Constructed wetlands (CWs) widely applied for wastewater treatment release significant greenhouse gases (GHGs), contributing to global warming. It is essential to characterize the comprehensive source-sink effects and affecting factors of GHGs in CWs, offering references and guidance for designing and operating CWs to better control GHG emissions. However, current reviews focus on individual GHG emission mechanisms. With the aid of the Web of Science Core Collection database, the relevant literature on carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) emissions in CWs after 2010 was collected and organized. As highlighted in the review, CWs can produce and transmit these GHGs into the atmosphere, forming sources of GHGs and sequestration CO₂ through plants photosynthesis, forming sinks of GHGs. Their overall performance depends on many factors. Hybrid CWs, Cyperus papyrus, Cyperus alternifolius, and Iris pseudacorus, adsorption substrates like Fe-C, low temperatures, and a C/N ratio of five are beneficial for GHG mitigation in CWs. Future studies should focus on in-depth research into the mechanisms and overall source-sink benefits of plants and microorganisms in relation to GHGs. This review provided a comprehensive understanding of the emission mechanisms and affecting factors of the major GHGs in CWs, bridging the research gap in this field, helping researchers to clarify the context, and providing valuable in-sights for further scientific investigations.

Urban wastewater, as the main influent type of Waste Water Treatment Plants (WWTPs), has the characteristic of low carbon to nitrogen ratio (C/N). In the biological nitrogen removal (BNR) process, insufficient carbon source often affects the nitrogen removal efficiency and leads to more N₂O emissions. We review recent researches on N₂O emissions in the BNR process of wastewater with low C/N. The availability of carbon sources affects heterotrophic denitrification (HD) and autotrophic nitrification/denitrification processes, which are the main reasons for N₂O emissions in BNR. For the sustainable development of BNR in WWTPs, we introduce strategies suitable for reducing N₂O emissions in the BNR process of low C/N wastewater from two aspects: traditional process innovation and new process development. These strategies mainly include carbon source addition, adjustment of aeration strategy, optimization of oxidation ditch and biofilm facilities, and application of Anammox related processes. In the future, it is still necessary to further deepen this research direction through the normalization of N₂O emission quantification standards, exploration of N₂O metabolism mechanisms, assessment of environmental effects of emission reduction strategies, and practical application of new processes.

Microbial flocculants (MBFs), a class of eco-friendly and biodegradable biopolymers produced by various microorganisms, have gained increasing attention as promising alternatives to conventional chemical flocculants in wastewater treatment and pollutant removal. This review presents a comprehensive overview of the current state of MBF research, encompassing their diverse sources (bacteria, fungi, and algae), major categories (polysaccharides, proteins, and glycoproteins), production processes, and flocculation performance and mechanisms. The wide-ranging applications of MBFs in removing suspended solids, heavy metals, dyes, and other pollutants from industrial and municipal wastewater are critically examined, highlighting their superior efficiency, selectivity, and environmental compatibility compared to traditional flocculants. Nonetheless, bioflocculants face significant challenges including high substrate costs, low production yields, and intricate purification methodologies, factors that impede their industrial scalability. Moreover, the risk of microbial contamination and the attendant health implications associated with the use of microbial flocculants (MBFs) necessitate thorough evaluation. To address the challenges of high production costs and variable product quality, strategies such as waste valorization, strain improvement, process optimization, and biosafety evaluation are discussed. Moreover, the development of multifunctional MBF-based flocculants and their synergistic use with other treatment technologies are identified as emerging trends for enhanced wastewater treatment and resource recovery. Future research directions are outlined, emphasizing the need for in-depth mechanistic studies, advanced characterization techniques, pilot-scale demonstrations to accelerate the industrial adoption of MBF, and moreover, integration with novel wastewater treatment processes, such as partial nitrification and the anammox process. This review is intended to inspire and guide further research and development efforts aimed at unlocking the full potential of MBFs as sustainable, high-performance, and cost-effective bioflocculants for addressing the escalating challenges in wastewater management and environmental conservation.

Constructed wetland (CW) is a critical ecological engineering for wastewater treatment and improvement of water quality. Nitrogen (N) removal is one of the vital functions of CWs during operation, and N treatment in CWs is mainly affected by aquatic plants and denitrification carried out by microbes. However, due to their low efficiency and instability in N removal, further applications of CWs are limited. The review provides a view of two basic characteristics of aquatic plants, radial oxygen loss (ROL) and root exudates, and their coupled effect on denitrification processes in CWs. First, the role of aquatic plants in denitrification is presented. The individual roles of ROL and root exudates in regulating denitrification, as well as their interaction in this process, have been discussed. Also, the limitation of conventional techniques to reveal interaction between the plant and the microbes has been highlighted. Further research on coupling regulatory mechanisms of ROL and root exudates may be conducted to develop an optimal wetland design and improve biological N removal. This review offers new insights and directions for improving N removal in CWs by utilizing the synergistic effects of plant ROL and root exudates.