Does the repeated freezing and thawing of the aerobic layer affect the anaerobic release of N2O from SWIS?

Abstract

Subsurface wastewater infiltration systems (SWIS) is an efficient, economical, and less temperature affected sewage treatment technology. Pollutants are removed by physical, chemical, and biological reactions such as filtration, adsorption, oxidation, and degradation. Under the conditions of limited carbon source and inactivation of nitrous oxide reductase, N2O, an important greenhouse gas, is released from the anaerobic layers of SWIS. However, is N2O release affected by repeated freeze-thawing of the upper aerobic layer? How does microbial population structure and denitrogenate activity in different profiles respond to the freeze-thaw cycle (FTC)? These questions have not yet been revealed. In this study, a SWIS simulator with in-situ regulation of FTC was first constructed. The re-distribution of N2O, microbial composition and denitrogenate activity were analyzed in response to FTC. Furthermore, potential bio-markers were screened and identified. The results revealed that the release of N2O was correlated with FTC, with the anaerobic layer being the main contributor throughout, accounting for 73.32–75.8 % of the total release. The limiting factor for N2O emissions was the NO3−-N concentration in the anaerobic zone, and there were no simple linear communications between total nitrogen and N2O generations. High throughput sequencing results showed the main markers of SWIS were Proteobacteria, Gemmatimonadetes, Firmicutes, Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi and Nitrospirae, accounting for 97.4 %–98.1 % of the total relative abundance. A significant positive correlation between Firmicutes and anaerobic release of N2O was observed, where Firmicutes abundance increased from 5 % to 21 % during the experimental cycle, while N2O concentration increased from 2.65 mg·L−1 to 18.88 mg·L−1. The results indicated that Firmicutes was an important biomarker of N2O release under freeze-thaw conditions.