Nitrogen Removal in Laboratory Model Leachfields with Organic‐Rich Layers

Abstract

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.

Process, performance, and pollution potential: A review of septic tank - soil absorption systems

“…Further work on the role of the riparian zone in the reduction of nitrates and the design of systems that incorporate C enriched layers (e.g. Bedessem et al 2005 (Walker et al 1973a) Five field SAS studied. Organic N retained in biomat zone.…”

Nitrate Removal Rates in a 15‐Year‐Old Permeable Reactive Barrier Treating Septic System Nitrate

“…Previous studies have examined a variety of carbonaceous solids and immiscible liquids for their ability to stimulate denitrification in PRBs treating nitrate. These have included cracked corn, molasses, cotton burr, newspaper, vegetable oil, leaf mulch, wheat straw, sawdust, and so forth (Erickson et al 1974;Volokita et al 1996aVolokita et al , 1996bSoares and Abeliovich 1998;Hunter 2001;Kim et al 2003;Bedessem et al 2005;Greenan et al 2006). However, most of these studies were laboratory tests that were less than a year in duration.…”

Nitrate Removal of Drainage Water with Barley Straw as a Bioreactor Filter

“…Typical soil infiltration systems removed an average 10 to 40% total N (USEPA, 2002). Bedessem et al (2005) showed that the average total N removal was 31% in the soil columns. Jensen and Siegrist (1990) reviewed numerous laboratory and field studies and found that denitrification accounts for an average of 20% of the total N lost from waste water infiltrating through the soil.…”

“…This reduction in the TP2 treatment was 67.10% and the effluent nitrate concentration decreased to 7.11 mg l -1 . Bedessem et al (2005) showed that the average total N removal was 67% in the organic layer consist of sawdust. In this period, the average nitrate decline in soil columns was approximately 16% and the effluent concentration declined to 30.55 mg l -1 .…”