N2O emissions in the subsurface wastewater infiltration systems: Influence of hydraulic loading on emission pathways

Subsurface wastewater infiltration system (SWIS) is an effective wastewater treatment technology, but it also releases nitrous oxide (N2O). N2O is an intermediate or by-product of nitrification and denitrification processes, its production and consumption are affected by freeze-thaw (FT). There is a dearth of comprehensive research on mechanisms and methods of N2O emission in SWIS under FT. Specifically, micro zone distribution (aerobic, facultative and anaerobic) in SWIS is crucial for N2O production and consumption. This study varied hydraulic loading rate (HLR) (5–15 cm/d) and simulated FT (−15 °C to 15 °C) to systematically analyze SWIS. Results showed that higher HLR and alternating FT enhanced denitrification, markedly increased total nitrogen removal in SWIS of 81.08 % (HLR = 15 cm/d), while N2O emission rate significantly decreased to a minimum of 0.027 mg/m2·h (HLR = 15 cm/d). However, bed micro zone structure contribution to N2O emission was not significantly correlated with HLR and FT, facultative and anaerobic were always major contributing zones, accounting for 79.6–90.68 % total release. This was related to robust nitric oxide reductase (NOR) activity in facultative and anaerobic layers, with 15.6 % higher NOR of 2.296 U/g in anaerobic than aerobic. High-throughput sequencing results showed that N2O release in SWIS was negatively correlated with Gemmatimonas. The abundance of Gemmatimonas increased 8.99-fold during the experiment, and Gemmatimonas is an important biomarker for N2O release in SWIS.