What is an EQ Basin? 

An Equalization (EQ) Basin is a crucial component in managing sanitary sewer overflows. During heavy rainfall, aging sewers can become overwhelmed with water infiltration, leading to potential overflows. These overflows can occur at designated locations or through unsecured manhole covers. EQ basins are large tanks or other elements designed to temporarily hold this potential spillover water.  When the sewer system reaches a certain level of capacity, a diversion structure within the system diverts flow to the EQ basin, typically through a pump station facility, for storage until the rain subsides, and it drains back to the system. This process prevents sewer water from contaminating streams, rivers and low-lying greenspace areas in the vicinity of sanitary systems. Engineers meticulously calculate the capacity of these basins to ensure they can handle the overflow. Recently, Metro Nashville Water engaged SSR to add two new EQ basins to their system. 

Davidson Branch 

The Davidson Branch project involved replacing an existing duty pump station while combining the upgrade with an EQ facility.    This project features a 6-million-gallon (6MG) tank and a new pumping station capable of conveying 3.8 million gallons per day (MGD) of duty flow and 11 MGD of equalization flow. Part of the property was reserved within the floodplain for community access to the Brookmeade Park and Greenway, and a long-term vision for new public facilities such as picnic areas and a canoe launch into Davidson Branch which would access the Cumberland River.  The project began in 2015 and was completed in the summer of 2023.  

Gibson Creek 

The Gibson Creek project features a 10-million-gallon (10MG) tank, diversion structure, and a new pumping station capable of conveying 22 MGD of equalization flow.  The site posed a significant challenge as it is entirely within the 100-year floodplain of Gibson Creek. To address this, all critical electrical equipment, including standby generators, was located above potential floodwaters.  The pumps, being wet-pit submersibles, were placed outside the pump station building, with the first floor used only for access to valves and flowmeters. Critical equipment was located on the second floor. Although costly, the savings from placing the electrical building near the pumps and elevated on stilts offset the expense and resulted in an aesthetically pleasing structure that blends with the neighborhood and the existing duty pump station. Construction was completed in early 2024. 

Successful Projects 

Since becoming operational, both EQ basins have successfully prevented sewer overflows in their respective areas. This success has had several positive environmental impacts: 

1. Protection of Water Bodies: By preventing sewer overflows, the EQ basins have significantly reduced the risk of contamination in local streams and rivers. This helps maintain the quality of these water bodies, supporting aquatic life and ensuring safer water for recreational activities. 
2. Improved Public Health: Reducing sewer overflows minimizes the potential for harmful pathogens to enter the environment, thereby protecting public health. Cleaner water bodies mean fewer health risks for the community. 
3. Enhanced Flood Management: The EQ basins help manage the excess stormwater that enters the sewer system.  While infiltration and inflow are not viewed positively for a sewer system, the fact that it happens routinely means that EQ related projects do contribute to the reduction of the environmental damage associated with flood events. 
4. Sustainable Water Management: These projects demonstrate a commitment to sustainable water management practices. By effectively managing stormwater and preventing overflows, Metro Water Services is contributing to a more resilient and sustainable urban environment. 

SSR One Efforts 

Both projects were SSR One efforts, involving numerous in-house disciplines to ensure success. In addition to providing water, civil/environmental, and process engineering, SSR also handled mechanical, electrical, plumbing, structural, and instrumentation and control designs. Our team also included building performance, controls integration, and sustainability experts, offering a comprehensive solution. 

Conclusion 

The addition of the Davidson Branch and Gibson Creek EQ basins marks a significant achievement for Metro Nashville Water. These projects not only enhance the capacity and reliability of the sewer system but also provide substantial environmental benefits. By preventing sewer overflows, protecting water bodies, and improving public health, these EQ basins contribute to a cleaner, safer, and more sustainable community for Nashville. The collaborative efforts of SSR and Metro Water Services highlight the importance of integrated engineering solutions in addressing complex urban infrastructure challenges. As these basins continue to operate successfully, they set a benchmark for future projects aimed at enhancing urban water management and environmental protection.