Cracking the Clay

A low-pressure drip design enables owners to build on a site with impermeable soils in northwest Colorado.
Cracking the Clay

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Owners razed a house to build a five-bedroom, 8,500-square-foot single-family home on 6.27 acres in Aspen, Colo. The existing 675 gpd onsite system was functioning, but old and too small for the new home.

The general contractor hired CHURCH Onsite Wastewater Consultants, a geological and civil engineering firm in Lakewood, to design a system suitable for soils with percolation rates higher than 400 minutes per inch. The owners would use the house just for the ski season: From March through December, only maintenance and cleaning staff would access it.

Robert (Bob) Wright, P.E., based the design on 130 gallons per person with two persons per bedroom. The following January, regulation changes decreased the load to 100 gallons per person. Although the revised system was 25 percent smaller than the original, it was still the largest one Wright had designed.

"I chose the Colorado low-pressure drip design because it works and because it is about 30 percent less expensive than pressurized drip tubing," he says. "Furthermore, installers can buy the components anywhere, and they don't need additional training to assemble PVC pipe."

The design has two septic tanks, a pump station, and perforated PVC laterals – one per 12-inch-deep trench, each 6 inches wide and 2 feet apart, leaving undisturbed soil on two sides of the pipe. Sizing of the drainfield is based on proprietary formulas.

"Most states believe that soils with a percolation rate of 120 minutes per inch are impermeable, and they won't issue building permits for those sites," says Wright. "This state-approved design enables development on such soils."

Site conditions

The site is an alpine meadow at elevation 9,000 feet with 6 percent slope. Soils are clay with percolation rates of 480 minutes per inch and no groundwater to a depth of 8 feet. Wright based the drainfield design on a rate of 180 or more minutes per inch.

System components

Wright designed the system to handle 1,000 gpd. The major components are:

1,500-gallon single-compartment and 1,000-gallon two-compartment concrete septic tanks by Grand Junction Pipe & Supply, Grand Junction, Colo.

48-inch manhole pump station with 1/2 hp Zoeller single-stage duplex pumps

Four-outlet distribution valve from K-Rain

12,160-square-foot Colorado low-pressure drip drainfield from CHURCH Onsite Wastewater Consultants

OSI S-series control panel, Orenco Systems

System operation

Wastewater flows 305 feet through a 2-inch PVC Schedule 40 discharge pipe to the first septic tank, then into the second, establishing the mandatory 30-hour retention time. The tanks have no effluent filter, although Wright specified an Orenco Biotube pump vault. "Most counties don't require effluent filters," he says. "Furthermore, contractors can change the design as long as it complies with the county code."

From the second tank, liquid flows to the manhole. Alternating,
on-demand pumps send 90 gallons to the distribution valve to dose one of four 32- by 95-foot drainfield zones. Each zone has four sections of four 2-inch PVC Schedule 40 laterals with 1/4-inch emitters on 8-inch centers facing down. An air/vacuum relief valve at the distribution valve allows
liquid to drain to the distribution pipes and back to the manhole.

According to Wright, the drainfield will probably never receive more than five doses per day. An annual evaporation rate of 50 inches enables the zones to dry quickly.

"Alternating and resting dispersal zones is a big part of avoiding overbuilding a biomat, especially when there is no effluent filter," says Wright. "Only 75 percent of the drainfield is operational, as one zone is always resting for a year. Each zone also has a distribution box with ball valves so laterals can be rested as needed."

Instead of cleanouts, one lateral in each zone comes to the surface. "The installer saws 1-inch-deep cuts on quadrants in the PVC cap to make it removable," says Wright. "We don't pressurize the system as part of the proprietary design criteria. The pump simply ensures that effluent reaches the end of each lateral, which is supposed to flow about half full and drain out."


A local contractor decommissioned the existing septic system and installed the new one. After he had difficulty with a 2,250-gallon two-
compartment mid-seam concrete tank, the 1,500- and 1,000-gallon tanks replaced it.

"The revised regulations allowed us to use a 2,000-gallon tank, but we like the added capacity to handle surges," says Wright. "We also have better treatment and more retention time. Tankage is cheap compared to the benefits." The tanks were bedded on 3 inches of sand.

The drainfield was supposed to be installed with a trencher to create a 6-inch-wide trench for a single lateral. However, Wright said the project instructions did not clearly communicate that requirement to the installer, who instead used a backhoe.

"We only allow people to use a backhoe if the site is too steep or the ground is too rocky for a trencher," says Wright. The result was 24-inch-wide trenches with a lateral on each side – still a functional system that met local regulations.


The design requires no maintenance contract. When the system was completed, Wright provided the owners with operation and maintenance guidelines in his design package, a copy of the Homeowner's Guide to Septic Systems from the U.S. EPA, and a checklist he created that includes rotating the zones annually. When Wright is in the vicinity (the office is 200 miles away), he checks the pumps and alarms. O


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