Perspective on inorganic electron donor-mediated biological denitrification process for low C/N wastewaters

Enterobacter sp. HIT-SHJ4 isolated from wetland with carbon, nitrogen and sulfur co-metabolism and its implication for bioremediation

Both autotrophic and heterotrophic denitrification are known as important bioprocesses of microbe-mediated nitrogen cycle in natural ecosystems. Actually, mixotrophic denitrification co-driven by organic matter and reduced sulfur substances are also common, especially in hypoxic environments such as estuarine sediments. However, carbon, nitrogen and sulfur co-metabolism during mixotrophic denitrification in natural water ecosystems has rarely been reported in detail. Therefore, this study investigated the co-metabolism of carbon, nitrogen and sulfur using samples collected from four distinct natural water ecosystems. Results demonstrated that samples from various sources all exhibited the ability for co-metabolism of carbon, nitrogen and sulfur. Microbial community analysis showed that Pseudomonas and Paracoccus were dominant bacteria ranging from 65.6% to 75.5% in mixotrophic environment. Enterobacter sp. HIT-SHJ4, a mixotrophic denitrifying strain which owned the capacity for co-metabolism of carbon, nitrogen and sulfur, was isolated and reported here for the first time. The strain preferred methanol as its carbon source and demonstrated remarkable efficiency for removing sulfide and nitrate with below 100 mg/L sulfide. Under weak acid conditions (pH 6.5-7.0), it exhibited enhanced capability in converting sulfide to elemental sulfur. Its bioactivity was evident within a temperature from 25 °C to 40 °C and C/N ratios from 0.75 to 3. This study confirmed the widespread presence of microbial-mediated synergistic carbon, nitrogen and sulfur metabolism in natural aquatic ecosystems. HIT-SHJ4 emerges as a novel strain, shedding light on carbon, nitrogen and sulfur co-metabolism in natural water bodies. Furthermore, it also serves as a promising candidate microorganism for in-situ ecological remediation, particularly in dealing with contamination posed by nitrate, sulfide, and organic matter.

Intra/extracellular electron transfer and metagenomic analysis elucidated the roles of magnetic iron powder (Fe3O4) on mixotrophic denitrification system

Elemental iron provides a viable strategy to improve the denitrification efficiency by expediting electron transport. However, the roles of magnetic iron powder (Fe3O4) on mixotrophic denitrification remains unknown. In this study, the intra/extracellular electron transfer (IET/EET) and microbial metabolism mechanisms were explored in a Fe3O4-mediated sulfide-autotrophic and heterotrophic denitrification system. The results showed that Fe3O4 promoted the formation of dense clump structure with filamentous cross-linking in activated sludge. Fe3O4 could increase the coenzyme Q activity in IET and the content of free riboflavin and cytochrome c in EET. Metagenomic analysis indicated that denitrification, sulfide oxidation and sulfate reduction were the main pathways of nitrogen and sulfur metabolism, and the enriched denitrifying bacteria (Halomonas and Hypobacterium) and sulfur-oxidizing bacteria (Marinicella) could stably support nitrate removal. This study expands our understanding of the IET/EET during Fe3O4-mediated mixotrophic denitrification process, providing a novel insight for nitrogen removal from marine recirculating aquaculture wastewater.

Unveiling synergistic enhancement mechanism of nitrogen removal in surface flow constructed wetlands: Utilizing iron scraps and elemental sulfur as integrated electron donors

Lacking electron donors generally causes poor denitrification performance in constructed wetlands (CWs). In this study, iron scraps (ISs) and elemental sulfur (S0) were employed as electron donors in different surface flow constructed wetlands (SFCWs): control (C-SF), ISs added (Fe-SF), S0 added (S-SF), and ISs and S0 combined (Fe + S-SF) to investigate the performance and mechanism of nitrogen (N) removal through continuous flow and batch experiments. The impact of hydraulic retention times (HRTs) and temperatures on N removal was explored. The combined use of ISs and S0 significantly improved nitrate (NO3- -N) removal in Fe + S-SF compared to the other SFCWs. During the 3-d HRT at 25 °C, the average NO3- -N removal efficiency in Fe + S-SF reached the highest value of 71.66 ± 12.54%, reducing NO3- -N concentrations from 12.03 mg/L to 3.47 mg/L. The results of the batch experiments revealed an N removal pattern that aligned with the findings of the continuous flow experiment. The microbial community analysis revealed a selective enrichment of key functional genera (e.g., Ferritrophicum and Dechloromonas), contributing to enhanced N removal in Fe + S-SF. These findings suggest that the synergistic use of ISs and S0 can achieve better denitrification efficiency and potentially be utilized for enhanced N removal from low C/N wastewater.

Effect of sand minerals on microbially induced carbonate precipitation by denitrification

Soil improvement techniques utilizing the metabolic functions of microorganisms, including microbially induced carbonate precipitation (MICP), have been extensively researched over the past few decades as part of bio-inspired geotechnical engineering research. Given that metabolic reactions in microorganisms produce carbonate minerals, an enhanced understanding of microbial interaction with soils could improve the effectiveness of MICP as a soil improvement technique. Therefore, this study investigated the effects of sands on MICP by denitrification to employ MICP for geotechnical soil improvement. Under the coexistence of natural sand and artificial silica sand, nitrate-reducing bacteria were cultured in a mixed liquid medium with nitrate, acetate, and calcium ions at 37 °C. Nitrate reduction occurred only in the presence of natural sand. However, the lack of chemical weathering of the composed minerals likely prevented the progress of bacterial growth and nitrate reduction in artificial silica sands. For natural sand, artificial chemical weathering by acid wash and ferrihydrite coating of the sand improved bacterial growth and accelerated nitrate reduction. The calcium carbonate formation induced by denitrification was also influenced by the state of the minerals in the soil and the nitrate reduction rate. The observed MICP enhancement is due to the involvement of coexisting secondary minerals like ferrihydrite with large specific surface areas and surface charges, which may improve the reaction efficiency by serving as adsorbents for bacteria and electron donors and acceptors in the solid phases, thereby promoting the precipitation and crystallization of calcium carbonate on the surfaces. This crystal formation in the minerals provides valuable insights for improving sand solidification via MICP. Considering the interactions between the target soil and microorganisms is essential to improving MICP processes for ground improvement.

Microbial communities and denitrification mechanisms of pyrite autotrophic denitrification coupled with three-dimensional biofilm electrode reactor

Denitrification is of great significance for low C/N wastewater treatment. In this study, pyrite autotrophic denitrification (PAD) was coupled with a three-dimensional biofilm electrode reactor (BER) to enhance denitrification. The effect of current on denitrification was extensively studied. The nitrate removal of the PAD-BER increased by 14.90% and 74.64% compared to the BER and the PAD, respectively. In addition, the electron utilization, extracellular polymeric substances secretion, and denitrification enzyme activity (NaR and NiR) were enhanced in the PAD-BER. The microbial communities study displayed that Dokdonella, Hydrogenophaga, Nitrospira, and Terrimonas became the main genera for denitrification. Compared with the PAD and the BER, the abundance of the key denitrification genes narG, nirK, nirS, and nosZ were all boosted in the PAD-BER. This study indicated that the enhanced autotrophic denitrifiers and denitrification genes were responsible for the improved denitrification in the PAD-BER. PRACTITIONER POINTS: PAD-BER displayed higher nitrate removal, EPS, NAR, and NIR activity. The three types of denitrification (HD, HAD, and PAD) and their contribution percentage in the PAD-BER were analyzed. HAD was dominant among the three denitrification processes in PAD-BER. Microbial community composition and key denitrification genes were tested to reveal the denitrification mechanisms.

Coordination of elemental sulfur and organic carbon source stimulates simultaneous nitrification and denitrification toward low C/N ratio wastewater

The feasibility of inducing simultaneous nitrification and denitrification (SND) by S0 for low carbon to nitrogen (C/N) ratio wastewater remediation was investigated. Compared with S0 and/or organics absent systems (-3.4 %∼5.0 %), the higher nitrogen removal performance (18.2 %∼59.8 %) was achieved with C/N ratios and S0 dosages increasing when S0 and organics added simultaneously. The synergistic effect of S0 and organics stimulated extracellular polymeric substances secretion and weakened intermolecular binding force of S0, facilitating S0 bio-utilization and reducing the external organics requirement. It also promoted microbial metabolism (0.16 ∼ 0.24 μg O2/(g VSS·h)) and ammonia assimilation (5.9 %∼20.5 %), thereby enhancing the capture of organics and providing more electron donors for SND. Furthermore, aerobic denitrifiers (15.91 %∼27.45 %) and aerobic denitrifying (napA and nirS) and ammonia assimilating genes were accumulated by this synergistic effect. This study revealed the mechanism of SND induced by coordination of S0 and organics and provided an innovative strategy for triggering efficient and stable SND.

Reason and control strategy for denitrification and anammox sludge flotation in nitrogen removal process: Mechanisms, strategies and perspectives

Anaerobic biological treatment technology, especially denitrification and anaerobic ammonia oxidation (anammox) technology as mainstream process, played dominant role in the field of biological wastewater treatment. However, the above process was prone to sludge floating during high load operation and thereby affecting the efficient and stable operation of the system. Excessive production of extracellular polymeric substance (EPS) was considered to be the main reason for anaerobic granular sludge flotation, but the summaries in this area were not comprehensive enough. In this review, the potential mechanisms of denitrification and anammox sludge floatation were discussed from the perspective of granular sludge structural characteristics, nutrient transfer, and microbial flora change respectively, and the corresponding control strategies were also summarized. Finally, this paper indicated that future research on sludge flotation should focus on reducing the negative effects of EPS in sludge particles.

Non-destructive monitoring and prediction of fouling by organic matters and residual anionic coagulant during membrane process

Physical fouling characteristics on silicon carbide (SiC) membranes induced by various organic matter compounds vary depending on the presence of calcium ions (Ca2+). Both destructive techniques (morphological surface analysis) and non-destructive techniques (fouling properties monitoring) were used to determine the fouling mechanisms and behavior during the membrane filtration systems. Destructive analysis and a modified Hermia model were employed to assess the fouling mechanisms. Fouling behavior was also analyzed through non-destructive monitoring techniques including optical coherence tomography (OCT) and three-dimensional laser scanning confocal microscopy (3D-LSM). At concentrations of 10, 30, and 100 mg/L without Ca2+, the flux decreased by 57-95% for humic acid (HA) and anionic polyacrylamide (APAM). APAM exhibited a notable removal rate of up to 56% without Ca2+. At concentration of 10, 30, and 100 mg/L in the absence of Ca2+, the flux decreased by 6-8% for sodium alginate (SA). However, the addition of Ca2+ led to a reduction in the flux for SA by up to 91% and resulted in a removal rate of 40%. Furthermore, addition of Ca2+ led to an alteration of the fouling characteristics of HA and SA. In the case of HA, higher concentrations resulted in elevated thickness and roughness with correlation coefficients of 0.991 and 0.992, respectively. For SA, increased SA concentration led to a thicker (correlation coefficient of 0.999) but smoother surfaces (correlation coefficients of 0.502). Monitoring of these physical characteristics of the fouling layer through non-destructive analysis is crucial for effective fouling management, optimization of the system performance and extending the lifespan of the membrane. By continuously assessing the fouling layer thickness and surface roughness, we expect to be able to provide insights on the fouling behavior, identify trends, that can help scientists and engineers to make informed decisions regarding fouling control strategies in future.

Spatial separation of nitrifiers and denitrifiers promotes selection and enrichment of polyhydroxyalkanoates storing mixed cultures fed by crude glycerol and propionate wastewater

Polyhydroxyalkanoates (PHA) recovery from industrial wastewater has been highlighted as a promising strategy for a circular bioeconomy. However, the high and varying level of nitrogen in wastewater makes enrichment of mixed microbial culture (MMC) low efficiency. In this study, spatial separation of nitrifiers and denitrifiers was adopted by adding biocarriers in MMC and decreasing the sludge retention time (SRT) to accelerate the enrichment of PHA-storing MMC fed by mixed wastewater containing glycerol and propionate. Nitrifiers and denitrifiers were sustained on biocarriers, obtaining a high total inorganic nitrogen removal and allowing a more efficient selective pressure of a high carbon and nitrogen ratio (C/N) under low SRT conditions. The maximum PHA cell content and relative abundance of PHA-storing bacteria were increased to 60.51 % (SRT 6 d) and 49.62 % (SRT 6 d) with the decrease of SRT, respectively. This study demonstrates an efficient way to highly enrich PHA-storing MMC from crude glycerol, which provide a relevant technical support for high-efficiency enrichment of PHA-storing bacteria in low C/N wastewater.

Microbially driven Fe-N cycle: Intrinsic mechanisms, enhancement, and perspectives

The iron‑nitrogen (FeN) cycle driven by microbes has great potential for treating wastewater. Fe is a metal that is frequently present in the environment and one of the crucial trace elements needed by microbes. Due to its synergistic role in the microbial N removal process, Fe goes much beyond the essential nutritional needs of microorganisms. Investigating the mechanisms behind the linked Fe-N cycle driven by microbes is crucial. The Fe-N cycle is frequently connected with anaerobic ammonia oxidation (anammox), nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), Feammox, and simultaneous nitrification denitrification (SND), etc. Although the main mechanisms of Fe-mediated biological N removal may vary depending on the valence state of the Fe, their similar transformation pathways may provide information on the study of certain element-microbial interactions. This review offers a thorough analysis of the facilitation effect and influence of Fe on the removal of nitrogenous pollutants in various biological N removal processes and summarizes the ideal Fe dosing. Additionally, the synergistic mechanisms of Fe and microbial synergistic N removal process are elaborated, covering four aspects: enzyme activity, electron transfer, microbial extracellular polymeric substances (EPS) secretion, and microbial community interactions. The methods to improve biological N removal based on the intrinsic mechanism were also discussed, with the aim of thoroughly understanding the biological mechanisms of Fe in the microbial N removal process and providing a reference and thinking for employing Fe to promote microbial N removal in practical applications.

Unveiling novel pathways and key contributors in the nitrogen cycle: Validation of enrichment and taxonomic characterization of oxygenic denitrifying microorganisms in environmental samples

Microorganisms play a crucial role in both the nitrogen cycle and greenhouse gas emissions. A recent discovery has unveiled a new denitrification pathway called oxygenic denitrification, entailing the enzymatic reduction of nitrite to nitric oxide (NO) by a putative nitric oxide dismutase (nod) enzyme. In this study, the presence of the nod gene was detected and subsequently enriched in anaerobic-activated sludge, farmland soil, and paddy soil samples. After 150 days, the enriched samples exhibited significant denitrification, and concomitant oxygen production. The removal efficiency of nitrite ranged from 64.6 % to 79.0 %, while the oxygen production rate was between 15.4 μL/min and 18.6 μL/min when exposed to a sole nitrogen source of 80 mg/L sodium nitrite. Additionally, batch experiments and kinetic analyses revealed the intricate pathways and underlying mechanisms governing the oxygenic denitrification reaction by using CARBOXY-PTIO, 18O-labelled water, and acetylene to unravel the intricacies of the reaction. The quantitative polymerase chain reaction (qPCR) results indicated a significant surge in the abundance of nod genes, escalating from 7.59 to 10.12-fold. Moreover, analysis of 16S ribosomal DNA (rDNA) amplicons revealed Proteobacteria as the dominant phylum and Thauera as the main genus, with the presumed affiliation. In this study, a new nitrogen conversion pathway, oxygenic denitrification, was discovered in environmental samples. This process provides the possibility for the control of nitrous oxide in the treatment of nitrogenous wastewater.

Application of a hybrid-fruit-peel (HFP) coagulant in low carbon source wastewater treatment as an external carbon source

The application of a hybrid-fruit-peel (HFP) coagulant used as an external carbon source (ECS) in both simulated water and real sewage having a low carbon source treated with sequencing batch reactor (SBR) was studied, compared with that of sodium acetate (NaAc). The impact of HFP on sludge properties (such as extracellular polymer substance (EPS), dehydrogenase activity (DHA), charged property, size, microscopic images and bacteria phase) was characterized. The results showed that as an ECS, HFP basically gave similar nitrogen removal to NaAc and also gave a similar developing trend of both dissolved oxygen (DO) and pH. HFP promoted more proliferation of microorganisms and posed higher levels of protein (PN) and polysaccharide (PS) than NaAc, but gave slightly lower DHA than NaAc. After HFP was added as an ECS, the types and quantities of microorganisms increased significantly, the effluent qualities were improved and the sludge size and extensibility became larger, which was conducive to direct contact and remove pollutants. HFP played a similar role to NaAc as ECS and can be used as a quality and slow-releasing ECS for low carbon source wastewaters.

Granulation of partial denitrification sludge: Advances in mechanism understanding, technologies development and perspectives

The high-rate and stably efficient nitrite generation is vital and still challenges the wide application of partial denitrification (PD) and anammox technology. Increasing attention has been drawn to the granulation of PD biomass. However, the knowledge of PD granular sludge is still limited in terms of granules characterization and mechanisms of biomass aggregation for high nitrite accumulation. This work reviewed the performance and granulation of PD biomass for high nitrite accumulation via nitrate reduction, including the system start-up, influential factors, granular characteristics, hypothetical mechanism, challenges and perspectives in future application. The physiochemical characterization and key influential factors were summarized in view of nitrite production, morphology analysis, extracellular polymer substance structure, as well as microbial mechanisms. The PD granules exhibit potential advantages of a high biomass density, good settleability, high hydraulic loading rates, and strong shock resistance. A novel granular sludge-based PD combined with anammox process was proposed to enhance the capability of nitrogen removal. In the future, PD granules utilizing different electron donors is a promising way to broaden the application of anammox technology in both municipal and industrial wastewater treatment.

The performance, mechanism and greenhouse gas emission potential of nitrogen removal technology for low carbon source wastewater

Excessive nitrogen can lead to eutrophication of water bodies. However, the removal of nitrogen from low carbon source wastewater has always been challenging due to the limited availability of carbon sources as electron donors. Biological nitrogen removal technology can be classified into three categories: heterotrophic biological technology (HBT) that utilizes organic matter as electron donors, autotrophic biological technology (ABT) that relies on inorganic electrons as electron donors, and heterotrophic-autotrophic coupling technology (CBT) that combines multiple electron donors. This work reviews the research progress, microbial mechanism, greenhouse gas emission potential, and challenges of the three technologies. In summary, compared to HBT and ABT, CBT shows greater application potential, although pilot-scale implementation is yet to be achieved. The composition of nitrogen removal microorganisms is different, mainly driven by electron donors. ABT and CBT exhibit the lowest potential for greenhouse gas emissions compared to HBT. N2O, CH4, and CO2 emissions can be controlled by optimizing conditions and adding constructed wetlands. Furthermore, these technologies need further improvement to meet increasingly stringent emission standards and address emerging pollutants. Common measures include bioaugmentation in HBT, the development of novel materials to promote mass transfer efficiency of ABT, and the construction of BES-enhanced multi-electron donor systems to achieve pollutant prevention and removal. This work serves as a valuable reference for the development of clean and sustainable low carbon source wastewater treatment technology, as well as for addressing the challenges posed by global warming.

Activation of indigenous denitrifying bacteria and enhanced nitrogen removal via artificial mixing in a drinking water reservoir: Insights into gene abundance, community structure, and co-existence model

Nitrogen pollution poses a severe threat to aquatic ecosystems and human health. This study investigated the use of water lifting aerators for in situ nitrogen reduction in a drinking water reservoir. The reservoir was thoroughly mixed and oxygenated after using water-lifting aerators for 42 days. The average total nitrogen concentration, nitrate nitrogen, and ammonium nitrogen-in all water layers-decreased significantly (P < 0.01), with a reduction efficiency of 35 ± 3%, 34 ± 2%, and 70 ± 6%, respectively. Other pollutants, including organic matter, phosphorus, iron, and manganese, were also effectively removed. Quantitative polymerase chain reactions indicated that bacterial nirS gene abundance was enhanced 26.34-fold. High-throughput sequencing, phylogenetic tree, and network analysis suggested that core indigenous nirS-type denitrifying bacteria, such as Dechloromonas, Simplicispira, Thauera, and Azospira, played vital roles in nitrogen and other pollutant removal processes. Furthermore, structural equation modeling revealed that nitrogen removal responded positively to WT, DO, and nirS gene abundance. Our findings provide a promising strategy for nitrogen removal in oligotrophic drinking water reservoirs with carbon deficiencies.

Promotion of nitrogen removal in a denitrification process elevated by zero-valent iron under low carbon-to-nitrogen ratio

The nitrogen removal efficiency and distribution of microbial community in a denitrification process aided by zero-valent iron (ZVI) under low carbon-to-nitrogen ratio (C/N) were assessed in this study. Experimental results demonstrated that the nitrogen removal efficiency (TNRE) increased to 96.4 ± 2.72% and 63.3 ± 4.02% after continuous addition of ZVI with molar ratio of ZVI to nitrate (NO3--N) (ZVI/N) of 6 at C/N of 3 and 2, respectively, which was 4% and 7.7% higher than the blank one. Meanwhile, extracellular polymeric substance (EPS) could be used as electron transfer medium and endogenous carbon source for denitrification system and also the production of which increased by 28.43% and 53.10% under ZVI stimulation compared to the control group. Finally, a symbiotic system composed by autotrophic and heterotrophic denitrification bacteria was formed by aid of ZVI. This study proposed new insights into denitrification process improved by ZVI.

The efficient utilization of thiocyanate on simultaneous removal of ammonium and nitrate through thiosulfate-driven autotrophic denitrifiers and anammox

The efficient utilization of thiocyanate remain be an important bottleneck in the low-cost nitrogen removal for wastewaters containing thiocyanate. The study aimed to investigate the feasibility of thiocyanate in removal of nitrate and ammonium through anammox (AN) and thiosulfate-driven autotrophic denitrifiers (TSAD). The results showed that removal of nitrate and ammonium were achieved rapidly utilizing thiocyanate, which was attributed to degradation of thiocyanate by TSAD and cooperation with AN. The utilization efficiency of thiocyanate in nitrogen removal was increased by 250% due to the microbial cooperation. Excess thiocyanate and ammonium did not influence the nitrogen removal amount. However, the nitrogen removal were affected obviously by the biomass ratio (X_AN/X_TSAD) between AN and TSAD Moreover, the dynamics related to removal of pollutants was described successfully by a modified Monod model with time constraints. These findings offer an insight for efficient utilization of thiocyanate in nitrogen removal via microbial cooperation.

Recent Advances in Biotechnologies for the Treatment of Environmental Pollutants Based on Reactive Sulfur Species

The definition of reactive sulfur species (RSS) is inspired by the reactivity and variable chemical valence of sulfur. Sulfur is an essential element for life and is a part of global geochemical cycles. Wastewater treatment bioreactors can be divided into two major categories: sulfur reduction and sulfur oxidation. We review the origins of the definition of RSS and related biotechnological processes in environmental management. Sulfate reduction, sulfide oxidation, and sulfur-based redox reactions are key to driving the coupled global carbon, nitrogen, and sulfur co-cycles. This shows the coupling of the sulfur cycle with the carbon and nitrogen cycles and provides insights into the global material-chemical cycle. We also review the biological classification and RSS metabolic mechanisms of functional microorganisms involved in the biological processes, such as sulfate-reducing and sulfur-oxidizing bacteria. Developments in molecular biology and genomic technologies have allowed us to obtain detailed information on these bacteria. The importance of RSS in environmental technologies requires further consideration.