Mechanistic Determination of Nitrogen Removal By Advanced Soil-Based Wastewater Treatment Systems Using 15n Isotopes

Abstract:

Current levels of nitrogen removal by onsite wastewater treatment systems (OWTS) are inadequate, with release of N from OWTS contributing to environmental N pollution, especially in coastal zones where aquatic ecosystems are sensitive to eutrophication. Current mechanistic understand of N removal are limited and mainly attributed to denitrification in the drainfield. Loss of N from N₂O production during nitrification, a sparsely researched topic, may be a significant mechanism in advanced OWTS systems that enhance O₂ diffusion by sand filter pre-treatment, shallow placement of infiltrative areas and timed dosing controls to prevent drainfield saturation. Replicate (n=3) intact soil mesocosms were used with ¹⁵N isotope to evaluate the effectiveness and mechanisms of N removal in drainfields with a conventional wastewater delivery (pipe-and-stone, P&S) compared to two advanced types of drainfields, pressurized shallow narrow drainfield (SND) and Geomat (GEO), a variation of a SND drainfield. Over the 11 day experiment, dissolved O₂ was 1.6 mg/L for P&S and 3.0 mg/L for SND and GEO. Removal of total N was 13.5% for P&S, 4.8% for SND and 5.4% for GEO. ¹⁵NH₄ labeled nitrogen inputs to drainfields were transformed primarily to ¹⁵NO₃ in all outputs. Consistent low ¹⁵N₂O levels were present in P&S, with increasing levels of N₂ peaking 48h after ¹⁵NH₄ injection, suggesting denitrification dominated N removal. By contrast, SND and GEO ¹⁵N₂O levels rose quickly, peaking 8h after ¹⁵NH₄ injection, suggesting N loss by nitrification. When the whole system is considered, including sand filter removal, 26 – 27% of total N was removed by the SND and GEO systems, whereas 14% of total N was removed in the P&S system. Our results suggest the SND and GEO systems as a whole are capable of removing a greater mass of N than the P&S system.