Combined Sewer Overflow (CSO) Program

Objectives

The Morris Forman Wastewater Treatment Plant (MFWTP) KPDES Permit required that MSD develop and implement a Combined Sewer Overflow (CSO) Abatement Program. The objective of the CSO Abatement Program is to reduce the pollutant loads caused by CSOs on receiving streams through compliance with the EPA CSO Control Policy. The initial phase of the program began in early 1991 and culminated in the development of a Combined Sewer Operational Plan (CSOP). Since that time, yearly updates to the original CSOP have been prepared.

The EPA CSO Control Policy, published in 1994, provided guidance on coordinating the planning, selection and implementation of CSO controls that meet the requirements of the Clean Water Act. The policy contained provisions for developing appropriate, site-specific NPDES permit requirements for combined sewer systems that overflow as a result of wet weather events. The policy established two main objectives for permittees: implementation of the Nine Minimum Controls (NMCs), and the development and implementation of a CSO Long-Term Control Plan (LTCP).

The intent of the NMCs is to secure the prompt implementation of control measures that will at least partially control wet weather CSO discharges. Per the CSO Control Policy, MSD’s NMCs were implemented on January 1, 1997. Selection and implementation of actual control measures was based on consideration of the specific combined sewer system and in many cases may address more than one of the NMCs.

The policy also directed the permittee to develop and implement a LTCP based on characterization, monitoring and modeling of the combined sewer system. The plan considers the site-specific nature of CSOs and gives highest priority to controlling overflows in sensitive areas. The policy provides two approaches for long term control of the CSOs – the presumptive and the demonstrative.

History

The sewer system owned, operated and maintained by MSD has evolved for almost a century and a half into an extensive network of both sanitary and combined sewers, diversion structures, mechanical regulators and other flow control devices, wastewater treatment plants, and pump stations. Prior to 1850, sewers in the City of Louisville were built primarily of cut stone and brick. The sewers appear to have been intended mainly for the purpose of storm water disposal and usually flowed towards the Ohio River or nearest surface drain. In was not until sometime in the 1860’s, with the introduction of a public water system, that the idea of constructing laterals to convey wastes directly from the houses into the sewers became adopted as common practice.

The development of Louisville’s combined sewer system was heavily influenced by the topography of the area. The western, southern and central portions of the city lie on an undulating plain or plateau, while the eastern portion rises rapidly and is cut by the valleys and forks of Beargrass Creek. This difference in topography caused the western and southern parts of the city to be sewered by a few large combined sewers at relatively great depths and gently slopes flowing west to the Ohio River. Four major combined sewers serve this part of the city – the Northwestern Interceptor, the Western Outfall, the Southern Outfall, and the Southwestern Outfall. In the central part of the city, numerous small diameter sewers were constructed flowing north to the Ohio River. Virtually every block between 1st Street and 12th Street is served in this manner. West of 12th Street, a large combined sewer discharging to the Ohio River at 34th Street serves this northern part of the city. In the eastern part of the city, numerous smaller sewers at steeper slopes and shallower depths were constructed, discharging to the two major forks of Beargrass Creek – the Middle Fork and the South Fork. These early sewers discharged all flow, sanitary plus storm, to the nearest stream or the Ohio River. No treatment was provided.

The current KPDES Permit for the MFWTP requires MSD to develop and implement a Combined Sewer Overflow Abatement Program. In July 1993, in partial fulfillment of that requirement, MSD submitted a CSOP to the Kentucky Division of Water (KDOW). The MFWTP Permit also requires that MSD prepare and submit a yearly report outlining the status of MSD’s Combined Sewer Overflow Abatement Program.

Program Elements

The CSO Abatement Program consist of flow monitoring, the Plumbing Modifications Program, development of a LTCP, compliance with the NMCs, and other system improvements and enhancements.

The following are CSO Program elements:

• Flow Monitoring
• Long-Term Control Plan
• Pretreatment