Nitrous oxide (N 2 O) emissions account for the majority of the carbon footprint of wastewater treatment plants (WWTPs). Many N 2 O mitigation strategies have since been developed while a holistic view is still missing. This article reviews the state-of-the-art of N 2 O mitigation studies in wastewater treatment. Through analyzing existing studies, this article presents the essential knowledge to guide N 2 O mitigations, and the logics behind mitigation strategies. In practice, mitigations are mainly carried out by aeration control, feed scheme optimization, and process optimization. Despite increasingly more studies, real implementation remains rare, which is a combined result of unclear climate change policies/incentives, as well as technical challenges. Five critical technical challenges, as well as opportunities, of N 2 O mitigations were identified. It is proposed that (i) quantification methods for overall N 2 O emissions and pathway contributions need improvement; (ii) a reliable while straightforward mathematical model is required to quantify benefits and compare mitigation strategies; (iii) tailored risk assessment needs to be conducted for WWTPs, in which more longterm full-scale trials of N 2 O mitigation are urgently needed to enable robust assessments of the resulting operational costs and impact on nutrient removal performance; (iv) current mitigation strategies focus on centralized WWTPs, more investigations are warranted for decentralised systems, especially decentralized activated sludge WWTPs; and (v) N 2 O may be mitigated by adopting novel strategies promoting N 2 O reduction denitrification or microorganisms that emit less N 2 O. Overall, we conclude N 2 O mitigation research is reaching a maturity while challenges still exist for a wider implementation, especially in relation to the reliability of N 2 O mitigation strategies and potential risks to nutrient removal performances of WWTPs.

Nitrous oxide (N 2 O) has been studied intensively in wastewater treatment as a detrimental greenhouse gas. However, increasingly more studies have adopted a contrasting objective, recovering N 2 O from wastewater as an energy resource. This article critically reviewed and analyzed the current status of N 2 O recovery research in wastewater treatment, to identify knowledge gaps and guide future research. Overall, N 2 O recovery is a promising research direction while still in active development. At present, unstable nitritation, the low energy potential, and potential environmental risks of N 2 O harvesting render the recovery of N 2 O from mainstream wastewater technically and economically challenging. High-strength wastewater treatment is more favorable for N 2 O recovery due to the high energy potential, established nitritation approaches, and significant carbon/aeration savings. The coupled aerobic−anoxic nitrous decomposition operation (CANDO) process is currently the most investigated and promising N 2 O recovery process. Nevertheless, more research is needed for its implementation on a large scale. Research opportunities for the CANDO process have been identified in this paper. Meanwhile, N 2 O recovery via autotrophic denitritation is a more recent concept, with limited studies hitherto. More experiments are needed to investigate its technological feasibility. Furthermore, other novel N 2 O recovery processes, e.g., truncated denitrification and chemical oxidation, should also be explored to facilitate the recovery of N 2 O from wastewater.

Achieving stable long-term mainstream nitrite oxidizing bacteria (NOB) suppression is the bottleneck for the novel partial nitrification (PN) process toward energy- and carbon-efficient wastewater treatment. However, long-term PN stability remains a challenge due to NOB adaptation. This study proposed and demonstrated a novel strategy for achieving NOB suppression by the primary treatment of mainstream wastewater with a forward osmosis (FO) membrane process, which facilitated two external NOB inhibition factors (salinity and free nitrous acid, FNA). To evaluate the proposed strategy, a lab-scale sequencing batch reactor was operated for 200 days. A stable PN operation was achieved with a nitrite accumulation ratio of 97.7 ± 2.8%. NOB were suppressed under the combined inhibition effect of NaCl (7.9 ± 0.2 g/L, as introduced by the FO direct filtration) and FNA (0.11 ± 0.02 mg of HNO2–N/L, formed as a result of the increased NH4 +–N concentration after the FO process). The two inhibition factors worked in synergy to achieve a more stable PN operation. The microbial analysis showed that the elevated salinity and accumulation of FNA reshaped the microbial community and selectively eliminated NOB. Finally, an economic and feasibility analysis was conducted, which suggests that the integration of an FO unit into PN/A is a feasible and economically viable wastewater treatment process.

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