Complete dechlorination and mineralization of pentachlorophenol (PCP) in a hydrogen-based membrane biofilm reactor (MBfR)

MnO2/TiO2-Catalyzed ozonolysis: enhancing Pentachlorophenol degradation and understanding intermediates

Pentachlorophenol is a pesticide widely known for its harmful effects on sewage, causing harm to the environment. In previous studies, our group identified adsorption as a crucial factor in catalytic ozonation processes, and subsequent observations revealed the catalyst's role in reducing toxicity during degradation. In this research, we quantified organochlorine intermediates and low molecular weight organic acids generated under optimal pH conditions (pH 9), with and without the catalyst. Additionally, we assessed the reactivity of these intermediates through theoretical calculations. Our findings indicate that the catalyst reduces the duration of intermediates. Additionally, the presence of CO2 suggests enhanced mineralization of pentachlorophenol, a process notably facilitated by the catalyst. Theoretical calculations, such as Fukui analysis, offer insights into potential pathways for the dechlorination of aromatic molecules by radicals like OH, indicating the significance of this pathway.

Use of nitrate, sulphate, and iron (III) as electron acceptors to improve the anaerobic degradation of linear alkylbenzene sulfonate: effects on removal potential and microbiota diversification

Linear alkylbenzene sulfonate (LAS) is a synthetic anionic surfactant that is found in certain amounts in wastewaters and even in water bodies, despite its known biodegradability. This study aimed to assess the influence of nitrate, sulphate, and iron (III) on LAS anaerobic degradation and biomass microbial diversity. Batch reactors were inoculated with anaerobic biomass, nutrients, LAS (20 mg L-1), one of the three electron acceptors, and ethanol (40 mg L-1) as a co-substrate. The control treatments, with and without co-substrate, showed limited LAS biodegradation efficiencies of 10 ± 2% and 0%, respectively. However, when nitrate and iron (III) were present without co-substrate, biodegradation efficiencies of 53 ± 4% and 75 ± 3% were achieved, respectively, which were the highest levels observed. Clostridium spp. was prominent in all treatments, while Alkaliphilus spp. and Bacillus spp. thrived in the presence of iron, which had the most significant effect on LAS biodegradation. Those microorganisms were identified as crucial in affecting the LAS anaerobic degradation. The experiments revealed that the presence of electron acceptors fostered the development of a more specialised microbiota, especially those involved in the LAS biodegradation. A mutual interaction between the processes of degradation and adsorption was also shown.

Pioneering Piezoelectric-Driven Atomic Hydrogen for Efficient Dehalogenation of Halogenated Organic Pollutants

The electrocatalytic hydrodehalogenation (EHDH) process mediated by atomic hydrogen (H*) is recognized as an efficient method for degrading halogenated organic pollutants (HOPs). However, a significant challenge is the excessive energy consumption resulting from the recombination of H* to H2 production in the EHDH process. In this study, a promising strategy was proposed to generate piezo-induced atomic H*, without external energy input or chemical consumption, for the degradation and dehalogenation of HOPs. Specifically, sub-5 nm Ni nanoparticles were subtly dotted on an N-doped carbon layer coating on BaTiO3 cube, and the resulted hybrid nanocomposite (Ni-NC@BTO) can effectively break C-X (X = Cl and F) bonds under ultrasonic vibration or mechanical stirring, demonstrating high piezoelectric driven dehalogenation efficiencies toward various HOPs. Mechanistic studies revealed that the dotted Ni nanoparticles can efficiently capture H* to form Ni-H* (Habs) and drive the dehalogenation process to lower the toxicity of intermediates. COMSOL simulations confirmed a "chimney effect" on the interface of Ni nanoparticle, which facilitated the accumulation of H+ and enhanced electron transfer for H* formation by improving the surface charge of the piezocatalyst and strengthening the interfacial electric field. Our work introduces an environmentally friendly dehalogenation method for HOPs using the piezoelectric process independent of the external energy input and chemical consumption.

Reduction of Chromate via Biotic and Abiotic Pathways in the Presence of Three Co-contaminating Electron Acceptors

Bioreduction of Cr(VI) to Cr(III) is a promising technology for removing Cr(VI), but Cr(VI) reduction alone cannot support microbial growth. This study investigated the reduction of Cr(VI) in the presence of three electron acceptors that typically coexist with Cr(VI): NO3-, SO42-, and Fe(III). All three systems could reduce Cr(VI) to Cr(III), but the fate of Cr, its impacts on reduction of the other acceptors, and its impact on the microbial community differed. Although Cr(VI) was continuously removed in the NO3--reduction systems, batch tests showed that denitrification was inhibited primarily through impeding nitrite reduction. The SO42- and Fe(III) reduction systems reduced Cr(VI) using a combination of biotic and abiotic processes. Across all three systems, the abundance of genera capable of reducing Cr(VI) increased following the introduction of Cr(VI). Conversely, the abundance of genera that cannot reduce or resist Cr(VI) decreased, leading to restructuring of the microbial community. Furthermore, the abundance of sulfide oxidizers and Fe(II) oxidizers substantially increased after the introduction of chromate. This study provides fundamental knowledge about how Cr(VI) bioreduction interacts with bioreductions of three other co-contaminating electron acceptors.

Biodegradation of nitrate and p-bromophenol using hydrogen-based membrane biofilm reactors in parallel

ABSTRACTP-bromophenol (4-BP) is a toxic halogenated phenolic organic compound. The conventional treatment processes for 4-BP elimination are costly and inefficient, with complete mineralization remaining a challenge for water treatment. To overcome these limitations, we investigated the treatment of 4-BP in a membrane biofilm reactor (MBfR) using hydrogen as an electron donor. The pathway of 4-BP degradation within the H2-MBfR was investigated through long-term operational experiments by considering the effect of nitrate and 4-BP concentrations, hydrogen partial pressure, static experiments, and microbial community diversity, which was studied using 16S rRNA. The results showed that H2-MBfR could quickly remove approximately 100% of 4-BP (up to 20 mg/L), with minimal intermediate product accumulation and 10 mg/L of nitrate continuously reduced. The microbial community structure showed that the presence of H2 created an anaerobic environment, and Thauera was the dominant functional genus involved in the degradation of 4-BP. The genes encoding related enzymes were further enhanced. This study provides an economically viable and environmentally friendly bioremediation technique for water bodies that contain 4-BP and nitrates.

Comparative insight of pesticide transformations between river and wetland systems

The widespread use of pesticides threatens the environment and ecosystems. Despite the positive effects of plant protection products, pesticides also have unexpected negative effects on nontarget organisms. The microbial biodegradation of pesticides is one of the major pathways for reducing their risks at aquatic ecosystems. The objective of this study was to compare the biodegradability of pesticides in simulated wetland and river systems. Parallel experiments were conducted with 17 pesticides based on the OECD 309 guidelines. A comprehensive analytical method, such as target screening combined with suspect and non-target screening, was performed to evaluate the biodegradation via identification of transformation products (TPs) using LC-HRMS. As evidence of biodegradation, we identified 97 TPs for 15 pesticides. Metolachlor and dimethenamid had 23 and 16 TPs, respectively, including Phase II glutathione conjugates. The analysis of 16S rRNA sequences for microbials characterized operational taxonomic units. Rheinheimera and Flavobacterium, which have the potential for glutathione S-transferase, were dominant in wetland systems. Estimation of toxicity, biodegradability, and hydrophobicity using QSAR prediction indicated lower environmental risks of detected TPs. We conclude that the wetland system is more favorable for pesticide degradation and risk mitigation mainly attributed to the abundance and variety of the microbial communities.

Biological reduction and hydrodechlorination of chlorinated nitroaromatic antibiotic chloramphenicol under H2-transfer membrane biofilm reactor

Chlorinated nitroaromatic antibiotic chloramphenicol (CAP) is a persistent pollutant that is widely present in environments. A H₂ transfer membrane biofilm reactor (H₂-MBfR) and short-term batch tests were setup to investigate the co-removal of CAP and NO₃^-. Results showed that the presence of CAP (<10 mg L^-1) has no effect on the denitrification process while 100% removal efficiency of CAP can be obtained when nitrate was absent. Nitroaromatic reduction and completely dechlorination were successfully realized when CAP was removed. The CAP transformation product p-aminobenzoic acid (PABA) was detected and batch tests revealed that the hydroxy carboxylation was far faster than nitroaromatic reduction when p-nitrobenzyl alcohol (PNBOH) was conversed to p-aminobenzoic acid (PABA). The path way of CAP degradation was proposed based on the intermediate's analysis. Microbial community analysis indicated that Pleomorphomonadaceae accounts for the dechlorination of CAP.

Removal of pentachlorophenol and phenanthrene from lignocellulosic biorefinery wastewater by a biocatalytic/biosurfactant system comprising cross-linked laccase aggregates and rhamnolipid

Synthesis and characterization of highly active cross-linked laccase aggregates (CLLAs) were performed and evaluated for removal of pentachlorophenol and phenanthrene from lignocellulosic biorefinery wastewater. Laccase from Tramates versicolor MTCC 138 was insolubilized as CLLAs via precipitation with 70% ammonium sulphate and simultaneous cross-linking with 5 mM glutaraldehyde to obtain activity recovery of 89.1%. Compared to the free laccase, the pH and thermal stability of the prepared CLLAs were significantly higher. At a high temperature of 60 °C, free laccase had a half-life of 0.25 h, while CLLAs had a half-life of 6.2 h. In biorefinery wastewater (pH 7.0), the free and CLLAs were stored for 3 day at a temperature of 30 °C. Free laccase completely lost their initial activity after 60 h; however, the CLLAs retained 39% activity till 72 h. Due to its excellent stability, free laccase and CLLAs were assessed for removing pentachlorophenol and phenanthrene in wastewater. CLLAs could remove 51-58% of pentachlorophenol (PCP) and phenanthrene (PHE) in 24 h. Biosurfactants, including surfactin, sophorolipid, and rhamnolipid, were assessed for their aptitude to improve the removal of organic contaminants in wastewater. Biorefinery wastewater incubated with all surfactants enhanced PCP and PHE removal compared to the no-surfactant controls. Further, 1 μM rhamnolipid significantly amplified pentachlorophenol and phenanthrene removal to 81-93% for free laccase and CLLAs, respectively.

Biofilm-based technology for industrial wastewater treatment: current technology, applications and future perspectives

The microbial community in biofilm is safeguarded from the action of toxic chemicals, antimicrobial compounds, and harsh/stressful environmental circumstances. Therefore, biofilm-based technology has nowadays become a successful alternative for treating industrial wastewater as compared to suspended growth-based technologies. In biofilm reactors, microbial cells are attached to static or free-moving materials to form a biofilm which facilitates the process of liquid and solid separation in biofilm-mediated operations. This paper aims to review the state-of-the-art of recent research on bacterial biofilm in industrial wastewater treatment including biofilm fundamentals, possible applications and problems, and factors to regulate biofilm formation. We discussed in detail the treatment efficiencies of fluidized bed biofilm reactor (FBBR), trickling filter reactor (TFR), rotating biological contactor (RBC), membrane biofilm reactor (MBfR), and moving bed biofilm reactor (MBBR) for different types of industrial wastewater treatment. Besides, biofilms have many applications in food and agriculture, biofuel and bioenergy production, power generation, and plastic degradation. Furthermore, key factors for regulating biofilm formation were also emphasized. In conclusion, industrial applications make evident that biofilm-based treatment technology is impactful for pollutant removal. Future research to address and improve the limitations of biofilm-based technology in wastewater treatment is also discussed.

Simultaneous removal of nitrate, nitrobenzene and aniline from groundwater in a vertical baffled biofilm reactor

The challenge of simultaneous removal of nitrobenzene (NB), aniline (AN) and nitrate from groundwater in a single bioreactor is mainly attributed to the persistence of AN to degradation with anoxic denitrification conditions. In this work, simultaneous removal of NB (100 μM), AN (100 μM) and nitrate (1 mM) was achieved within 8 h with a COD/N ratio of 8 in a vertical baffled biofilm reactor (VBBR). By setting DO concentration at 0.4-0.5 mg L^-1 to create a micro-aerobic condition, NB removal rate was accelerated without accumulation of AN, and AN could serve as electron donors for denitrification after ring cleavage. High-throughput sequencing showed that biofilm was predominated by denitrifiers (Luteimonas, Planctomyces, Thiobacillus, Thauera and so on) and NB-degrading bacteria (Pseudomonas), and biodiversity varied vertically along the height of the reactor. A dominantly anaerobic pathway for reducing NB to AN was identified by PICRUSt analysis, as the predicted genes involved in aerobic transformation of NB were several magnitudes lower than those in the anaerobic pathway. This study provided a new insight to the role of oxygen in robust bioremediation groundwater contaminated with NB, AN and nitrate.

Enhanced removal of diclofenac via coupling Pd catalytic and microbial processes in a H2-based membrane biofilm reactor: Performance, mechanism and biofilm microbial ecology

Diclofenac (DCF) is a most widely used anti-inflammatory drug, which has attracted worldwide attention given its low biodegradability and ecological damage, especially toxic effects on mammals including humans. In this study, a H₂-based membrane biofilm reactor (H₂-MBfR) was constructed with well-dispersed Pd nanoparticles generated in situ. The Pd-MBfR was applied for catalytic reductive dechlorination of DCF. In batch tests, DCF concentration had significantly effect on the rate and extent DCF removal, and NO₃^- had negative impact on DCF reductive dechlorination. Over 67% removal of 0.5 mg/L DCF and 99% removal of 10 mg/L NO₃^--N were achieved in 90 min, and the highest removal of 97% was obtained at 0.5 mg/L DCF in the absence of NO₃^-. Over 78 days of continuous operation, the highest steady-state removal flux of DCF was 0.0097 g/m²/d. LC-MS analysis indicated that the major product was 2-anilinephenylacetic acid (APA). Dechlorination was the main removal process of DCF mainly owing to the catalytic reduction by PdNPs, microbial reduction, and the synergistic reduction of microbial and PdNPs catalysis using direct delivery of H₂. Moreover, DCF reductive Dechlorination shifted the microbial community in the biofilms and Sporomusa was responsible for DCF degradation. In summary, this work expands a remarkable feasibility of sustainable catalytic removal of DCF.

Humin and biochar accelerated microbial reductive dechlorination of 2,4,6-trichlorophenol under weak electrical stimulation

The extracellular electron transfer (EET) is regarded as one of the crucial factors that limit the application of the bioelectrochemical system (BES). In this study, two different solid-phase redox mediators (RMs), biochar (1.2 g/L, T-B) and humin (1.2 g/L, T-H) were used for boosting the microorganisms accessing the electrons required for 2,4,6-TCP dechlorination under weak electrical stimulation (-0.278 V vs. Standard hydrogen electrode). BES with dissolved RM anthraquinone-2,6-disulfonate (AQDS 0.5 mmol/L, T-A) was used as a comparison. The results showed that dechlorination of 2,4,6-TCP could be greatly accelerated by biochar (1.78 d^-1) and humin (1.50 d^-1) than AQDS (0.24 d^-1) and no RM control (T-M, 0.27 d^-1). Moreover, phenol became the predominant dechlorination product in T-H (78.5 %) and T-B (63.0 %) instead of 4-CP in T-M (67.1 %) and T-A (89.8 %). Pseudomonas, Sulfurospirillum, Desulfuromonas, Dehalobacter, Anaeromyxobacter, and Dechloromonas belonging to Proteobacteria or Firmicutes rather than Chloroflexi might be responsible for the dechlorination activity. Notably, different RMs tended to stimulate distinct electroactive bacteria. Pseudomonas was the most abundant microorganism in T-M (41.92 %) and T-A (17.24 %), while Rhodobacter was most prevalent in T-H (20.04 %) and Azonexus was predominant in T-B (48.48 %). This study is essential in advancing the understanding of EET in BES for microbial degradation of organohalide contaminants under weak electrical stimulation.

Long-term effects of Cu(II) on denitrification in hydrogen-based membrane biofilm reactor: Performance, extracellular polymeric substances and microbial communities

Divalent copper (Cu(II)) frequently coexists with nitrate (NO₃^-) in industrial wastewater and the effect of Cu(II) on the autotrophic denitrification system using H₂ as the electron donor remains unknown. In this study, the hydrogen-based membrane biofilm reactor (H₂-MBfR) was operated continuously over 150 days to explore the effect of Cu(II) on the performance of autotrophic denitrification system and understand the key roles of EPS and microbial community. More than 95% of 20 mg-N/L NO₃^- was removed at 1-5 mg/L Cu(II), and the removal rate of NO₃^--N was stabilized to 82% at 10 mg/L Cu(II) after a short period, while NH₄⁺ and NO₂^- in effluent were hardly detected, indicated that high concentration of Cu(II) did not permanently inhibit the denitrification performance in H₂-MBfR. Colorimetric determination showed that Cu(II) stimulated the secretion of EPS, in which the protein (PN) content was much higher than polysaccharide (PS). The PN/PS ratios increased from 0.93 to 1.99, and the PN was more sensitive to copper invasion. The results of three-dimensional excitation-emission matrix illustrated that tryptophan was the main component of EPS chelating Cu(II) to reduce toxicity. The results of Fourier-transform infrared demonstrated that hydroxyl, carboxyl, and protein amide groups bound and reduced Cu(II). Furthermore, Cu(II) was effectively removed (>80%), and the results of distribution and morphology analysis of Cu(II) show that the electron-dense deposits of monovalent copper (Cu(I)) were found in EPS and biofilms and the reduction of Cu(II) to Cu(I) was an obvious self-defense reaction of biofilm to copper stress. The microbial richness and diversity decreased with the long-term exposure to Cu(II), while the relative abundance of denitrifiers Azospira and Dechloromonas increased. This study provides a scientific basis for the optimal design of treatment system for removal of nitrate and recovery of heavy metals simultaneously.

Concurrent anaerobic chromate bio-reduction and pentachlorophenol bio-degradation in a synthetic aquifer

Chromate [Cr(VI)] and pentachlorophenol (PCP) coexist widely in the environment and are highly toxic to public health. However, whether Cr(VI) bio-reduction is accompanied by PCP bio-degradation and how microbial communities can keep long-term stability to mediate these bioprocesses in aquifer remain elusive. Herein, we conducted a 365-day continuous column experiment, during which the concurrent removals of Cr(VI) and PCP were realized under anaerobic condition. This process allowed for complete Cr(VI) bio-reduction and PCP bio-degradation at an efficiency of 92.8 ± 4.2% using ethanol as a co-metabolic substrate. More specifically, Cr(VI) was reduced to insoluble chromium (III) and PCP was efficiently dechlorinated with chloride ion release. Collectively, Acinetobacter and Spirochaeta regulated Cr(VI) bio-reduction heterotrophically, while Pseudomonas mediated not only Cr(VI) bio-reduction but also PCP bio-dechlorination. The bio-dechlorinated products were further mineralized by Azospira and Longilinea. Genes encoding proteins for Cr(VI) bio-reduction (chrA and yieF) and PCP bio-degradation (pceA) were upregulated. Cytochrome c and intracellular nicotinamide adenine dinucleotide were involved in Cr(VI) and PCP detoxification by promoting electron transfer. Taken together, our findings provide a promising bioremediation strategy for concurrent removal of Cr(VI) and PCP in aquifers through bio-stimulation with supplementation of appropriate substrates.

Influence of nitrate concentration on trichloroethylene reductive dechlorination in weak electric stimulation system

The co-existence of volatile chlorinated hydrocarbons (VCHs) and nitrate pollution in groundwater is prominent, but how nitrate exposure affects weak-electrical stimulated bio-dechlorination activity of VCH is largely unknown. Here, by establishing weak-electrical stimulated trichloroethylene (TCE) dechlorination systems, the influence on TCE dechlorination by exposure to the different concentrations (25-100 mg L^-1) of nitrate was investigated. The existence of nitrate in general decreased TCE dechlorination efficiency to varying degrees, and the higher nitrate concentration, the stronger the inhibitory effects, verified by the gradually decreased transcription levels of tceA. Although the TCE dechlorination kinetic rate constant decreased by 36% the most, under all nitrate concentration ranges, TCE could be completely removed within 32 h and no difference in generated metabolites was found, revealing the well-maintained dechlorination activity. This was due to the quickly enriched bio-denitrification activity, which removed nitrate completely within 9 h, and thus relieved the inhibition on TCE dechlorination. The obvious bacterial community structure succession was also observed, from dominating with dechlorination genera (e.g., Acetobacterium, Eubacterium) to dominating with both dechlorination and denitrification genera (e.g., Acidovorax and Brachymonas). The study proposed the great potential for the in situ simultaneous denitrification and dehalogenation in groundwater contaminated with both nitrate and VCHs.

Mainstream nitrogen removal in membrane aerated biofilm reactor at minimal lumen pressure

Nitrogen removal via anammox is a promising and sustainable solution in mainstream wastewater treatment. To maintain stable anammox process, competitors of anammox bacteria should be suppressed while cooperators need to be favoured. This study demonstrated a synchronous aerobic and anaerobic ammonium removal process in a membrane aerated biofilm reactor (MABR) under minimal lumen pressure. By adjusting the lumen pressure, aerobic and anaerobic ammonium oxidation rate can be synchronized to minimize interference of nitrite oxidizing bacteria (NOB) by limiting NOB's access to both oxygen and nitrite. Long-term performance indicated that PN/A in MABR could be achieved at zero positive aeration pressure. Furthermore, by connecting two MABRs in series, high total nitrogen (TN) removal efficiency of 71.1% ± 5.3% was attained with a TN removal rate of 30.1 ± 3.2 mg-N/L/d. The organic carbon present in the wastewater reduced the nitrate concentration in the effluent while not affecting the overall nitrogen removal efficiency and rate. Real-time qPCR analysis suggested that the abundance of amoA gene was relatively stable while K-strategist Nitrospira 16S rRNA gene did not surge in the long-term operation. High throughput sequencing showed that Candidatus Brocadia and uncultured anaerobic ammonium oxidizing bacteria from Chloroflexi were the most abundant anammox taxa. Denitrifiers, such as Denitratisoma may be responsible to reduce the nitrate in the effluent.

Destruction of 4-chlorophenol by the hydrogen-accelerated catalytic Fenton system enhanced by Pd/NH2-MIL-101(Cr)

4-chlorophenol (4-CP) could be rapidly mineralized by using Fenton reaction. However, massive iron sludge will be generated because of the excessive consumption of iron salt and poor recycling of Fe^III back to Fe^II. In this paper, by introducing hydrogen gas and solid catalyst Pd/NH₂-MIL-101(Cr) to classic Fenton reactor, the novel system named MHACF-NH₂-MIL-101(Cr) was constructed. Much less Fe^II was needed in this system because the hydrogen could significantly accelerate the regeneration of Fe^II. The catalyst improved the utilization of H₂. The degradation reaction of 4-CP could be driven by using only trace amount of Fe^II. It could be rapidly degraded by the hydroxyl radical detected by the 4-Hydroxy-benzoicacid which is the oxidative product of benzoic acid and hydroxyl radical. The effects of dosage of ferrous salt, H₂O₂ and catalyst, H₂ flow, Pd content, and initial pH of and concentration of 4-CP aqueous solution were investigated. The robustness and morphology changes of this catalytic material were also systematically analysed. By clarifying the role of this solid MOFs material in this hydrogen-mediated Fenton reaction system, it will provide a new direction for the research and development of advanced oxidation processes with high efficiency and low sludge generation in future.

Iron carriers promote biofilm formation and p-nitrophenol degradation

Vertical baffled biofilm reactors (VBBR) equipped with Plastic-carriers and Fe-carriers were employed to explore the effect of biofilm carriers on biofilm formation and p-nitrophenol (PNP) degradation. The results showed that Fe-carriers enhanced biofilm formation and PNP degradation. The maximum thickness of biofilm grown on the Fe-carriers was 1.5-fold higher than that on the Plastic-carriers. The Fe-VBBR reached a maximum rate of PNP removal at 13.02 μM L^-1 h^-1 with less sodium acetate addition (3 mM), while the maximum rate of PNP removal was 11.53 μM L^-1 h^-1 with more sodium acetate addition (6 mM) in the Plastic-based VBBR. High-throughput sequencing suggested that the Fe-VBBR had a higher biodiversity of the bacterial community in evenness, and the Achromobacter genus and Xanthobacteraceae family were as main PNP degraders. Kyoto Encyclopedia of Genes and Genomes (KEGG) Orthology analysis suggested more abundances of iron uptake genes were expressed to transport iron into the cytoplasm under an iron-limited condition in two VBBRs, and the metabolic pathway of PNP degradation went through 4-nitrocatechol and 1,2,4-benzenetriol. Our results provide a new insight for iron enhancing biofilm formation and PNP degradation.

How phenol stresses anammox for the treatment of ammonia-rich wastewater: Phenomena, microbial community evolution and molecular modeling

Phenol is a biotoxic organic compound and found in large quantities in ammonia-rich wastewater discharged from coking and petrochemical industries. In this work, phenol was fed to the system of anaerobic ammonia oxidation (anammox), and the possible inhibitory mechanism was speculated using the characterization of granular sludge, analysis of microbial community and molecular docking simulations. The results showed that phenol (0-300 mg/L) did not significantly inhibit anammox. However, phenol did activate denitrification, which increased the nitrogen removal rate (NRR) by 0.94 kg N/(m³·d). Moreover, when phenol concentration reached t400 mg/L, the NRR was inhibited by 70%, while the extracellular polymeric substance (EPS) of granular sludge was reduced. Phenol resulted in the reduction of Candidatus_Kuenenia and promoted the proliferation of phenol-degrading denitrifying bacteria, Azoarcus and Thauera. Molecular docking indicated that phenol, 2-nitrophenol and 4-nitrophenol could bind the nitrite reductase (NirS), which prevented the first step of the anammox reaction.

NOB suppression strategies in a mainstream membrane aerated biofilm reactor under exceptionally low lumen pressure

Integrating the aeration-efficient membrane aerated biofilm reactor (MABR) with anaerobic ammonium oxidation (anammox) could yield further reduction in energy in wastewater treatment facilities. However, nitrite oxidizing bacteria (NOB) suppression remained challenging due to the absence of intrinsic inhibition factors in mainstream conditions. This study investigated selective NOB suppression strategies in MABR under <5 kPa lumen pressure. Three MABRs were seeded from different seeding sludge, and operated under various ammonium loading rates, aeration pressure, and temporary inhibitory shock conditions. The three reactors were operated for 170-456 days depending on studied parameters. The results showed that higher ammonium loading could create a substrate-oxygen imbalance and quickly contain emergent NOB activity when aeration pressure was not excessive. In addition, lowering of aeration pressure reversed nitrite oxidizing activities without affecting ammonium oxidizing bacteria (AOB). Cultivating partial nitritation biofilm under zero positive aeration pressure slowed down the growth of NOB yet resulted in self-induced anammox activities. With the aid of temporary free ammonia (FA)/free nitrous acid (FNA) treatment, full-nitrifying biofilm could be transformed to stable partial nitritation biofilm. More than 84% nitrite accumulation ratio (NAR) was sustained during stable operation in each reactor together with an ammonium removal rate of more than 100 mg-N/L/d. Microbial analysis revealed that Nitrosomonas was the main AOB taxon in the three reactors while K-strategist Nitrospira showed presence despite low nitrite oxidizing activities. Under zero positive pressure, proliferation of Nitrospira was much slower while Candidatus Brocadia was self-induced. Furthermore, Nitrospira showed downturn after temporary inhibition treatment.

Co-removal of 2,4-dichlorophenol and nitrate using a palladized biofilm: Denitrification-promoted microbial mineralization following catalytic dechlorination

The effects of nitrate on 2,4-dichlorophenol (2,4-DCP) dechlorination and biodegradation in a hydrogen (H₂)-based palladized membrane biofilm reactor (Pd-MBfR) were studied. The Pd-MBfR was created by synthesizing palladium nanoparticle (Pd⁰NPs) that spontaneously associated with the biofilm to form a Pd⁰-biofilm. Without input of nitrate, the Pd-MBfR had rapid and stable catalytic hydrodechlorination: 93% of the 100-μM influent 2,4-DCP was continuously converted to phenol, part of which was then fermented via acetogenesis and methanogenesis. Introduction of nitrate enabled phenol mineralization via denitrification with only a minor decrease in catalytic hydrodechlorination. Phenol-degrading bacteria capable of nitrate respiration were enriched in the Pd⁰-biofilm, which was dominated by the heterotrophic genera Thauera and Azospira. Because the heterotrophic denitrifiers had greater yields than autotrophic denitrifiers, phenol was a more favorable electron donor than H₂ for denitrification. This feature facilitated phenol mineralization and ameliorated denitrification inhibition of catalytic dechlorination through competition for H₂. Increased nitrite loading eventually led to deterioration of the dechlorination flux and selectivity toward phenol. This study documents simultaneous removal of 2,4-DCP and nitrate in the Pd-MBfR and interactions between the two reductions.

Recent advances in membrane biofilm reactor for micropollutants removal: Fundamentals, performance and microbial communities

The occurrence of micropollutants (MPs) in water and wastewater imposes potential risks on ecological security and human health. Membrane biofilm reactor (MBfR), as an emerging technology, has attracted much attention for MPs removal from water and wastewater. The review aims to consolidate the recent advances in membrane biofilm reactor for MPs removal from the standpoint of fundamentals, removal performance and microbial communities. First, the configuration and working principles of MBfRs are reviewed prior to the discussion of the current status of the system. Thereafter, a comprehensive review of the MBfR performance for MPs elimination based on literature database is presented. Key information on the microbial communities that are of great significance for the removal performance is then synthesized. Perspectives on the future research needs are also provided in this review to ensure the development of MBfRs for more cost-effective elimination of MPs from water and wastewater.

Hydrodefluorination of Perfluorooctanoic Acid in the H2-Based Membrane Catalyst-Film Reactor with Platinum Group Metal Nanoparticles: Pathways and Optimal Conditions

PFAAs (perfluorinated alkyl acids) have become a concern because of their widespread pollution and persistence. A previous study introduced a novel approach for removing and hydrodefluorinating perfluorooctanoic acid (PFOA) using palladium nanoparticles (Pd⁰NPs) in situ synthesized on H₂-gas-transfer membranes. This work focuses on the products, pathways, and optimal catalyst conditions. Kinetic tests tracking PFOA removal, F^- release, and hydrodefluorination intermediates documented that PFOA was hydrodefluorinated by a mixture of parallel and stepwise reactions on the Pd⁰NP surfaces. Slow desorption of defluorination products lowered the catalyst's activity for hydrodefluorination. Of the platinum group metals studied, Pd was overall superior to Pt, Rh, and Ru for hydrodefluorinating PFOA. pH had a strong influence on performance: PFOA was more strongly adsorbed at higher pH, but lower pH promoted defluorination. A membrane catalyst-film reactor (MCfR), containing an optimum loading of 1.2 g/m² Pd⁰ for a total Pd amount of 22 mg, removed 3 mg/L PFOA during continuous flow for 90 days, and the removal flux was as high as 4 mg PFOA/m²/d at a steady state. The EPA health advisory level (70 ng/L) also was achieved over the 90 days with the influent PFOA at an environmentally relevant concentration of 500 ng/L. The results document a sustainable catalytic method for the detoxification of PFOA-contaminated water.

Bioelectrochemical systems with a cathode of stainless-steel electrode for treatment of refractory wastewater: Influence of electrode material on system performance and microbial community

The large-scale application of the bioelectrochemical system (BES) is limited by the cost-effective electrode materials. In this study, five kinds of stainless-steel materials were used as the cathode of the BES coupled with anaerobic digestion (BES-AD) for the treatment of diluted N, N-dimethylacetamide (DMAC) wastewater. Compared with a carbon-cloth cathode, BES-AD with a stainless-steel cathode had more engineering due to its low cost, although the operating efficiencies were slightly inferior. Stainless-steel mesh with a 100 µm aperture (SSM-100 μm) was the most cost-effective electrode and the implanted BES exhibited better COD removal efficiency, electrochemical performance and biodegradability. Analysis of microbial community revealed the synergetic effect between exoelectrogen and fermentative bacteria had been strengthened in the SSM-100 μm cathode biofilm. Function analysis of the microbial community based on PICRUSt predicted metagenomes revealed that the metabolic pathways of xenobiotics biodegradation and metabolism in the SSM-100 μm cathode were stimulated.

Efficacy of a novel electrochemical membrane-aerated biofilm reactor for removal of antibiotics from micro-polluted surface water and suppression of antibiotic resistance genes

An electrochemical membrane-aerated biofilm reactor (EMABR) was developed for removing sulfamethoxazole (SMX) and trimethoprim (TMP) from contaminated water. The exertion of electric field greatly enhanced the degradation of SMX and TMP in the EMABR (~60%) compared to membrane-aerated biofilm reactor (MABR, < 10%), due to the synergistic effects of the electro-oxidation (the generation of reactive oxygen species) and biological degradation. Microbial community analyses demonstrated that the EMABR enriched the genus of Xanthobacter, which was potentially capable of degrading aromatic intermediates. Moreover, the EMABR had a lower relative abundance of antibiotic resistance genes (ARGs) (0.23) compared to the MABR (0.56), suggesting the suppression of ARGs in the EMABR. Further, the SMX and TMP degradation pathways were proposed based on the detection of key intermediate products. This study demonstrated the potential of EMABR as an effective technology for removing antibiotics from micro-polluted surface water and suppressing the development of ARGs.

Groundwater contaminated with short-chain chlorinated paraffins and microbial responses

The vertical migrations of toxic and persistent short-chain chlorinated paraffins (SCCPs) in soils as well as the microbial responses have been reported, however, there is a paucity of data on the resulting groundwater contamination. Here, we determined the concentration and congener profile of SCCPs in the groundwater beneath a production plant of chlorinated paraffins (CPs) and characterized the microbial community to explore their responses to SCCPs. Results showed that SCCPs ranged from not detected to 70.3 μg/L, with C₁₃-CPs (11.2-65.8%) and Cl₇-CPs (27.2-50.6%), in mass ratio, as the dominant groups. Similar to the distribution pattern in soils, SCCPs in groundwater were distributed in hotspot pattern. CP synthesis was the source of SCCPs in groundwater and the entire contamination plume significantly migrated downgradient, while there was an apparent hysteresis of C₁₃-CP migration. Groundwater microbial community was likely shaped by both hydrogeological condition (pH and depth) and SCCPs. Specifically, the microbial community responded to the contamination by forming a co-occurrence network with "small world" feature, where Desulfobacca, Desulfomonile, Ferritrophicum, Methylomonas, Syntrophobacter, Syntrophorhabdus, Syntrophus, and Thermoanaerobaculum were the keystone taxa. Furthermore, the interrelations between bacterial taxa and SCCPs indicated that the microbial community might cooperate to achieve the dechlorination and mineralization of SCCPs through either anaerobic organohalide respiration mainly functioned by the keystone taxa, or cometabolic degradation processes functioned by Aquabacterium and Hydrogenophaga. Results of this study would provide a better understanding of the environmental behavior and ecological effects of SCCPs in groundwater systems.

Continuous and efficient immobilization of heavy metals by phosphate-mineralized bacterial consortium

Traditional sewage treatment technology cannot remove heavy metals, which needs to be improved urgently. Lysinibacillus with the function of bio-mineralization was screened and loaded on granular sludge to form a phosphate-mineralized bacterial consortium, which demonstrated the ability of self-regulating pH and automatic solid-liquid separation. Heavy metals could be fixed on the bacterial consortium to produce stable and harmless phosphate minerals. The highest removal efficiency of Pb(Ⅱ), Cd(Ⅱ), and Ni(Ⅱ) were 97.9%, 70%, and 40%, respectively. Organic matter and other metal ions in actual polluted water had little effect on the Pb(Ⅱ) removal efficiency. Mechanism analysis was conducted through 3D-EEM, XRD, SEM-EDS, XPS, FTIR, and high-throughput sequencing analyses. The bacterial consortium was a multi-species coexistence system, but Lysinibacillus played a major role in removing Pb(Ⅱ). C-O and O-H bonds of tyrosine and phosphorous organics were broken by enzyme catalysis and the metal-oxygen bond (Pb-O) was formed. Mineral crystals in the reactor accumulated, transforming from the initial phase non-crystalline structure to the metaphase Pb₃(PO₄)₂ and eventually to the Pb₅(PO₄)₃OH. This research obtained a promising technique for immobilizing Pb(Ⅱ) or other hazardous metals continuously and efficiently.

WITHDRAWN: Progress in the removal of organic microcontaminants from wastewater using high retention membrane bioreactors: A critical review

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Performance of photocatalytic ozonation process for pentachlorophenol (PCP) removal in aqueous solution using graphene-TiO2 nanocomposite (UV/G-TiO2/O3)

The aim of this study was to evaluate the efficiency of photocatalytic ozonation process using graphene-dioxide titanium nanocomposite in removing Pentachlorophenol (PCP) from aqueous solutions. In this study, nanocomposites with graphene to TiO2(G/T) ratios of 1:10 and 1:20 were synthesized by hydrothermal method, and its characteristics were assessed using various analyses, SEM, XRD, FTIR, TEM, BET and TGA. In this process, the effects of parameters including O3 concentration (0.25-1.25 mg/L), nanocomposite concentration (50-500 mg/L), initial PCP concentration (10-100 mg/L), and time (10-60 min), were studied. The results showed that PCP removal efficiency was increased by decreasing solute concentration. Increasing nanocomposite dose to 100 mg/L was led to an increase in efficiency (99.1%), but then a decreasing trend was observed. Increasing the concentration of ozone, up to specific value, also enhanced the efficiency but then had a negative effect on process efficiency. Furthermore, the optimum ratio of the catalyst was determined to be 1:20. The highest efficiency of the process for initial pentachlorophenol concentration of 100 mg/L was obtained 98.82% in optimum conditions (catalyst dose of 100 mg/L and 60 min). It is concluded that the photocatalytic ozonation process using graphene-dioxide titanium nanocomposite had the highest efficiency in removal and mineralization of PCP.

Nitrate Removal and Dynamics of Microbial Community of A Hydrogen-Based Membrane Biofilm Reactor at Diverse Nitrate Loadings and Distances from Hydrogen Supply End

The back-diffusion of inactive gases severely inhibits the hydrogen (H2) delivery rate of the close-end operated hydrogen-based membrane biofilm reactor (H2-based MBfR). Nevertheless, less is known about the response of microbial communities in H2-based MBfR to the impact of the gases’ back-diffusion. In this research, the denitrification performance and microbial dynamics were studied in a H2-based MBfR operated at close-end mode with a fixed H2 pressure of 0.04 MPa and fed with nitrate (NO3−) containing influent. Results of single-factor and microsensor measurement experiments indicate that the H2 availability was the decisive factor that limits NO3− removal at the influent NO3− concentration of 30 mg N/L. High-throughput sequencing results revealed that (1) the increase of NO3− loading from 10 to 20–30 mg N/L resulted in the shift of dominant functional bacteria from Dechloromonas to Hydrogenophaga in the biofilm; (2) excessive NO3− loading led to the declined relative abundance of Hydrogenophaga and basic metabolic pathways as well as counts of most denitrifying enzyme genes; and (3) in most cases, the decreased quantity of N metabolism-related functional bacteria and genes with increasing distance from the H2 supply end corroborates that the microbial community structure in H2-based MBfR was significantly impacted by the gases’ back-diffusion.

Dynamic monitoring and regulation of pentachlorophenol photodegradation process by chemiluminescence and TiO2/PDA

Pentachlorophenol (PCP), a highly toxic halogenated aromatic compound, and its direct photolysis or TiO₂ photocatalysis may generate toxic intermediates and induce secondary pollution in the environment. It is urgently needed to design a strategy to inhibit the toxic intermediates in the photodegradation of PCP. To achieve this, polydopamine (PDA), a non-toxic substance, modified TiO₂ (P25/PDA) nanoparticles were synthesized and used to improve the PCP photodegradation process. The dynamic tracking of toxic intermediates tetrachloro-1,4-benzoquinone (TCBQ) and trichlorohydroxy-1,4-benzoquinone (OH-TrCBQ) produced in the PCP photodegradation process were obtained by continuous flow chemiluminescence. Combined with reactive oxygen species (ROS) measurements, P25/PDA could approximatively depress 70 % TCBQ and 40 % OH-TrCBQ generation through the regulation of ROS especially the generation of a fairly large amount of H₂O₂ (about 30 μM) and O₂^- (about 20 μM) on the surface of the P25/PDA. The toxicity evaluation showed that the photodegradation of PCP by P25/PDA was a safer and green approach. Therefore, it was instructive to inhibit the formation of highly toxic intermediates in the photodegradation of environmental contaminants by regulating the ROS generated on the surface of the photocatalysts.

Boosting photocatalytic chlorophenols remediation with addition of sulfite and mechanism investigation by in-situ DRIFTs

Sulfite is recently found to be promising in enhancing photocatalytic pollutants degradation, which is a byproduct from flue gas desulfuration process. Herein, 4-chlorophenol (4-CP) photodegradation was systematically investigated in a sulfite mediated system with g-C₃N₄ as photocatalyst. The degradation efficacy was improved by about 3 times with addition of 25 mM Na₂SO₃. The dominant responsible reactive oxygen species for chlorophenols remediation in the presence of sulfite included O₂^·-, SO₃^·-, and SO₄^·- as confirmed by radical quenching experiments and electron spin resonances technology. In-situ DRIFTs results indicated the improved cleavage of CCl and CH bonds with the simultaneous formation of CO and CC bonds when bisulfite was added. Degradation intermediates such as 4-chlorocatechol, hydroquinone, and muconic acid were detected by HPLC-MS. Furthermore, the photodegradation mechanisms of 4-CP were tentatively discussed . Other chlorophenols (phenol, 2-CP, 2,4-DCP, and their mixture) were also efficiently removed in the system, suggesting that sulfite could be universally applied in photocatalytic wastewater purification.

Diversity of bacteria and archaea in the groundwater contaminated by chlorinated solvents undergoing natural attenuation

Chlorinated solvents (CS)-contaminated groundwater poses serious risks to the environment and public health. Microorganisms play a vital role in efficient remediation of CS. In this study, the microbial community (bacterial and archaeal) composition of three CS-contaminated groundwater wells located at an abandoned chemical factory which covers three orders of magnitude in concentration (0.02-16.15 mg/L) were investigated via 16S rRNA gene high-throughput sequencing. The results indicated that Proteobacteria and Thaumarchaeota were the most abundant bacterial and archaeal groups at the phylum level in groundwater, respectively. The major bacterial genera (Flavobacterium sp., Mycobacterium sp. and unclassified Parcubacteria taxa, etc.) and archaeal genera (Thaumarchaeota Group C3, Miscellaneous Crenarchaeotic Group and Miscellaneous Euryarchaeotic Group, etc.) might be involved in the dechlorination processes. In addition, Pearson's correlation analyses showed that alpha diversity of the bacterial community was not significantly correlated with CS concentration, while alpha diversity of archaeal community greatly decreased with the increased contamination of CS. Moreover, partial Mantel test indicated that oxidation-reduction potential, dissolved oxygen, temperature and methane concentration were major drivers of bacterial and archaeal community composition, whereas CS concentration had no significant impact, indicating that both indigenous bacterial and archaeal community compositions are capable of withstanding elevated CS contamination. This study improves our understanding of how the natural microbial community responds to high CS-contaminated groundwater.

Optimization ofS/Fe ratio for enhanced nitrobenzene biological removal in anaerobicSystem amended withSulfide-modified nanoscale zerovalent iron

Anaerobic reduction of nitrobenzene (NB) can be efficiently enhanced bySupplementing withSulfide-modified nanoscale zerovalent iron (S-nZVI). In thisStudy,S/Fe ratio ofS-nZVI was further optimized for enhancing biological NB removal in anaerobicSystem amended withS-nZVI and inoculated by anaerobicSludge. The results indicated that the performance andStability of the coupled anaerobicSystem for NB reduction and aniline formation were remarkably improved byS-nZVI atS/Fe molar ratio of 0.3 (0.3S-nZVI). TheSecretion of extracellular polymericSubstances (EPS), transformation of volatile fatty acids (VFAs), yield of methane and activity ofSeveral key enzymes could be efficiently improved by 0.3S-nZVI. Furthermore,Species related to NB reduction, fermentation, electroactivity and methanogenesis could be enriched in 0.3S-nZVI coupled anaerobicSystem, with remarkable improvement in the biodiversity observed. ThisStudy demonstrated thatSulfidation would be a promising method to improve the performance of nZVI in coupled anaerobicSystems for the removal of recalcitrant nitroaromatic compounds from wastewater.

Removal of Organic Micro-Pollutants by Conventional Membrane Bioreactors and High-Retention Membrane Bioreactors

The ubiquitous presence of organic micropollutants (OMPs) in the environment as a result of continuous discharge from wastewater treatment plants (WWTPs) into water matrices—even at trace concentrations (ng/L)—is of great concern, both in the public and environmental health domains. This fact essentially warrants developing and implementing energy-efficient, economical, sustainable and easy to handle technologies to meet stringent legislative requirements. Membrane-based processes—both stand-alone or integration of membrane processes—are an attractive option for the removal of OMPs because of their high reliability compared with conventional process, least chemical consumption and smaller footprint. This review summarizes recent research (mainly 2015–present) on the application of conventional aerobic and anaerobic membrane bioreactors used for the removal of organic micropollutants (OMP) from wastewater. Integration and hybridization of membrane processes with other physicochemical processes are becoming promising options for OMP removal. Recent studies on high retention membrane bioreactors (HRMBRs) such as osmotic membrane bioreactor (OMBRs) and membrane distillation bioreactors (MDBRs) are discussed. Future prospects of membrane bioreactors (MBRs) and HRMBRs for improving OMP removal from wastewater are also proposed.

Effect of O2, Ni0 coatings, and iron oxide phases on pentachlorophenol dechlorination by zero-valent iron

This study explores the zero-valent iron (ZVI) dechlorination of pentachlorophenol (PCP) and its dependence on the dissolved oxygen (O₂), presence/formation of iron oxides, and presence of nickel metal on the ZVI surface. Compared to the anoxic system, PCP dechlorination was slower in the presence of O₂, which is a potential competitive electron acceptor. Despite O₂ presence, Ni⁰ deposited on the ZVI surfaces catalyzed the hydrogenation reactions and enhanced the PCP dechlorination by Ni-coated ZVI bimetal (Nic/Fe). The presence of O₂ led to the formation of passivating oxides (maghemite, hematite, lepidocrocite, ferrihydrite) on the ZVI and Nic/Fe bimetallic surfaces. These passive oxides resulted in greater PCP incorporation (sorption, co-precipitation, and/or physical entrapment with the oxides) and decreased PCP dechlorination in the oxic systems compared to the anoxic systems. As received ZVI comprised of a wustite film, and in the presence of O₂, only ≈ 17% PCP dechlorination observed after 25 days of exposure with tetrachlorophenol being detected as the end product. Wustite remained as the predominant oxide on as received ZVI during the 25 days of reaction with PCP under oxic and anoxic conditions. ZVI acid-pretreatment resulted in the replacement of wustite with magnetite and enhanced PCP degradation (e.g. ≈ 52% of the initial PCP dechlorinated after 25 days under oxic condition) with accumulation of mixtures of tetra-, tri-, and dichlorophenols. When the acid-washed ZVI was rinsed in NiSO₄/H₂SO₄ solution, Ni⁰ deposited on the ZVI surface and all the wustite were replaced with magnetite. After 25 days of exposure to the Nic/Fe, ≈ 78% and 97% PCP dechlorination occurred under oxic and anoxic conditions, respectively, producing predominantly phenol. Wustite and magnetite are respectively electrically insulating and conducting oxides and influenced the dechlorination and H₂ production. In conclusion, this study clearly demonstrates that the dissolved oxygen present in the aqueous solution decreases the PCP dechlorination and increases the PCP incorporation when using ZVI and Nic/Fe bimetallic systems. The findings provide novel insights towards deciphering and optimizing the performance of complex ZVI and bimetallic systems for PCP dechlorination in the presence of O₂.

Nitrification and denitrification processes for mitigation of nitrous oxide from waste water treatment plants for biovalorization: Challenges and opportunities

Nitrous oxide (N₂O) is a potent greenhouse gas. Even though its emissions is much lesser than CO₂ but its global warming potential (GWP) is 298 times more than CO₂. N₂O emissions from wastewater treatment plants was caused due to incomplete nitrification or incomplete denitrification catalyzed by ammonia-oxidizing bacteria and heterotrophic denitrifiers. Low dissolved oxygen, high nitrite accumulation, change in optimal pH or temperature, fluctuation in C/N ratio, short solid retention time and non-availability of Cu ions were responsible for higher N₂O leakage. Regulation of enzyme metabolic pathways involved in N₂O production and reduction has also been reviewed. Sequential bioreactors, bioscrubbers, membrane biofilters usage have helped microbial nitrification-denitrification processes in succumbing N₂O production in wastewater treatment plants. Reduction of N₂O negativity has been studied through its valorization for the formation of value added products such as biopolymers has led to biorefinery approaches as an upcoming mitigation strategy.

Hydrogenotrophic Microbial Reduction of Oxyanions With the Membrane Biofilm Reactor

Oxyanions, such as nitrate, perchlorate, selenate, and chromate are commonly occurring contaminants in groundwater, as well as municipal, industrial, and mining wastewaters. Microorganism-mediated reduction is an effective means to remove oxyanions from water by transforming oxyanions into harmless and/or immobilized forms. To carry out microbial reduction, bacteria require a source of electrons, called the electron-donor substrate. Compared to organic electron donors, H₂ is not toxic, generates minimal secondary contamination, and can be readily obtained in a variety of ways at reasonable cost. However, the application of H₂ through conventional delivery methods, such as bubbling, is untenable due to H₂'s low water solubility and combustibility. In this review, we describe the membrane biofilm reactor (MBfR), which is a technological breakthrough that makes H₂ delivery to microorganisms efficient, reliable, and safe. The MBfR features non-porous gas-transfer membranes through which bubbleless H₂ is delivered on-demand to a microbial biofilm that develops naturally on the outer surface of the membranes. The membranes serve as an active substratum for a microbial biofilm able to biologically reduce oxyanions in the water. We review the development of the MBfR technology from bench, to pilot, and to commercial scales, and we elucidate the mechanisms that control MBfR performance, particularly including methods for managing the biofilm's structure and function. We also give examples of MBfR performance for cases of treating single and co-occurring oxyanions in different types of contaminated water. In summary, the MBfR is an effective and reliable technology for removing oxyanion contaminants by accurately providing a biofilm with bubbleless H₂ on demand. Controlling the H₂ supply in accordance to oxyanion surface loading and managing the accumulation and activity of biofilm are the keys for process success.

Response of the reactor performance and microbial community to a shift of ISDD process from micro-aerobic to anoxic condition

Micro-aerobic condition has proven to be effective in enhancing sulfide oxidation to elemental sulfur (S⁰) during integrated simultaneous desulfurization and denitrification process (ISDD). In this study we investigated and compared the performance and microbial community of ISDD process operating under initially anoxic, then micro-aerobic and finally switch back to anoxic condition. For all the three tested scenarios, comparable bioreactor performance in terms of sulfate (95.0 ± 4.4%, 90.6 ± 3.8%, 89.8 ± 3.5%) and nitrate (∼100%) removal was achieved. However, a shift of ISDD bioreactor from micro-aerobic to anoxic environment clearly increased the S⁰ production (30.6%), relative to that at initial anoxic condition (14.2%). Further anoxic bioreactor operation with different influent nitrate concentrations also obtained satisfactory performance particularly in terms of S⁰ production. Microbial community analysis results showed that functional microorganisms selectively enriched at micro-aerobic condition, particularly sulfide-oxidizing bacteria (SOB), could also function well and enhance S⁰ production when bioreactor switching from micro-aerobic to anoxic environment. We proposed that micro-aerobic strategy could function as a bio-selector and provide a new idea in functional microorganisms selectively enrichment for wastewater treatment.

Managing microbial communities in membrane biofilm reactors

Membrane biofilm reactors (MBfRs) deliver gaseous substrates to biofilms that develop on the outside of gas-transfer membranes. When an MBfR delivers electron donors hydrogen (H₂) or methane (CH₄), a wide range of oxidized contaminants can be reduced as electron acceptors, e.g., nitrate, perchlorate, selenate, and trichloroethene. When O₂ is delivered as an electron acceptor, reduced contaminants can be oxidized, e.g., benzene, toluene, and surfactants. The MBfR's biofilm often harbors a complex microbial community; failure to control the growth of undesirable microorganisms can result in poor performance. Fortunately, the community's structure and function can be managed using a set of design and operation features as follows: gas pressure, membrane type, and surface loadings. Proper selection of these features ensures that the best microbial community is selected and sustained. Successful design and operation of an MBfR depends on a holistic understanding of the microbial community's structure and function. This involves integrating performance data with omics results, such as with stoichiometric and kinetic modeling.