Natural Resources Science
wastewater; nitrogen removal; denitrification; water-filled pore space
Onsite wastewater treatment systems (OWTS), or septic systems, release nitrogen (N), which can be detrimental to aquatic ecosystems. The final step in the treatment of wastewater is dispersal onto a drainfield, where it percolates through the soil. Part of the N is removed from wastewater and released into the atmosphere as N2 and N2O by denitrification, which requires anoxic conditions. Previous studies looking at the effect of soil water-filled pore space (WFPS) on denitrification using clean water with a high level of dissolved O2 (DO) have identified a minimum of 60% WFPS for denitrification to take place. We examined the effect of WFPS on N2O production in drainfield soil using wastewater with low or zero DO. We used replicated (n=3) microcosms with fine or coarse textured soil and adjusted the WFPS using deionized water (DI; DO=8 mg/L), sand-filter effluent (SFE; DO=3 mg/L), or septic tank effluent (STE; DO=0 mg/L). We found that N2O production using STE or DI did not change with WFPS for either of the two soil textures. For SFE we observed N2O production that increased with WFPS, and that N2O was produced even at the lowest WFPS. Additionally, we saw no significant difference in the minimum WFPS resulting in denitrification between the two soil textures. Our results suggest that a high concentration of nitrate in SFE – and its absence in DI and STE – appeared to control the effects of WFPS on N2O production in both soils. Future experiments will examine the effects of nitrate addition to DI and STE on the effects of WFPS on N2O production.