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Journal of Environmental Quality Abstract - Waste Management

Nitrogen Removal in Laboratory Model Leachfields with Organic-Rich Layers


This article in JEQ

  1. Vol. 34 No. 3, p. 936-942
    Received: Jan 20, 2004

    * Corresponding author(s): bedessem@uwyo.edu
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  1. Marjorie E. Bedessem *,
  2. Thomas V. Edgar and
  3. Robert Roll
  1. Department of Civil and Architectural Engineering, 1000 East University Avenue, Department 3295, University of Wyoming, Laramie, WY 82071


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.

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