Monday, September 9, 2024
11:30 AM - 12:00 PM
Location Name
Overcoming Tie-In And Commissioning Challenges At Nashville’s Largest Treatment Plant

The Central Wastewater Treatment Plant (CWWTP) Capacity Improvements and Combined Sewer Overflow (CSO) Reduction project aimed to diminish the frequency and volume of CSO overflow occurrences at the CWWTP, the largest wastewater facility in Middle Tennessee. The objective was realized through augmenting the wet weather treatment capacity of the CWWTP, enhancing the overall capacity of the Central Pumping Station, and constructing the Plant’s first true Headworks Facility. Garney was the subcontractor for the civil, structural, architectural, process, and yard pipe construction of the new 440 MGD headworks facility. Though the site was isolated from the remaining plant during construction, the execution of the tie-ins, and startup and commissioning of the headworks facility, affected multiple points within the plant and off-site collection system, including sewage pump stations. Twelve critical tie-ins transferring the four existing 60”-66” PCCP plant influent force mains to the four new DIP influent lines ranging from 60”-64” and four steel effluent lines ranging from 60”-84” connecting to the plant at various points were made. Prior to starting construction of the facility, Garney unearthed each tie-in point to confirm depth, alignment, pipe condition, and restraint, thus confirming final alignment, restraint needs, and defining a plan for connection to the aged PCCP. As no existing connection points were available on the existing pipes and adding valves at tie-in points was cost and time prohibitive, the influent tie-ins were made in an irreversible fashion using unforgiving restrained joint pipe. Two of the existing lines were being connected to both influent and effluent lines in the new system, creating a sequencing issue and minimizing facility effluent capacity during the initial tie-ins, while multiple influent lines crossed at the tie-in locations. Additionally, two of these lines cross Nashville and have various pump stations tied into them along the way. There was no plan at the start of construction for these additional flow sources that needed accounting for. As it was impractical to simulate the volume of design flow, system performance testing and commissioning was completed using raw sewage. From the start of the project, Garney’s management team, in cooperation with MWS plant personnel, barge design, and the CMAR Team investigated and designed a flow routing plan, inclusive of multiple collection system pump stations, existing plant influent lines, and headworks facility to ensure the tie-in points would not be under flow, allowing adequate flow for testing & commissioning, and to not exceed the phased effluent pipeline capacity. Garney’s team put in months of preparation to ensure the right piping materials were on site, collection system were locked out, and headworks systems were operational to meet the tight tie-in window. The original schedule had the tie-ins and testing scheduled in the low flow summer months, but due to delays, was moved to the unpredictable fall months. In the end, Garney crews worked around-the-clock for 15 days to minimize the risk of overflows and maintain plant operations while 300+ MGD of flow capacity was transferred from the old system to the new headworks.