The sewer system in Nashville dates back to the late 1800s and originally consisted of a combined sewer system, later transitioning to separate sanitary and storm sewers. A combined sewer system consists of a single set of pipes that conveys both sanitary sewage and stormwater. Combined sewer systems were common in cities that developed in the 19th century to address public health problems caused by lack of proper sanitation. Since there were no facilities for treating wastewater in that era, it was common practice that sewage and stormwater were both discharged directly to the rivers and streams.
The treatment of wastewater began in the 20th century when pipelines were constructed to convey sanitary sewage along with stormwater to treatment plants to improve water quality. Wastewater treatment plants have limitations, however, to the volume of flow that can be effectively treated during rainfall events. Intense rainfall often leads to flows of stormwater in the combined sewer system that exceed treatment plant capacity. These high flows of primarily stormwater are discharged without treatment and referred to as combined sewer overflows or CSOs. The CSOs are permitted by EPA and TDEC under the terms of a permit issued under the Clean Water Act.
Separate sanitary sewers are intended to convey only sanitary sewage, but the piping systems deteriorate over time and allow rainwater to leak into the lines. During extreme rainfall events, the volume of rainwater entering these older sewers can overwhelm the capacity of the system, leading to sanitary sewer overflows (SSOs).
Metro Water Services began an aggressive program of infrastructure improvements in 1990 to reduce the number of CSOs and SSOs, making tremendous progress toward improving water quality in the Cumberland River and its tributaries. However, despite these improvements, a significant number of overflows still remained, prompting state and federal regulatory agencies to approach Metro in 2007 about the need for additional work within the sewer system.
In March 2009, a Consent Decree between the United States of America, the State of Tennessee, and the Metropolitan Government of Nashville and Davidson County was approved and entered with the United States District Court for the Middle District of Tennessee. The Consent Decree was filed on behalf of the United States Environmental Protection Agency (EPA) and the Tennessee Department of Environment and Conservation (TDEC), and it requires Metro to use its best efforts to achieve the following goals:
- Full compliance with National Pollutant Discharge Elimination System (NPDES) permits, the Clean Water Act, the Tennessee Water Quality Control Act, and their regulations.
- Addressing the conditions contributing to sanitary sewer overflows (SSOs), with the goal of eliminating the 27 overflows listed in the Consent Decree.
- Compliance with EPA’s combined sewer overflow (CSO) Control Policy.
The schedule for the Consent Decree was amended in August 2010 to provide additional time because of the impacts of the May 2010 flood. The amended schedule includes the following requirements:
- 2 ½ years to develop a Corrective Action Plan/Engineering Report (CAP/ER) to address conditions causing overflows in the separate sanitary sewer system with the goal of eliminating the 27 overflows listed in the Consent Decree. The CAP/ER was submitted to EPA and TDEC in September 2011.
- 2 ½ years to develop an updated Long Term Control Plan (LTCP) to mitigate overflows from the combined sewer system and reduce water quality impacts to the Cumberland River. The LTCP was submitted to EPA and TDEC in September 2011.
- 11 years after approval of each of these documents to complete the recommended improvements
The Consent Decree also includes provisions for civil penalties for past violations and stipulated penalties for violations that may occur in the future. In lieu of the full civil penalty, Metro agreed to conduct and has completed Supplemental Environmental Projects to improve public health and the environment.
At an estimated cost of $1.0–1.5 billion, the Clean Water Nashville Overflow Abatement Program represents a major investment in the community and provides the following benefits:
- Renews and improves aging infrastructure
- Enhances the environment for Nashville
- Improves water quality in the Cumberland River and its tributary watersheds throughout Davidson County
- Provides major engineering and construction projects to boost the local economy
Corrective Action Plan/Engineering Report (CAP/ER)
To meet Consent Decree requirements, Metro Water Services developed a Corrective Action Plan / Engineering Report, addressing conditions causing overflows in their sanitary sewer system. These sanitary sewer overflows, known as SSOs, have the potential to contribute to the impairment of Nashville's creeks, streams, and rivers and potentially pose a risk to public health.
The CAP/ER development began with a characterization of Metro's sanitary sewer system through extensive monitoring and modeling to understand the existing system's limitations. The need for improvements to address both current and future sewer capacity needs was then assessed, and potential alternatives were evaluated with the most efficient and cost effective solutions recommended for construction. These recommended projects, which include infrastructure rehabilitation, additional conveyance capacity, and storage of wet weather flows, will be implemented through the Clean Water Nashville Overflow Abatement Program.
Long Term Control Plan
As a result of the Consent Decree, Metro Water Services completed an update of the Long Term Control Plan for the combined sewer system which provides a plan for reducing overflows into the Cumberland River.
The focus of the planning was to reduce the occurrence and impact of overflows into the Cumberland River during wet weather events. MWS followed EPA's CSO Policy in implementing a rigorous engineering, quantitative, and scientific process for identifying and evaluating alternatives to reduce combined sewer overflows. Consideration included financial and engineering analyses to develop recommended improvements in conjunction with four key objectives that were established early in the planning process:
- Improve the water quality of the Cumberland River by reducing impacts from combined sewer overflows.
- Provide a level of CSO controls that results in improvements in water quality that are consistent with the community’s use of the Cumberland River.
- Align investment in CSO controls to be commensurate with the contribution of CSOs to water quality relative to other sources.
- Consider the impact of the overall program cost on the ratepayers in the current economic climate.
A map of Nashville’s combined sewer system and overflow discharge locations can be found here.
These goals and objectives were developed based on feedback provided by representatives from MWS, local government, and the community through a public engagement process developed to solicit input from affected stakeholders.