Simultaneously advanced removal of nitrogen and phosphorus in a biofilter packed with ZVI/PHBV/sawdust composite: Deciphering the succession of dominant bacteria and keystone species

Groundwater denitrification enhanced by a hydrogel immobilized iron/solid carbon source: impact on denitrification and substrate release performance

Encapsulating a solid carbon source and zero-valent iron (ZVI) within a hydrogel can prevent direct contact with groundwater, thereby extending the lifespan of their released active substrates. It is currently unclear whether the solid carbon source and ZVI will mutually influence each other's active substrate release process and the corresponding denitrification patterns, necessitating further investigation. In this study a hydrogel encapsulating different weight ratios of micron-sized zero-valent iron (mZVI, as ZVI) and polyhydroxybutyrate (PHB, as a solid carbon source) was synthesized. The aim was to investigate the influence of PHB on the release of dissolved iron from mZVI and denitrification mechanism. Results indicated that PHB was consumed at a higher rate than mZVI, and more mZVI active sites could be exposed after PHB consumption. Meanwhile, PHB increased the porosity of the hydrogel, allowing more active sites of mZVI to be exposed and thus releasing more dissolved iron. Furthermore, PHB enhanced the rate of microbial corrosion of mZVI, which further increased the release of dissolved iron. Higher PHB content in the hydrogel reduced the oxidation of the released dissolved iron, resulting in a microbial community dominated by heterotrophic microorganisms. Conversely, lower PHB content led to significant Fe(II) oxidation and a considerable relative abundance of mixotrophic microorganisms in the microbial community. Microorganisms with iron reduction potential were also detected. This study provides theoretical support for the precise control of mixed nutrient denitrification based on hydrogel immobilization and lays the foundation for its further practical application in groundwater.

Significantly Enhanced Nitrate and Phosphorus Removal by Pyrite/Sawdust Composite-Driven Mixotrophic Denitrification with Boosted Electron Transfer: Comprehensive Evaluation of Water-Gas-Biofilm Phases during a Long-Term Study

Further reducing total nitrogen (TN) and total phosphorus (TP) in the secondary effluent needs to be realized effectively and in an eco-friendly manner. Herein, four pyrite/sawdust composite-based biofilters were established to treat simulated secondary effluent for 304 days. The results demonstrated that effluent TN and TP concentrations from biofilters under the optimal hydraulic retention time (HRT) of 3.5 h were stable at <2.0 and 0.1 mg/L, respectively, and no significant differences were observed between inoculated sludge sources. The pyrite/sawdust composite-based biofilters had low N2O, CH4, and CO2 emissions, and the effluent's DOM was mainly composed of five fluorescence components. Moreover, mixotrophic denitrifiers (Thiothrix) and sulfate-reducing bacteria (Desulfosporosinus) contributing to microbial nitrogen and sulfur cycles were enriched in the biofilm. Co-occurrence network analysis deciphered that Chlorobaculum and Desulfobacterales were key genera, which formed an obvious sulfur cycle process that strengthened the denitrification capacity. The higher abundances of genes encoding extracellular electron transport (EET) chains/mediators revealed that pyrite not only functioned as an electron conduit to stimulate direct interspecies electron transfer by flagella but also facilitated EET-associated enzymes for denitrification. This study comprehensively evaluates the water-gas-biofilm phases of pyrite/sawdust composite-based biofilters during a long-term study, providing an in-depth understanding of boosted electron transfer in pyrite-based mixotrophic denitrification systems.

Amorphous Fe substrate enhances nitrogen and phosphorus removal in sulfur autotrophic process

The autotrophic denitrification of coupled sulfur and natural iron ore can remove nitrogen and phosphorus from wastewater with low C/N ratios. However, the low solubility of crystalline Fe limits its bioavailability and P absorption capacity. This study investigated the effects of amorphous Fe in drinking water treatment residue (DWTR) and crystalline Fe in red mud (RM) on nitrogen and phosphorus removal during sulfur autotrophic processes. Two types of S-Fe cross-linked filler particles with three-dimensional mesh structures were obtained by combining sulfur with the DWTR/RM using the hydrogel encapsulation method. Two fixed-bed reactors, sulfur-DWTR autotrophic denitrification (SDAD) and sulfur-RM autotrophic denitrification (SRAD), were constructed and stably operated for 236 d Under a 5-8-h hydraulic retention time, the average NO3--N, TN, and phosphate removal rates of SDAD and SRAD were 99.04 %, 96.29 %, 94.03 % (SDAD) and 97.33 %, 69.97 %, 82.26 % (SRAD), respectively. It is important to note that fermentative iron-reducing bacteria, specifically Clostridium_sensu_stricto_1, were present in SDAD at an abundance of 58.17 %, but were absent from SRAD. The presence of these bacteria facilitated the reduction of Fe (III) to Fe (II), which led to the complete denitrification of the S-Fe (II) co-electron donor to produce Fe (III), completing the iron cycle in the system. This study proposes an enhancement method for sulfur autotrophic denitrification using an amorphous Fe substrate, providing a new option for the efficient treatment of low-C/N wastewater.

Boosting the denitrification efficiency of iron-based constructed wetlands in-situ via plant biomass-derived biochar: Intensified iron redox cycle and microbial responses

Considering the unsatisfied denitrification performance of carbon-limited wastewater in iron-based constructed wetlands (ICWs) caused by low electron transfer efficiency of iron substrates, utilization of plant-based conductive materials in-situ for improving the long-term reactivity of iron substrates was proposed to boost the Fe (III)/Fe (II) redox cycle thus enhance the nitrogen elimination. Here, we investigated the effects of withered Iris Pseudacorus biomass and its derived biochar on nitrogen removal for 165 days in ICWs. Results revealed that accumulate TN removal capacity in biochar-added ICW (BC-ICW) increased by 14.7 % compared to biomass-added ICW (BM-ICW), which was mainly attributed to the synergistic strengthening of iron scraps and biochar. The denitrification efficiency of BM-ICW improved by 11.6 % compared to ICWs, while its removal capacity declined with biomass consumption. Autotrophic and heterotrophic denitrifiers were enriched in BM-ICW and BC-ICW, especially biochar increased the abundance of electroactive species (Geobacter and Shewanella, etc.). An active iron cycle exhibited in BC-ICW, which can be confirmed by the presence of more liable iron minerals on iron scraps surface, the lowest Fe (III)/Fe (II) ratio (0.51), and the improved proportions of iron cycling genes (feoABC, korB, fhuF, TC.FEV.OM, etc.). The nitrate removal efficiency was positively correlated with the nitrogen, iron metabolism functional genes and the electron transfer capacity (ETC) of carbon materials (P < 0.05), indicating that redox-active carbon materials addition improved the iron scraps bioavailability by promoting electron transfer, thus enhancing the autotrophic nitrogen removal. Our findings provided a green perspective to better understand the redox properties of plant-based carbon materials in ICWs for deep bioremediation in-situ.

Coupling of polyhydroxybutyrate and zero-valent iron for enhanced treatment of nitrate pollution within the Permeable Reactive Barrier and its downgradient aquifer

Permeable Reactive Barriers (PRBs) have been utilized for mitigating nitrate pollution in groundwater systems through the use of solid carbon and iron fillers that release diverse nutrients to enhance denitrification efficiency. We conduct laboratory column tests to evaluate the effectiveness of PRBs in remediating nitrate pollution both within the PRB and in the downgradient aquifer. We use an iron-carbon hydrogel (ICH) as PRB filler, which has different weight ratios of polyhydroxybutyrate (PHB) and microscale zero-valent iron (mZVI). Results reveal that denitrification in the downgradient aquifer accounts for at least 19.5 % to 32.5 % of the total nitrate removal. In the ICH, a higher ratio of PHB to mZVI leads to higher contribution of the downgradient aquifer to nitrate removal, while a lower ratio results in smaller contribution. Microbial community analysis further reveals that heterotrophic and mixotrophic bacteria dominate in the downgradient aquifer of the PRB, and their relative abundance increases with a higher ratio of PHB to mZVI in the ICH. Within the PRB, autotrophic and iron-reducing bacteria are more prevalent, and their abundance increases as the ratio of PHB to mZVI in the ICH decreases. These findings emphasize the downgradient aquifer's substantial role in nitrate removal, particularly driven by dissolved organic carbon provided by PHB. This research holds significant implications for nutrient waste management, including the prevention of secondary pollution, and the development of cost-effective PRBs.

Reductive sequestration of Cr (VI) by phosphorylated nanoscale zerovalent iron

The cracked phosphorylated nanoscale zerovalent iron (p-nZVI) has a strong electron selectivity towards the reductive removal of many heavy metal ions in waters. However, the unintended environmental risk after interactions with impurities or wastewater are not involved. Therefore, in this study, the phosphate group was successfully adsorbed into p-nZVI, and the cracked p-nZVI was successfully prepared with an optimal P/Fe ratio of 0.5%. The dosages of p-nZVI and temperatures were positively correlated with the removal rates. The removal process of Cr(VI) was more suitable by the Langmuir isothermal model（R2 > 0.99). The process of Cr (VI) (10, 20 and 40 mg/L) removal more fitted the pseudo first-order reaction model, while the process of Cr (VI) (60, 80 mg/L) removal more fitted the pseudo second-order reaction model. The Cr (VI) removal rates gradually decreased when the pH was increased. Dissolved oxygen slowed nanoiron reaction rates. The order of inhibition on the reactivity towards Cr(VI) was SiO32- > SO42- > PO43- > NO3- > HCO3-.The facilitation followed the order of Cd2+>Cu2+>Mg2+>Mn2+>Ca2+. Ca2+ showed an inhibitory effect, but all other cations showed different degrees of facilitation. The promotion effect is relatively similar in presence of Mn2+ or Mg2+. HA had a significant inhibitory effect. Environmental friendly p-nZVI had a good effect in simulated groundwater, seawater, river water and secondary effluent of the urban sewage treatment plant. The main pathway to remove Cr (VI) was in situ reduction by p-nZVI. The improved adsorption and reduction effect of p-nZVI on heavy metal ions in water was due to the structural change and the phosphate group.

Effects of disinfectant type and dosage on biofilm's activity, viability, microbiome and antibiotic resistome in bench-scale drinking water distribution systems

Drinking water distribution systems (DWDSs) are important for supplying high-quality water to consumers and disinfectant is widely used to control microbial regrowth in DWDSs. However, the disinfectant's influences on microbial community and antibiotic resistome in DWDS biofilms and the underlying mechanisms driving their dynamics remain elusive. The study investigated the effects of chlorine and chloramine disinfection on the microbiome and antibiotic resistome of biofilms in bench-scale DWDSs using metagenomics assembly. Additionally, the biofilm activity and viability were monitored based on adenosine triphosphate (ATP) and flow cytometer (FCM) staining. The results showed that both chlorine and chloramine disinfectants decreased biofilm ATP, although chloramine at a lower dosage (1 mg/L) could increase it. Chloramine caused a greater decrease in living cells than chlorine. Furthermore, the disinfectants significantly lowered the microbial community diversity and altered microbial community structure. Certain bacterial taxa were enriched, such as Mycobacterium, Sphingomonas, Sphingopyxis, Azospira, and Dechloromonas. Pseudomonas aeruginosa exhibited high resistance towards disinfectants. The disinfectants also decreased the complexity of microbial community networks. Some functional taxa (e.g., Nitrospira, Nitrobacter, Nitrosomonas) were identified as keystones in chloramine-treated DWDS microbial ecological networks. Stochasticity drove biofilm microbial community assembly, and disinfectants increased the contributions of stochastic processes. Chlorine had greater promotion effects on antibiotic resistance genes (ARGs), mobile genetic elements (MGEs) and ARG hosts than chloramine. The disinfectants also selected pathogens, such as Acinetobacter baumannii and Klebsiella pneumonia, and these pathogens also harbored ARGs and MGEs. Overall, this study provides new insights into the effects of disinfectants on biofilm microbiome and antibiotic resistome, highlighting the importance of monitoring and managing disinfection practices in DWDSs.

Uncovering interactions among ternary electron donors of organic carbon source, thiosulfate and Fe0 in mixotrophic advanced denitrification: Proof of concept from simulated to authentic secondary effluent

To offset the imperfections of higher cost and emission of CO2 greenhouse gas in heterotrophic denitrification (HDN) as well as longer start-up time in autotrophic denitrification (ADN), we synergized the potential ternary electron donors of organic carbon source, thiosulfate and zero-valent iron (Fe0) to achieve efficient mixotrophic denitrification (MDN) of oligotrophic secondary effluent. When the influent chemical oxygen demand to nitrogen (COD/N) ratio ascended gradually in the batch operation with sufficient sulfur to nitrogen (S/N) ratio, the MDN with thiosulfate and Fe0 added achieved the highest TN removal for treating simulated and authentic secondary effluents. The external carbon is imperative for initiating MDN, while thiosulfate is indispensable for promoting TN removal efficiency. Although Fe0 hardly donated electrons for denitrification, the suitable circumneutral environment for denitrification was implemented by OH- released from Fe0 corrosion, which neutralized H+generated during thiosulfate-driven ADN. Meanwhile, Fe0 corrosion consumed the dissolved oxygen (DO) and created the low DO environment suitable for anoxic denitrification. This process was further confirmed by the continuous flow operation for treating authentic secondary effluent. The TN removal efficiency achieved its maximum under the combination condition of influent COD/N ratio of 3.1-3.5 and S/N ratio of 2.0-2.1. Whether in batch or continuous flow operation, the coordination of thiosulfate and Fe0 maintained the dominance of Thiobacillus for ADN, with the dominant heterotrophic denitrifiers (e.g., Plasticicumulans, Terrimonas, Rhodanobacter and KD4-96) coexisting in MDN system. The interaction insights of ternary electron donors in MDN established a pathway for realizing high-efficiency nitrogen removal of secondary effluent.

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.

Simultaneously enhanced autotrophic-heterotrophic denitrification in iron-based ecological floating bed by plant biomass: Metagenomics insights into microbial communities, functional genes and nitrogen metabolic pathways

In this study, the ecological floating bed supporting with zero-valent iron (ZVI) and plant biomass (EFB-IB) was constructed to improve nitrogen removal from low-polluted water. The effects of ZVI coupling with plant biomass on microbial community structure, metabolic pathways and functional genes were analyzed by metagenomic sequencing, and the mechanism for nitrogen removal was revealed. Results showed that compared with mono-ZVI system (EFB-C), the denitrification efficiencies of EFB-IB were effectively enhanced, with the higher average NO3--N removal efficiencies of 22.60-59.19%. Simultaneously, the average NH4+-N removal efficiencies were 73.08-91.10%. Metagenomic analyses showed that EFB-IB enriched microbes that involved in iron cycle, lignocellulosic degradation and nitrogen metabolism. Plant biomass addition simultaneously increased the relative abundances of autotrophic and heterotrophic denitrifying bacteria. Network analysis showed the cooperation between autotrophic and heterotrophic denitrifying bacteria in EFB-IB. Moreover, compared with EFB-C, plant biomass addition increased the relative abundances of genes related to iron cycle, lignocellulose degradation and glycolysis processes, ensuring the production of autotrophic and heterotrophic electron donors. Therefore, the relative abundances of key enzymes and functional genes related to denitrification were higher in EFB-IB, being beneficial to the NO3--N removal. Additionally, the correlation analysis of nitrogen removal and functional genes verified the synergistic mechanism of iron-based autotrophic denitrification and plant biomass-mediated heterotrophic denitrification in EFB-IB. In summary, plant biomass has excellent potential to improve the nitrogen removal of iron-based EFB from low-polluted water.

Iron-based multi-carbon composite and Pseudomonas furukawaii ZS1 co-affect nitrogen removal, microbial community dynamics and metabolism pathways in low-temperature aquaculture wastewater

Aerobic denitrification is the key process in the elimination of nitrogen from aquaculture wastewater, especially for wastewater with high dissolved oxygen and low carbon/nitrogen (C/N) ratio. However, a low C/N ratio, especially in low-temperature environments, restricts the activity of aerobic denitrifiers and decreases the nitrogen elimination efficiency. In this study, an iron-based multi-solid carbon source composite that immobilized aerobic denitrifying bacteria ZS1 (IMCSCP) was synthesized to treat aerobic (DO > 5 mg/L), low temperature (<15 °C) and low C/N ratio (C/N = 4) aquaculture wastewater. The results showed that the sequencing batch biofilm reactor (SBBR) packed with IMCSCP exhibited the highest nitrogen removal performance, with removal rates of 95.63% and 85.44% for nitrate nitrogen and total nitrogen, respectively, which were 33.03% and 30.75% higher than those in the reactor filled with multi-solid carbon source composite (MCSC). Microbial community and network analysis showed that Pseudomonas furukawaii ZS1 successfully colonized the SBBR filled with IMCSCP, and Exiguobacterium, Cellulomonas and Pseudomonas were essential for the nitrogen elimination. Metagenomic analysis showed that an increase in gene abundance related to carbon metabolism, nitrogen metabolism, extracellular polymer substance synthesis and electron transfer in the IMCSCP, enabling denitrification in the SBBR to be achieved via multiple pathways. The results of this study provided new insights into the microbial removal mechanism of nitrogen in SBBR packed with IMCSCP at low temperatures.

Promoting aerobic denitrification in reservoir water with iron-activated carbon: Enhanced nitrogen and organics removal efficiency, and biological mechanisms

Carbon scarcity limits denitrification in micropolluted water, especially in drinking water reservoirs. Therefore, a Fe-activated carbon (AC) carrier was used in this study to enhance the nitrogen removal capacity of aboriginal denitrification in drinking water reservoirs under aerobic conditions. Following carrier addition, total nitrogen (TN) and permanganate index (CODMn) removal efficiencies reached 81.89% and 72.66%, respectively, and were enhanced by 40.45% and 39.65%. Nitrogen balance analysis indicated that 77.86% of the initial TN was converted into gaseous nitrogen. Biolog analysis suggested that the metabolic activity of denitrifying bacteria was substantially enhanced. 16S rRNA gene sequencing indicated that organic degradation bacteria, hydrogen-consuming, Fe-oxidizing, and Fe-reducing denitrifying bacteria (e.g., Arenimonas, Hydrogenophaga, Zoogloea, Methylibium, and Piscinibacter) evolved into the dominant species. Additionally, napA, nirS, nirK, and nosZ genes were enriched by 3.17, 6.68, 0.40, and 6.70 folds, respectively, which is conducive to complete denitrification. These results provide a novel pathway for the use of Fe-AC to promote aerobic denitrification in micropolluted drinking water reservoirs.

Seasonal variations of biofilm C, N and S cycling genes in a pilot-scale chlorinated drinking water distribution system

Biofilms in drinking water distribution systems (DWDS) host diverse microorganisms. However, the functional attributes of DWDS biofilms and their associations with seasonality remain unclear. This study aims to characterize variations in the microbial metabolic traits of DWDS biofilms collected during different seasons, using a pilot-scale DWDS in dark under plug-flow conditions during one-year operation period. Network analysis was used to predict the functional gene hosts. The overall functional attributes determined by shotgun metagenomics exhibited significant differences among seasons. Genes associated with aromatic metabolism, fatty acid biosynthesis and degradation, and capsular extracellular polymeric substance (EPS) were significantly upregulated in summer owing to the higher temperatures and chlorine in the influent of the DWDS. Moreover, the pathways associated with nitrogen, sulfur, glycolysis, and tricarboxylic acid (TCA) cycling, as well as carbon fixation were reconstructed and displayed according to the sampling season. Nitrogen reduction pathways [dissimilatory nitrate reduction to ammonium (DNRA) 73 %, assimilatory nitrate reduction to ammonium (ANRA) 21 %] were identified in DWDS biofilms, but nitrogen oxidation pathways were not. Sulfur cycling were involved in diverse pathways and genes. Glycolysis and TCA cycling offered electron donors and energy sources for nitrogen and sulfur reduction in biofilms. Carbon fixation was observed in DWDS biofilms, with the predominant pathway for fixing carbon dioxide being the reductive citrate cycle (38 %). Constructed functional gene networks composed of carbon, nitrogen, and sulfur cycling-related genes demonstrated synergistic effects (Positive proportion: 63.52-71.09 %). In addition, from spring to autumn, the network complexity decreased and network modularity increased. The assembly mechanism of carbon, nitrogen and sulfur cycling-related genes was driven by stochastic processes for all samples. These results highlight the diverse functional genes in DWDS biofilms, their synergetic interrelationships, and the seasonality effect on functional attributes.

Novel slow-release carbon source improves anodic denitrification and electricity generation efficiency in microbial fuel cells

MFC anodic denitrification is more suitable for the coexistence of organic matter and nitrate in actual sewage, but the traditional carbon source has some problems such as high cost and difficulty of dosage control in MFC. Herein, corncob and polycaprolactone (PCL) were mechanically pulverized and mixed in the system of polyvinyl alcohol and sodium alginate, and cross-linked to prepare slow-release carbon source fillers (CPSP), which were added to the MFC anolyte to realize the coupling of solid-phase denitrification and anodic denitrification. Results showed the start-up period of MFC experimental group (MFC-C) with CPSP was slightly longer than the control group (MFC-0), but MFC-C's maximum output voltage (648.4 mV) and power density (2738 mW/m3) could be increased by 5% and 15% higher than that of MFC-0 (P < 0.05). The degradation process of MFC substrate in unit cycle was mainly divided into nitrogen removal stage (0-8 h) and electricity generation stage (8-48 h). The NO3--N and COD degradation and power generation kinetic processes of MFC conformed to the Han-Levenspiel model. Kinetics experiments showed CPSP can improve the affinity and tolerance of MFC to NO3--N, also it can alleviate the pressure of electron competition in anolyte and improve coulombic efficiency. In addition, microbial communities were significantly changed under the effect of CPSP (P < 0.001). Meanwhile, CPSP can promote the synthesis of denitrification functional genes. This study provides a new strategy to improve the performance of MFC by the addition of novel denitrification carbon source.

Performances and mechanisms of simultaneous removal of nitrate and phosphate by biofilter assembled with sponge iron/copper and corn cobs

Sponge iron (SI) is a potential material for removing nitrate and phosphate from water. We decorated the SI with copper (Cu) to enhance its removal performance. To gain insight into the nitrate and phosphate removal utilizing SI/Cu and microbial coupling systems, three biofilters filled with corn cob (CC), corn cob + sponge iron (CS) and corn cob + sponge iron/copper (CSCu) were constructed. The results showed that the effluent NO3--N and PO43--P concentrations of CSCu remained consistently below 1 and 0.1 mg/L. The introduction of SI/Cu led to the enrichment of the Dechloromonas genus, making it the dominant microbial group, occupying 42.65% of the effective sequences. Modification of SI with Cu increased nitrogen cycle-related functional genes abundance in CSCu, with a 634% increase in nirS compared to CS. These findings proved that SI/Cu was a promising material, providing an approach to concomitantly removing nitrate and phosphate.

Start-up of solid-phase denitrification process for treatment of nitrate-rich water in recirculating mariculture system: Carbon source selection and nitrate removal mechanism

Efficient nitrate removal from recirculating mariculture system (RMS) water is of significance since high concentration of nitrate would cause chronic health effects on aquatic organisms and eutrophication. Solid-phase denitrification (SPD) is a safer and more sustainable approach than conventional heterotrophic denitrification by dosing liquid carbon sources. Thus, its application for treating nitrate-rich RMS water was investigated in this study. Poly 3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) was identified with the best nitrate removal among four kinds of carbon sources. PHBV-filled reactors started with mariculture, municipal and mixing sludges (at the ratio of 1:1) and fed with 200 mg L-1 nitrate-rich RMS water all achieved over 81% nitrate removals with a HRT of 4 days. The dissolved organic carbon concentrations of the reactors were in the range of 3-9 mg L-1. Arcobacter, Halomonas, and Psedomonas were dominant genera responsible for nitrate removal in different reactors. Metagenomic analyses indicate that both denitrification and assimilatory nitrate reduction (ANR) are the main contributors to nitrate removals. Metagenomic results illustrated nirB/D cooperated with nasA may perform ANR pathway, which transformed nitrate to ammonia for biosynthesis. These results indicate that SPD could be a safer alternative for treating nitrate-rich RMS water, and provide new insights into nitrogen metabolism pathways in SPD process.

Ecological risk assessment and identification of the distinct microbial groups in heavy metal-polluted river sediments

To assess the health of river ecosystems, it is essential to quantify the ecological risk of heavy metals in river sediments and the structure of microbial communities. As important tributaries of the Tuo River in the upper reaches of the Yangtze River, the Mianyuan River and the Shiting River, are closely related to the economic development and human daily life in the region. This study assessed the ecological risks of heavy-metal-polluted river sediments, the heavy-metal-driven bacterial communities were revealed, and the relationships between the ecological risks and the identical bacterial communities were discussed. The Cd content was significantly greater than the environmental background value, leading to a serious pollution and very high ecological risk at the confluence of the two rivers and the upper reaches of the Mianyuan River. Microbial community analysis showed that Rhodobacter, Nocardioides, Sphingomonas, and Pseudarthrobacter were the dominant bacterial genera in the sediments of the Shiting River. However, the dominant bacterial genera in the Mianyuan River were Kouleothrix, Dechloromonas, Gaiella, Pedomicrobium, and Hyphomicrobium. Mantel test results showed (r = 0.5977, P = 0.005) that the Cd, As, Zn, Pb, Cr, and Cu were important factors that influenced differences in the distribution of sediment bacterial communities Mianyuan and Shiting rivers. A correlation heatmap showed that heavy metals were negatively correlated for most bacterial communities, but some bacterial communities were tolerant and showed a positive correlation. Overall, the microbial structure of the river sediments showed a diverse spatial distribution due to the influence of heavy metals. The results will improve the understanding of rivers contaminated by heavy metals and provide theoretical support for conservation and in situ ecological restoration of river ecosystems.

Simultaneous removal of heterocyclic drugs and total nitrogen from biochemical tailwater by peracetic acid/cobalt-loaded ceramsite-based denitrification biofilter

It is difficult to achieve simultaneous and efficient removal of heterocyclic drugs (HCDs) and total nitrogen (TN) in conventional denitrification biofilter (DNBF). Inspired by the effective degradation of refractory organic matter by cobalt-based advanced oxidation process and the need for in-situ upgrading of DNBF, peracetic acid (PAA)/cobalt-loaded ceramsite-based DNBF system was constructed for the first time to treat biochemical tailwater containing HCDs. Results showed that PAA/Co-DNBF had relatively high removal rates for the four HCDs with the order of CBZ > TMP > SDZ > SMX, and the optimal DNBF was H2 with 150 μg L^-1of PAA. Overall, TN and HCDs removal increased by 178%-455% and 2.50%-40.99% respectively. When the influent concentration of NO₃^--N, COD and each HCDs of 20 mg/L, 60 mg/L and 20 μg/L, below 15 mg/L of effluent TN and the highest average removal rate of SMX (67.77%) could be achieved, under HRT of 4 h in H2. More even distribution of microbial species and low acute toxicity of effluent were also achieved. More even distribution of microbial species and low acute toxicity of effluent were also achieved. In addition, high extracellular polymeric substance (EPS) content and Gordonia after the addition of PAA contributed to the degradation of HCDs. This study supplied a potentially effective strategy for the treatment of biochemical tailwater containing HCDs and provided new insight into the advance of denitrification technology.

Eggshell based biochar for highly efficient adsorption and recovery of phosphorus from aqueous solution: Kinetics, mechanism and potential as phosphorus fertilizer

Development of an efficient and green adsorbent is of great significance for phosphorus removal and recovery from eutrophic water. This work prepared an eggshell modified biochar (ESBC) by co-pyrolysis of eggshells and corn stalk. ESBC exhibited an excellent performance for phosphorus adsorption over a wide pH range (5-13), and achieved a maximum adsorption of 557.0 mg P/g. The adsorption process was well fitted by pseudo-second-order model (R² > 0.962) and Sips model (R² > 0.965), and it was endothermic (ΔH⁰ > 0) and spontaneous (ΔG⁰ < 0) according to thermodynamic analysis. The column experiment confirmed the feasibility of ESBC as a filter media for phosphorus removal in flow condition, and obtained a P removal of 460.0 mg/g. Soil burial tests indicated P-laden ESBC has a good P slow-release performance (maintained for up to 25 days). Overall, ESBC has a promising application potential as an efficient adsorbent for phosphorus recovery and subsequently as a slow-release fertilizer.

Synchronous N and P Removal in Carbon-Coated Nanoscale Zerovalent Iron Autotrophic Denitrification─The Synergy of the Carbon Shell and P Removal

Fe⁰ is a promising electron donor for autotrophic denitrification in the simultaneous removal of nitrate and phosphorus in low C/N wastewater. However, P removal may inevitably inhibit bio-denitrification. It has not been well recognized and led to an overdose of iron materials. This study employed carbon-coated zerovalent iron (Fe⁰@C) to support autotrophic denitrification to mitigate the inhibition effects of P removal and enhance both N and P removal. The critical role of the carbon shell in Fe⁰@C was to block the direct contact between Fe⁰ and P and NO₃^--N, to maintain the Fe⁰ activity. Besides, P inhibited the chemical reduction of NO₃^--N by competing for Fe⁰ active sites. This indirectly boosted H₂ generation and promoted bio-denitrification. P removal displayed negligible effects on microbial species but indirectly enhanced the nitrogen metabolic activities because of promoted H₂ in Fe⁰@C-based autotrophic denitrification. Bio-denitrification, in turn, strengthened Fe-P co-precipitation by promoting the formation of ferric hydroxide as a secondary adsorbent for P removal. This study demonstrated an efficient method for simultaneous N and P removal in autotrophic denitrification and revealed the synergistic interactions among N and P removal processes.

Air Pollutants Removal Using Biofiltration Technique: A Challenge at the Frontiers of Sustainable Environment

Air pollution is a central problem faced by industries during the production process. The control of this pollution is essential for the environment and living organisms as it creates harmful effects. Biofiltration is a current pollution management strategy that concerns removing odor, volatile organic compounds (VOCs), and other pollutants from the air. Recently, this approach has earned vogue globally due to its low-cost and straightforward technique, effortless function, high reduction efficacy, less energy necessity, and residual consequences not needing additional remedy. There is a critical requirement to consider sustainable machinery to decrease the pollutants arising within air and water sources. For managing these different kinds of pollutant reductions, biofiltration techniques have been utilized. The contaminants are adsorbed upon the medium exterior and are metabolized to benign outcomes through immobilized microbes. Biofiltration-based designs have appeared advantageous in terminating dangerous pollutants from wastewater or contaminated air in recent years. Biofiltration uses the possibilities of microbial approaches (bacteria and fungi) to lessen the broad range of compounds and VOCs. In this review, we have discussed a general introduction based on biofiltration and the classification of air pollutants based on different sources. The history of biofiltration and other mechanisms used in biofiltration techniques have been discussed. Further, the crucial factors of biofilters that affect the performance of biofiltration techniques have been discussed in detail. Finally, we concluded the topic with current challenges and future prospects.

Aeration Biofilter Filler Screening and Experimental Research on Nitrogen and Phosphorus Purification in Rural Black Water

In rural toilets, black water still remains polluted by nitrogen and phosphorus after being pre-treated by septic tanks. This study uses aerated biofilters to purify black water, screen the biofilter filler, and determine its effect on nitrogen and phosphorus purification in rural black water. This study introduced the concept of the “shape factor” into the Langmuir and Freundlich equations and optimized the isotherm adsorption model to better fit the actual dynamics of nitrogen and purification in black water. Combined with the first-order kinetic equation, the double constant equation, and the Elovich equation, the adsorption performance of seven kinds of biofilter fillers (i.e., zeolite, volcanic rock, sepiolite, ceramsite, anthracite, vermiculite, and peat) was studied. Then, the biofilter was constructed using a combination of fillers with better adsorption properties, and its ability to purify rural black water was studied. Results showed that vermiculite and zeolite had little effect on nitrogen and a high saturated adsorption of 654.50 and 300.89 mg·kg−1, respectively; peat and ceramsite had little effect on phosphorus and a high saturated adsorption of 282.41 mg·kg−1 and 233.89 mg·kg−1, respectively. The adsorption rate of nitrogen from fast to slow was vermiculite > peat > zeolite > volcanic rock > sepiolite > ceramsite > anthracite. The adsorption rate of phosphorus from fast to slow was peat > ceramsite > zeolite > sepiolite > vermiculite > volcanic rock > anthracite. Four combined biological filter fillers aided the removal of nitrogen and phosphorus from rural high-concentration black water. The combination of zeolite and ceramsite filler had a good nitrogen and phosphorus removal effect in high-concentration black water. After the system was stable, the nitrogen removal rate attained 71–73%, and the phosphorus removal rate attained 73–76% under the influent condition of total nitrogen and phosphorus concentrations of 150–162 and 10–14 mg·L−1, respectively. This study provides technical support and reference for the purification and treatment of rural black water.