Incorporating decentralized wastewater design concepts into centralized infrastructure

    Infiltrator IM-1060 tanks were selected to provide ease of handling on the difficult Appalachian sites with steep slopes and limited backyard access. Photo: Infiltrator Water Technologies

    The decentralized wastewater treatment industry serving much of the nation’s wastewater needs is expanding to include centralized approaches and incorporate existing centralized infrastructure. It can be a benefit to towns without public sewer systems or adequate onsite septic systems to employ decentralized strategies that tie into a neighboring existing centralized system. To do so, engineers have taken various approaches, including septic tank effluent pump (STEP) systems.

    One such case is Surgoinsville, a low-income community in the Appalachian area of northeastern Tennessee with a population of just over 1,800. Similar to small towns throughout the country, Surgoinsville was without a public sewer system, limiting economic development opportunities. Existing homes were being served by subsurface sewage disposal systems that were failing and in many cases were compromised by other factors, including structures built over these systems. Additionally, the current package plant in operation since 1959 that served the Surgoinsville elementary and middle schools was at the end of its useful life.

    The town decided to pursue a solution that could handle current needs with capacity for the future to open up economic opportunity. This project involved development of a new public sanitary sewer system. The first phase of the system development serves the elementary and middle schools and approximately 247 properties in the town. The system was designed with the additional capacity to serve approximately 700 properties.

    Feasibility study

    The installer had a staging yard to assemble and store tanks and deliver them as needed to each site. Photo: Infiltrator Water Technologies
    The installer had a staging yard to assemble and store tanks and deliver them as needed to each site. Photo: Infiltrator Water Technologies

    A feasibility study was conducted to shed light on the design alternatives available and to help narrow the scope of the project by identifying the best scenario. In this case, the owner of the current system, the Town of Surgoinsville, contracted with Tysinger, Hampton and Partners of Johnson City, Tenn., to conduct the study in order to move the project forward in the most sustainable way for the town.

    Project Leader Jill Workman, P.E., led the team in a comparative life-cycle analysis evaluating the economically feasible options for development of a sewer system. Alternatives evaluated included:

    • onsite treatment plant with drip disposal using a STEP system for conveyance;
    • conventional gravity system with discharge offsite at the nearby Church Hill wastewater treatment plant (WWTP);
    • STEP system with discharge offsite to the Church Hill WWTP; and
    • low-pressure grinder pump system with discharge offsite to the Church Hill WWTP.

    The analysis demonstrated that a low-pressure STEP collection system was the best alternative to meet the town’s needs. Low-pressure systems are beneficial for areas with low-density population because they have minimal inflow/infiltration as compared with gravity sewer systems. Published in 2014, the U.S. Environmental Protection Agency’s “Guide for Estimating Infiltration and Inflow” states, “Virtually every sewer system has some infiltration and/or inflow. This cannot be discounted in terms of increased load, increased cost, and energy usage.”

    STEP systems also typically have less odor issues than grinder pump systems since they do not convey solids, and the replacement costs are less for STEP than grinder pump systems.

    Following the analysis, the town selected the low-pressure STEP collection solution because it was the most cost-effective solution and could provide them with projected future capacity. Not only will the STEP system allow for possible economic growth, but the capacity will allow the town to possibly apply for grant funding to annex additional existing subdivisions with an eye toward increasing its tax base. The solution also provided homeowners with a way to address problems with existing subsurface sewage disposal systems.


    However, to move forward, Surgoinsville had some big challenges, as all small communities do without previous experience with wastewater infrastructure. These included securing and maintaining funding sources, sewer rate analysis, guidance for the start-up of a new public utility, sewer line routing within a limited public right-of-way, hydraulics, customer identification, extensive permitting, and diligent coordination with local utility providers and government officials.

    Funding was ultimately secured and included a $1.3 million Rural Development Community Programs Grant, $500,000 Community Development Block Grant, $500,000 from the Appalachian Regional Commission, $485,000 from the U.S. EPA, a $1.5 million U.S. Department of Agriculture Rural Development loan, and $200,000 in local funds.

    System details

    The $4.5 million project serves 247 residences, two schools, and nine businesses. Installed on lots with existing homes, outbuildings, driveways, and landscaping, the project was designed with the additional capacity to serve approximately 700 properties. The low-pressure sewer system delivers wastewater to the City of Church Hill’s wastewater plant. The system is comprised of approximately nine miles of low-pressure sanitary sewer collection lines, two pump stations, and individual STEP services at more than 200 residences that include Infiltrator IM-1060 plastic tanks.

    Engineers from Tysinger, Hampton & Partners guided the community through the design and construction process and subcontracted with CTI Engineers, Inc. of Chattanooga, Tenn., for assistance with the feasibility study led by Ron Key, P.E., senior project manager at CTI. This also included computerized hydraulic modeling of the system, assistance with final pipe size selection, and preparation of technical specifications.

    The installer, Mike Smith Pump Service, had a staging yard to assemble and store tanks and deliver them as needed to each site. A boom truck or heavy equipment was not required. The Infiltrator IM-1060 tanks were selected to provide ease of handling on these difficult Appalachian sites with steep slopes and limited backyard access to existing homes.

    An even greater concern was the installation scheduling because of a strict construction timetable that could not be jeopardized. Rather than leaving scheduling in someone else’s hands, it became paramount to have scheduling under the installer’s control. Since the installer had an onsite staging yard, it allowed its staff to assemble and deliver tanks when the weather prohibited onsite work and excavation, thus keeping staff working fulltime instead of being moved to another job or sent home to wait on the weather to clear. This was a plus for overall project scheduling and delivery.

    Existing concrete tanks were either removed or crushed and filled in place and then the new tanks were installed. Each tank was tested in place for water tightness. A pump vault package supplied by Quanics, Inc. included the pump, floats, wiring, control panel, and vault ready to be inserted into the tank and hooked up to power.


    The project recently received a 2016 Rebuild Tennessee Award from the Tennessee Development District Association. The award is for the best infrastructure project in the First Tennessee Development District of Northeast Tennessee. Twenty-four projects were considered for the award.

    Implementation of an unbiased feasibility study at the outset of the project resulted in Surgoinsville benefitting from the best possible solution to its wastewater treatment challenges at a cost they could manage with the help of funding sources. It is an excellent example of successfully incorporating decentralized design concepts into centralized infrastructure.

    Dennis Hallahan, P.E., technical director at Infiltrator Water Technologies (, has more than 25 years of experience with onsite wastewater treatment system design and construction. Currently, he is responsible for technology transfer between Infiltrator and the regulatory and design communities and consults on product research and testing for universities and private consultants.