Research Studies

Study #1 - Shallow Gravelless Drainfield Performs at Very High Loading Rates

A gravelless drainfield at Orenco's facility in Roseburg, Oregon has been receiving effluent from various experimental sand filters for six years. Ten inches deep, 12 inches wide, and 40' long, it is loaded at an average of more than 600 gpd... over 15 gpd/ft². The soil is 28" of silt loam underlain by a claypan, with wintertime groundwater at a depth of 18". This drainfield exhibits no visible evidence that wastewater is being applied.

Study #2 & #3 - Shallow Drainfields Reduce Nitrogen

Two studies clearly showed that significant nitrogen loss can take place when pretreated, highly nitrified sand filter effluent is distributed in small doses in the top few inches of an organic soil:

Study 2 - From September 1993 to November 1994, a suction lysimeter pulled samples of sand filter effluent from 12" below a drip system's tubing. (The drip system was buried 3" below grade). 44 data points were generated. The average nitrate of the sand filter effluent was 86 mg/L. After passing through the 12" of soil in the drip bed drainfield, the average nitrate of the effluent was 50 mg/L... a 42% reduction in average nitrate. (Additional facts: sewage source was an office building; total kjeldahl nitrogen leaving the septic tank ranged from 90 to 190 mg/L; the drip bed was 3' x 50'; soil was clay loam covered by native lawn grass; the average daily flow was 370 gallons; the average soil loading rate was 2.5 gpd/ft².)

Study 3 - From November 1993 to September 1994, a suction lysimeter pulled samples of polished trickling filter effluent from 12" below a shallow gravelless drainfield trench. 40 data points were generated. The average nitrate of the polished trickling filter effluent was 13.7 mg/L. After passing through the 12" of soil below the shallow gravelless drainfield, the average nitrate of the effluent was 5.9 mg/L... a 57% reduction in average nitrate. (Additional facts: sewage source was a two-bedroom home; total kjeldahl nitrogen leaving the septic tank was 66 mg/L; the shallow gravelless drainfield was 12" (w) x 20' (long) x 12" deep; the soil was a silty clay loam with no vegetative covering; the average daily flow was 115 gpd; the average soil loading rate was 5.75 gpd/ft².)

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These studies show that the widely used practice of placing pretreated effluent in deeper drainfield soil results in a "missed opportunity" for additional wastewater treatment and nutrient removal. See also Wert, S. and R.C. Paeth, 1985. "Performance of disposal trenches charged with recirculating sand filter effluent." In: 5th Northwest On-Site Waste Water Treatment Short Course "On-Site Waste Water Treatment: Environmental Significance." State of Washington Department of Social and Health Services, Olympia, Washington.