King of the Road

A decentralized system with recirculating trickling filter and drip dispersal provides reliable treatment for a rural community in eastern Tennessee.
King of the Road

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Wastewater erupting from drainfields serving 33 homes, a church, and a cabin rental office along King Branch Road in Sevier County, Tenn., flowed into a small mountain stream paralleling the road.

The state Department of Environment and Conservation banned recreational activities and fishing on the Little Pigeon River below its juncture with the stream after finding extreme levels of fecal bacteria in the water. The river is a tourist attraction.

The county Environmental Health Department and Southeast Environmental Engineering in Knoxville explored decentralized solutions. "Funding and permitting were the greatest challenges," says designer Michael Hines, P.E., and the company's founding principal. "At one point, it appeared that an act of Congress might be required to cross a corner of the National Park Service's property, but it all finally came together in 2007."

The decentralized system has a septic tank effluent pumping (STEP) system, a force main, a primary settling tank, a recirculating trickling biofilter, a dose tank, disinfection and drip dispersal.

Site conditions

Soils are 24 to 35 inches of Ranger/Junaluska sandy silt above bedrock. The area, part of the Great Smoky Mountains, has steep, heavily forested slopes bisected by numerous small streams. Residential lots are small.

System components

Based on analysis of expected contributors to the system, Hines designed the system to handle 11,000 gpd. Major components are:

28 1,000-gallon concrete STEP tanks. All tanks from C.R. Barger & Sons in Lenoir City, Tenn., have concentric pump vaults, integral 24-inch riser rings, and rubber inlet and outlet boots.

Five 1,500-gallon concrete STEP tanks.

33 1/2 hp 10 gpm high-head turbine STEP pumps from AY McDonald.

5,650 feet of 4-inch SDR11 HDPE force main.

5,000-gallon primary settling tank.

11,000-gallon Model 30/24/2300 Bioclere recirculating trickling filter with clarifier from Aquapoint, an OBEH Company.

3,000-gallon dose tank with dual 1 hp, 30 gpm high-head AY McDonald turbine pumps.

Arkal 2-inch x 2 Spin Klin disc filter supplied by JNM Technologies, Inc., Bryan, Texas.

Dual 40 gpm Sanitron UV units in series from Atlantic Ultraviolet.

8,300 feet of Netafim Subsurface Drip Dispersal Tubing.

Drip programmable logic controller with remote telemetry from
Adenus Technologies; STEP control panels from Tennessee Pump and Controls.

System operation

Wastewater flows through 4-inch PVC laterals to the septic tanks, then is pumped through 1.25-inch HDPE pipes to the force main. The force main discharges to the primary settling tank at the treatment site. Effluent in the tank flows to the trickling filter, where it is pumped to the top and sprinkled over packed plastic media.

After effluent trickles through the media to the clarifier at the bottom, a pump recirculates it. Effluent flowing into the treatment unit displaces an equal amount of liquid in the clarifier, sending it by gravity to the dose tank. Each hour, a pump in the clarifier runs for two minutes, sucking up and
discharging settled solids to the primary settling tank for additional digestion.

Every 30 minutes, alternating pumps in the dose tank run for 15 minutes, sending 600 gallons through 100-micron disc filters and UV disinfection to the drip field. The field has two 35,000-square-foot zones, each with 500 feet of tubing in loops on 10-foot centers and orifices spaced 12 inches apart.

The pressure-compensated emitters drip at 0.61 gallons per hour. "We try to keep the pressure less than 45 psi to avoid expansion stress on the tubing and potential failure of insert fittings.," says Hines.


Project manager Mickey Barger from C.R. Barger & Sons oversaw the sewer and onsite crews. "Sandstone is horribly hard, and shale is as bad or worse," he says. "To lay more than a mile of force main, my six guys needed a large rock trencher with stinger to dig the ditch alongside the road."

Installing the new septic tanks with cleanouts went faster because workers often used the original holes after ripping out the existing tanks. "They were usually in the one location that had enough soil to set a tank," says Barger. "In some places, we installed lowboy tanks to avoid hammering out more rock with large power hammers attached to tracked excavators."

The onsite crew cut down the point of a ridge in a mountain for the treatment site. "We had to stop on the road, then reverse up a 30 to 45 percent sloped access road because there was no room at the top to turn around and drive out," says Barger.

It took three days to excavate the holes for the recirculation, treatment, and dose tanks, set in series and bedded on 6 inches of gravel. To maintain gravity flow, the last two holes were 15 feet deep.

Workers placed a 4-inch-thick, 8-foot-square precast concrete pad on top of the gravel for the 17-foot-high treatment unit, which protruded 2 feet above grade. They set the conical part on the pad, then chained the unit to eyebolts in the concrete before backfilling with No. 57 gravel up to the cylindrical portion. The remaining backfill was soil. All tank inlets and outlets were bedded with gravel to avoid cracking or dislocating the pipes.

The crew installed the drip field above the treatment site on 1.5 wooded acres. They cut a ditch up the mountain, installed the dosing header with a 2-inch solenoid valve at the bottom of each zone, then dragged the tubing through the forest, laying it on the contour while nestling it against shrubs, trees or rocks.

"A return header with solenoid valve at the bottom of the hill allows us to backflush the lines once a week," says Hines. "A computer runs all the operations." The installation took a year.


Tennessee Wastewater Systems owns and operates the system. Southeast Environmental Engineering inspects the treatment site twice a month. "Animals gnaw on the tubing, and we fix the leaks," says Hines. "We also clean the UV sleeves, check pump performance, maintain the biological process control, and provide maintenance." Solids in the primary settling tank are checked annually and pumped when necessary.


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