Mitigating NO emissions in land treatment systems: Mechanisms, influences, and future directions.

Land treatment systems (LTS) are widely used in decentralized domestic wastewater treatment due to low energy requirements and effective treatment outcomes. However, LTS operations are also a significant source of NO emissions, a potent greenhouse gas threatening the ozone layer and posing risks to human health. Despite the importance of understanding and controlling NO emissions, existing literature lacks comprehensive analyses of the mechanisms driving NO generation and effective control strategies within LTS. This study addresses this gap by reviewing current research and identifying key factors influencing NO emissions in LTS. This review reveals that in addition to traditional nitrification and denitrification processes, co-denitrification and complete ammonia oxidation are crucial for microbial nitrogen removal in LTS. Plant selection is primarily based on their nitrogen absorption capacity while using materials such as biochar and iron can provide carbon sources or electrons to support microbial activities. Optimizing operational parameters is essential for reducing NO emissions and enhancing nitrogen removal efficiency in LTS. Specifically, the carbon-to‑nitrogen ratio should be maintained between 5 and 12, and the hydraulic loading rate should be kept within 0.08-0.2 m/(m·d). Dissolved oxygen and oxidation-reduction potential should be adjusted to meet the aerobic or anaerobic conditions the microorganisms require. Additionally, maintaining a pH range of 6.5-7.5 by adding alkaline substances is crucial for sustaining nitrous oxide reductase activity. The operating temperature should be maintained between 20 and 30 °C to support optimal microbial activity. This review further explores the relationship between environmental factors and microbial enzyme activity, community structure changes, and functional gene expression related to NO production. Future research directions are proposed to refine NO flux control strategies. By consolidating current knowledge and identifying research gaps, this review advances LTS management strategies that improve wastewater treatment efficiency while mitigating the environmental and health impacts of NO emissions.