Field study on N 2 O emission from subsurface wastewater infiltration system under variable loading rates and drying-wetting cycles.

In this field study, the impacts of influent loadings and drying-wetting cycles on N2 O emission in a subsurface wastewater infiltration (SWI) system were investigated. N2 O emitted under different operation conditions were quantified using static chamber and gas chromatograph techniques. N2 O conversion rate decreased from 6.6 ± 0.1% to 2.7 ± 0.1% with an increase in hydraulic loading (HL) from 0.08 to 0.24 m3 /m2 ·d. By contrast, N2 O conversion rate increased with increasing pollutant loading (PL) up to 8.2 ± 0.5% (PL 4.2 g N/m2 ·d) above which conversion rate decreased, confirming that N2 O production was under the interaction of nitrification and denitrification. Taking into consideration the pollutants (chemical oxygen demand (COD), NH4 + -N, NO3 - -N and total nitrogen (TN)) removal ratio and N2 O emission, optimal loading ranges and drying-wetting modes were suggested as HL 0.08-0.12 m3 /m2 ·d, PL 3.2-3.7 g N/m2 ·d and 12 h:12 h, respectively. The results revealed that in SWI systems, conversion ratio of influent nitrogen to N2 O could be between 4.5% and a maximum of 7.0%.