High-Class Subdivision Calls for High-End Septic Installation

A STEP system with trickling biofilter serves a high-end lakefront subdivision on a site with substantial change in elevation.
High-Class Subdivision Calls for High-End Septic Installation
At the left, the 35,000-gallon poured-in-place circular dosing tank; at the right, the treatment tank with shrubs planted around it. (Photos courtesy of Dart Kendall)

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A developer wanted to build 108 weekend lake homes in an exclusive subdivision in Dandridge, Tenn. Dart Kendall, owner of Advanced Septic in Acworth, Ga., won the bid to design and install the private wastewater treatment plant.

Kendall worked with Bob Faulhaber, P.E., of Faulhaber Engineering and Sustainability in Cookeville, Tenn., to resolve site challenges. "The elevation rose 200 feet from one end of the subdivision to the drip fields, and some homes were a mile away from them," says Kendall.

The autonomous-redundant solution involved septic tank effluent pumping (STEP) systems, a trickling biofilter, high-pressure drip dosing, and programmable logic controllers (PLCs). The installation took four months.

Site conditions

Soils are moderate angular blocky structure with 0.24 gallons per square foot per day loading rate. The steep and rocky site borders Douglas Lake at the foot of Great Smoky Mountains National Park.

System components

Kendall and Faulhaber designed the system to handle 26,460 gpd. Major components are:

  • 1,000-gallon dual-compartment one-piece precast septic tank with Polylok effluent filter and two Polylok risers. Concrete tanks from Hommel Concrete, Newport, Tenn.
  • 1,000-gallon single-compartment one-piece precast pump tank with 1/2 hp Myers high-head effluent pump
  • 240 CF-1900 AccuPac Cross Flow trickling filter blocks from Brentwood Industries
  • 35,000-gallon concrete dosing tank with duplex 1.5 hp Goulds sewage pumps, duplex Myers 1/2 hp high-head effluent pumps, and duplex 1.5 hp Myers filtered effluent (drip) pumps
  • 1.5 hp high-pressure Goulds booster pump
  • Three 2-inch disc filters from Arkal Filtration Systems
  • Three elevated tanks from ChemTank holding a combined 20,000 gallons
  • 40,000 feet of Geoflow drip tubing with 10 Rain Bird 2-inch solenoid valves
  • Flowmeter from SeaMetrics
  • DirectLOGIC 205 PLC system from Koyo Electronics Industries Co. (AutomationDirect)

System operation

Effluent from the STEP pump tanks enters a 1.25- to 4-inch PVC Schedule 40 force main discharging to the dosing tank. At 10,000 gallons, the alternating high-head effluent pumps send 2,000 gallons to the elevated tanks 70 feet higher than the treatment plant. Hydrostatic pressure then feeds water through disc filters in the control room to eight mister sprayers totaling 3 gpm in eight risers on the treatment tank.

"Running the misters continually feeds the microorganisms and produces much cleaner effluent for less energy," says Kendall. "Digestion is so thorough that there is very little biomass, which sloughs off, drains to the dosing tank, and is pumped out eventually. Our BOD levels are less than 2 mg/L."

The spray system also has 16 mid-size sprayers totaling 30 gpm that activate as needed, and eight sprayers delivering a combined 300 gpm to dispense shock loads. The latter run only with the plant at capacity.

The 2- by 2- by 4-foot media blocks, stacked four deep inside the treatment tank and surrounding a hollow center column, sit on adjustable bases with cut-to-length stanchions that make sure the blocks clear the ceiling by 12 inches. Two 1/3 hp fans in the tank pull 30,000 cfm of air down through the blocks and exhaust it out the column. Intake and exhaust pipes have carbon scrubbers to prevent odors during power failures.

Each block has 48 square feet of surface area per cubic foot. After trickling through the media, effluent drips out the bottom of the stacks to the floor drain and gravity flows to the dosing tank. When the system is at capacity, the sewage recirculating pumps will run every five minutes. An effluent pump sends water to the 3.5-acre drip field via a 3-inch PVC pipe sized to reduce friction loss.

Because the field is 100 feet of head above the plant, the booster pump in the control room cycles with the effluent pump to supply enough pressure. All the pumps run daily for a minute to prevent corrosion. Solid-state relays switch them on and off.

The drip field has 10 zones with 20 lines of tubing 200 feet long and 2.5 feet apart. Each emitter delivers one gallon per hour. Solenoid valves with pressure-regulators control
the zones. Pressures average 20 psi, but reach 180 psi going to the highest points.

Dose sizes vary depending on soil absorption rates. The computer reads the flowmeter to check for blown tubes or leaks. When they occur, the computer turns off the zone, bypasses it, and sends Kendall a text message.

Two 12-volt backup batteries run the plant during power outages, ensuring that the elevated tanks feed the sprayers. If the power is out for more than a day, Kendall brings a generator.

Installation

Subcontractors installing STEP systems as homes are built follow a specification booklet written by Kendall. Another subcontractor installed the force main.

Kendall's crew cleared trees before digging 50-foot-diameter, 11-foot-deep holes for the 35,000-gallon circular underground tanks and control room. A Caterpillar excavator with rock teeth on the bucket enabled the operator to flake out shale, which he struck at 3 feet below the surface.

"We made the circular tank forms," says Kendall. "They're a little more difficult to pour, but the structure is much stronger than square tanks. We needed that strength with all the rock in the soil." The 40-foot-diameter, 9-foot-deep tanks have 8-inch-thick walls of 4,000 psi fiber mesh concrete with steel rebar.

Kendall hauled each media block through a hatch in the treatment tank lid and down a ladder. Beginning at the center column and working out, he placed the bottom layer cantilevered across and at right angles to the 8-inch-wide AccuPier supports on 24-inch centers. He set additional layers at 90 degrees to the one below. "The pattern maximizes mixing and distribution, while increasing strength and stability," he says.

After workers laid piping for the sprayers on top of the tank and hung the spray heads in the risers, they covered the structure with 18 inches of insulating wood chips made from the cleared trees. A second team cleared a place in a wooded hollow and set the three elevated tanks.

The crew targeted softwoods when clearing some trees from the drip field. "We prefer installing drip fields in woods because hardwood trees uptake 22 mg/L of total nitrogen, and the highest we've seen from this system is 3 mg/L," says Kendall.

Workers used a custom-built 16-inch disc-cutting saw that fit between tightly spaced trees to install drip tubing 9 inches deep on slopes with up to 60 percent grade. A 6.5 hp chain saw engine powered the saw. They buried pressure mains from the plant to the drip field using a tracked Caterpillar 226 skid-steer loader with Bradco trencher (Paladin Construction Group).

For additional stability, Kendall's son, Cliff, mounting dual wheels on the downhill side of the LM42 Vermeer walk-beside vibratory plow with 50 hp turbo diesel. "Even then, we still occasionally chained the skid-steer or backhoe to the plow to prevent it from rolling down the hill," says Kendall. Installing a zone took four to five days.

Workers fenced the drip field and posted warning signs. They also landscaped the area around the underground complex.

Maintenance

Kendall's Aqua Green Utility, a Tennessee wastewater utility, owns and operates the system. To eliminate human error and reduce maintenance calls, he built PLC control panels and had software written for them. The autonomous system notifies Kendall via text messages if mechanical devices fail, enabling him to send a replacement with the technician on weekly visits. "From an operational standpoint, this saves a tremendous amount of money," says Kendall.

Until the replacement arrives, the computer turns on the redundant component. If technicians forget to turn on the pumps after servicing them, the computer activates them in eight hours. When pressure differential switches on the disc filters indicate they are clogging, the computer turns on the drip pump to backflush them. "We put excessive time into designing efficiency," says Kendall.



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