer_overflow_bypass_treatment_id

Each table contains three columns: "permit_identifier", "program_report_form_set_id", and "program_report_form_id". The combination of these columns represent a unique "event report' and can also be used to link the central "sewer_overflow_bypass_report_events" table to the other "sewer_overflow_bypass..." tables. EPA generated a "sewer_overflow_bypass_event_key", as the column in order to link the "sewer_overflow_bypass..." tables.

Entity Relationship Diagram

The primary key columns are bolded in the linked diagram.

Data Elements by Table

The primary key columns are bolded in each table.

 Permits (COLLECTION_SYSTEM_PERMITS.csv)

 Sewer Overflow Events (SEWER_OVERFLOW_BYPASS_REPORT_EVENTS.csv)

 Causes (SEWER_OVERFLOW_BYPASS_CAUSES.csv)

 Corrective Actions (SEWER_OVERFLOW_BYPASS_CORRECTIVE_ACTIONS.csv)

 Impacts (SEWER_OVERFLOW_BYPASS_IMPACTS.csv)

 Receiving Waters (SEWER_OVERFLOW_BYPASS_RECEIVING_WATERS.csv)

 Types (SEWER_OVERFLOW_BYPASS_TYPES.csv)

 Treatment Codes (SEWER_OVERFLOW_TREATMENT_CODES.csv)

Data Element Definitions and Reference Codes

The NPDES Sewer Overflow and Bypass Event Metadata (CSV) (60 KB) provides definitions of each data element. This file is also included with the data in the ZIP file.

Reference codes used for describing sewer overflow and bypass events (e.g., "Corrective Action" codes) are provided on the following EPA webpage regarding sewer overflow and POTW payloads.