The interplay of hematite and photic biofilm triggers the acceleration of biotic nitrate removal.

The soil-water interface is replete with photic biofilm and iron minerals; however, the potential of how iron minerals promote biotic nitrate removal is still unknown. This study investigates the physiological and ecological responses of photic biofilm to hematite (Fe2O3), in order to explore a practically feasible approach for in-situ nitrate removal. The nitrate removal by photic biofilm was significantly higher in the presence of Fe2O3 (92.5%) compared to the control (82.8%). Results show that the presence of Fe2O3 changed the microbial community composition of the photic biofilm, facilitates the thriving of Magnetospirillum and Pseudomonas, and promotes the growth of photic biofilm represented by the extracellular polymeric substance (EPS) and the content of chlorophyll. The presence of Fe2O3 also induces oxidative stress (•O2-) in the photic biofilm, which was demonstrated by electron spin resonance spectrometry. However, the photic biofilm could improve the EPS productivity to prevent the entrance of Fe2O3 to cells in the biofilm matrix and mitigate oxidative stress. The Fe2O3 then promoted the relative abundance of Magnetospirillum and Pseudomonas and the activity of nitrate reductase, which accelerates nitrate reduction by the photic biofilm. This study provides an insight into the interaction between iron minerals and photic biofilm and demonstrates the possibility of combining biotic and abiotic methods to improve the in-situ nitrate removal rate.

Microbial interactions facilitating efficient methane driven denitrification via in-situ utilization of short chain fatty acids.

High nitrate pollution in agriculture and industry poses a challenge to emerging methane oxidation coupled denitrification. In this study, an efficient nitrate removal efficiency of 100 % was achieved at an influent loading rate of 400 mg-N/L·d, accompanied by the production of short chain fatty acids (SCFAs) with a maximum value of 80.9 mg/L. Batch tests confirmed that methane was initially converted to acetate, which then served as a carbon source for denitrification. Microbial community characterization revealed the dominance of heterotrophic denitrifiers, including Simplicispira (22.8 %), Stappia (4.9 %), and the high­nitrogen-tolerant heterotrophic denitrifier Diaphorobacter (19.0 %), at the nitrate removal rate of 400 mg-N/L·d. Notably, the low abundance of methanotrophs ranging from 0.24 % to 3.75 % across all operational stages does not fully align with the abundance of pmoA genes, suggesting the presence of other functional microorganisms capable of methane oxidation and SCFAs production. These findings could facilitate highly efficient denitrification driven by methane and contributed to the development of denitrification using methane as an electron donor.

Genetic control of root morphology in response to nitrogen across rapeseed diversity.

Rapeseed (Brassica napus L.) is an oil-containing crop of great economic value but with considerable nitrogen requirement. Breeding root systems that efficiently absorb nitrogen from the soil could be a driver to ensure genetic gains for more sustainable rapeseed production. The aim of this study is to identify genomic regions that regulate root morphology in response to nitrate availability. The natural variability offered by 300 inbred lines was screened at two experimental locations. Seedlings grew hydroponically with low or elevated nitrate levels. Fifteen traits related to biomass production and root morphology were measured. On average across the panel, a low nitrate level increased the root-to-shoot biomass ratio and the lateral root length. A large phenotypic variation was observed, along with important heritability values and genotypic effects, but low genotype-by-nitrogen interactions. Genome-wide association study and bulk segregant analysis were used to identify loci regulating phenotypic traits. The first approach nominated 319 SNPs that were combined into 80 QTLs. Three QTLs identified on the A07 and C07 chromosomes were stable across nitrate levels and/or experimental locations. The second approach involved genotyping two groups of individuals from an experimental F2 population created by crossing two accessions with contrasting lateral root lengths. These individuals were found in the tails of the phenotypic distribution. Co-localized QTLs found in both mapping approaches covered a chromosomal region on the A06 chromosome. The QTL regions contained some genes putatively involved in root organogenesis and represent selection targets for redesigning the root morphology of rapeseed.

[Effect of intestinal nitrate on growth of Klebsiella pneumoniae and its regulatory mechanism].

OBJECTIVE: To explore the effect of intestinal nitrates on the growth of Klebsiella pneumoniae and its regulatory mechanisms. METHODS: K. pneumoniae strains with nitrate reductase narG and narZ single or double gene knockout or with NarXL gene knockout were constructed and observed for both aerobic and anaerobic growth in the presence of KNO3 using an automated bacterial growth analyzer and a spectrophotometer, respectively. The mRNA expressions of narG and narZ in K. pneumoniae in anaerobic cultures in the presence of KNO3 and the effect of the binary regulatory system NarXL on their expresisons were detected using qRT-PCR. Electrophoretic mobility shift assays (EMSA) and MST analysis were performed to explore the specific regulatory mechanisms of NarXL in sensing and utilizing nitrates. Competitive experiments were conducted to examine anaerobic growth advantages of narG and narZ gene knockout strains of K. pneumoniae in the presence of KNO3. RESULTS: The presence of KNO3 in anaerobic conditions, but not in aerobic conditions, promoted bacterial growth more effectively in the wild-type K. pneumoniae strain than in the narXL gene knockout strain. In anaerobic conditions, the narXL gene knockout strain showed significantly lowered mRNA expressions of narG and narZ (P < 0.0001). EMSA and MST experiments demonstrated that the NarXL regulator could directly bind to narG and narZ promoter regions. The wild-type K. pneumoniae strain in anaerobic cultures showed significantly increased expressions of narG and narZ mRNAs in the presence of KNO3 (P < 0.01), and narG gene knockout resulted in significantly attenuated anaerobic growth and competitive growth abilities of K. pneumoniae in the presence of KNO3 (P < 0.01). CONCLUSION: The binary regulatory system NarXL of K. pneumoniae can sense changes in intestinal nitrate concentration and directly regulate the expression of nitrate reductase genes narG and narZ to promote bacterial growth.

[Construction and characterization of a modA gene mutant strain of Klebsiella pneumoniae].

OBJECTIVE: To construct a mutant strain of Klebsiella pneumoniae NTUH- K2044 with modA gene deletion and its complementary strain and explore the role of modA gene in modulating anaerobic nitrate respiratory growth and phenotypes of K. pneumoniae. METHODS: The modA deletion mutant K. pneumoniae strain was constructed by homologous recombination using the suicide vector pKO3-Km. To obtain the complementary strain C-modA, the whole sequence fragment containing the promoter, open reading frame and terminator regions of modA was cloned into pGEM-T-easy and electrically transformed into the modA deletion mutant. The NTUH-K2044 wild-type strain, modA gene deletion mutant and complementary strain were compared by measuring in vitro anaerobic nitrate respiration growth, competitiveness index, biofilm quantification, mucoviscosity assay and morphological measurement using Image J. RESULTS: The modA deletion mutant strain ΔmodA and the complementary strain C-modA were successfully constructed. The modA gene knockout strain showed inhibited anaerobic nitrate respiratory growth compared with the wild- type and C-modA strains with significantly weakened competitiveness, reduced capacity of biofilm synthesis during anaerobiosis, and lowered mucoviscosity under anaerobic conditions. The ΔmodA strain showed a spherical morphology in anaerobic conditions as compared with the normal short rod-like morphology of K. pneumoniae, with also distinctly shorter length than the wild-type and C-modA strains. CONCLUSION: The molybdate transport system encoding gene modA is associated with the pathogenic capacity of K. pneumoniae by modulating its anaerobic nitrate respiration, competitiveness, biofilm formation, hypermucoviscous phenotype and morphology.

Ammonium-derived nitrous oxide is a global source in streams.

Global riverine nitrous oxide (N2O) emissions have increased more than 4-fold in the last century. It has been estimated that the hyporheic zones in small streams alone may contribute approximately 85% of these N2O emissions. However, the mechanisms and pathways controlling hyporheic N2O production in stream ecosystems remain unknown. Here, we report that ammonia-derived pathways, rather than the nitrate-derived pathways, are the dominant hyporheic N2O sources (69.6 ± 2.1%) in agricultural streams around the world. The N2O fluxes are mainly in positive correlation with ammonia. The potential N2O metabolic pathways of metagenome-assembled genomes (MAGs) provides evidence that nitrifying bacteria contain greater abundances of N2O production-related genes than denitrifying bacteria. Taken together, this study highlights the importance of mitigating agriculturally derived ammonium in low-order agricultural streams in controlling N2O emissions. Global models of riverine ecosystems need to better represent ammonia-derived pathways for accurately estimating and predicting riverine N2O emissions.

Microbiome structure and function in parallel full-scale aerobic granular sludge and activated sludge processes.

Aerobic granular sludge (AGS) and conventional activated sludge (CAS) are two different biological wastewater treatment processes. AGS consists of self-immobilised microorganisms that are transformed into spherical biofilms, whereas CAS has floccular sludge of lower density. In this study, we investigated the treatment performance and microbiome dynamics of two full-scale AGS reactors and a parallel CAS system at a municipal WWTP in Sweden. Both systems produced low effluent concentrations, with some fluctuations in phosphate and nitrate mainly due to variations in organic substrate availability. The microbial diversity was slightly higher in the AGS, with different dynamics in the microbiome over time. Seasonal periodicity was observed in both sludge types, with a larger shift in the CAS microbiome compared to the AGS. Groups important for reactor function, such as ammonia-oxidising bacteria (AOB), nitrite-oxidising bacteria (NOB), polyphosphate-accumulating organisms (PAOs) and glycogen-accumulating organisms (GAOs), followed similar trends in both systems, with higher relative abundances of PAOs and GAOs in the AGS. However, microbial composition and dynamics differed between the two systems at the genus level. For instance, among PAOs, Tetrasphaera was more prevalent in the AGS, while Dechloromonas was more common in the CAS. Among NOB, Ca. Nitrotoga had a higher relative abundance in the AGS, while Nitrospira was the main nitrifier in the CAS. Furthermore, network analysis revealed the clustering of the various genera within the guilds to modules with different temporal patterns, suggesting functional redundancy in both AGS and CAS. KEY POINTS: • Microbial community succession in parallel full-scale aerobic granular sludge (AGS) and conventional activated sludge (CAS) processes. • Higher periodicity in microbial community structure in CAS compared to in AGS. • Similar functional groups between AGS and CAS but different composition and dynamics at genus level.

Effect of pyrite particle size on the denitrification performance of autotrophic or split-mixotrophic bioreactors supported by pyrite/polycaprolactone.

In this study, a pyrite-based autotrophic denitrification (PAD) system, a polycaprolactone (PCL)-supported heterotrophic denitrification (PHD) system, and a pyrite+PCL-based split-mixotrophic denitrification (PPMD) system were constructed. The pyrite particle size was controlled in 1-3, 3-5, or 5-8 mm in both the PAD and PPMD systems to investigate the effect of pyrite particle size on the denitrification performance of autotrophic or split-mixotrophic bioreactors. It was found that the PAD system achieved the best denitrification efficiency with an average removal rate of 98.98% in the treatment of 1- to 3-mm particle size, whereas it was only 19.24% in the treatment of 5- to 8-mm particle size. At different phases of the whole experiment, the nitrate removal rates of both the PHD and PPMD systems remained stable at a high level (>94%). Compared with the PAD or PHD system, the PPMD system reduced the concentrations of sulfate and chemical oxygen demand in the final effluent efficiently. The interconnection network diagram explained the intrinsic metabolic pathways of nitrogen, sulfur, and carbon in the three denitrification systems at different phases. In addition, the microbial community analysis showed that the PPMD system was beneficial for the enrichment of Firmicutes. Finally, the impact mechanism of pyrite particle size on the performance of the PPMD system was proposed. PRACTITIONER POINTS: The reduction of pyrite particle size was beneficial for improving the efficiency of the PAD process. The change in particle size had an effect on NO2 --N accumulation in the PAD system. The accumulation of NH4 +-N in the PPMD system increased with the decrease in particle size. The reduction of pyrite particle size increased the production of SO4 2- in the PAD and PPMD systems. The correlations among the effluent indicators of the PAD and PPMD systems could be well explained.

Recent mechanistic developments for cytochrome c nitrite reductase, the key enzyme in the dissimilatory nitrate reduction to ammonium pathway.

Cytochrome c nitrite reductase, NrfA, is a soluble, periplasmic pentaheme cytochrome responsible for the reduction of nitrite to ammonium in the Dissimilatory Nitrate Reduction to Ammonium (DNRA) pathway, a vital reaction in the global nitrogen cycle. NrfA catalyzes this six-electron and eight-proton reduction of nitrite at a single active site with the help of its quinol oxidase partners. In this review, we summarize the latest progress in elucidating the reaction mechanism of ammonia production, including new findings about the active site architecture of NrfA, as well as recent results that elucidate electron transfer and storage in the pentaheme scaffold of this enzyme.

Redox regulation in primary nitrate response: Nitric oxide in the spotlight.

Nitrogen (N) is the main macronutrient of plants that determines growth and productivity. Nitrate is the major source form of N in soils and its uptake and assimilatory pathway has been extensively studied. The early events that occur after the perception of nitrate is known as primary nitrate response (PNR). In this review, new findings on the redox signal that impacts PNR are discussed. We will focus on the novel role of Nitric Oxide (NO) as a signal molecule and the mechanisms that are involved to control NO homeostasis during PNR. Moreover, the role of Reactive Oxygen Species (ROS) and the possible interplay with NO in the PNR are discussed. The sources of NO during PNR will be analyzed as well as the regulation of its intracellular levels. Furthermore, we explored the relevance of the direct action of NO through the S-nitrosation of the transcription factor NLP7, one of the master regulators in the nitrate signaling cascade. This review gives rise to an interesting field with new actors to mark future research directions. This allows us to increase the knowledge of the physiological and molecular fine-tuned modulation during nitrate signaling processes in plants. The discussion of new experimental data will stimulate efforts to further refine our understanding of the redox regulation of nitrate signaling.

Insights into feasibility and microbial characterizations on simultaneous elimination of dissolved methane from anaerobic effluents and nitrate/nitrite reduction in a conventional anoxic reactor with magnetite.

Discovery of nitrate/nitrite-dependent anaerobic methane oxidation (DAMO) challenges the conventional biological treatment processes, since it provides a possibility of simultaneously mitigating dissolved methane emissions from anaerobic effluents and reducing additional carbon sources for denitrification. Due to the slow growth of specialized DAMO microbes, this possibility has been just practiced with biofilms in membrane biofilm reactors or granular sludge in membrane bioreactors. In this study, simultaneous elimination of dissolved methane from anaerobic effluents and nitrate/nitrite reduction was achieved in a conventional anoxic reactor with magnetite. Calculations of electron flow balance showed that, with magnetite the eliminated dissolved methane was almost entirely used for nitrate/nitrite reduction, while without magnetite approximately 52 % of eliminated dissolved methane was converted to unknown organics. Metagenomic sequencing showed that, when dissolved methane served as an electron donor, the abundance of genes for reverse methanogenesis and denitrification dramatically increased, indicating that anaerobic oxidation of methane (AOM) coupled to nitrate/nitrite reduction occurred. Magnetite increased the abundance of genes encoding the key enzymes involved in whole reverse methanogenesis and Nir and Nor involved in denitrification, compared to that without magnetite. Analysis of microbial communities showed that, AOM coupled to nitrate/nitrite reduction was proceeded by syntrophic consortia comprised of methane oxidizers, Methanolinea and Methanobacterium, and nitrate/nitrite reducers, Armatimonadetes_gp5 and Thauera. With magnetite syntrophic consortia exchanged electrons more effectively than that without magnetite, further supporting the microbial growth.

Nitrate-dependent anaerobic methane oxidation coupled to Fe(III) reduction as a source of ammonium and nitrous oxide.

'Candidatus Methanoperedens nitroreducens' is an archaeal methanotroph with global importance that links carbon and nitrogen cycles and great potential for sustainable operation of wastewater treatment. It has been reported to mediate the anaerobic oxidation of methane through a reverse methanogenesis pathway while reducing nitrate to nitrite. Here, we demonstrate that 'Ca. M. nitroreducens' reduces ferric iron forming ammonium (23.1 %) and nitrous oxide (N2O, 46.5 %) from nitrate. These results are supported with the upregulation of genes coding for proteins responsible for dissimilatory nitrate reduction to ammonium (nrfA), N2O formation (norV, cyt P460), and multiple multiheme c-type cytochromes for ferric iron reduction. Concomitantly, an increase in the N2O-reducing SJA-28 lineage and a decrease in the nitrite-reducing 'Candidatus Methylomirabilis oxyfera' are consistent with the changes in 'Ca. M. nitroreducens' end products. These findings demonstrate the highly flexible physiology of 'Ca. M. nitroreducens' in anaerobic ecosystems with diverse electron acceptor conditions, and further reveals its roles in linking methane oxidation to global biogeochemical cycles. 'Ca. M. nitroreducens' could significantly affect the bioavailability of nitrogen sources as well as the emission of greenhouse gas in natural ecosystems and wastewater treatment plants.

Advancing towards electro-bioremediation scaling-up: On-site pilot plant for successful nitrate-contaminated groundwater treatment.

The potential of nitrate electro-bioremediation has been fully demonstrated at the laboratory scale, although it has not yet been fully implemented due to the challenges associated with scaling-up bioelectrochemical reactors and their on-site operation. This study describes the initial start-up and subsequent stable operation of an electro-bioremediation pilot plant for the treatment of nitrate-contaminated groundwater on-site (Navata site, Spain). The pilot plant was operated under continuous flow mode for 3 months, producing an effluent suitable for drinking water in terms of nitrates and nitrites (<50 mg NO3- L-1; 0 mg NO2- L-1). A maximum nitrate removal rate of 0.9 ± 0.1 kg NO3- m-3 d-1 (efficiency 82 ± 18 %) was achieved at a cathodic hydraulic retention time (HRTcat) of 2.0 h with a competitive energy consumption of 4.3 ± 0.4 kWh kg-1 NO3-. Under these conditions, the techno-economic analysis estimated an operational cost of 0.40 € m-3. Simultaneously, microbiological analyses revealed structural heterogeneity in the reactor, with denitrification functionality concentrated predominantly from the centre to the upper section of the reactor. The most abundant groups were Pseudomonadaceae, Rhizobiaceae, Gallionellaceae, and Xanthomonadaceae. In conclusion, this pilot plant represents a significant advancement in implementing this technology on a larger scale, validating its effectiveness in terms of nitrate removal and cost-effectiveness. Moreover, the results validate the electro-bioremediation in a real environment and encourage further investigation of its potential as a water treatment.

Low-voltage stimulated denitrification performance of high-salinity wastewater using halotolerant microorganisms.

Nitrate is a common contaminant in high-salinity wastewater, which has adverse effects on both the environment and human health. However, conventional biological treatment exhibits poor denitrification performance due to the high-salinity shock. In this study, an innovative approach using an electrostimulating microbial reactor (EMR) was explored to address this challenge. With a low-voltage input of 1.2 V, the EMR reached nitrate removal kinetic parameter (kNO3-N) of 0.0166-0.0808 h-1 under high-salinities (1.5 %-6.5 %), which was higher than that of the microbial reactor (MR) (0.0125-0.0478 h-1). The mechanisms analysis revealed that low-voltage significantly enhanced microbial salt-in strategy and promoted the secretion of extracellular polymeric substances. Halotolerant denitrification microorganisms (Pseudomonas and Nitratireductor) were also enriched in EMR. Moreover, the EMR achieved a NO3-N removal efficiency of 73.64 % in treating high-salinity wastewater (salinity 4.69 %) over 18-cycles, whereas the MR only reached 54.67 %. In summary, this study offers an innovative solution for denitrification of high-salinity wastewater.

Competitive partitioning of denitrification pathways during arrested methanogenesis: Implications in ammonium recovery, N2O emission, and volatile fatty acid production.

The complex interaction between nitrate (NO3-) reduction and fermentation is poorly understood when high levels of NO3- are introduced into anaerobic systems. This study investigated the competitive distribution between conventional denitrification (DEN) and dissimilatory nitrate reduction to ammonium (DNRA) during simultaneous denitrification and fermentation in arrested methanogenesis. Up to 62% of initial NO3- (200 mg-N/L) was retained as ammonium through DNRA at a chemical oxygen demand (COD)/N ratio of 25. Significant N2O emission occurred (1.7 - 8.0% of the initial NO3-) with limited carbon supply (≤1600 mg COD/L) and sludge concentration (≤3000 mg COD/L). VFA composition shifted predominantly towards acetic acid (>50%) in the presence of nitrate. A novel kinetic model was developed to predict DNRA vs. DEN partitioning and NO2- accumulation. Overall, NO3- input, organic loading, and carbon source characteristics independently and collectively controlled competitive DNRA vs. DEN partitioning.

Feasibility of in-situ sludge fermentation coupled with partial denitrification: Key roles of initial organic matters and alkaline pH.

Achieving partial denitrification (PD) by using fermentation products extracted from waste activated sludge (WAS) rather than commercial organic matters is a promising approach for providing nitrite for anammox, while sludge reduction could also be realized by WAS reutilization. This study proposed an In-situ Sludge Fermentation coupled with Partial Denitrification (ISFPD) system and explored its performance under different conditions, including initial pH, nitrate concentrations, and organic matters. Results showed that nitrite production increased with the elevation of initial pH (from 6 to 9), and the highest nitrate-to-nitrite transformation ratio (NTR) reached 77% at initial pH 9. The PD rates and NTR were observed to be minimally influenced by initial nitrate concentrations. Acetate was preferred by denitrifying bacteria, while macromolecules such as proteins necessitated be hydrolyzed to be suitable for further utilization. The insights gained through this study paved the way for efficient nitrite production and sustainable WAS reutilization in harmony.

Anaerobic oxidation of ammonium and short-chain gaseous alkanes coupled to nitrate reduction by a bacterial consortium.

The bacterial species "Candidatus Alkanivorans nitratireducens" was recently demonstrated to mediate nitrate-dependent anaerobic oxidation of short-chain gaseous alkanes (SCGAs). In previous bioreactor enrichment studies, the species appeared to reduce nitrate in two phases, switching from denitrification to dissimilatory nitrate reduction to ammonium (DNRA) in response to nitrite accumulation. The regulation of this switch or the nature of potential syntrophic partnerships with other microorganisms remains unclear. Here, we describe anaerobic multispecies cultures of bacteria that couple the oxidation of propane and butane to nitrate reduction and the oxidation of ammonium (anammox). Batch tests with 15N-isotope labelling and multi-omic analyses collectively supported a syntrophic partnership between "Ca. A. nitratireducens" and anammox bacteria, with the former species mediating nitrate-driven oxidation of SCGAs, supplying the latter with nitrite for the oxidation of ammonium. The elimination of nitrite accumulation by the anammox substantially increased SCGA and nitrate consumption rates, whereas it suppressed DNRA. Removing ammonium supply led to its eventual production, the accumulation of nitrite, and the upregulation of DNRA gene expression for the abundant "Ca. A. nitratireducens". Increasing the supply of SCGA had a similar effect in promoting DNRA. Our results suggest that "Ca. A. nitratireducens" switches to DNRA to alleviate oxidative stress caused by nitrite accumulation, giving further insight into adaptability and ecology of this microorganism. Our findings also have important implications for the understanding of the fate of nitrogen and SCGAs in anaerobic environments.

Effects of nitrate reduction on the biotransformation of 1H-1,2,4-triazole: Mechanism and community evolution.

Due to the refractory of 1 H-1,2,4-triazole (TZ), conventional anaerobic biological treatment technology is usually restricted by low removal efficiency and poor system stability. In this study, TZ biodegradation and nitrate reduction was coupled to improve the removal efficiency of TZ from polluted wastewater. Batch assay was performed with pure culture strain Raoultella sp. NJUST42, which was reported to have the capability to degrade TZ in our previous study. Based on batch assay result, complete removal of TZ could be achieved in the presence of nitrate, whereas only 50% of TZ could be removed in the control system. Long-term stability experiment indicated that the relative abundance of microorganisms (Bacteroidetes_vadinHA17, Georgenia, Anaerolinea, etc) was obviously enhanced under nitrate reduction condition. During long-term period, major intermediates for TZ biodegradation such as [1,2,4]Triazolidine-3,5-diol, hydrazine dibasic carboxylic acid and carbamic acid were detected. A novel TZ biotransformation approach via hydration, TZ-ring cleavage, deamination and oxidation was speculated. PICRUSt1 and KEGG pathway analyses indicated that hydration (dch), oxidation (adhD, oah, pucG, fdhA) of TZ and nitrate reduction (Nar, napA, nrfA, nirBK, norB, nosZ) were significantly enhanced in the presence of nitrate. Moreover, the significant enrichment of TCA cycle (gab, sdh, fum, etc.) indicated that carbon and energy metabolism were facilitated with the addition of nitrate, thus improved TZ catabolism. The proposed mechanism demonstrated that TZ biodegradation coupled with nitrate reduction would be a promising approach for efficient treatment of wastewater contaminated by TZ.

Novel and innovative approaches to partial denitrification coupled with anammox: A critical review.

The partial denitrification (PD) coupled with anaerobic ammonium oxidation (Anammox) (PD/A) process is a unique biological denitrification method for sewage that concurrently removes nitrate (NO3--N) and ammonium (NH4+-N) in sewage. Comparing PD/A to conventional nitrification and denitrification technologies, noticeable improvements are shown in energy consumption, carbon source demand, sludge generation and emissions of greenhouse gasses. The PD is vital to obtaining nitrites (NO2--N) in the Anammox process. This paper provided valuable insight by introduced the basic principles and characteristics of the process and then summarized the strengthening strategies. The functional microorganisms and microbial competition have been discussed in details, the S-dependent denitrification-anammox has been analyzed in this review paper. Important factors affecting the PD/A process were examined from different aspects, and finally, the paper pointed out the shortcomings of the coupling process in experimental research and engineering applications. Thus, this research provided insightful information for the PD/A process's optimization technique in later treating many types of real and nitrate-based wastewater. The review paper also provided the prospective economic and environmental position for the actual design implementation of the PD/A process in the years to come.

Downflow sponge biofilm reactors for polluted raw water treatment: Performance optimisation, kinetics, and microbial community.

Water outages caused by elevated ammonium (NH4+-N) levels are a prevalent problem faced by conventional raw water treatment plants in developing countries. A treatment solution requires a short hydraulic retention time (HRT) to overcome nitrification rate limitation in oligotrophic conditions. In this study, the performance of polluted raw water treatment using a green downflow sponge biofilm (DSB) technology was evaluated. We operated two DSB reactors, DSB-1 and DSB-2 under different NH4+-N concentration ranges (DSB-1: 3.2-5.0 mg L-1; DSB-2: 1.7-2.6 mg L-1) over 360 days and monitored their performance under short HRT (60 min, 30 min, 20 min, and 15 min). The experimental results revealed vertical segregation of organic removal in the upper reactor depths and nitrification in the lower depths. Under the shortest HRT of 15 min, both DSB reactors achieved stable NH4+-N and chemical oxygen demand removal (≥95%) and produced minimal effluent nitrite (NO2--N). DSB system could facilitate complete NH4+-N oxidation to nitrate (NO3--N) without external aeration energy requirement. The 16S rRNA sequencing data revealed that nitrifying bacteria Nitrosomonas and Nitrospira in the reactor were stratified. Putative comammox bacteria with high ammonia affinity was successfully enriched in DSB-2 operating at a lower NH4+-N loading rate, which is advantageous in oligotrophic treatment. This study suggests that a high hydraulic rate DSB system with efficient ammonia removal could incorporate ammonia treatment capability into polluted raw water treatment process and ensure safe water supply in many developing countries.