Overlooked nitrogen-cycling microorganisms in biological wastewater treatment

• AOA and comammox bacteria can be more abundant and active than AOB/NOB at WWTPs. • Coupled DNRA/anammox and NO_x-DAMO/anammox/comammox processes are demonstrated. • Substrate level, SRT and stressors determine the niches of overlooked microbes. • Applications of overlooked microbes in enhancing nitrogen removal are promising. Nitrogen-cycling microorganisms play key roles at the intersection of microbiology and wastewater engineering. In addition to the well-studied ammonia oxidizing bacteria, nitrite oxidizing bacteria, heterotrophic denitrifiers, and anammox bacteria, there are some other N-cycling microorganisms that are less abundant but functionally important in wastewater nitrogen removal. These microbes include, but not limited to ammonia oxidizing archaea (AOA), complete ammonia oxidation (comammox) bacteria, dissimilatory nitrate reduction to ammonia (DNRA) bacteria, and nitrate/nitrite-dependent anaerobic methane oxidizing (NO_x-DAMO) microorganisms. In the past decade, the development of high-throughput molecular technologies has enabled the detection, quantification, and characterization of these minor populations. The aim of this review is therefore to synthesize the current knowledge on the distribution, ecological niche, and kinetic properties of these “overlooked” N-cycling microbes at wastewater treatment plants. Their potential applications in novel wastewater nitrogen removal processes are also discussed. A comprehensive understanding of these overlooked N-cycling microbes from microbiology, ecology, and engineering perspectives will facilitate the design and operation of more efficient and sustainable biological nitrogen removal processes.     

The combined effects of biomass and temperature on maximum specific ammonia oxidation rate in domestic wastewater treatment

• Actual SAORs was determined using MLVSS and temperature. • Measured SAOR decreased with increasing MLVSS 1.1‒8.7 g/L. • Temperature coefficient (θ) decreased with increasing MLVSS. • Nitrification process was dynamically simulated based on laboratory-scale SBR tests. • A modified model was successfully validated in pilot-scale SBR systems. Measurement and predicted variations of ammonia oxidation rate (AOR) are critical for the optimization of biological nitrogen removal, however, it is difficult to predict accurate AOR based on current models. In this study, a modified model was developed to predict AOR based on laboratory-scale tests and verified through pilot-scale tests. In biological nitrogen removal reactors, the specific ammonia oxidation rate (SAOR) was affected by both mixed liquor volatile suspended solids (MLVSS) concentration and temperature. When MLVSS increased 1.6, 4.2, and 7.1-fold (1.3‒8.9 g/L, at 20°C), the measured SAOR decreased by 21%, 49%, and 56%, respectively. Thereby, the estimated SAOR was suggested to modify when MLVSS changed through a power equation fitting. In addition, temperature coefficient (θ) was modified based on MLVSS concentration. These results suggested that the prediction of variations ammonia oxidation rate in real wastewater treatment system could be more accurate when considering the effect of MLVSS variations on SAOR.     

Less attention paid to waterborne SARS-CoV-2 spreading in Beijing urban communities

• A survey on individual’s perception of SARS-CoV-2 transmission was conducted. • Waterborne transmission risks are far less perceived by individuals. • Precautions of preventing wastewater mediated transmission are implemented. • The precautions for wastewater transmission are less favored by the public. • Education level differs the most regarding to waterborne transmission perception. SARS-CoV-2 has been detected in various environmental media. Community and individual-engaged precautions are recommended to stop or slow environmentally-mediated transmission. To better understand the individual’s awareness of and precaution to environmental dissemination of SARS-CoV-2, an online survey was conducted in Beijing during March 14–25, 2020. It is found that the waterborne (especially wastewater mediated) spreading routes are far less perceived by urban communities. The precautions for wastewater transmission are less favored by the public than airborne and solid waste mediated spreading routes. Such risk communication asymmetry in waterborne transmission will be further enlarged in places with fragile water system. Furthermore, education level is the most significant attribution (Sig.<0.05) that causes the difference of awareness and precautions of the waterborne transmission among the respondents, according to the variance analysis results. Our survey results emphasize the urgent need for evidence-based, multifactorial precautions for current and future outbreaks of COVID-19.     

Research progress and prospects of complete ammonia oxidizing bacteria in wastewater treatment

• Comammox bacteria have unique physiological characteristics. • Comammox bacteria are widely distributed in natural and artificial systems. • Comammox bacteria have the potential to reduce N₂O emissions. • Coupling comammox bacteria with DEAMOX can be promoted in wastewater treatment. • Comammox bacteria have significant potential for enhancing total nitrogen removal. Complete ammonia oxidizing bacteria, or comammox bacteria (CAOB), can oxidize ammonium to nitrate on its own. Its discovery revolutionized our understanding of biological nitrification, and its distribution in both natural and artificial systems has enabled a reevaluation of the relative contribution of microorganisms to the nitrogen cycle. Its wide distribution, adaptation to oligotrophic medium, and diverse metabolic pathways, means extensive research on CAOB and its application in water treatment can be promoted. Furthermore, the energy-saving characteristics of high oxygen affinity and low sludge production may also become frontier directions for wastewater treatment. This paper provides an overview of the discovery and environmental distribution of CAOB, as well as the physiological characteristics of the microorganisms, such as nutrient medium, environmental factors, enzymes, and metabolism, focusing on future research and the application of CAOB in wastewater treatment. Further research should be carried out on the physiological characteristics of CAOB, to analyze its ecological niche and impact factors, and explore its application potential in wastewater treatment nitrogen cycle improvement.     

Ammonia removal from low-strength municipal wastewater by powdered resin combined with simultaneous recovery as struvite

• Powdered resin was employed for ammonia recovery from municipal wastewater. • Powdered resin achievedefficient ammonia removal under various working conditions. • Co-existing cations indicated competitive adsorption of ammonia. • Ammonia was recoveredby two-stage crystallization coupled with ion exchange. Low-strength municipal wastewater is considered to be a recoverable nutrient resource with economic and environmental benefits. Thus, various technologies for nutrient removal and recovery have been developed. In this paper, powdered ion exchange resin was employed for ammonia removal and recovery from imitated low-strength municipal wastewater. The effects of various working conditions (powdered resin dosage, initial concentration, and pH value) were studied in batch experiments to investigate the feasibility of the approach and to achieve performance optimization. The maximum adsorption capacity determined by the Langmuir model was 44.39 mg/g, which is comparable to traditional ion exchange resin. Further, the effects of co-existing cations (Ca²⁺, Mg²⁺, K⁺) were studied. Based on the above experiments, recovery of ammonia as struvite was successfully achieved by a proposed two-stage crystallization process coupled with a powdered resin ion exchange process. Scanning electron microscopy (SEM) and X-ray diffractometry (XRD) results revealed that struvite crystals were successfully gained in alkaline conditions (pH= 10). This research demonstrates that a powdered resin and two-stage crystallization process provide an innovative and promising means for highly efficient and easy recovery from low-strength municipal wastewater.     

Electricity-driven ammonia oxidation and acetate production in microbial electrosynthesis systems

• MES was constructed for simultaneous ammonia removal and acetate production. • Energy consumption was different for total nitrogen and ammonia nitrogen removal. • Energy consumption for acetate production was about 0.04 kWh/g. • Nitrate accumulation explained the difference of energy consumption. • Transport of ammonia and acetate across the membrane deteriorated the performance. Microbial electrosynthesis (MES) is an emerging technology for producing chemicals, and coupling MES to anodic waste oxidation can simultaneously increase the competitiveness and allow additional functions to be explored. In this study, MES was used for the simultaneous removal of ammonia from synthetic urine and production of acetate from CO₂. Using graphite anode, 83.2%±5.3% ammonia removal and 28.4%±9.9% total nitrogen removal was achieved, with an energy consumption of 1.32 kWh/g N for total nitrogen removal, 0.45 kWh/g N for ammonia nitrogen removal, and 0.044 kWh/g for acetate production. Using boron-doped diamond (BDD) anode, 70.9%±12.1% ammonia removal and 51.5%±11.8% total nitrogen removal was obtained, with an energy consumption of 0.84 kWh/g N for total nitrogen removal, 0.61 kWh/g N for ammonia nitrogen removal, and 0.043 kWh/g for acetate production. A difference in nitrate accumulation explained the difference of total nitrogen removal efficiencies. Transport of ammonia and acetate across the membrane deteriorated the performance of MES. These results are important for the development of novel electricity-driven technologies for chemical production and pollution removal.     

Mechanistic insight into the biofilm formation and process performance of a passive aeration ditch (PAD) for decentralized wastewater treatment

• A Passive Aeration Ditch was developed to treat decentralized wastewater. • A model was developed to describe the process performance. • A high C/N ratio facilitates microbial growth but nitrification deteriorates. • A high salinity decreases both organic and nitrogen contaminants removal. Decentralized wastewater containing elevated salinity is an emerging threat to the local environment and sanitation in remote coastal communities. Regarding the cost and treatment efficiencies, we propose a passive aeration ditch (PAD) using non-woven polyester fabric as a feasible bubbleless aerator and biofilm carrier for wastewater treatment. Consideration has been first given to PAD’s efficacy in treating saline decentralized wastewater, and then to the impact of chemical oxygen demand-to-nitrogen (C/N) ratio and salinity on biofilm formation. A multispecies model incorporating the salinity effect has been developed to depict the system performance and predict the microbial community. Results showed that the PAD system had great capacity for pollutants removal. The biofilm thickness increased at a higher C/N ratio because of the boost of aerobic heterotrophs and denitrifying bacteria, which consequently improved the COD and total nitrogen removal. However, this led to the deterioration of ammonia removal. Moreover, while a higher salinity benefited the biofilm growth, the contaminant removal efficiencies decreased because the salinity inhibited the activity of aerobic heterotrophs and reduced the abundance of nitrifying bacteria inside the biofilm. Based on the model simulation, feed water with salinity below 2% and C/N ratio in a range of 1 to 3 forms a biofilm that can reach relatively high organic matter and ammonia removal. These findings not only show the feasibility of PAD in treatment of saline decentralized wastewater, but also offer a systematic strategy to predict and optimize the process performance.     

Sludge fermentation liquid addition attained advanced nitrogen removal in low C/N ratio municipal wastewater through short-cut nitrification-denitrification and partial anammox

• Sludge fermentation liquid addition resulted in a high NAR of 97.4%. • Extra NH₄⁺-N from SFL was removed by anammox in anoxic phase. • Nitrogen removal efficiency of 92.51% was achieved in municipal wastewater. • The novel system could efficiently treat low COD/N municipal wastewater. Biological nitrogen removal of wastewater with low COD/N ratio could be enhanced by the addition of wasted sludge fermentation liquid (SFL), but the performance is usually limited by the introducing ammonium. In this study, the process of using SFL was successfully improved by involving anammox process. Real municipal wastewater with a low C/N ratio of 2.8–3.4 was treated in a sequencing batch reactor (SBR). The SBR was operated under anaerobic-aerobic-anoxic (AOA) mode and excess SFL was added into the anoxic phase. Stable short-cut nitrification was achieved after 46d and then anammox sludge was inoculated. In the stable period, effluent total inorganic nitrogen (TIN) was less than 4.3 mg/L with removal efficiency of 92.3%. Further analysis suggests that anammox bacteria, mainly affiliated with Candidatus_Kuenenia, successfully reduced the external ammonia from the SFL and contributed approximately 28%–43% to TIN removal. Overall, this study suggests anammox could be combined with SFL addition, resulting in a stable enhanced nitrogen biological removal.     

Distribution of antibiotic resistance genes and their association with bacteria and viruses in decentralized sewage treatment facilities

• Distribution of ARGs in decentralized sewage facilities were investigated. • Bacitracin-ARGs were most predominant ARGs in rural wastewater. • ARGs were identified in bacterial and viral community. • ARGs of rpoB, drfE, gyrA and parC were both correlated with bacteria and phages. • More attention should be paid to the risk of spreading ARG by phages. The distribution of antibiotic resistance genes (ARGs) has been intensively studied in large-scale wastewater treatment plants and livestock sources. However, small-scale decentralized sewage treatment facilities must also be explored due to their possible direct exposure to residents. In this study, six wastewater treatment facilities in developed rural areas in eastern China were investigated to understand their risks of spreading ARGs. Using metagenomics and network analysis tools, ARGs and bacterial and viral communities were identified in the influent (INF) and effluent (EFF) samples. The dominant ARGs belonged to the bacitracin class, which are different from most of municipal wastewater treatment plants (WWTPs). The dominant hosts of ARGs are Acidovorax in bacterial communities and Prymnesiovirus in viral communities. Furthermore, a positive relationship was found between ARGs and phages. The ARGs significantly correlated with phages were all hosted by specific genera of bacteria, indicating that phages had contributed to the ARG’s proliferation in sewage treatment facilities. Paying significant concern on the possible enhanced risks caused by bacteria, viruses and their related ARGs in decentralized sewage treatment facilities is necessary.     

Wastewater treatment meets artificial photosynthesis: Solar to green fuel production,water remediation and carbon emission reduction

• Mitigating energy utilization and carbon emission is urgent for wastewater treatment. • MPEC integrates both solar energy storage and wastewater organics removal. • Energy self-sustaining MPEC allows to mitigate the fossil carbon emission. • MPEC is able to convert CO₂ into storable carbon fuel using renewable energy. • MPEC would inspire photoelectrochemistry by employing a novel oxidation reaction. Current wastewater treatment (WWT) is energy-intensive and leads to vast CO₂ emissions. Chinese pledge of “double carbon” target encourages a paradigm shift from fossil fuels use to renewable energy harvesting during WWT. In this context, hybrid microbial photoelectrochemical (MPEC) system integrating microbial electrochemical WWT with artificial photosynthesis (APS) emerges as a promising approach to tackle water-energy-carbon challenges simultaneously. Herein, we emphasized the significance to implement energy recovery during WWT for achieving the carbon neutrality goal. Then, we elucidated the working principle of MPEC and its advantages compared with conventional APS, and discussed its potential in fulfilling energy self-sustaining WWT, carbon capture and solar fuel production. Finally, we provided a strategy to judge the carbon profit by analysis of energy and carbon fluxes in a MPEC using several common organics in wastewater. Overall, MPEC provides an alternative of WWT approach to assist carbon-neutral goal, and simultaneously achieves solar harvesting, conversion and storage.     

Bioinspired cellulose-based membranes in oily wastewater treatment

• Cellulose-based membrane separates oily wastewater mimicking the living things. • The three central surface mechanisms were reviewed. • Preparation, performance, and mechanism are critically evaluated. • First review of wettability based cellulose membrane as major material. • The current and future importance of the research are discussed. It is challenging to purify oily wastewater, which affects water-energy-food production. One promising method is membrane-based separation. This paper reviews the current research trend of applying cellulose as a membrane material that mimics one of three typical biostructures: superhydrophobic, underwater superoleophobic, and Janus surfaces. Nature has provided efficient and effective structures through the evolutionary process. This has inspired many researchers to create technologies that mimic nature’s structures or the fabrication process. Lotus leaves, fish scales, and Namib beetles are three representative structures with distinct functional and surface properties: superhydrophobic, underwater superoleophobic, and Janus surfaces. The characteristics of these structures have been widely studied and applied to membrane materials to improve their performance. One attractive membrane material is cellulose, which has been studied from the perspective of its biodegradability and sustainability. In this review, the principles, mechanisms, fabrication processes, and membrane performances are summarized and compared. The theory of wettability is also described to build a comprehensive understanding of the concept. Finally, future outlook is discussed to challenge the gap between laboratory and industrial applications.     

Submerged arc plasma system combined with ozone oxidation for the treatment of wastewater containing non-degradable organic compounds

• Submerged arc plasma was introduced in terms of wastewater treatment. • Ozone oxidation was coupled with submerged arc plasma system. • Ozone was converted into O and O₂ by submerged arc plasma. • Decomposition rate was accelerated by submerged arc plasma. • Introduction of ozone led to significant increase in mineralization. Submerged arc plasma technology was assessed for the removal of phenols from wastewater. The OH radicals generated from the boundary between the plasma and waste solution were considered as a significant factor on the degradation reaction. In this study, the effects of highly energetic electrons released from the submerged arc plasma were mainly studied. The highly energetic electrons directly broke the strong chemical bond and locally increased the reaction temperatures in solution. The effects of the submerged-arc plasma on the decomposition of phenol are discussed in terms of the input energy and initial concentration. The single use of submerged arc plasma easily decomposed the phenol but did not increase the mineralization efficiency. Therefore, the submerged arc plasma, coupled with the ozone injection, was investigated. The submerged arc plasma combined with ozone injection had a synergic effect, which led to significant improvements in mineralization with only a small increase in input energy. The decomposition mechanism of phenol by the submerged arc plasma with the ozone was analyzed.     

Partial anammox achieved in full scale biofilm process for typical domestic wastewater treatment

• A full scale biofilm process was developed for typical domestic wastewater treatment. • The HRT was 8 h and secondary sedimentation tank was omitted. • Candidatus Brocadia were enriched in the HBR with an abundance of 2.89%. • Anammox enabled a stable ammonium removal of ~15% in the anoxic zone. The slow initiation of anammox for treating typical domestic wastewater and the relatively high footprint of wastewater treatment infrastructures are major concerns for practical wastewater treatment systems. Herein, a 300 m³/d hybrid biofilm reactor (HBR) process was developed and operated with a short hydraulic retention time (HRT) of 8 h. The analysis of the bacterial community demonstrated that anammox were enriched in the anoxic zone of the HBR process. The percentage abundance of Candidatus Brocadia in the total bacterial community of the anoxic zone increased from 0 at Day 1 to 0.33% at Day 130 and then to 2.89% at Day 213. Based upon the activity of anammox bacteria, the removal of ammonia nitrogen (NH₄⁺-N) in the anoxic zone was approximately 15%. This showed that the nitrogen transformation pathway was enhanced in the HBR system through partial anammox process in the anoxic zone. The final effluent contained 12 mg/L chemical oxygen demand (COD), 0.662 mg/L NH₄⁺-N, 7.2 mg/L total nitrogen (TN), and 6 mg/L SS, indicating the effectiveness of the HBR process for treating real domestic wastewater.     

Mechanisms for simultaneous ozonation of sulfamethoxazole and natural organic matters in secondary effluent from sewage treatment plant

• SMX was mainly degraded by hydrolysis, isoxazole oxidation and double-bond addition. • Isoxazole oxidation and bond addition products were formed by direct ozonation. • Hydroxylated products were produced by indirect oxidation. • NOM mainly affected the degradation of SMX by consuming ^•OH rather than O₃. • Inhibitory effect of NOM on SMX removal was related to the components’ aromaticity. Sulfamethoxazole (SMX) is commonly detected in wastewater and cannot be completely decomposed during conventional treatment processes. Ozone (O₃) is often used in water treatment. This study explored the influence of natural organic matters (NOM) in secondary effluent of a sewage treatment plant on the ozonation pathways of SMX. The changes in NOM components during ozonation were also analyzed. SMX was primarily degraded by hydrolysis, isoxazole-ring opening, and double-bond addition, whereas hydroxylation was not the principal route given the low maximum abundances of the hydroxylated products, with m/z of 269 and 287. The hydroxylation process occurred mainly through indirect oxidation because the maximum abundances of the products reduced by about 70% after the radical quencher was added, whereas isoxazole-ring opening and double-bond addition processes mainly depended on direct oxidation, which was unaffected by the quencher. NOM mainly affected the degradation of micropollutants by consuming ^•OH rather than O₃ molecules, resulting in the 63%–85% decrease in indirect oxidation products. The NOM in the effluent were also degraded simultaneously during ozonation, and the components with larger aromaticity were more likely degraded through direct oxidation. The dependences of the three main components of NOM in the effluent on indirect oxidation followed the sequence: humic-like substances>fluvic-like substances>protein-like substances. This study reveals the ozonation mechanism of SMX in secondary effluent and provides a theoretical basis for the control of SMX and its degradation products in actual water treatment.     

A review on anammox process for the treatment of antibiotic-containing wastewater: Linking effects with corresponding mechanisms

• Anammox is promising for nitrogen removal from antibiotic-containing wastewater. • Most antibiotics could inhibit the anammox performance and activity. • Antibiotic pressure promoted the increase in antibiotic resistance genes (ARGs). • Antibiotic-resistance mechanisms of anammox bacteria are speculated. Antibiotic is widely present in the effluent from livestock husbandry and the pharmaceutical industry. Antibiotics in wastewater usually have high biological toxicity and even promote the occurrence and transmission of antibiotic resistant bacteria and antibiotic resistance genes. Moreover, most antibiotic-containing wastewater contains high concentration of ammonia nitrogen. Improper treatment will lead to high risk to the surrounding environment and even human health. The anaerobic ammonium oxidation (anammox) with great economic benefit and good treatment effect is a promising process to remove nitrogen from antibiotic-containing wastewater. However, antibiotic inhibition has been observed in anammox applications. Therefore, a comprehensive overview of the single and combined effects of various antibiotics on the anammox system is conducted in this review with a focus on nitrogen removal performance, sludge properties, microbial community, antibiotic resistance genes and anammox-involved functional genes. Additionally, the influencing mechanism of antibiotics on anammox consortia is summarized. Remaining problems and future research needs are also proposed based on the presented summary. This review provides a better understanding of the influences of antibiotics on anammox and offers a direction to remove nitrogen from antibiotic-containing wastewater by the anammox process.     

Enhancing the efficiency of nitrogen removing bacterial population to a wide range of C:N ratio (1.5:1 to 14:1) for simultaneous C & N removal

• Simultaneous C & N removal in Methammox occurs at wide C:N ratio. • Biological Nitrogen Removal at wide C:N ratio of 1.5:1 to 14:1 is not reported. • Ammonia removal shifted from mixotrophy to heterotrophy at high C:N ratio. • Acetogenic population compensated for ammonia oxidizers at high C:N ratio. • Methanogens increase the plasticity of nitrogen removers at high C:N ratio. High C:N ratio in the wastewater limits biological nitrogen removal (BNR), especially in anammox based technologies. The present study attempts to improve the COD tolerance of the BNR process by associating methanogens with nitrogen removing bacterial (NRB) populations. The new microbial system coined as ‘Methammox’, was investigated for simultaneous removal of COD (C) and ammonia (N) at C:N ratio 1.5:1 to 14:1. The ammonia removal rate (11.5 mg N/g VSS/d) and the COD removal rates (70.6 mg O/g VSS/d) of Methammox was close to that of the NRB (11.1 mg N/g VSS/d) and the methanogenic populations (77.9 mg O/g VSS/d), respectively. The activities established that these two populations existed simultaneously and independently in ‘Methammox’. Further studies in biofilm reactor fetched a balanced COD and ammonia removal (55%–60%) at a low C:N ratio (≤2:1) and high C:N ratio (≥9:1). The population abundance of methanogens was reasonably constant, but the nitrogen removal shifted from mixotrophy to heterotrophy as the C:N ratio shifted from low (C:N≤2:1) to high (C:N≥9:1). The reduced autotrophic NRB (ammonia- and nitrite-oxidizing bacteria and Anammox) population at a high C:N ratio was compensated by the fermentative group that could carry out denitrification heterotrophically. The functional plasticity of the Methammox system to adjust to a broad C:N ratio opens new frontiers in biological nitrogen removal of high COD containing wastewaters.     

The main anammox-based processes,the involved microbes and the novel process concept from the application perspective

• The PNA, denitratation/anammox, and DAMO/anammox process are reviewed together. • Denitratation/anammox-based process is promising in mainstream treatment. • DAMO and denitratation processes realize the higher nitrogen removal efficiency. • The utilization of metabolism diversity of functional microbe is worth exploring. • An effective waste treatment system concept is proposed. Anammox technology has been widely researched over the past 40-year from the laboratory-scale to full-scale. It is well-known that in actual applications, the solo application of anammox is not feasible. Since both ammonium and nitrite are prerequisites based on the reaction mechanism, the pre-treatment of wastewater is necessary. With the combination of anammox process and other pre-treatment processes to treat the actual wastewater, many types of anammox-based processes have been developed with distinct nitrogen removal performance. Thus, in order to heighten the awareness of researchers to the developments and accelerate the application of these processes to the treatment of actual wastewater, the main anammox-based processes are reviewed in this paper. It includes the partial nitritation/anammox process, the denitratation/anammox (PD/A) process, the denitrifying anaerobic methane oxidation/anammox (DAMO/A) process, and more complex deuterogenic processes. These processes have made the breakthroughs in the application of the anammox technology, such as the combination of nitrification and PD/A process can achieve stability and reliability of nitrogen removal in the treatment of mainstream wastewater, the PD/A process and the DAMO/A have brought about further improvements in the total nitrogen removal efficiency of wastewater. The diversity of functional microbe characteristics under the specific condition indicate the wide application potential of anammox-based processes, and further exploration is necessary. A whole waste treatment system concept is proposed through the effective allocation of above mentioned processes, with the maximum recovery of energy and resources, and minimal environmental impact.     

One-pot hydrothermal fabrication of BiVO4/Fe3O4/rGO composite photocatalyst for the simulated solar light-driven degradation of Rhodamine B

• BiVO₄/Fe₃O₄/rGO has excellent photocatalytic activity under solar light radiation. • It can be easily separated and collected from water in an external magnetic field. • BiVO₄/Fe₃O₄/0.5% rGO exhibited the highest RhB removal efficiency of over 99%. • Hole (h⁺) and superoxide radical (O₂^•−) dominate RhB photo-decomposition process. • The reusability of this composite was confirmed by five successive recycling runs. Fabrication of easily recyclable photocatalyst with excellent photocatalytic activity for degradation of organic pollutants in wastewater is highly desirable for practical application. In this study, a novel ternary magnetic photocatalyst BiVO₄/Fe₃O₄/reduced graphene oxide (BiVO₄/Fe₃O₄/rGO) was synthesized via a facile hydrothermal strategy. The BiVO₄/Fe₃O₄ with 0.5 wt% of rGO (BiVO₄/Fe₃O₄/0.5% rGO) exhibited superior activity, degrading greater than 99% Rhodamine B (RhB) after 120 min solar light radiation. The surface morphology and chemical composition of BiVO₄/Fe₃O₄/rGO were studied by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV–visible diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. The free radicals scavenging experiments demonstrated that hole (h⁺) and superoxide radical (O₂^•−) were the dominant species for RhB degradation over BiVO₄/Fe₃O₄/rGO under solar light. The reusability of this composite catalyst was also investigated after five successive runs under an external magnetic field. The BiVO₄/Fe₃O₄/rGO composite was easily separated, and the recycled catalyst retained high photocatalytic activity. This study demonstrates that catalyst BiVO₄/Fe₃O₄/rGO possessed high dye removal efficiency in water treatment with excellent recyclability from water after use. The current study provides a possibility for more practical and sustainable photocatalytic process.     

Mechanism of dichloromethane disproportionation over mesoporous TiO2 under low temperature

• Mechanism of DCM disproportionation over mesoporous TiO₂ was studied. • DCM was completely eliminated at 350℃ under 1 vol.% humidity. • Anatase (001) was the key for disproportionation. • A competitive oxidation route co-existed with disproportionation. • Disproportionation was favored at low temperature. Mesoporous TiO₂ was synthesized via nonhydrolytic template-mediated sol-gel route. Catalytic degradation performance upon dichloromethane over as-prepared mesoporous TiO₂, pure anatase and rutile were investigated respectively. Disproportionation took place over as-made mesoporous TiO₂ and pure anatase under the presence of water. The mechanism of disproportionation was studied by in situ FTIR. The interaction between chloromethoxy species and bridge coordinated methylenes was the key step of disproportionation. Formate species and methoxy groups would be formed and further turned into carbon monoxide and methyl chloride. Anatase (001) played an important role for disproportionation in that water could be dissociated into surface hydroxyl groups on such structure. As a result, the consumed hydroxyl groups would be replenished. In addition, there was another competitive oxidation route governed by free hydroxyl radicals. In this route, chloromethoxy groups would be oxidized into formate species by hydroxyl radicals transfering from the surface of TiO₂. The latter route would be more favorable at higher temperature.     

Integrated energy view of wastewater treatment: A potential of electrochemical biodegradation

• Energy is needed to accelerate the biological wastewater treatment. • Electrical energy input in traditional technology is indirect and inefficient. • Direct injection of electricity can be a game changer to maximize energy efficiency. • Microbial electrochemical unit for decentralized wastewater treatment is proposed. It has been more than one century since the activated sludge process was invented. Despite its proven stability and reliability, the energy (especially the electrical energy) use in wastewater treatment should evolve to meet the increasingly urgent demand of energy efficiency. This paper discusses how the energy utilized in conventional biological wastewater treatment can be altered by switching the indirect energy input to a direct electricity injection, which is achieved by the electrode integration providing extra thermodynamic driving force to biodegradation. By using electrodes instead of oxygen as terminal electron acceptors, the electrical energy can be utilized more efficiently, and the key of direct use of electrical energy in biodegradation is the development of highly active electroactive biofilm and the increase of electron transfer between microbes and the electrode. Furthermore, the synergy of different microbial electrochemical units has additional benefit in energy and resource recovery, making wastewater treatment more sustainable.