Hooking up more and more units to a North Carolina trailer park prompted installation of a new advanced treatment system employing recirculating compartments and denitrification.


As demand for sites at a mobile home park in Cedar Point, N.C., developed, the owners added more spaces with sewer connections. The increasing volume eventually overwhelmed the low-pressure pipe drainfield in the center of 96 lots.

Pumping the 5,000-gallon septic tank every other day prevented ponding and odors. During the system’s permit renewal, the Carteret County Health Department inspector identified the situation and issued a consent decree.

President Greg Mayfield of Southern Water and Soil in Zephyrhills, Fla., worked with Orenco Systems’ distributor Steve Barry of AQWA in Wilson, N.C., to design a replacement system. They chose AdvanTex technology partially due to space restrictions, but primarily to reduce nutrient loading to protect sensitive aquatic life in the adjacent Intercoastal Waterway.

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Kevin Davidson, P.E., of Agri-Waste Technology in Raleigh, N.C., was the certifying engineer.

“Total nitrogen from the denitrification system had to be less than 20 mg/L,” says Mayfield. “The AdvanTex AX-Max matched up well, averaging 80 percent nitrogen reduction.”

According to Barry, the installation was the first of its kind in the state.

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Site Conditions

Soils are fine sand with a long-term acceptance rate of 1.0 gpd per square foot and high water table at 6 feet.

System Components

Major components of the 12,000 gpd system are:

  • Primary 8,000-gallon dual-compartment concrete septic tank (All tanks from Shoaf Precast Septic Tank, Lexington, N.C.)
  • Secondary 8,000-gallon dual-compartment septic tank with two 15-inch Biotube pump vaults, Orenco
  • Existing 5,000-gallon single-compartment concrete tank converted to a tertiary septic tank
  • 5,000-gallon dose tank with alternating 1/2 hp effluent pumps, Orenco
  • 12,000-gallon AdvanTex AX-Max packed bed filter module, Orenco
  • 12,000-gallon AdvanTex AX-Max denitrification module, Orenco
  • 9,600 feet of 1/2-inch drip tubing, Netafim Irrigation
  • TCOM control panel, Orenco

System operation

A lift station on the property pumps wastewater through a 2-inch force main to the septic tanks. The primary tank has effluent return lines from the recirculation pump in the treatment module and in the denitrification module. “When denitrifying, the septic tanks provide additional residence time to reduce nitrogen,” says Mayfield.

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At preset intervals, pumps in the dose tank deliver 136 gallons to the three-compartment, 42-foot-long treatment module. As influent enters the 5,500-gallon recirculation-blending chamber, it mixes with filtrate dripping from the hanging textile media. A pump in a walled-off area sends 150 gallons per minute to a manifold above the media. A portion of each dose drips into the recirculation-filtrate chamber.

A baffle divides the flow between the recirculation-blending and recirculation-filtrate chambers, and a recirculation return valve controls liquid levels. No wastewater passes into the second chamber without first flowing through the media.

Effluent gravity flows through a 6-inch drainpipe to the denitrification module packed with 10 cubic yards of 1-inch tire chips. Bacterial action on the aggregate reduces nitrate to nitrogen gas released through the ventilation assembly. Recirculation is timed. When activated, the 1/2 hp recirculation pump sends 20 percent of the liquid back to the septic tank.

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Every 100 minutes, the pump doses the 11,360-square-foot drainfield, feeding 500 gallons at 25 gpm to two of the four 30- by 100-foot zones. Each zone has 24 laterals on 14.2-inch centers. Laterals combine in a manifold before returning to the headworks in the rear of the treatment module. To handle seasonal flows, the operator changes the rest cycle time. The system also has an emergency generator.

Installation

Mayfield and a crew of seven staged their equipment in a boat storage area after removing the vessels. The original drainfield provided a 150- by 250-foot-wide work area.

While waiting for the modules to arrive, the team used a rented Kubota U25 compact excavator and two Bobcat T190 compact excavators to remove 30 inches of existing drainfield and stockpile the good sand. “After pumping the system for a year, the substandard 12-inch stone had no biomat,” says Mayfield. “Using stone undersized by six inches probably caused some of the system’s problems. However, the curtain drain was working, so we kept it.”

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W.A. Page and Sons in Swansboro, N.C., removed unsuitable soil to a landfill, then brought in 40 loads of quality beach sand to build the new drainfield adjacent to the old one. To eliminate stratification, workers mixed the beach sand and stockpiled sand together with the T190s, then operator/project manager Drexyl Brewer used the Doosan DX140LC excavator to homogenize it further. They compressed each layer of sand spread on the drainfield until the bed was 6 to 8 inches above grade. After laying the drip tubing, they covered it with 12 inches of sand.

In total, installing the drainfield took two weeks. Mayfield’s father-in-law fielded questions from park residents and kept them a safe distance away. “They were fascinated with the project and excited about having a system that worked,” says Mayfield.

The crew returned two weeks later to set the tanks in the old drainfield. “The health department allowed us to set them above ground to avoid excavation problems below the water table,” says Mayfield.Using the Doosan, Brewer excavated 4.5-foot-deep holes and partially filled them with gravel. Phillip Shoaf brought the tanks and Superior Cranes in Rockingham, N.C., set them. After the septic tanks were installed, the 100-ton crane became stuck in the sand while changing locations. Stabilization mats under the wheels freed the vehicle, then workers repositioned them until the crane reached the AX-Max installation site.

“It took from 9:30 a.m. to 3 p.m. to set and backfill the tanks to the lids with compacted sand,” says Mayfield. “We worked until 9 p.m. with a team from AQWA installing the pre-plumbed treatment units. For a system this size, though, the installation was easy.”

The modules shipped with integrated lugs and lifting cables. After Brewer excavated 3.5-foot-deep holes and laborers bedded them with 12 inches of compacted gravel, Superior Cranes set the containers. Working under lights, Brewer used the Doosan to lift 1-ton bags of tire chips over the tank in the denitrification module. AQWA workers standing on the module’s scaffolding, cut open the bags, then Brewer moved the bucket back and forth to spread the aggregate as it fell out. Each bag contained two cubic yards.

Mayfield’s team backfilled with sand around the modules to final grade. After the system was finished, park management fenced off the area.

Maintenance

AQWA holds the two-year maintenance agreement. The state requires a certified operator with a subsurface operator license and backup operator for the system. Dan Fortin, a Grade IV wastewater operator, visits the system periodically to check the nozzles and clean the Biotube filters. “While the AX-Max is extremely robust, all wastewater treatment systems require a certain level of operation and maintenance to achieve their treatment goals,” says Barry.

More Information

Doosan Infracore America Corporation - 770/831-2200 - www.doosanequipment.com

Netafim USA - 888/638-2346 - www.netafimusa.com

Orenco Systems, Inc. - 800/348-9843 - www.orenco.com


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