Controlling carbon dioxide-to-hydrogen ratio to improve hydrogen utilization and denitrification rates of hydrogenotrophic autotrophic denitrification through homoacetogenesis-heterotrophic denitrification pathway

Enhanced and sustainable advanced nitrogen removal in mixotrophic systems using pyrite and solid carbon source

Utilizing widespread minerals/solid wastes as electron donors for denitrification is conducive to sustainable wastewater treatment. The current denitrification technologies based on single pyrite/solid carbon sources have problems of limited removal efficiency or unstable carbon release. In this study, two continuous biofilters, pyrite-corncob mixotrophic system (RPCM) and pyrite-polybutylene succinate mixotrophic system (RPPM), were conducted and operated steadily for a long period (>326 d). The mixotrophic systems achieved advanced removal of NO3--N (18 mg L-1) and a small amount of NH4+-N (2.5 mg L-1), with stabilized effluent TIN less than 2 mg L-1 at HRT of 4 h. Additionally, the systems demonstrated several distinct advantages, including no additional alkalinity requirement and a low risk of secondary contamination. RPCM could achieve advanced nitrogen removal at a higher nitrogen loading rate (93.6 mg L-1 d-1) but demanded periodic replenishment of corncob. In contrast, the organic matter release and nitrogen removal performance of RPPM exhibited stability throughout the operation. The increased abundance of functional microorganisms related to C, N, S, and Fe metabolism was essential for advanced nitrogen removal through synergistic effects. This study will provide implications for developing novel wastewater treatment processes emphasizing both nitrogen removal and waste valorization.

Dual-layer elemental sulfur-packed denitrification reactor to control sulfate generation and sulfide discharge by two-point inlet and internal recirculation

In this study, an elemental sulfur (S0) autotrophic denitrification reactor (SADR) and a wood chunk and S0 mixotrophic denitrification reactor (WSMDR) were constructed with dual-layers to effectively remove nitrate from water using two-inlets and internal recirculation. The denitrification rates were 66-114 and 70-104 g-N/(m3·d) for the SADR and WSMDR, respectively. Sulfate production was 5.5-5.9 and 3.2-4.5 mg SO42-/mg reduced N in the SADR and WSMDR, respectively, being lower than theoretical value. In addition, there was no sulfide emission from either reactor. Chlorobium, Chlorobaculum, Ignavibacterium, Sulfuritalea, and Thiobacillus were involved in nitrate reduction in both reactors. Chlorobiaceae had the highest abundance and played an essential role in maintaining the integrity of the co-occurrence pattern. The abundance of functional genes positively correlated with the denitrification performance. This study demonstrates that the operation of two-inlets and internal recirculation can effectively reduce byproduct generation, thereby promoting the practical application of the SADR and WSMDR.

Improved anaerobic digestion of food waste under ammonia stress by side-stream hydrogen domestication

High ammonia concentration inhibits archaea's activity, causing the accumulation of H2 and acetate, which suppresses methane production in anaerobic digestion (AD). The study aimed to enhance microbial hydrogen metabolism through a side-stream hydrogen domestication (SHD) strategy, which involves applying hydrogen stimulation to a portion of the sludge separately. SHD maintained a stable methane yield of 407.5 mL/g VS at a high total ammonia nitrogen (TAN) concentration of 3.1 g/L. In contrast, the control group gradually decreased and stopped methane production at a TAN concentration of 2.3 g/L. Further analysis using enzyme activity assays, flow cytometry, and metagenomics explored the mechanisms underlying ammonia tolerance of SHD-treated group. SHD reshaped the microbial community, enriching homoacetogens and Methanosaeta-dominated methanogenic archaea. Key metabolic pathways including homoacetogenesis, butyrate degradation, propionate degradation, and methane production were enhanced. The activity of related enzymes also increased. Gene abundance in energy-generating pathways, such as glycolysis, was enhanced, ensuring adequate ATP production. Additionally, the high gene abundance of ion transport systems contributed to regulating proton imbalance and supplementing intracellular K+. This study provides important insights and practical guidance for developing novel techniques in the field of anaerobic digestion.

Optimization of "sulfur-iron-nitrogen" cycle in constructed wetlands by adjusting siderite/sulfur (Fe/S) ratio

Sulfur-siderite autotrophic denitrification (SSAD) has been proved to solve the key problem of low nitrogen removal efficiency caused by the shortage of carbon source in constructed wetlands (CWs). In this study, five vertical flow constructed wetlands (VFCWs) were constructed with different Fe/S ratios (0/0, 0/1, 1/1, 2/1 and 1/2) to optimizing SSAD process, labeled S.0, S.1, S.2, S.3 and S.4. The results showed that the best NO3--N and TN removal rates were achieved with a Fe/S ratio of 2:1 (S.3), which were 96.26 ± 1.40% and 93.63 ± 3.12%, respectively. The abundance of denitrification genes (nirS, nirK and nosZ) in S.3 was significantly increased. Illumina high-throughput sequencing analysis indicated that the abundance and diversity of microorganisms involved in the "Sulfur-Iron-Nitrogen" cycle were enriched in S.3. The current study provided that the "Sulfur-Iron-Nitrogen" cycle in CWs was optimized by adjusting Fe/S ratio, and more types of denitrifying bacteria could be enriched, thereby enhancing nitrogen removal.

Influence of oxygen partial pressure on homoacetogenesis and promotion of acetic acid accumulation through low pH regulation under microaerobic conditions

Homoacetogenesis is an important pathway for bio-utilization of CO2; however, oxygen is a key environmental influencing factor. This study explored the impact of different initial oxygen partial pressures (OPPs) on homoacetogenesis, while implementing low pH regulation enhanced acetic acid (HAc) accumulation under microaerobic conditions. Results indicated that cumulative HAc production increased by 18.2% in 5% OPP group, whereas decreases of 31.3% and 56.0% were observed in 10% and 20% OPP groups, respectively, compared to the control group. However, hydrogenotrophic methanogens adapted to microaerobic environment and competed with homoacetogens for CO2, thus limiting homoacetogenesis. Controlling influent pH 5.0 per cycle increased cumulative HAc production by 18.3% and 18.2% in 5% and 10% OPP groups, respectively, compared with the control group. Consequently, regulating low pH effectively inhibited methanogenic activity under microaerobic conditions, thus increasing HAc production. This study was expected to expand the practical application of homoacetogenesis in bio-utilization of CO2.

Biological bromate reduction coupled with in situ gas fermentation in H2/CO2-based membrane biofilm reactor

Bromate, a carcinogenic contaminant generated in water disinfection, presents a pressing environmental concern. While biological bromate reduction is an effective remediation approach, its implementation often necessitates the addition of organics, incurring high operational costs. This study demonstrated the efficient biological bromate reduction using H2/CO2 mixture as the feedstock. A membrane biofilm reactor (MBfR) was used for the efficient delivery of gases. Long-term reactor operation showed a high-level bromate removal efficiency of above 95 %, yielding harmless bromide as the final product. Corresponding to the short hydraulic retention time of 0.25 d, a high bromate removal rate of 4 mg Br/L/d was achieved. During the long-term operation, in situ production of volatile fatty acids (VFAs) by gas fermentation was observed, which can be regulated by controlling the gas flow. Three sets of in situ batch tests and two groups of ex situ batch tests jointly unravelled the mechanisms underpinning the efficient bromate removal, showing that the microbial bromate reduction was primarily driven by the VFAs produced from in situ gas fermentation. Microbial community analysis showed an increased abundance of Bacteroidota group from 4.0 % to 18.5 %, which is capable of performing syngas fermentation, and the presence of heterotrophic denitrifiers (e.g., Thauera and Brachymonas), which are known to perform bromate reduction. Together these results for the first time demonstrated the feasibility of using H2/CO2 mixture for bromate removal coupled with in situ VFAs production. The findings can facilitate the development of cost-effective strategies for groundwater and drinking water remediation.