Combined Sewer Overflow (CSO) Program

A Combined Sewer System (CSS) is a wastewater collection system owned by a State or municipality which conveys sanitary wastewaters (domestic, commercial and industrial wastewaters) and storm water through a single pipe system to a Publicly Owned Treatment Works (POTW). A Combined Sewer Overflow (CSO) is the discharge from a CSS at a point prior to the POTW. CSO discharges occur when rainfall or snowmelt occurs, causing the system to be overwhelmed and discharge untreated sewage to rivers and streams, potentially posing a threat to human health and the environment. CSOs are point sources subject to NPDES permit requirements including both technology-based and water quality-based requirements of the Clean Water Act.

All CSO communities in Indiana are required to implement a Long Term Control Plan (LTCP) to reduce/eliminate discharges from CSOs. As these plans can often entail significant infrastructure improvements and significant costs, some plans can take up to 20 years to fully implement. IDEM staff track compliance with LTCP implementation schedules, in addition to performing routine audits at CSO communities. IDEM also reviews the performance of the CSO Long Term Control Plans, after they are fully implemented, to ensure the implemented plans achieve the level of CSO control they were designed to achieve.

Indiana CSO Outfall Map

IDEM has created an interactive map that allows individuals to see the location of every CSO outfall in Indiana. For each outfall, the map provides information on the NPDES number, CSO number, receiving stream, outfall status, and other variables.

About the CSO Program

The Combined Sewer Overflow program augments the NPDES Municipal permitting program by implementing a strategy for the maintenance and management of combined sewer collection systems. The primary objective of this group is to insure the minimization of impacts to waters of the state from combined sewer overflows (CSOs).

CSO Background Information

IDEM CSO Guidance Documents:
Great Lakes Basin Enhanced Public Notification Rule Guidance Documents:
U.S. EPA CSO Guidance Documents:
CSO Related Statutes
Nonrule Policy Documents

The following documents are available on the IDEM Nonrule Policies page:

  • Application of Existing Use Concept in Conducting Use Attainability Analyses for Long Term Control Plan Communities for Primary Contact Recreational Uses - NRPD Water-014
  • CSO-Related Bypass in Long Term Control Plans (Maximizing Treatment at the Existing POTW Treatment Plant) - NRPD Water-015
  • CSO Treatment Facilities – Policy Number - NRPD Water-016

CSO Monthly Report of Operations (MRO) Form and Instructions

The following forms and supplemental content are available on the IDEM Forms page:

  • CSO Monthly Report of Operations (MRO)
  • 2013 CSO Reporting Memo
  • CSO MRO Instructions
  • Example Form **

The CSO MRO Form is in Excel format and must be filled out with your computer. The CSO MROs must be saved as pdf documents to be attached to the DMR submittal through NetDMR. The CSO MRO excel file should be submitted electronically at the end of the year to your CSO Project Manager.

**The data and parameters shown on this document are for an example form only and do not necessarily reflect all of the required parameters and limitations on your NPDES permit.

As of August 1, 2013, the new CSO MRO is the only accepted form for reporting wet-weather CSO overflows. Along with the written instructions located on the this webinar is provided to supply permittee's with additional assistance in filling out the new form.

Monthly Monitoring Report (MMR) for Wet Weather Treatment Facilities (WWTF) Form

The Monthly Monitoring Report (MMR) for Wet Weather Treatment Facilities (WWTF) Form is now available on the IDEM Forms page.

The MMR for WWTF form is in Excel format and must be filled out with your computer. It should be noted that the pages of the MMR for WWTF should be saved as a PDF document to be attached to the DMR submittal through NetDMR. This form contains detailed instructions regarding its completion.

As of January 1, 2017, the new MMR for WWTF is the only accepted form for reporting wet weather treatment facility discharges.

Combined Sewer Overflow Operational Plan (CSOOP)

Audit Program

IDEM intends to monitor each CSO community’s compliance progress throughout their LTCP implementation schedule. IDEM staff have initiated a program in which we will conduct site visits to CSO communities at various times throughout their LTCP implementation period to document the implementation of approved LTCP tasks. You may have already received a letter from IDEM indicating the date of our site visit and requesting your assistance in completing background information in preparation for our visit. If you have not been contacted regarding such a visit, please note that IDEM staff are actively contacting communities to schedule these visits. We look forward to working with each CSO community throughout the LTCP implementation period in a shared goal of improving Indiana’s water quality.

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