The Ho-Chunk Nation Environmental Health Department found rising nitrate levels in potable water wells serving the Blue Wing community in Tomah, Wis. The village has 20 homes, a day care center, a community center, and a meal center for senior citizens. A preliminary study by the Indian Health Service cited failing onsite systems, sandy soils, and shallow groundwater as contributing factors.

After a soil and site investigation, the Ayres Associates engineering firm of Madison, Wis., recommended a gravity collection system and recirculating gravel filter with denitrification and drip dispersal.

“We wanted to demonstrate nitrogen reduction with a modified gravel filter, and how the design performs in a northern climate,” says project manager and designer Rick Apfel. Although the system is not operating at design capacity, BOD is less than 2 mg/l and TSS is 5 mg/l. Nitrate levels fluctuate from 14 mg/l in winter to 6 to 8 mg/l in summer, for a total nitrogen removal of 65 to 85 percent.

Site conditions

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Soils are sandy with a loading rate of 0.5 gallons per square foot per day. The seasonal water table is 2 to 4 feet below the surface. Part of the five-acre site was a red pine plantation.

System components

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Apfel designed the system to handle 20,000 gpd, including flows from three undeveloped multi-family dwellings, each with 18 bedrooms. Major components are:

• 25,000-gallon two-compartment septic tank (all tanks from Wieser Concrete, Maiden Rock, Wis.)
• 20,000-gallon two-compartment septic tank
• 12,000-gallon denitrification tank with stacked media blocks from Petersen Products
• 18,000-gallon recirculation tank
• 15,000-gallon dose tank
• 7,040-square-foot recirculating gravel filter in eight 10- by 88-foot zones
• Perc-Rite drip system components from American Manufacturing include 20,160 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. Equipment supplied by Petersen Products, Fredonia, Wis.

System operation

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Wastewater flows from the sewer to a lift station and is pumped through a 3-inch force main to the first septic tank. It flows by gravity to the second septic tank and anoxic denitrification tank. Valving enables the operator to bypass the first tank for pumping and other maintenance.

A submersible circulation pump conveys mixed effluent and filtrate from the bottom of the denitrification tank to distribution pipes at the top of two 8- by 6-foot-square blocks of corrugated PVC crossflow media totaling 576 square feet of surface. All components are submerged to avoid introducing oxygen. The tank has a minimum 8-hour detention time.

Nitrified filtrate from the gravel filter supplements the effluent. “We want the bacteria to extract the oxygen from the nitrate,” says Apfel. “The process produces nitrogen gas that dissipates to the atmosphere and reduces total nitrogen in the effluent.”

Effluent flows from the denitrification tank to the recirculation tank. Alternating pumps cycle every hour, dosing the gravel filter with 2,080 gallons in 18 minutes. Electrically actuated butterfly valves in the control building sequence the dose through 2-inch distribution piping to the active zones.

Effluent drains through the gravel filter in about 30 minutes. A float in the recirculation tank controls a three-way plug valve that directs the filtrate to either the dose tank for dispersal to the drainfield or back to the denitrification tank for recirculation. Filtrate is recirculated through the gravel filter at 3:1 to 5:1 ratios.

The drainfield has two banks, each with four 36- by 140-foot zones. The hydraulic unit, with two 3 hp suction-lift alternating STA-RITE pumps (Pentair Water), 120 micron filters, and manifold, is in the control building. Every hour, the pump runs for 40 minutes, drawing 835 gallons of filtrate through the filters and sending it to one of five active zones. The drip tubing, on 2-foot centers, has emitters 24 inches apart. Each one disperses 0.92 gallons per hour at 7 to 60 psi. The lines drain via gravity after every dose.

Installation

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Mike Stroik of Heartland Construction in Slinger, Wis., installed the onsite system. Randy Rudisill of Heartland Utilities in Baraboo installed the 8-inch sewer mains and manholes.

Members of the Ho-Chunk community cut the timber. Stroik’s crew cleared and grubbed the site in late October and, after scarifying the area, added 6 to 14 inches of mound sand to keep the required 3-foot separation to seasonal groundwater. They laid the drip tubing and covered it with 18 inches of sandy loam and 6 inches of topsoil.

Setting the tanks required trenching the excavations at a 1:1 ratio. The men also built the control building, formed the berm for the gravel filter, and installed the underground piping. “We needed three days above 40 degrees F to electrostatically weld the synthetic liner sheets, so we shut down operations until spring,” says Apfel.

Rudisill completed the sewer work during winter. When Stroik’s crew returned in April, they installed the liner with the edges keyed into the berm, added the underdrain piping on 12 inches of coarse stone, covered it with 24 inches of pea gravel, laid the laterals, and covered them with 6 inches of pea gravel.

The challenge was adding the media without damaging the liner or piping. “Mike’s backhoe operator placed as much gravel as he could, and then they brought in a truck-mounted telescoping belt conveyor with discharge chute,” says Apfel. “It was slick. The operator spread the gravel in exactly the right places at the correct depth and discharged it from about three feet to minimize the impact of the weight.”

The men covered the outside berm slopes with topsoil and seeded it, then built a 7-foot security fence with barbed wire and a locked gate around the system. Heartland Utilities abandoned the existing septic tanks, extended new 4-inch laterals from the houses, and connected them to the sewer.

Maintenance

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A Class I wastewater operator from Petersen Management remotely monitors daily performance. Once a month, he collects data for analysis, checks all components, and evaluates sludge levels to determine if the tanks need pumping. Twice a year, he rotates the active zones for the gravel filter and drainfield and flushes the filter’s laterals.