Smyrna Wastewater Treatment Plant Expansion
Smyrna, TN

Expansion of the Smyrna Wastewater Treatment Plant (WWTP) consisted of upgrading all components of the existing wastewater treatment to process 9.0-million gallons per day, from 5.85-million gallons per day. Improvements included new and expanded components for the headworks screening-grease removal, influent lift station, anaerobic basin-flow splitter cells, oxidation basins-biological treatment processes, clarifiers, scum pumps and screening, tertiary filtration, UV disinfection, post aeration, returned activated sludge, waste activated sludge, aerobic digestion, centrifuge solids dewatering, and SCADA (Supervisory Control and Data Acquisition) system.


Smyrna is located along Interstate 24, between Nashville and Murfreesboro in Middle Tennessee. The region has experienced tremendous growth in recent years, and the WWTP expansion was necessary as to allow the Town to accommodate its thriving residential, industrial, and commercial sectors. The Town’s major employer, Nissan North America, employees approximately 8,500 workers, many of which reside in the Town of Smyrna. The WWTP expansion will not only serve Nissan, it will also support suppliers that keep the facility operational twenty-four hours a day, seven days a week. The Town is home to several other industrial enterprises such Taylor Farms Food Packaging Company, Vijon Personal Hygiene Company, and Smyrna/Rutherford County Airport along with commercial and retail establishments that will benefit from the expansion. The WWTP will serve the continued growth and future population of the Town, from its current population of approximately 45,000 to a projected population of 70,000 by year 2040.

  • Read More

    The expansion was designed based on flows of an average daily flow of 9.0-million gallons per day, peak daily flow of 16.0-million gallons per day, and peak hourly flow of 21.0-million gallons per hour. The project required applying for and obtaining an expanded NPDES (National Pollutant Discharge Elimination System) permit to allow an increased discharge into an existing 303d listed stream which has a 0.0 cubic feet per second, 7Q10 flow (lowest seven-day average stream flow occurring every ten years).  

    The key to keeping the project on track was facilitating constant communication with state regulators in implementing an advanced nutrient removal process. This process kept the same existing NPDES permit’s nutrient waste load allocations/discharge poundage. To achieve this, an engineering design alternatives analysis report was performed to determine the most viable process that could achieve the required lower nutrient discharge concentrations of both total nitrogen and phosphorous.

    After evaluating numerous treatment processes, the biological treatment process selected for the expanded portion (3.0-million gallons per day) of the plant was the five-stage Bardenpho Advanced Nutrient Removal process. In addition, the existing WWTP components consisted of the extended-aeration activated sludge process which required an extensive conversion to a three-stage Bardenpho Advanced Nutrient Removal process, for a design flow rate of 6.0-million gallons per day. The effluent flows from each of these two similar processes are blended after secondary clarification is complete and prior to the tertiary filtration, to have a combined design flow of 9.0-million gallons per day.

    The Smyrna Wastewater Treatment Plant Expansion was completed by Dempsey, Dilling, & Associates, now an affiliate of Thomas & Hutton.