The Lewis and Clark Caverns State Park in Whitehall, Mont., was building a visitor center that could accommodate 400 tourists per day. Interstate Engineering, a civil engineering contractor in Billings, hired Travis West, RS/REHS, of Land Development Solutions LLC in Columbus, Mont., to design the onsite system.
West’s first concern was handling the high-strength waste (180 mg/l total nitrogen), as no graywater would dilute the volume. His second concern was the cold climate and its effect on the biological treatment process, since the park turned off the water from October through April, rendering the system dormant. Furthermore, the onsite code required a replacement drainfield, and space was limited.
“We needed a product that would reduce our drainfield footprint and require no maintenance or attention from the park staff,” says West. After researching options, he chose the pre-engineered Eliminite system, a fixed-film bioreactor invented by Thomas Kallenbach, P.E., of Three Forks and Justin Buchanan of Bozeman, Mont.
The technology achieved aerobic nitrification, anaerobic denitrification, solids settling, and BOD reduction in a single tank. The onsite system functions perfectly with minimal maintenance.
Site conditions
Soils are sandy clay loam with a percolation rate of 32 minutes per inch and a loading rate of 0.3 gpd per square foot. The slope is 5.4 percent. The center is next to the Jefferson River, a noted blue-ribbon fishing and recreational stream.
System components
West designed the system to treat 2,000 gpd. Its major components are:
• Two 2,400-gallon dual-compartment concrete septic tanks made by Three Forks Concrete, Three Forks, Mont.
• Two A100-8 effluent filters (Polylok), one per tank.
• 4,000-gallon Eliminite 620C bioreactor and two 24-inch Polylok risers.
• 1 hp Hydromatic SKHS 100 recirculation pump, Hydromatic, Pentair Pump Group, Ashland, Ohio.
• 1/2 hp Hydromatic SHEF 50 discharge pump.
• 1,200 feet of 1.5-inch pressurized pipe inside 36-inch Quick4 chambers from Infiltrator Systems Inc.
• Siemens Logo Control Panel from SJE-Rhombus.
System operation
Wastewater gravity flows through 4-inch Schedule 40 PVC pipe to the two septic tanks, then into the bioreactor pump basin. The recirculation pump in the basin is set to recirculate four times before discharging to the drainfield. When activated, it draws 150 gallons of effluent into a spray bar with four heads that evenly distribute it over 620 cubic feet of MetaRocks.
“MetaRocks are spheres of closed-cell polyurethane resins with three deep-contoured channels that provide large, open pores for passive air transfer,” says Kallenbach. “A coating of coarse sand and finely crushed recycled glass enables a thin liquid film to cover the surface and promote even, consistent growth of microorganisms.”
The 9-foot-deep packed media bed needs no cleaning or replacement. Nitrified liquid collects at the bottom of the tank. Each time the recirculating pump engages to dose the filter, it returns a portion of the fluid through the denitrification line to the first septic tank.
At peak capacity and after the fourth cycle, the rising liquid at the bottom of the tank engages a float-activated discharge pump in the pump basin. The pump sends 327 gallons through a 2-inch 160 psi DR11 HDPE Schedule 40 force main to the drainfield six times per day.
An automatic distribution valve alternately doses three zones, each 400 by 100 feet. Each zone has 1.5-inch pressurized laterals suspended with plastic snap-tie fasteners inside the chambers. A 1.5-inch Valterra knife valve on each lateral provides equal hydraulic distribution.
The Lung, a proprietary air entrainment device driven by the recirculation pump, supplies oxygen to the microorganisms. Every time the recirculation pump runs, it discharges air drawn into the spray bar from the previous cycle into the bioreactor. When the pump shuts off, liquid drains by gravity back to the pump chamber, simultaneously drawing air into the pipe.
During winter, the recirculation pump continues to cycle effluent over the MetaRocks, but none is discharged to the drainfield. When the visitor center opens in spring, the system resumes where it left off in fall.
Installation
David McKeever from Diamond Construction Inc. in Helena excavated the holes, set the tanks, and dug the drainfield trenches 32 inches deep to escape the frost. Tony DeGidio, Eliminite production manager, and Dustin Paulson then insulated the bioreactor tank with two layers of 2-inch-thick blue board foam and plumbed it.
Meanwhile, McKeever excavated a 6-foot-deep trench for the horizontal section of the force main to prevent freezing. He inserted its vertical section length into a 6-inch PVC sleeve filled with water-resistant foam insulation.
“Even with a 50 percent reduction of the drainfield’s footprint, the two beds barely fit on the slope,” says West. “Using Squirt, the pressurized drainfield design program written by Thomas to size the pipes, orifices, and pumps, made it easy to see the result of different combinations and find the correct solutions.”
McKeever drilled 1/8-inch orifices five feet apart in the pressurized pipe. Every fourth hole faced down and had a Polylok diffuser. The 100-foot-long laterals had 20 orifices. After installation, West pressure-tested the zones, and then McKeever backfilled after placing two layers of 2-inch styrofoam board over the manifolds. It took one day to set the tanks and two to complete the drainfield.
Maintenance
EcoSentry LLC in Bozeman has the two-year service contract. The state Department of Environmental Quality requires effluent testing twice per year for the first two years, then annually. Septic tank pumpouts are scheduled every three to five years. When the center closes for the season, the service provider switches the system to the suspended mode. It restarts automatically in spring.
