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Biological denitrification is the most widely used method for nitrogen removal in water treatment. Compared with heterotrophic and autotrophic denitrification, mixotrophic denitrification is later studied and used. Because mixotrophic denitrification can overcome some shortcomings of heterotrophic and autotrophic denitrification, such as a high carbon source demand for heterotrophic denitrification and a long start-up time for autotrophic denitrification. It has attracted extensive attention of researchers and is increasingly used in biological nitrogen removal processes. However, so far, a comprehensive review is lacking. This paper aims to review the current research status of mixotrophic denitrification and provide guidance for future research in this field. It is shown that mixotrophic denitrification processes can be divided into three main kinds based on different kinds of electron donors, mainly including sulfur-, hydrogen-, and iron-based reducing substances. Among them, sulfur-based mixotrophic denitrification is the most widely studied. The most concerned influencing factors of mixotrophic denitrification processes are hydraulic retention times (HRT) and ratio of chemical oxygen demand (COD) to total inorganic nitrogen (C/N). The dominant functional bacteria of sulfur-based mixotrophic denitrification system are Thiobacillus, Azoarcus, Pseudomonas, and Thauera. At present, mixotrophic denitrification processes are mainly applied for nitrogen removal in drinking water, groundwater, and wastewater treatment. Finally, challenges and future research directions are discussed., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Thiosulfate is frequently used as an energy source and electron donor in autotrophic denitrification (AD) for removing nitrate from wastewater. However, transforming pathways of S 2 O 3 2- in this process is unclear. Herein, the aim of this study is to explore possible transforming pathways of sulfur compounds in thiosulfate-based AD process. After measuring the variation of NO 3 - , NO 2 - , and various sulfur compounds such as S 0 , SO 4 2- , S 2 O 3 2- , acid volatile sulfide (AVS), and S 2- in the presence and absence of S 2 O 3 2- , the variation process of S 2 O 3 2- and the contribution of various sulfur compounds were analyzed. The results indicated that S 0 , AVS, and S 2- were the intermediate products when S 2 O 3 2- was applied as an electron donor. All S 2 . Furthermore, sulfur transformation and balance in nitrate removal process was also investigated. This suggests the transforming pathways of sulfur compounds in denitrification process. Nitrogen removal and sulfur conversion process are dependent on the presence of microorganisms in the sludge.3 2- , S 0 , AVS, and S 2- could act as electron donors in the nitrate removal process with the final products of SO 4 2- . The utilization priority of these four sulfur sources was presumed in the following order: S 2-  > S 2 O 3 2-  > AVS ≈ S 0 . Furthermore, sulfur transformation and balance in nitrate removal process was also investigated. This suggests the transforming pathways of sulfur compounds in denitrification process. Nitrogen removal and sulfur conversion process are dependent on the presence of microorganisms in the sludge., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Ecological soil systems (ESSs) are usually used to remove nitrogen from wastewater. Due to the poor denitrification performance of traditional ecological soil systems (ESSs), this study proposes a two-stage water distribution system to improve the nitrogen removal. The effects of different distribution ratios on the system treatment effect were studied in an intermittent operation mode. After determining the optimal distribution ratio and intermittent operation conditions, the dynamics of system inflow, outflow, and nitrogen removal were monitored. Theoretical analysis of the denitrification mechanism was carried out. The results showed that the optimum water distribution ratio was 2: 1, and a mean total nitrogen removal rate of 60.42% was achieved, which is 23.09% greater than that is typically achieved by the single-section ecological system. Under optimum distribution ratio conditions, the system also demonstrated effective removal of chemical oxygen demand (COD), total phosphorus (TP) and ammonia nitrogen (NH 4 + -N), allowing the effluent to satisfy China's urban sewage treatment plant level B emission standards., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Autotrophic and mixotrophic denitrification, two approaches of biological denitrification, have drawn more and more attention among the techniques to remove nitrogen from the aquatic environment. This study investigated the influence of phosphorus on the denitrification performance and bacterial community structure in the autotrophic and mixotrophic denitrification reactors. The activity test was applied to evaluate the variation of denitrification activity of autotrophic and mixotrophic sludge before and after phosphorus addition. High-throughput sequencing was used to analyze the change of bacterial community structure. The results showed that NO 3 -N removal efficiency of autotrophic and mixotrophic denitrification process increased by 40 and 35%, respectively, after phosphorus addition. The sludge denitrification activity of autotrophic and mixotrophic sludge was enhanced significantly. And phosphorus addition could greatly improve the proportion of denitrifying bacteria in both autotrophic (from 11.83 to 64.31%) and mixotrophic denitrifying sludge (from 13.59 to 45.12%). Overall, phosphorus addition could greatly improve the autotrophic and mixotrophic denitrification ability in the phosphorus deficient surface water.- -N removal efficiency of autotrophic and mixotrophic denitrification process increased by 40 and 35%, respectively, after phosphorus addition. The sludge denitrification activity of autotrophic and mixotrophic sludge was enhanced significantly. And phosphorus addition could greatly improve the proportion of denitrifying bacteria in both autotrophic (from 11.83 to 64.31%) and mixotrophic denitrifying sludge (from 13.59 to 45.12%). Overall, phosphorus addition could greatly improve the autotrophic and mixotrophic denitrification ability in the phosphorus deficient surface water., (Copyright © 2018 Elsevier B.V. All rights reserved.)

[Display omitted] • Heavy metals significantly affect denitrification processes in water treatment. • Effects of heavy metals vary greatly with their types, valence, and concentrations. • Influence mechanisms of heavy metals on denitrification processes were discussed. • Regulatory strategies to reduce the effects of heavy metals were summarized. • The challenges and perspectives were proposed for further works. Nitrate is a global contaminant in water bodies due to its high-water solubility, excess nitrate poses a significant threat to aquatic ecosystems and human health. Biological denitrification is considered as an environmentally friendly technology for nitrate pollution treatment, but it is sensitive to environmental conditions. Heavy metals are a common and important environmental factor for affecting biological denitrification. Its effects vary greatly with different types, valence, and concentrations. This article provides the first comprehensive review on the current research status of the effects of heavy metals on denitrification processes in water treatment, with emphasis on different denitrification processes and the corresponding denitrifying bacteria species, as well as the responding strategies. Finally, the challenges and perspectives are discussed for future studies on effects of heavy metals on different denitrification processes. [ABSTRACT FROM AUTHOR]

The calamus type surface-flow constructed wetland (SFCW) is widely used for treating low-polluted water bodies, such as effluents from wastewater treatment plants. The calamus biomass is considered a potential carbon source for denitrification, integrating plant biomass into the carbon cycle in SFCWs. This study conducted experiments on the calamus biomass decomposition and evaluated its potential application for denitrification in an SFCW system. We showed that decomposition of the biomass could release a high ratio of total organic carbon (TOC) to total nitrogen (TN), reaching 26.9, 27.7, and 23.6 at biomass doses of 0.1, 0.5, and 1.0 g L−1. A continuous-flow experiment on the SFCW system revealed that higher TN removal efficiency and load were obtained after the addition of biomass, approximately three times before the biomass addition. Microbial community analysis revealed an increase in heterotrophic denitrifying bacteria (i.e., Flavobacterium , Dechloromonas , and Bacillus) content throughout the period of biomass addition. This study demonstrated an excellent enhancement of the denitrification efficiency in the SFCW caused by the addition of calamus biomass, suggesting its potential application as an excellent plant carbon source for biological denitrification. Image 1 • A high C/N ratio (26.9) was released from decomposing 0.1 g L−1 of calamus biomass. • Calamus biomass can be used as carbon source in denitrification processes. • Denitrification efficiency was enhanced by about 200% with the addition of biomass. • Content of denitrifying bacteria was increased by 8 times via adding the biomass. [ABSTRACT FROM AUTHOR]