Mound in a Box

An aerobic bacterial generator and raised mound system help a designer and installer save a bed-and-breakfast in a resort town in northeast Wisconsin
Mound in a Box

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A ruling by the Door County (Wis.) Sanitarian Department to remove steel holding tanks more than 20 years old left owners of a seasonal three-bedroom bed-and-breakfast in Sturgeon Bay scrambling.

Setbacks from property lines, wetlands, and the house left a 32- by 28-foot area for the drainfield. Chris Fellner, owner of Advanced Soil and Wastewater in Sturgeon Bay, recommended a SludgeHammer aerobic bacterial generator to reduce the drainfield footprint by 20 percent. When combined with a mound-in-a-box absorption bed designed by Dave Labott of Sturgeon Bay, the system just fit in the available area.

 

Site conditions

Soils are sandy with a loading rate of 1.6 gpm. The water table is 20 inches below grade. The one-acre lot fronts on Lake Michigan, and 60 percent of it is wetland.

 

System components

Chris Fellner designed the system to handle 450 gpd. Its major components are:

Three-compartment concrete septic tank from Premier Concrete, Appleton, Wis. (Northeast Asphalt)

S-86 SludgeHammer aerobic bacterial generator supplied by Advanced Soil and Wastewater

8-inch Orenco effluent filter in second compartment

Simplex 1/3 hp Hydromatic (Pentair) SHEF 40 effluent pump with control floats from Septic Products (SPI) in the third compartment

120 feet of Flowtech geosynthetic aggregate from ICC Technologies in two 9- by 20-foot cells

Concrete blocks from Premier Concrete

Existing alarm system

 

System operation

Wastewater drains through a 4-inch PVC lateral to the tank’s 1,000-gallon septic tank compartment. The 36-inch-tall aerobic generator, sitting on the bottom, has 150 square feet of surface area colonized by proprietary bacteria.

An air pump at the surface introduces oxygen into the tank at 3.5 cfm/2.0 psi. The resulting vigorous circulation directs wastewater through the generator at 15,000 to 20,000 gpd, ensuring that the liquid is processed 20 times in 24 hours.

Treated clear water flows into the 328-gallon settling compartment with effluent filter before entering the 757-gallon pump chamber. The on-demand pump runs about seven times per day, delivering 61.53 gallons to the mound in each dose. A double manifold loads both cells equally. The microbes digest the nutrients in the effluent so completely that nothing remains to create a biomat.

 

Installation

Doug Fellner and crew from Fellner Soil & Septic in Sturgeon Bay removed the 3,000-gallon holding tank, then extended the hole to accommodate the 13- by 8- by 5-foot-high septic tank. “We dug down eight feet with a steel-tracked Kobelco 115SR DZ excavator,” says Fellner. “The additional depth was necessary to accommodate three feet of fill on top of the tank to raise the ground level and counteract the tank’s buoyancy.”

The excavator hit groundwater at 2.5 feet. Fellner dewatered the hole using two electric 1 hp Tsurumi pumps powered by electricity from the inn. After bedding the excavation with 8 inches of screened 3/4-inch stone, they set the tank and filled the first compartment with water to prevent flotation.

“As soon as Door County sanitarian John Teichtler gave us the nod, we backfilled with more stone,” says Fellner. “Within five hours, the owner could flush the toilets and use water.” Workers removed 12 to 18 inches of topsoil and fill soil from the drainfield area so no organic material remained to clog the mound sand.

After creating the depression and compacting the soil for the walls, Fellner used the excavator to set the first row of 3,000-pound interlocking concrete blocks. “The first row of the box is below ground, but the second row is visible,” he says. “The owner ordered a decorative river-washed stone facing that we applied.”

Workers poured and leveled 30 inches of mound sand inside the box, then laid out the two drainfield cells, each with 60 feet of geosynthetic aggregate in three 20-foot bundles of three 12-inch-diameter modules lying side by side. The center module has a 1.25-inch PVC distribution lateral with 5/32-inch orifices hand-drilled on 2-foot centers for 10 holes per pipe.

“We use a very sharp bit to bore the orifices and keep the drill running as it breaks through the pipe,” says Fellner. “A smooth bore is critical, as tailings will eventually clog the laterals, even with a SludgeHammer reducing fines.”

Each 10-foot-long module holds 30 gallons of water, replaces more than 1,000 pounds of gravel, is 1.6 times more efficient, and is easily joined using internal couplers. A 3-foot-wide wall of mound sand separates and holds the bundles in place. The 2-inch supply manifold feeds the 1.5-inch manifold at each cell.

Workers rolled geotextile fabric over the bundles, then cut slits to install inspection ports on either end of the drainfield. They added 12 inches of topsoil and seeded it with grass. “Any shallow-rooted vegetation is permissible, but flowers should be annual and planted around the edges,” says Fellner. “The mound in a box is a simple, economical, low-tech solution that works very well.”

 

Maintenance

The Door County Sanitarian Department requires a maintenance contract on pretreatment systems. Chris Fellner maintains the system. He annually changes the microbe stick, cleans the effluent filter, and checks the pump and floats.



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