[Nitrogen Removal Characteristics and Metabolism Mechanism of High-Efficiency Cold-Tolerant Heterotrophic Nitrification-Aerobic Denitrification Bacterium Glutamicibacter sp. WS1 for Various Nitrogen Sources at Low Temperature]

For resolving the problems of poor nitrogen removal efficiency and substandard effluent quality in wastewater treatment plants during winter, a cold-tolerant strain Glutamicibacter sp. WS1, with heterotrophic nitrification-aerobic denitrification ability, was isolated from activated sludge. The functional genes for nitrogen conversion of strain WS1 were amplified by PCR, and the nitrogen removal characteristics of the strain were verified under different nitrogen sources at 15℃. In addition, the effects of environmental factors on the aerobic denitrification performance of the strain were explored at low temperature. Finally, a reasonable nitrogen metabolism pathway of strain WS1 was resolved based on functional genes and nitrogen balance analysis. The results showed that strain WS1 contained functional genes related to nitrogen conversion, including amoA, napA, nirS, and nirK genes. Notably, nirS and nirK genes coexisted in the strain. At the low temperature of 15℃, with NH4+-N, NO3--N, NO2--N+NO3--N, and NH4+-N+NO3--N as nitrogen sources, the corresponding removal efficiencies of strain WS1 were 100%, 98.10%, 99.87%+100%, and 100%+94.92%, respectively. The optimal denitrification performance of the strain was achieved with sodium citrate as the carbon source, C/N of 16, pH of 8, DO of 4.5-6.8 mg·L-1, and temperature of 30℃. In addition, the NO3--N removal efficiency of strain WS1 reached 92.50% under low temperature (15℃) and low C/N (10) conditions. Based on the results of PCR amplification and nitrogen balance analysis, heterotrophic nitrification-aerobic denitrification/aerobic denitrification and assimilation were the main pathways for nitrogen substrate removal by strain WS1, in which most of the inorganic nitrogen (47%-56%) was converted to gaseous nitrogen through heterotrophic nitrification-aerobic denitrification/aerobic denitrification. Strain WS1 has broad application prospects in the treatment of low-temperature nitrogenous wastewater.

[Mechanism of trehalose-enhanced metabolism of heterotrophic nitrification-aerobic denitrification community under high-salt stress]

Heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria are aerobic microorganisms that can remove nitrogen under high-salt conditions, but their performance in practical applications are not satisfactory. As a compatible solute, trehalose helps microorganisms to cope with high salt stress by participating in the regulation of cellular osmotic pressure, and plays an important role in promoting the nitrogen removal efficiency of microbial populations in the high-salt environment. We investigated the mechanism of exogenous-trehalose-enhanced metabolism of HN-AD community under high-salt stress by starting up a membrane aerobic biofilm reactor (MABR) to enrich HN-AD bacteria, and designed a C₁₅₀ experimental group with 150 μmol/L trehalose addition and a C₀ control group without trehalose. The reactor performance and the community structure showed that NH₄⁺-N, total nitrogen (TN) and chemical oxygen demand (COD) removal efficiency were increased by 29.7%, 28.0% and 29.1%, respectively. The total relative abundance of salt-tolerant HN-AD bacteria (with Acinetobacter and Pseudofulvimonas as the dominant genus) in the C₁₅₀ group reached 66.8%, an 18.2% increase compared with that of the C₀ group. This demonstrated that trehalose addition promoted the enrichment of salt-tolerant HN-AD bacteria in the high-salt environment to enhance the nitrogen removal performance of the system. In-depth metabolomics analysis showed that the exogenous trehalose was utilized by microorganisms to improve proline synthesis to increase resistance to high-salt stress. By regulating the activity of cell proliferation signaling pathways (cGMP-PKG, PI3K-Akt), phospholipid metabolism pathway and aminoacyl-tRNA synthesis pathway, the abundances of phosphoethanolamine, which was one of the glycerophospholipid metabolites, and purine and pyrimidine were up-regulated to stimulate bacterial aggregation and cell proliferation to promote the growth of HN-AD bacteria in the high-salt environment. Meanwhile, the addition of trehalose accelerated the tricarboxylic acid (TCA) cycle, which might provide more electron donors and energy to the carbon and nitrogen metabolisms of HN-AD bacteria and promote the nitrogen removal performance of the system. These results may facilitate using HN-AD bacteria in the treatment of high-salt and high-nitrogen wastewater.

[Advances in denitrification microorganisms and processes]

Denitrification is an indispensable part of most sewage treatment systems. The biological denitrification process has attracted much attention in the past decades due to the advantages such as cost-effectiveness, process simplicity, and absence of secondary pollution. This review summarized the advances on biological denitrification processes in recent years according to the different physiological characteristics and denitrification mechanisms of denitrification microorganisms. The pros and cons of different biological denitrification processes developed based on nitrifying bacteria, denitrifying bacteria, and anaerobic ammonia-oxidizing bacteria were compared with the aim to identify the best strategy for denitrification in a complex wastewater environment. The rapid development of synthetic biology provides possibilities to develop highly-efficient denitrifying strains based on mechanistic understandings. Combined with the applications of automatic simulation to obtain the optimal denitrification conditions, cost-effective and highly-efficient denitrification processed can be envisioned in the foreseeable future.

Role of heterotrophic nitrifiers and aerobic denitrifiers in simultaneous nitrification and denitrification process: A non-conventional nitrogen removal pathway in wastewater treatment

Bacterial species capable of performing both nitrification and denitrification in a single vessel under similar conditions have gained significance in the wastewater treatment scenario considering their unique character of performing the above reactions under heterotrophic and aerobic conditions respectively. Such a novel strategy often referred to as simultaneous nitrification and denitrification (SND) has a tremendous potential in dealing with various wastewaters having low C:N content, considering that the process needs very little or no external carbon source and oxygen supply thus adding to its cost-effective and environmentally friendly nature. Though like other microorganisms, heterotrophic nitrifiers and aerobic denitrifiers convert inorganic or organic nitrogen-containing substances into harmless dinitrogen gas in the wastewater, their ecophysiological role in the global nitrogen cycle is still not yet fully understood. Attempts to highlight the role played by the heterotrophic nitrifiers and aerobic denitrifiers in dealing with nitrogen pollution under various environmental operating conditions will help in developing a mechanistic understanding of the SND process to address the issues faced by the traditional methods of aerobic autotrophic nitrification-anaerobic heterotrophic denitrification.

[Removal of Urea by Heterotrophic Nitrification-Aerobic Denitrification Mixed Strains and Effects of Heavy Metals and Salinity]

To explore the effect of heterotrophic nitrification-aerobic denitrification mixed strains on urea removal, the removal characteristics of urea and effects of heavy metals and salinity on urea removal by mixed strains (DM01+YH01+YH02) were investigated. The results showed that urea could be efficiently degraded by mixed strains at 24 h in 200.0 mg·L^-1 urea wastewater when the carbon source was sodium citrate, C/N was 10, temperature was 30℃, pH was 7, and rotation speed was 130 r·min^-1. Heavy metal ions (Ni²⁺, Cd²⁺, Cu²⁺, and Zn²⁺) can reduce the removal efficiency of urea by mixed strains, and the degree of influence was Cd²⁺ > Cu²⁺ > Ni²⁺ > Zn²⁺. Fe²⁺ (<20.0 mg·L^-1) in urea wastewater can enhance the removal efficiency of urea by mixed strains. Furthermore, a salinity of more than 10.0 mg·L^-1 in urea wastewater can reduce the removal efficiency of urea by mixed strains.

Role of heterotrophic aerobic denitrifying bacteria in nitrate removal from wastewater

With the increase in industrial and agricultural activities, a large amount of nitrogenous compounds are released into the environment, leading to nitrate pollution. The perilous effects of nitrate present in the environment pose a major threat to human and animal health. Bioremediation provides a cost-effective and environmental friendly method to deal with this problem. The process of aerobic denitrification can reduce nitrate compounds to harmless dinitrogen gas. This review provides a brief view of the exhaustive role played by aerobic denitrifiers for tackling nitrate pollution under different ecological niches and their dependency on various environmental parameters. It also provides an understanding of the enzymes involved in aerobic denitrification. The role of aerobic denitrification to solve the issues faced by the conventional method (aerobic nitrification-anaerobic denitrification) in treating nitrogen-polluted wastewaters is elaborated.

[Advances in heterotrophic nitrification-aerobic denitrifying bacteria for nitrogen removal under extreme conditions]

Heterotrophic nitrification-aerobic denitrification (HN-AD) is an enrichment and breakthrough theory of traditional autotrophic nitrification heterotrophic denitrification. Heterotrophic nitrification-aerobic denitrifiers with the feature of wide distribution, strong adaptability and unique metabolic mechanism have many special advantages, including fast-growing, rapid biodegradability and long lasting activity, which can rapidly remove ammonia nitrogen, nitrate nitrogen (NO₃⁻-N) and nitrite nitrogen (NO₂⁻-N) under aerobic conditions simultaneously. Therefore, HN-AD bacteria show the important potential for denitrification under extreme conditions with high-salt, low-temperature or high-ammonia nitrogen environment, and HN-AD bacteria attract extensive attention in the field of biological denitrification of wastewater. In this review, we first introduce the previously reported HN-AD bacterial species which have denitrification performance in the extreme environments and state their typical metabolic mechanism. Then, we systematically analyze the nitrogen removal characteristics and potential under extreme conditions. We also briefly describe the progress in the application of HN-AD bacterial. Finally, we outlook the application prospects and research directions of HN-AD denitrification technology.

[Aerobic Denitrification and Microbial Community Shift in SBR Bioaugmented with Strains YH01 and YH02]

The enhanced aerobic denitrification capability of the mixed strains YH01+YH02 in utilizing potassium nitrate, and the dynamic changes in the microbial community component during the period of operation, were evaluated. The microbial community in different stages of the SBR was analyzed by using high-throughput sequencing technology after inoculation with YH01+YH02. The results showed that the NO₃^--N, TN, and COD removal efficiencies increased by 12.1%, 9.2%, and 9.4%, respectively. The relative abundances of the microbes in the microbial community increased at the genus level, and the diversity in the microbial community decreased after enhancement. Principal component analysis and UPGMA analysis revealed that the period of SBR operation was roughly divined into four phases. The relative abundances of Delftia and Acidovorax increased during the period of operation, and YH01+YH02 exhibited excellent compatibility with the SBR ecosystem and played an important part in aerobic denitrification.