A Type IV advanced treatment design with pressurized at-grade distribution is the answer after a real estate inspection fail in Minnesota


Finding a dry well ended Daren Hofschulte’s point-of-sale inspection of a two-bedroom single-family home in Pine Island, Minnesota. According to state code, dry wells are noncompliant.
Hofschulte, owner of Hofschulte Backhoe and Septic in Elgin, Minnesota, frequently designs mound systems with leaching chambers for such properties, but not this time. The house, set into a hill, was 20 feet from the street. A shared potable well eliminated using the area west of the house, and the hill rose behind it in a 10 percent slope.

“That left pretreatment, and only licensed engineers may design Type IV advanced treatment systems,” says Hofschulte. For assistance, he turned to Chris Priebe, advanced designer at G-Cubed Engineering in Chatfield, Minnesota.

“Pretreatment reduces the vertical separation to the seasonal high water table from 36 inches to 18 inches, but we found bedrock at 12 inches,” says Priebe. “However, adding UV disinfection enabled us to reduce the separation to 12 inches, and design a pressurized at-grade system.”

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Site conditions

Soils are 12 to 18 inches of silt loam over limestone bedrock with a loading rate of 0.78 gpd per square foot. The lot is 0.8 acres.

System components

Priebe designed the system to handle 300 gpd. Major components are:

  • A four-chamber  2,080-gallon tank from Wieser Precast Steps housing a Hoot Systems H-600A 600-gpd aeration unit and clarifier
  • 4/10 hp effluent pump, A.Y. McDonald Mfg. Co.
  • 3G ultraviolet disinfection unit, SALCOR Inc.
  • SJE-Rhombus control panel
  • 36 EZflow by Infiltrator Water Technologies geosynthetic aggregate module

System operation

Wastewater flows 20 feet through a 4-inch PVC lateral to the 400-gallon pretreatment compartment, then to the 715-gallon aeration chamber. Aeration stones on the bottom of the chamber releases finely diffused bubbles to keep the bacteria alive and solids in motion. Liquid then rises up through the 205-gallon clarifier, where solids settle out before flowing to the 760-gallon pump chamber.

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The pump cycles six times per day for 7.4 minutes. Each 50-gallon dose passes through the UV tube, then gains 18 feet in elevation as it travels 95 feet through a 2-inch force main to the drainfield. An additional 66 gallons is drainback. Only the 4-inch distribution pipe in the middle horizontal triple module configuration receives effluent. Water from the distribution pipe then flows by gravity through the remaining four rows of bundles in the 32- by 12-foot drainfield.

Installation

Hofschulte’s crew installed the tank and drainfield simultaneously. After the original septic tank was pumped, son Corey Hofschulte and laborer Jeff Rahman crushed and abandoned it with a John Deere 75D tracked excavator.

The 14- by 7- by 5-foot ATU tank fit between the property line setbacks east of the house and parallel to the road. As Hofschulte and Rahman dug through sand, sandstone and bedrock, the walls caved in. Soon the excavator arm’s 21-foot reach wasn’t enough to clean out the sand. Daren Hofschulte called for reinforcements — his John Deere 160 LC track excavator with 29-foot reach. By the time Rahman cleaned the corners of the hole, it measured 25 by 18 by 11 feet.

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“We hauled away the rocky material and kept the sandy soil for backfill,” says Daren Hofschulte. “Spoil was piled behind us, on both sides of the road, and on the road, but we left enough room for traffic to drive around it.”

Meanwhile, operator Britt Coulson used the 75D to slope into the hill for the drainfield, grade the area to drain runoff, and scarify the soil. “The west side of the house had ample space to maneuver our equipment around the retaining walls and up the hill,” says Hofschulte.

The preassembled bundles — three side-by-side 10-foot by 12-inch-diameter modules — snap together to form the required absorption area. “I would use them every day if they magically appeared on the job when we arrived,” says Hofschulte. “They work great on steep slopes, but transporting them was the most difficult part of this install.”

Hofschulte strapped the bundles on a utility trailer, then stopped several times to retie them as wind loosened the load. “You can’t strap them down as if they were plastic pipe, because that would compress the Styrofoam peanuts,” he says. “Break a bundle and millions of little peanuts fly all over. In hindsight, we should have used a dump truck with a
rolling tarp.”

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To achieve a 32-foot-long absorption area, Rahman and Corey Hofschulte snapped three bundles together to equal 30 feet, then added a 5-foot-long bundle on either end to make up differences in distances and extend the drainfield’s capacity slightly. They installed five rows, then covered them with geotextile fabric. Coulson used a John Deere 333 skid-loader to backfill with sandy loam and 6 inches of topsoil.

Digging the 3-foot-deep trench and laying the force main was straightforward. “We install an onsite system every day, and often on steep slopes,” says Hofschulte. “This was an ordinary installation for us.”

Maintenance

Another company maintains the manufacturer’s required service contract. Biannually, the technician observes the mechanical and electrical components, collects an effluent sample for fecal coliform bacteria before cleaning the UV bulb, and replaces it every two years.

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