Heavy clays and restrictive set-backs rendered an acre lot unacceptable for a conventional onsite permit, putting the plans for building a vacation home on hold for a couple in Hollywood, S.C.
They contacted Robert Gross, P.E., of Beaufort Group LLC in Beaufort, S.C., an engineering firm that specializes in alternative treatment designs. Working with Gross, the couple selected a peat-fiber biofiltration pretreatment system because it can handle intermittent usage and the maintenance requirements are minimal.
State code specifies that alternative systems must have a replacement drainfield. As space was limited, Gross selected a subsurface discharge drip-dispersal system. Upon reviewing the plans, the state still considered the site denied, and placed liability on the homeowners for any treatment problems or system failures. Gross’s design is the first in Charleston County to incorporate peat biofiltration with drip dispersal.
Site conditions
Soils consist of Class III and IV (expansive clays) beneath 12 to 17 inches of humus-rich topsoil. The soil has a long-term acceptance rate of 0.25 gpd per square foot. The seasonal high zone of saturation was estimated at 2 to 5 inches below the surface.
System components
Gross designed the system to handle 480 gpd. Its major components are:
• 1,000-gallon, two-compartment concrete septic tank with A300 Zabel effluent filter, a product of Polylok. All precast tanks made by Knight’s Septic Tanks, Summerville, S.C.
• 1,000-gallon concrete pump tank with 0.4-hp simplex pump from Red Jacket Water Products ITT, Seneca Falls, N.Y.
• Four 150-gpd Puraflo peat biofilter modules from Bord na Mona Corp.
• 1,000-gallon concrete dosing tank with 1/2-hp high-head turbine pump in a 6-inch-diameter Cool Guide pump vault from American Manufacturing Co. Inc.
• Perc-Rite drip system components from American Manufacturing, including 1,050 feet of 1/2-inch pressure-compensating drip tubing, hydraulic unit with auto-backwashing disk filters, and PLC-driven combination pump control panel with Web-based telemetry.
System operation
Wastewater gravity flows into the septic tank, then to the pump tank. Every two hours, the pump in the tank runs for 60 seconds, sending 40 gallons of effluent through a 2-inch Schedule 40 PVC force main to the biofilters. A manifold in each module evenly doses the peat using a distribution grid.
Purification occurs as the liquid percolates through the media over 36 to 48 hours. The peat also suppresses odors. The units produce effluent averaging less than 10 mg/l TSS and BOD, and 99 percent reduction of fecal coliforms with no pathogens.
“When the house is vacant for extended periods, the microorganisms living on the media form spores and go dormant,” says Chris Keiger, P.G., business development manager for Bord na Mona. “But shortly after the homeowners return, the peat system reactivates to optimal performance levels.”
The treated effluent gravity flows from the bottom of the modules through 2-inch Schedule 40 PVC drain lines to a sample chamber, then to the top of the dosing tank. The pump cycles every four hours, sending 120 gallons in 12 minutes to the drainfield.
The drainfield has 14 drip lines, 75 feet long on 2-foot centers. The 525 pressure-compensating drip emitters, spaced 2 feet on center, deliver 0.61 gallons per hour per emitter at 50 psi. The high pressure prevents root intrusions. When the pumping cycle ends, air-release valves rapidly drain the drip tubing through the emitters and prevent fine sands from being drawn into them.
The Perc-Rite system automatically forward flushes the drip tubing at more than 2 feet per second at adjustable intervals to prevent slime growth. The hydraulic unit’s disk filters are automatically backflushed at the start of each cycle and every five minutes into the pump run time. The fully automated system is maintenance free.
Installation
Ernest Rentz of Rentz Inc. in Hollywood installed the system. Gross required a 200-foot-long French (curtain) drain buried 3 feet deep along the up-gradient side of the property to minimize surface runoff into the drainfield. “It had to go in first, and the difficult soils made installing the corrugated pipe challenging,” Gross notes.
After clearing the site of brush, Rentz’s men used a small backhoe on rubber tires to excavate the trench for the drain. “We were working below the water table and in clay that you could make pottery out of,” says Rentz. “The trench stayed open for only a short time, so we had to work fast and in short segments. It was the most difficult part of the installation, and it took two days.”
Because of the high water table, Rentz expected to dewater the holes for the tanks and biofilter modules, but the clay next to the house was so tightly packed that the excavations didn’t fill as quickly as anticipated. The tanks and modules were a snug fit between the house and 80- by 50-foot replacement area, offset from the French drain by 5 feet. The distance from house to drain was 30 feet, and the tanks were offset 5 feet from the house.
Each 7- by 4.5-foot by 32-inch-high module weighs 1,800 pounds. Rentz used his backhoe to set them on a 6-inch-deep gravel support bed. Once the modules were connected, the system was blown out with the cap off both ends of the manifold, and a pumping drawdown test determined the correct dosing volume.
As Rentz’s men graded the drainfield area to provide a 10-foot-long elevated taper with 1:10 slope, someone discovered a potable well on the neighbor’s property — and it was too close to the drainfield. To meet the setbacks from a well and nearby drainage ditch, Gross modified the drainfield area from eight lines 100 feet long with a 130- by 46-foot footprint to 14 lines 75 feet long with a 28- by 75-foot footprint.
After excavating 6-inch-wide trenches for the drip tubing, the men added 12 inches of Class 1 medium sand to provide the required separation between the emitters and shallowest part of the seasonal high groundwater. The drip tubing was backfilled with 18 inches of lightly compacted sand, and the cap was planted with grass seed.
Keiger spoke to the homeowners about the importance of directing downspouts away from the tanks and drainfield. The couple had the house contractor install the gutters last and comply with Keiger’s recommendation.
Maintenance
The owners entered into a one-year renewable maintenance agreement with Rentz. Maintenance is a yearly inspection involving opening the septic and pump tanks to check the scum, sludge, and water levels. The effluent filter is also cleaned annually.
The service provider runs a cycle on each pump, inspects the peat media for grease or ponding, and looks for solids or particulates in the biofilter effluent. He also checks the control panel, floats, valves, gauges, and pressures in the drip equipment. The peat should last for 15 years or more, after which it will be vacuumed out and replaced.
