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TECHNICAL REPORTS

Waste Management

Nitrogen Removal in Laboratory Model Leachfields with Organic-Rich Layers

Department of Civil and Architectural Engineering, 1000 East University Avenue, Department 3295, University of Wyoming, Laramie, WY 82071

^* Corresponding author (bedessem{at}uwyo.edu)

Received for publication January 20, 2004. Septic system leachfields can release dissolved nitrogen in the form of nitrate into ground water, presenting a significant source of pollution. Low cost, passive modifications, which increase N removal in traditional leachfields, could substantially reduce the overall impact on ground water resources. Bench-scale laboratory models were constructed to evaluate the effect of placing an organic layer below the leachfield on total N removal. The organic layer provides a carbon source for denitrification. Column units representing septic leachfields were constructed with sawdust–native soil organic layers placed 0.45 m below the influent line and with thicknesses of 0.0, 0.3, 0.6, and 0.9 m. Using a synthetic septic tank effluent, NO₃–N concentrations at 3.8 m below the influent line were consistently below 1 mg L^–1 during 10 months of operation compared with a NO₃–N concentration of nearly 12 mg L^–1 in the control column. The average total N removal increased from 31% without the organic layer to 67% with the organic layer. Total N removal appeared limited by the extent of organic N oxidation and nitrification in the 0.45-m aerobic zone. Design modifications targeted at improving nitrification above the organic layer may further increase total N removal. Increased organic layer thicknesses from 0.3 m to 0.9 m did not significantly improve average total N removal, but caused a shift in residual nitrogen from organic N to ammonia N. Results indicate that addition of a layer of carbon source material at least 0.3 m thick below a standard leachfield substantially improves total N removal.

Abbreviations: BOD₅, five-day biochemical oxygen demand • MCL, maximum contaminant level • TKN, total Kjeldahl nitrogen

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JEQ 2005 34: ix. [Full Text]