Nitrogen removal in laboratory model leachfields with organic-rich layers.
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, NO(3)-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 NO(3)-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.
Bedessem
Marjorie E
ME
Department of Civil and Architectural Engineering, University of Wyoming, Laramie, WY 82071, USA. bedessem@uwyo.edu
Edgar
Thomas V
TV
Roll
Robert
R
D000641:Ammonia / Q000032:analysis / Q000737:chemistry; D002244:Carbon; D009584:Nitrogen / Q000737:chemistry / Q000302:isolation & purification*; D012722:Sewage; D012989:Soil Pollutants / Q000032:analysis*; D012995:Solubility; D014873:Water Pollutants / Q000302:isolation & purification*
D012722:Sewage; D012989:Soil Pollutants; D014873:Water Pollutants; D002244:Carbon; D000641:Ammonia; D009584:Nitrogen