Innovative sewer project eyed for Groseclose

By DAN KEGLEY/Staff

A sewer technology new to Smyth County, but in use in other places for decades, is being considered for the Groseclose area.
John Neel, president of the Christiansburg engineering and design firm Gay and Neel, told the county supervisors last week about the merits and drawbacks of decentralized sewer systems.
The centralized kind is familiar. It’s the type Smyth County and its towns use. Sewer lines from all of the area served by Chilhowie’s system, for example, converge at the intake side of the central sewage treatment plant. Cleaned to high tolerances, the outgoing water is discharged into the nearby river.
Decentralized sewer has, in effect, a number of small units instead of one big operation, making it highly flexible and adaptable, Neel said.
Neel outlined for the supervisors the specific differences between the traditional and the newer sewer.
Decentralized sewers serve “a narrower sewer-shed area,” perhaps of five square miles, he said, that is further divided into smaller areas, each of which has a treatment facility. Centralized sewers may serve a 15-square-mile area, Neel said.
The systems employ smaller diameter pipes, 1.25 to 2-inches in diameter, compared to eight inches for centralized systems.
They handle smaller volumes. They are less susceptible to infiltration and inflow – water leaking into the system or storm water channeled into the system. That means they do not have to be designed to handle peak storm volumes as centralized systems must be, he said.
Smaller volumes mean smaller components, including not only pipes but features like pumps. Interceptor tanks, one per house or neighborhood, pretreat the volume so that it can be carried through smaller pipes. Neel said an interceptor tank’s pump can operate on 110-volt, or typical household, current.
Of more petite, flexible infrastructure, decentralized sewer systems can be fit into difficult terrain more easily than centralized systems.
“The topography in Southwest Virginia can present significant challenges for conventional sewer design,” Neel said.
They can more economically accommodate one or two houses whose distance from a centralized system’s line would make connecting them infeasible.
“An eight-inch line is prohibitive to run up a hollow for one or two houses,” Neel said, but not for decentralized systems.
Waste can be discharged into suitable soils “for septic tank-like dispersal,” he said.
Decentralized systems’ operation is labor-efficient, according to Neel. Where conventional plants need continuous onsite attendance by an operator, sites in a decentralized system can be visited once daily.
They can be constructed at half the cost of conventional systems, but the trade-off is in higher operation and maintenance costs and in the number of moving parts, like interceptor tank pumps and vacuum-operated valves, that need repair and replacement.
Conventional systems’ costs are front-loaded. They are “constructed before the first connection comes on” and begin to generate revenue, Neel said.
Decentralized systems’ treatment is scalable, Neel said, and can be tailored to the needs of actual clients. “You install only what’s needed,” he said.
But decentralized systems have their downside, too, mainly in the number of moving parts, like interceptor tank pumps and vacuum-operated valves, that need repair and replacement.
There is also an ownership question that must be ironed out, Neel said. Who owns and is responsible for the electrical cost of pumping from the interceptor tanks?
The flexibility of a decentralized system, according to Neel, makes it appropriate for consideration at Groseclose where Interstate 81and the Appalachian Trail would challenge the design of a conventional sewer.
Presence of a sewer system in that area would speed development, he said. How much development is desired is a factor for consideration.
Neel said communities in Oregon have used decentralized systems since the 1970s.
Elliston put in a decentralized system 20 years ago, and the Tom’s Creek section of Blacksburg built one starting in 2000.
“The decentralized concept has been around quite a while, and it has tested well,” he said. He said his firm does not want to “shoehorn decentralized in” at Groseclose if it is not the best solution.
The supervisors retained Gay and Neel to conduct a $31,800 sewer engineering report. The cost is reimbursable through a State Travel Assistance Grant, according to County Engineer Scott Simpson.

dkegley@wythenews.com