Bye-Bye Railcars! Nashville's 350 MGD UV Disinfection Facility is Online!

Introduction Nashville Metro Water Services' (MWS) Central Water Reclamation Facility (WRF) has disinfected its treated plant effluent using gaseous chlorine since the facility was originally commissioned in 1958. Sixty-five years later, after 10+ years of planning, design, and construction, a new 350 MGD ultraviolet (UV) disinfection facility is online, and the last 90-ton rail cars were removed from the site. As of January 2024, the new UV facility has been operating as expected for six months, with E.coli values well within permit limits. The new UV facility is one of largest UV disinfection installation in the United States. Planning Driven by its Consent Decree, MWS commissioned the Central WRF Optimization Study (COPT Study) which recommended improvements to cost-effectively and reliably increase the peak wet weather capacity of its largest treatment facility from 220 MGD to 350 MGD via optimization; rather than by adding expensive new process tankage (Yarosz and Taylor, 2015). Figure 1 is an aerial photo of the Central WRF, which is located less than 2 miles from the heart of downtown Nashville. Since the Central WRF's existing gaseous chlorine disinfection system was unable to treat the large increase in capacity and MWS continued to have concerns with transporting, storing, and handling gaseous chemicals, MWS decided to evaluate alternative disinfection technologies as part of the COPT Study. The Central WRF also had two disinfection locations and desired to simplify operations by disinfecting optimized plant secondary effluent flows in a single location. A business case evaluation of multiple disinfection technologies was performed using both cost and non-cost considerations to determine the most beneficial long-term solution for MWS (BC, 2014). Capital costs, 15-year net present values (NPV) and 20-year NPVs were developed Non-cost factors include constructability, operability, reliability, safety, impact to traffic, and impact of potential future regulatory issues. UV disinfection was selected over sodium hypochlorite (bulk and onsite generation), peracetic acid (PAA), ozone. Table 1 includes the final results of this business case evaluation. Design of UV Facility Consistent with the project's goal to utilize existing structures, the design team determined it was feasible to construct the new UV disinfection facility within the footprint of the existing north chlorine contact tanks (CCT), which were designed to facilitate chlorine disinfection of 150 MGD. The following bullets briefly discuss some of the key design considerations. These and many other design considerations will be fully detailed in the full technical paper, if selected. The existing pile foundation of the CCTs was sufficient to support the new UV facility. The addition of concrete was offset by the reduction in water volume. The existing piles could also accommodate an enclosed pre-engineered metal building for shelter. CFD modeling was performed to confirm there was adequate mixing of north and south secondary effluent flows and determine a UV channel configuration that would not require flow straightening baffles as MWS has experienced algae issues with them at another WRF. Figure 2 shows output from the CFD model, which indicated that the flow split was within 2% of ideal at peak flow. There was limited headloss available to fit the UV facility into the Central WRFs hydraulic profile. Piping modifications upstream of the facility were required to reduce headloss. UV equipment O&M features were developed collaboratively in workshops with O&M staff where the engineers could listen, understand, and incorporate MWS' ideas and preferences into the design. The UV facility was designed around Trojan Technologies UVSigna(R) equipment because Trojan was able to propose multiple configurations that fit within the limited footprint of the existing chlorine contact tank structure, whereas several other leading UV manufacturers could not. Tables 2 and 3 include Design Flows and Permit Values and UV System Design Parameters, respectively. Additional discussion and supporting data related to the parameters in these tables will be included in the full technical paper, if selected. Construction Sequencing The UV facility layout and structural design had to facilitate construction sequencing that maintained plant effluent disinfection with flow routed through the structure throughout the duration of construction, except for several 2-to-5-day long shutdowns that generally occurred between different phases of construction. With the assistance of the Construction Manager at Risk (CMAR), a sequencing plan was developed in which the existing CCT structure was converted to the UV facility one half at a time. Figures 3 through 7 illustrate the sequence of construction, which was completed in approximately 3 years. Figures 8 is a photo from inside the UV facility. Status of Project/Significance of the Investigation All phases of this 10+ year long project are complete and the $35 million UV facility has been operating successfully for 6 months. This technical paper will include performance data, O&M experience, and 'if we could do it over again' insight from the Central WRF plant manager and the design engineer. Utilities, consultants, contractors, and others who are preparing to upgrade their water resource recovery facility (WRRF) disinfection systems can greatly benefit from this project's experience.