Biochar addition supports high digestion performance and low membrane fouling rate in an anaerobic membrane bioreactor under low temperatures

Enhanced methane production and biofouling mitigation by Fe2O3 nanoparticle-biochar composites in anaerobic membrane bioreactors

Conductive materials present an innovative approach to enhancing methane production while facilitating sludge reduction and recovering resources in anaerobic digestion systems. However, the synergistic mechanisms by which composite materials influence the performance of anaerobic membrane bioreactor (AnMBR)-particularly in improving methane production and mitigating membrane fouling, remain underexplored. To address this, Fe2O3 nanoparticle-biochar composites (Fe2O3-BC) were synthesized to enhance electrical conductivity and promote efficient electron transfer in AnMBRs system. These results demonstrated that Fe2O3-BC exhibit high electron donor and electron acceptor capacities, increasing electron transport system (ETS) activity and conductivity by 1.4-fold and 1.7-fold, respectively. This enhancement accelerated the degradation of organic matter during the hydrolysis-acidification stage and boosted the activity of key enzymes (CytC and F420) in the methanogenic phase, resulting in a 42 % increase in methane production. Microbial community analysis indicated that Fe2O3-BC strengthened the methanogenic pathway by fostering efficient metabolic interactions between acidogenic bacteria (e.g., norank_f_Rikenellaceae) and methanogens (e.g., Methanosaeta). Long-term experiments with the Fe3BC25-AnMBR reactor showed a significant reduction in the accumulation of soluble microbial products (SMP) and extracellular polymeric substances (EPS) on membrane surfaces, along with a decline in fouling-related bacteria (e.g., Bacteroidota), mitigating membrane fouling by approximately 20-65 %. Furthermore, Fe3BC25 inhibited biofilm-related quorum sensing (QS) signals (e.g., C6-HSL, AI-1, and AI-2), reducing microbial adhesion and biofouling. Simultaneously, it enhanced methanogenesis by upregulating QS signals associated with methane production (e.g., C10-HSL). Fe2O3-BC is expected to offer a promising strategy for advancing energy-driven AnMBR processes.

Advanced performance and operational strategies of partitioned anaerobic membrane bioreactor combined with ferromagnetic biochar for pesticide wastewater treatment at low-temperature and insufficient hydraulic load

Accompanied by high energy consumption and the difficulty in maintaining process stability, the treatment of pesticide wastewater in cold climates remain a significant challenge due to its low biodegradability and complex composition. Therefore, this study developed a hybrid anaerobic membrane bioreactor (HAnMBR) that incorporates ferromagnetic biochar and a partitioned upflow anaerobic sludge blanket structure. Its performance was evaluated and compared to that of a conventional anaerobic membrane bioreactor (CAnMBR) under varying organic loading rate (OLR) and hydraulic loading rate (HLR) at 10 °C. Results indicated that the maximum average removal rates of COD, pyridine, and trichlorfon in HAnMBR were 17.3 %, 11.2 %, and 6.9 % higher than those in CAnMBR, respectively. The volatile fatty acid (VFA) concentration decreased by 14.8-55.4 %, and the electron transport system activity (ETSA) increased by 1.3-2.6 times. HAnMBR significantly reduced the sludge yield by 21.5-51.0 % and decreased the SVI by 13.3-39.4 %. The operational strategies were subsequently determined, including: an influent COD concentration of 8.0 g/L and a HLR of 0.9 m3/(m2·d); an OLR of 5 kg COD/(m3·d) and a HLR of 0.7 m3/(m2·d); and a HLR of 1.0 m3/(m2·d) and an OLR of 7 kg COD/(m3·d). This study offered a new process for effectively mitigating the impact of load shocks from high-strength wastewater at low temperatures, potentially expanding the application of AnMBRs.

Applicability analysis of algae biochar for anaerobic membrane bioreactors in wastewater treatment: A review from a sustainability assessment perspective

The incorporation of biochar can significantly enhance the performance of anaerobic membrane bioreactors (AnMBRs), achieving up to a 95 % increase in pollutant removal efficiency and an 86 % improvement in methane production. Algae biochar, in particular, shows great promise as an effective additive in AnMBR systems because of its low cost (approximately $0.470/kg) and the abundance of raw material sources. This paper presents a comprehensive applicability analysis of algae biochar-AnMBRs from a sustainability assessment perspective, addressing technical, environmental, economic, and social dimensions. Key technical benefits include a reduction in membrane fouling by 92.1 % and an enhancement of energy recovery by 58.7 % compared to conventional AnMBRs. Following this, the paper evaluates algae biochar-AnMBRs from environmental, economic, and social viewpoints to emphasize the practical applicability and potential of this process. Finally, this review addresses the limitations related to the full-scale implementation of this technology and proposes strategic approaches to overcome these challenges. Overall, the review provides valuable insights into the practical application of algae biochar-AnMBR systems, with a strong focus on sustainability.

An in-situ biochar-enhanced anaerobic membrane bioreactor for swine wastewater treatment under various organic loading rates

A biochar-assisted anaerobic membrane bioreactor (BC-AnMBR) was conducted to evaluate the performance in treating swine wastewater with different organic loading rates (OLR) ranging from 0.38 to 1.13 kg-COD/(m3.d). Results indicated that adding spent coffee grounds biochar (SCG-BC) improved the organic removal efficiency compared to the conventional AnMBR, with an overall COD removal rate of > 95.01%. Meanwhile, methane production of up to 0.22 LCH4/gCOD with an improvement of 45.45% was achieved under a high OLR of 1.13 kg-COD/(m3.d). Furthermore, the transmembrane pressure (TMP) in the BC-AnMBR system was stable at 4.5 kPa, and no irreversible membrane fouling occurred within 125 days. Microbial community analysis revealed that the addition of SCG-BC increased the relative abundance of autotrophic methanogenic archaea, particularly Methanosarcina (from 0.11% to 11.16%) and Methanothrix (from 16.34% to 24.05%). More importantly, Desulfobacterota and Firmicutes phylum with direct interspecific electron transfer (DIET) capabilities were also enriched with autotrophic methanogens. Analysis of the electron transfer pathway showed that the concentration of c-type cytochromes increased by 38.60% in the presence of SCG-BC, and thus facilitated the establishment of DIET and maintained high activity of the electron transfer system even at high OLR. In short, the BC-AnMBR system performs well under various OLR conditions and is stable in the recovery energy system for swine wastewater.

Anaerobic membrane bioreactors for municipal wastewater: Progress in resource and energy recovery improvement approaches

Anaerobic membrane bioreactor (AnMBR) offer promise in municipal wastewater treatment, with potential benefits including high-quality effluent, energy recovery, sludge reduction, and mitigating greenhouse gas emissions. However, AnMBR face hurdles like membrane fouling, low energy recovery, etc. In light of net-zero carbon target and circular economy strategy, this work sought to evaluate novel AnMBR configurations, focusing on performance, fouling mitigation, net-energy generation, and nutrients-enhancing integrated configurations, such as forward osmosis (FO), membrane distillation (MD), bioelectrochemical systems (BES), membrane photobioreactor (MPBR), and partial nitrification-anammox (PN/A). In addition, we highlight the essential role of AnMBR in advancing the circular economy and propose ideas for the water-energy-climate nexus. While AnMBR has made significant progress, challenges, such as fouling and cost-effectiveness persist. Overall, the use of novel configurations and energy recovery strategies can further improve the sustainability and efficiency of AnMBR systems, making them a promising technology for future sustainable municipal wastewater treatment.

Synergistic effects of biochar addition and filtration mode optimization on mitigating membrane fouling in high-solid anaerobic membrane bioreactors

In this study, a high-solid anaerobic membrane bioreactor was established for treating food waste, and membrane fouling rates were regulated through multivariate modulation. The anaerobic membrane bioreactor operated stably at a high organic loading rate of 28.75 gCOD/L/d achieved a methane production rate of 8.03 ± 0.61 L/L/d. Experimental findings revealed that the most effective control of membrane fouling was achieved at a filtration- relaxation ratio (F/R) of 10/90 s. This indicates that a higher relaxation frequency provided improved the mitigation of membrane fouling. Compared with single F/R modulation, the combined modulation of biochar and F/R provided enhanced control over membrane fouling. Moreover, the addition of biochar altered the sludge properties of the reactor, thereby preventing the formation of a dense cake layer. Additionally, biochar enhanced the sheer force of the fluid on the membrane surface and facilitated the separation of pollutants during the relaxation stage, thereby contributing to improved control of membrane fouling.

Deciphering the behavior and potential mechanism of biochar at different pyrolysis temperatures to alleviate membrane biofouling

Biofouling limits applications of membrane technology in wastewater treatment, but dosing additives to membrane tanks is an effective method to alleviate biofouling. In this study, biochar derived from corncob and pyrolyzed at 300, 500, and 700°C was dosed to determine the underlying anti-biofouling mechanism. The effects of the biochar on the membrane properties and foulant behavior were systematically investigated. The results showed that biochar delayed the occurrence of the fouling transition (0.5-3.0 h), and decreased the flux decline rate, thus achieving a higher water flux (3.1-3.7 times of the control group). Biochar altered membrane surface properties, and increased the membrane surface charge, roughness, and hydrophilicity, which all contributed to higher membrane permeability. Moreover, adding biochar reduced the number of foulants in the fouling layer, particularly protein substances. The flux model fit and the XDLVO theory further revealed the mitigating effect of biochar on membrane biofouling. At the initial intermediate-blocking stage, the effect of biochar on membrane fouling was determined by its properties, and adsorption capacity to the foulants, BC500 presented the best mitigation performance. At the later cake-filtration stage, the role of biochar in membrane fouling was strongly associated with protein content in the fouling layer, and the minimum rate of flux decline occurred in BC300. This study promotes the understanding and development of biochar to alleviate membrane biofouling.

Biochar mediated high-rate anaerobic bioreactors: A critical review on high-strength wastewater treatment and management

Treatment of high-strength wastewater is critical for the aquatic environment and receiving water bodies around the globe. Untreated or partially treated high-strength wastewater may cause severe damage to the existing water bodies. Various high-rate anaerobic bioreactors have been developed in the last decades for treating high-strength wastewater. High-rate anaerobic bioreactors are effective in treating industrial wastewater and provide energy in the form of methane as well. However, the physical or chemical properties of high-strength industrial wastewater, sometimes, disrupt the functioning of a high-rate anaerobic bioreactor. For example, the disintegration of granular sludge in up flow anaerobic sludge blanket reactor or membrane blocking in an anaerobic membrane bioreactor are the results of a high-strength wastewater treatment which hamper the proper functioning and may harm the wastewater treatment plant economically. Biochar, if added to these bioreactors, may help to alleviate the ill-functioning of high-rate anaerobic bioreactors. The primary mechanisms by biochar work in these bioreactors are direct interspecies electron transfer, microbial immobilization, or gene level alternations in microbial structure. The present article explores and reviews the recent application of biochar in a high-rate anaerobic bioreactor treating high-strength industrial wastewater.

Insights into current bio-processes and future perspectives of carbon-neutral treatment of industrial organic wastewater: A critical review

With the rise of the concept of carbon neutrality, the current wastewater treatment process of industrial organic wastewater is moving towards the goal of energy conservation and carbon emission reduction. The advantages of anaerobic digestion (AD) processes in industrial organic wastewater treatment for bio-energy recovery, which is in line with the concept of carbon neutrality. This study summarized the significance and advantages of the state-of-the-art AD processes were reviewed in detail. The application of expanded granular sludge bed (EGSB) reactors and anaerobic membrane bioreactor (AnMBR) were particularly introduced for the effective treatment of industrial organic wastewater treatment due to its remarkable prospect of engineering application for the high-strength wastewater. This study also looks forward to the optimization of the AD processes through the enhancement strategies of micro-aeration pretreatment, acidic-alkaline pretreatment, co-digestion, and biochar addition to improve the stability of the AD system and energy recovery from of industrial organic wastewater. The integration of anaerobic ammonia oxidation (Anammox) with the AD processes for the post-treatment of nitrogenous pollutants for the industrial organic wastewater is also introduced as a feasible carbon-neutral process. The combination of AnMBR and Anammox is highly recommended as a promising carbon-neutral process for the removal of both organic and inorganic pollutants from the industrial organic wastewater for future perspective. It is also suggested that the AD processes combined with biological hydrogen production, microalgae culture, bioelectrochemical technology and other bio-processes are suitable for the low-carbon treatment of industrial organic wastewater with the concept of carbon neutrality in future.

Dominant factors analyses and challenges of anaerobic digestion under cold environments

With the development of fermentation technology and the improvement of efficiency, anaerobic digestion (AD) has been playing an increasingly primary role in waste treatment and resource recovery. Temperature is undoubtedly the most important factor because it shapes microbial habitats, changes the composition of the microbial community structure, and even affects the expression of related functional genes. More than half of the biosphere is in a long-term or seasonal low-temperature environment (<20 °C), which makes psychrophilic AD have broad application prospects. Therefore, this review discusses the influencing factors and enhancement strategies of psychrophilic AD, which may provide a corresponding reference for future research on low-temperature fermentation. First, the occurrence of AD has been discussed. Then, the adaptation of microorganisms to the low-temperature environment was analyzed. Moreover, the challenges of psychrophilic AD have been reviewed. Meanwhile, the strategies for improving psychrophilic AD are presented. Further, from technology to application, the current situation of psychrophilic AD in pilot-scale tests is described. Finally, the economic and environmental feasibility of psychrophilic AD has been highlighted. In summary, psychrophilic AD is technically feasible, while economic analysis shows that the output benefits cannot fully cover the input costs, and the large-scale practical application of psychrophilic AD is still in its infancy. More research should focus on how to improve fermentation efficiency and reduce the investment cost of psychrophilic AD.

Sludge granulation in PN/A enhances nitrogen removal from mainstream anaerobically pretreated wastewater

Treating anaerobically pretreated wastewater using partial nitritation/anammox (PN/A) process faces severe challenges because of the complex syntrophic and competitive relationship among various bacteria. Results of this study suggested a continuous low dissolved oxygen (DO) concentration failed to sustain NH4+ removal (<80 %), whereas moderate DO concentrations with high aerobic periods suppressed anammox reaction. Through implementing a moderate DO concentration with low aerobic periods (MDO-LA), NH4+ and total nitrogen removal efficiency reached 91.5 ± 5.5 % and 71.3 ± 2.8 % respectively. The specific activities of ammonium-oxidizing bacteria (AOB) and anaerobic ammonium-oxidizing bacteria (AnAOB) reached 0.942 ± 0.030 and 0.277 ± 0.010 g nitrogen per gram mixed liquor volatile suspended solids, respectively, mainly because MDO-LA favored Thiothrix (filamentous bacteria) wash-out and promoted Nitrosomonas growth. Moreover, sludge granules covered by a thin exterior rim with abundant AOB were formed, favoring Ca. Brocadia growth (5.4 % to 13.2 %) and mass transfer between AOB and AnAOB, which consequently increased the expression of genes coding hydroxylamine oxidase and hydrazine synthase. Overall, achievements in this study provide a promising operating strategy for PN/A treating anaerobically pretreated wastewater.

Performance and economic evaluation of a pilot scale embedded ends-free membrane bioreactor (EEF-MBR)

In this work, an embedded ends-free membrane bioreactor (EEF-MBR) has been developed to overcome the fouling problem. The EEF-MBR unit has a novel configuration where a bed of granular activated carbon is placed in the bioreactor tank and fluidized by the aeration system. The performance of pilot-scale EEF-MBR was assessed based on flux and selectivity over 140 h. The permeate flux fluctuated between 2 and 10 L.m^-2.h^-1 under operating pressure of 0.07-0.2 bar when EEF-MBR was used to treat wastewater containing high organic matter. The COD removal efficiency was more than 99% after 1 h of operating time. Results from the pilot-scale performance were then used to design a large-scale EEF-MBR with 1200 m³.day^-1 capacity. Economic analysis showed that this new MBR configuration was cost-effective when the permeate flux was set at 10 L.m^-2.h^-1. The estimated additional cost for the large-scale wastewater treatment was about 0.25 US$.m^-3 with a payback period of 3 years. KEY POINTS: • Performance of new MBR configuration, EEF-MBR, was assessed in long term operation. • EEF-MBR shows high COD removal and relatively stable flux. • Cost estimation of large scale shows the cost effective EEF-MBR application.

Effects of biochar addition towards the treatment of blackwater in anaerobic dynamic membrane bioreactor (AnDMBR): Comparison among room temperature, mesophilic and thermophilic conditions

Effects of biochar addition on the performance of anaerobic dynamic membrane bioreactor (AnDMBR) under different temperatures for blackwater treatment was investigated. When the organic load ratios (OLR) was 1.0 g COD/L·d, the specific methane yield for the three biochar-amended reactors were 125.7, 148.0 and 182.3 mLCH₄/g COD_removed, respectively. Compare to those digesters without biochar participation, the methane production in the thermophilic reactor with biochar increased by 12% while the other two digesters increased by 6-10%. An analysis of membrane filtration resistance showed a reduction in total resistance (R_t) of 6.2 × 10¹¹-7.3 × 10¹¹ m^-1 when biochar was added to the three reactors. The thermophilic reactors with biochar increased the relative abundance of Methanothermobacter and promoted gene expression of metabolic pathways related to hydrolysis, acid production and methane production. Overall, biochar showed great potential as an inexpensive conductive material to increase methane production with reduced membrane fouling in AnDMBR systems.

Evaluating performance of nano-Fe3O4 modified granular activated carbon assisted wastewater treatment in anaerobic fluidized membrane bioreactor

Magnetic granular activated carbon (MGAC), a nano-Fe₃O₄ modified granular activated carbon, was used as the carrier in an anaerobic fluidized-bed membrane bioreactor (AFMBR) to promote domestic wastewater treatment efficiency and alleviate membrane biofouling. Chemical oxygen demand (COD) removal reached 89 ± 2.6% with the effluent concentration of 20 ± 3.9 mg/L in the MGAC-AFMBR, while it was 28 ± 5.2 mg/L in AFMBR at hydraulic retention time (HRT) of 4 h. Total nitrogen (TN) removal was also enhanced by 4.0% with MGAC. An increased proportion of Chloroflexi and Bacteroidetes in the sludge may be responsible for improved treatment performance. MGAC reduced the protein and polysaccharide content in extracellular polymeric substances (EPS) by 9.8% and 8.1%, respectively. Besides, Bacteroidetes and Proteobacteria abundance decreased by 4.0% and 16.6% in the membrane cake layer with MGAC addition. Therefore, the high-quality effluent and low membrane biofouling of AFMBR was sustained by MGAC.

In-situ membrane fouling control and performance improvement by adding materials in anaerobic membrane bioreactor: A review

Anaerobic membrane bioreactor (AnMBR) is a promising treatment technique for various types of wastewaters, and is preferred over other conventional aerobic and anaerobic methods. However, membrane fouling is considered a bottleneck in AnMBR system, which technically blocks membrane pores by numerous inorganics, organics, and other microbial substances. Various materials can be added in AnMBR to control membrane fouling and improve anaerobic digestion, and studies reporting the materials addition for this purpose are hereby systematically reviewed. The mechanism of membrane fouling control including compositional changes in extracellular polymeric substances (EPSs) and soluble microbial products (SMPs), materials properties, stimulation of antifouling microbes and alteration in substrate properties by material addition are thoroughly discussed. Nonetheless, this study opens up new research prospects to control membrane fouling of AnMBR, engineered by material, including compositional changes of microbial products (EPS and SMP), replacement of quorum quenching (QQ) by materials, and overall improvement of reactor performance. Regardless of the great research progress achieved previously in membrane fouling control, there is still a long way to go for material-mediated AnMBR applications to be undertaken, particularly for materials coupling, real scale application and molecular based studies on EPSs and SMPs, which were proposed for future researches.

Biochar-assisted anaerobic membrane bioreactor towards high-efficient energy recovery from swine wastewater: Performances and the potential mechanisms

A high-efficient energy recovery system of biochar-assisted anaerobic membrane bioreactor (BC-AnMBR) was established for swine wastewater treatment. Comparing with a conventional AnMBR, biochar addition accelerated volatile fatty acids (VFA) degradation during start-up stage, thereby shortened start-up duration by 44.0 %. Under a high organic loading rate (OLR) of 21.1 gCOD/L/d, BC-AnMBR promoted COD removal efficiency from 90.1 % to 95.2 %, and maintained a high methane production rate of 4.8L CH₄/L/d. The relative abundance of Methanosaeta declined from 53.9 % in conventional AnMBR to 21.0 % in BC-AnMBR, whereas that of Methanobrevibacter dramatically increased from 10.3 % to 70.9 %, respectively. Metabolic pathway analysis revealed that biochar not only strengthened hydrogenotrophic methanogenesis pathway, but also upregulated the genes encoding electron transfer carriers and riboflavin metabolism, suggesting the role of biochar facilitating direct interspecies electron transfer for syntrophic methanogenesis. The excellent energy yield performances under high OLR confirmed BC-AnMBR as an advanced system for high-strength swine wastewater treatment.

Biochar Facilitated Direct Interspecies Electron Transfer in Anaerobic Digestion to Alleviate Antibiotics Inhibition and Enhance Methanogenesis: A Review

Efficient conversion of organic waste into low-carbon biofuels such as methane through anaerobic digestion (AD) is a promising technology to alleviate energy shortages. However, issues such as inefficient methane production and poor system stability remain for AD technology. Biochar-facilitated direct interspecies electron transfer (DIET) has recently been recognized as an important strategy to improve AD performance. Nonetheless, the underlying mechanisms of biochar-facilitated DIET are still largely unknown. For this reason, this review evaluated the role of biochar-facilitated DIET mechanism in enhancing AD performance. First, the evolution of DIET was introduced. Then, applications of biochar-facilitated DIET for alleviating antibiotic inhibition and enhancing methanogenesis were summarized. Next, the electrochemical mechanism of biochar-facilitated DIET including electrical conductivity, redox-active characteristics, and electron transfer system activity was discussed. It can be concluded that biochar increased the abundance of potential DIET microorganisms, facilitated microbial aggregation, and regulated DIET-associated gene expression as a microbial mechanism. Finally, we also discussed the challenges of biochar in practical application. This review elucidated the role of DIET facilitated by biochar in the AD system, which would advance our understanding of the DIET mechanism underpinning the interaction of biochar and anaerobic microorganisms. However, direct evidence for the occurrence of biochar-facilitated DIET still requires further investigation.

Anaerobic membrane bioreactor for carbon-neutral treatment of industrial wastewater containing N, N-dimethylformamide: Evaluation of electricity, bio-energy production and carbon emission

The feasibility of anaerobic membrane bioreactor (AnMBR) for the treatment of N, N-dimethylformamide (DMF)-containing wastewater was theoretically compared with the conventional activated sludge (CAS) process in this study. The electricity consumption and expenditure, bio-energy production and CO₂ emission were investigated using the operational results of a lab-scale AnMBR operated in a long-term operation. The AnMBR was capable of producing bio-methane from wastewater and generated 3.45 kWh/m³ of electricity as recovered bio-energy while the CAS just generated 1.17 kWh/m³ of electricity from the post-treatment of excessive sludge disposal. The large quantity of bio-methane recovered by the AnMBR can also be sold as sustainable bioresource for the use of household natural gas with a theoretical profit gain of 29,821 US$/year, while that of the CAS was unprofitable. The AnMBR was also demonstrated to significantly reduce the carbon emission by obtaining a theoretical negative CO₂ production of -2.34 kg CO₂/m³ with the recycle of bio-energy while that for the CAS was 4.50 kg CO₂/m³. The results of this study demonstrate that the AnMBR process has promising potential for the carbon-neutral treatment of high-strength DMF-containing wastewater in the future.

Material mass balance and elemental flow analysis in a submerged anaerobic membrane bioreactor for municipal wastewater treatment towards low-carbon operation and resource recovery

The anaerobic membrane bioreactor (AnMBR) has gained huge attention as a municipal wastewater (MWW) treatment process that combined high organics removal, a low sludge yield and bioenergy recovery. In this study, a 20 L AnMBR was set up and operated steadily for 70 days in temperate conditions with an HRT of 6 h and a flux of 12 LMH for the treatment of real MWW, focusing on the behavior of the major elements (C, N, P and S) from an elemental balance perspective. The results showed that the AnMBR achieved more than 85 % COD removal, a low sludge yield (0.081 gVSS/gCOD_removed) and high methane production (0.31 L-CH₄/gCOD_removed) close to the theoretical value. The elemental flow analysis revealed that the AnMBR converted 77 % of the influent COD to methane (57 % gaseous and 20 % dissolved) and 6 % of the COD for sludge production. In addition, the AnMBR converted 34 % of the total carbon to energy-generated carbon, and only 3 % was in the form of CO₂ in the biogas for further upgradation, which was in line with the concept of carbon neutrality. Since little nitrogen or phosphorus were removed, the permeate was nutrient-rich and further treatment to recover the nutrients would be required. This study illustrates the superior performance of the AnMBR for MWW treatment with a microscopic view of elemental behavior and provides a reference for implementing the mainstream AnMBR process in carbon-neutral wastewater treatment plants.

Aerobic granular sludge cultivated from Fe-loaded activated carbon as carrier working low-strength wastewater conditions by bioreactor

This study proposes a new method to promote the granulation process while accelerating the degradation efficiency of nutrients. The new strategy could involve preparing Fe-loaded activated carbon (FAC) before start-up of granular cultivation and then cultivating the process of aerobic granular sludge (AGS) with such materials. In addition, this experiment could further comprehend how the preparation and characteristics of FAC affect the formation and properties of AGS. The conclusions showed that compared with the control, FAC enhanced the sedimentation performance and significant removal efficiency. Meanwhile, the values of protein (PN) and polysaccharide (PS) also increased significantly in the addition of FAC, indicating the production of substances were induced by FAC. Molecular biology methods indicated that the rapid production of granulation and removal of nutrients were considered as the abundance of various microbes and denitrifying bacteria at the addition of FAC. This research showed that the presence of FAC is a useful strategy for the initiation of sludge particle formation to promote the treatment of wastewater, containing COD and NH₄⁺ at about 150-100 and 30 mg L^-1.

Effect of asparagine, corncob biochar and Fe(II) on anaerobic biological treatment under low temperature: Enhanced performance and microbial community dynamic

To ensure the efficiency of anaerobic biological treatment technology at lower temperature will expand the application of anaerobic reactor in practical industrial wastewater treatment. Through a batch experiment, asparagine, corncob biochar and Fe²⁺ were selected as strengthening measures to analyze the effects on the anaerobic sludge characteristics, microbial community and functional genes in the low temperature (15 °C). Results showed that after 21 days, asparagine began to promote chemical oxygen demand (COD) removal by the anaerobic treatment, with highest COD removal rate (81.65%) observed when the asparagine concentration was 1 mmol/L. When adding 3 g biochar, 25 mg/L Fe²⁺, and the combination of biochar and Fe²⁺, the COD removal rates reached to 82%, 92% and 97%, respectively. In the presence of asparagine, both biochar and Fe²⁺ alone or in combination increased the activity of protease (16.35%-120.71%) and coenzyme F₄₂₀ (5.63%-130.2%). The relative abundance of Proteobacteria and Methanobacterium increased in the presence of biochar and Fe²⁺. In addition, the KEGG results showed that the combined addition of biochar and Fe²⁺ enhanced bacterial replication and repair and promoted amino acid metabolism of archaea.

Application of biochar immobilized microorganisms for pollutants removal from wastewater: A review

Microbial immobilization technology (MIT) has been rapidly developed and used to remove pollutants from water/wastewater in recent years, owing to its high stability, rapid reaction rate, and high activity. Microbial immobilization carrier with low cost and high removal efficiency is the key of MIT. Biochar is considered to be an efficient carrier for microbial immobilization because of its high porosity and good adsorption effect, which can provide a habitat for microorganisms. The use of biochar immobilized microorganisms to treat different pollutants in wastewater is a promising treatment method. Compared with the other biological treatment technology, biochar immobilized microorganisms can improve microbial abundance, repeated utilization ratio, microbial metabolic capacity, etc. However, current research on this method is still in its infancy. Little attention has been paid to the interaction mechanisms between biochar and microorganisms, and many studies are only carried out in the laboratory. There are still problems such as difficult recovery after use and secondary pollution caused by residual pollutants after biochar adsorption, which need further clarification. To have comprehensive digestion and an in-depth understanding of biochar immobilized microorganisms technology in wastewater treatment, the wastewater treatment methods based on biochar are firstly summarized in this review. Then the mechanisms of immobilized microorganisms were explored, and the applications of biochar immobilized microorganisms in wastewater were systematically reviewed. Finally, suggestions and perspectives for future research and practical application are put forward.

Biochar enhances the biotransformation of organic micropollutants (OMPs) in an anaerobic membrane bioreactor treating sewage

The removal of emerging organic micropollutants (OMPs) in anaerobic membrane bioreactors (AnMBRs) has garnered considerable attention owing to the rapid development of AnMBR technology and the increased environmental risk caused by OMP discharge. We investigated the removal efficiency of 22 typical OMPs from sewage being treated in an AnMBR, and implemented and evaluated an upgrading strategy by adding biochar. The average removal efficiency of OMPs was only 76.8% due to hydrophilic OMPs containing electron-withdrawing groups (ketoprofen, ibuprofen, diclofenac, and carbamazepine) being poorly removed. Biochar addition (5.0 g/L) promoted the removal of recalcitrant OMPs by 45%, leading to an enhanced removal efficiency of 88.7%. Although biochar has a high adsorption capacity to different OMPs, the biotransformation rather than sorption removal efficiency of 13 of the 22 OMPs was largely enhanced, suggesting that adsorption-biotransformation was the main approach by which biochar enhances the OMP removal. Biotransformation test and microbial analysis revealed that the enrichment of species (Flavobacterium, Massilia, Acinetobacter, and Cloacibacterium) involved in OMP biotransformation on biochar contributed largely to the enhanced biotransformation removal efficiency of OMPs. In this way, the enhanced electron transfer activity and syntrophic metabolism between hydrogenotrophic methanogens and species that oxidize acetate to H₂/CO₂ on biochar jointly contributed to the stable CH₄ production and OMP biotransformation. This study provides a promising strategy to enhance the OMP removal in AnMBRs and improves our understanding of the underlying mechanism of biochar-amended OMP removal in anaerobic treatment systems.

Carbon-neutral treatment of N, N-dimethylformamide-containing industrial wastewater by anaerobic membrane bioreactor (AnMBR): Bio-energy recovery and CO2 emission reduction

High-strength industrial wastewater containing approximately 2000 mg/L of N, N-dimethylformamide (DMF) was treated by the anaerobic membrane bioreactor (AnMBR) during a long-term operation with the concept of carbon neutrality in this study. Bio-methane was recovered as bio-energy or bio-resource from DMF-containing wastewater along with the CO₂ emission reduction. The results are clear evidence of the feasibility of carbon-neutral treatment of DMF-containing wastewater by the AnMBR. With an effective degradation under the organic loading rate of 6.53 COD kg/m³/d at the HRT of 12 h, the AnMBR completely covered the energy consumption during long-term operation by saving electricity of 4.16 kWh/m³ compared with the conventional activated sludge process. The CO₂ emission of the AnMBR was just 1.06 kg/m³, remarkably reducing 1.45 kg/m³ of CO₂. The treatment of DMF-containing wastewater by the AnMBR perfectly realized the goal of carbon neutrality, and was considered as an alternative to the conventional activated sludge process.

Insights into the role of biochar on the acidogenic process and microbial pathways in a granular sulfate-reducing up-flow sludge bed reactor

In this study, the effect of biochar on sulfate reduction and anaerobic acidogenic process was explored in a granular sulfate-reducing up-flow sludge bed reactor in both long-term operation and batch tests. Both bioreactors had a high sulfate reduction efficiency of over 95% during the long-term operation, while the reactor with biochar addition showed higher sulfate reduction efficiency and stronger robustness against volatile fatty acids accumulation with a higher organic loading and sulfate loading rate. Batch tests showed that adding biochar significantly lessened the lag phase of the sulfate-reducing process, accelerated the adaption of acidogens, and facilitated both production and utilization of volatile fatty acids. The microbial pathways proved that biochar could regulate the acidification fermentation pathway and facilitate the enrichment of assimilative desulfurization bacteria. Overall, this study revealed that the acidogenic sulfate-reducing metabolic pathway could be enhanced by biochar, offering a potential application for effective sulfate-laden wastewater treatment.

Pilot plant demonstration of temperature impacts on the methanogenic performance and membrane fouling control of the anaerobic membrane bioreactor in treating real municipal wastewater

A 5,000-L anaerobic membrane bioreactor (AnMBR) fed with actual municipal wastewater was employed to study the impact of temperature drops on methanogenic performance and membrane fouling. With temperature dropped from 25 °C to 15 °C, the methane yield decreased from 0.244 to 0.205 NL-CH₄/g-COD_removal and the dissolved methane increased from 29% to 43%, resulted in the methanogenic performance reduced by 25%. The membrane rejection offset the deteriorated anaerobic digestion at low temperatures and ensured the stable COD removal efficiency of 84.5%-90.0%. The synergistic effects of the increased microbial products and viscosity and the residual inorganic foulants aggravated the membrane fouling at lower temperatures. As the organic fouling was easily removed by NaClO, the inorganics related to the elements of S, Ca and Fe were the stubborn membrane foulants and required the enhanced acid membrane cleaning. These findings obtained under the quasi-practical condition are expected to promote the practical applications of mainstream AnMBR.

A review on anaerobic membrane bioreactors for enhanced valorization of urban organic wastes: Achievements, limitations, energy balance and future perspectives

Sustainable urban development is threatened by an impending energy crisis and large amounts of organic wastes generated from the municipal sector among others. Conventional waste management methods involve greenhouse gas (GHG) emission and limited resource recovery, thus necessitating advanced techniques to convert such wastes into bioenergy, bio-fertilizers and valuable-added products. Research and application experiences from different scale applications indicate that the anaerobic membrane bioreactor (AnMBR) process is a kind of high-rate anaerobic digester for urban organic wastes valorization including food waste and waste sludge, while the research status is still insufficiently summarized. Through compiling recent achievements and literature, this review will focus on the following aspects, including AnMBR treatment performance and membrane fouling, technical limitations, energy balance and techno-economic assessment as well as future perspectives. AnMBR can enhance organic wastes treatment via complete retention of functional microbes and suspended solids, and timely separation of products and potential inhibitory substances, thus improving digestion efficiency in terms of increased organics degradation rates, biogas production and process robustness at a low footprint. When handling high-solid organic wastes, membrane fouling and mass transfer issues can be the challenges limiting AnMBR applications to a wet-type digestion, thus countermeasures are required to pursue extended implementations. A conceptual framework is proposed by taking various organic wastes disposal and final productions (permeate, biogas and biosolids) utilization into consideration, which will contribute to the development of AnMBR-based waste-to-resource facilities towards sustainable waste management and more economic-environmental benefits output.

A comprehensive review on the use of conductive materials to improve anaerobic digestion: Focusing on landfill leachate treatment

Globally, around 70% of waste is disposed of in open dumps or landfill sites, with the leachate generated from these sites containing high concentrations of organic and inorganic compounds, which will adversely affect aquatic environments if discharged without proper treatment. Anaerobic digestion of landfill leachate is an environmentally-friendly method that efficiently converts organic compounds into methane-rich biogas. However, the widespread application of anaerobic digestion has been hindered by poor system stability, low methanogenic activity and a high level of volatile fatty acids (VFAs) accumulation, increasing the operational costs of treatment. Conductive materials can be added to the digester to improve the performance of anaerobic digestion in landfill leachate treatment systems and studies reporting the use of conductive materials for this purpose are hereby thoroughly reviewed. The mechanism of microbial growth and enrichment by conductive materials is discussed, as well as the subsequent effect on waste metabolism, methane production, syntrophic relationships and interspecies electron transfer. The porous structure, specific surface area and conductivity of conductive materials play vital roles in the facilitation of syntrophic relationships between fermentative bacteria and methanogenic archaea. In addition, the mediation of direct interspecies electron transfer (DIET) by conductive materials increases the methane content of biogas from 16% to 60% as compared to indirect interspecies electron transfer (IIET) in conventional anaerobic digestion systems. This review identifies research gaps in the field of material-amended anaerobic systems, suggesting future research directions including investigations into combined chemical-biological treatments for landfill leachate, microbial management using conductive materials for efficient pollutant removal and the capacity for material reuse. Moreover, findings of this review provide a reference for the efficient and large-scale treatment of landfill leachate by anaerobic digestion with conductive materials.

Seasonal temperatures impact on the mass flows in the innovative integrated process of anaerobic membrane bioreactor and one-stage partial nitritation-anammox for the treatment of municipal wastewater

A pilot-scale anaerobic membrane bioreactor (AnMBR) integrated with a one-stage partial nitritation-anammox (PN/A) reactor was operated for the treatment of municipal wastewater (MWW) at seasonal temperatures of 15-25 °C. The removal efficiencies of COD and total nitrogen (TN) were always > 90% and > 75% respectively. The methanogenesis and PN/A were identified as the primary removal pathways of COD and TN, respectively, and were suppressed at low temperatures. With the temperature dropped from 25 °C to 20 °C to 15 °C, the methane-accounted COD decreased from 63.1% to 59.6% to 48.4%, and the PN/A-accounted TN decreased from 58.1% to 51.7% to 45.3%. The AnMBR and PN/A mutually complement each other in this combined process, as the AnMBR removed 8.5%-16.1% of TN by sludge entrainment and the PN/A reactor removed 2.6%-3.4% of COD by denitrification and aerobic oxidation. These results highlighted the strong feasibility of applying the AnMBR-PN/A process to the treatment of MWW in temperate climate.

Unveiling the characterization and development of prokaryotic community during the start-up and long-term operation of a pilot-scale anaerobic membrane bioreactor for the treatment of real municipal wastewater

The anaerobic membrane bioreactor (AnMBR) is a promising sustainable process and technology for the treatment of municipal wastewater from the perspective of carbon neutrality. In this study, a large pilot-scale AnMBR was constructed and the microbial community development of the anaerobic digested sludge in the AnMBR was determined during the treatment of municipal wastewater. The AnMBR system was conducted for 217 days during a long-term operation with the feed of real municipal wastewater. The characterization and dynamics of the microorganisms revealed that a stable prokaryotic community was gradually achieved. In the community of methane-producing archaea (or methanogens), the acetotrophic methanogen Methanosaeta was significantly enriched at an ambient temperature of 25 °C with an overwhelming relative abundance in the entire community. The abundance of Methanosaeta was even higher than the most abundant bacterial phyla Chloroflexi, Firmicutes, Proteobacteria and Bacteroidetes. This phenomenon is quite different from that found in other typical anaerobic systems. The massive enrichment of methanogens is the key to maintaining stable methane production in the treatment of municipal wastewater by the AnMBR. The interspecies cooperation of major functional bacterial groups including protein/carbohydrate/cellulose-degrading (genera Anaerovorax, Aminomonas, Levilinea, Flexilinea and Ruminococcus etc.), sulfate-reducing (Desulfovibrio and Desulfomicrobium etc.) and syntrophic (Syntrophorhabdus and Syntrophus etc.) bacteria with acetotrophic and hydrogenotrophic archaea enhances the stability of reactor operation and help to acclimate the entire prokaryotic community to the characteristics of real municipal wastewater.

In-situ monitoring of membrane fouling migration and compression mechanism with improved ultraviolet technique in membrane bioreactors

An improved UV spectrum in-situ monitoring system was applied to explore the membrane fouling behavior in membrane bioreactors (MBRs). The changes in absorbance curve illustrated that the formation of a stubborn fouling layer includes the migration and compression of membrane surface foulants. The initial flux negatively correlates with the migration degree (unevenness) of membrane fouling, while fiber length is positively correlated. In further experiments, ultrasonic thickness measurement excludes fouling layer compression caused by spatial collapse under external force. Moisture content measurement tests demonstrated that the moisture content changed from 52% to 31% after fouling layer compression, which confirmed that the fouling layer compression is mainly caused by the "high pressure dehydration effect". Finally, a membrane backwashing strategy based on fouling layer compression theory indicated that the backwashing process should be carried out at a stage where the accumulation of membrane fouling is constant but the fouling layer is not compressed.

When dewatered swine manure-derived biochar meets swine wastewater in anaerobic digestion: A win-win scenario towards highly efficient energy recovery and antibiotic resistance genes attenuation for swine manure management

This work explored the feasibility of dewatered swine manure-derived biochar (DSMB) as an additive to facilitate anaerobic digestion (AD) of swine wastewater for energy recovery and antibiotic resistance genes (ARG) attenuation enhancements. With 20 g/L DSMB assistance, the methanogenic lag time of swine wastewater was shortened by 17.4-21.1%, and the maximum CH₄ production rate increased from 40.8 mL/d to 48.3-50.5 mL/d, among which DSMB prepared under 300 °C exhibited a better performance than that prepared under 500 °C and 700 °C. Integrated analysis of DSMB electrochemical properties, microbial electron transfer system activity, and microbial community succession revealed the potential of DSMB-300 to act as redox-active electron transfer mediators between syntrophic microbes to accelerate syntrophic methanogenesis via potential direct interspecies electron transfer. Meanwhile, DSMB preparation by pyrolysis dramatically reduced ARG abundance by almost 4 logs. Adding DSMB into AD not only strengthened the attenuation efficiency of ARG in the original swine wastewater, but also effectively controlled the potential risk of horizontal gene transfer by mitigating 74.8% of the mobile gene elements abundance. Accordingly, we proposed a win-win scenario for bio-waste management in swine farms, highlighting the more advanced energy recovery and ARG attenuation compared to the current status.

Mitigation of acidogenic product inhibition and elevated mass transfer by biochar during anaerobic digestion of food waste

Anaerobic digestion (AD) of food waste is widely accepted as a promising technology for both waste disposal and resource recovery. With the advancing of AD technology, to exploit the capacity of organic waste for maximum energy/resource recovery becomes the new focus and hence, improve the viability of this technology for practical application. Product inhibition and mass transfer are the common limitations encountered during AD of putrescible organic waste. Biochar materials have been widely used to promote AD process in recent years. This review summarizes the mechanism and regulation strategies of biochar and its modified derivatives in promoting AD of solid waste (mainly food waste) from the three aspects of hydrolysis, syntrophic acetogenesis, and methane production. At the same time, the relationship between carbon materials and electron transfer among anaerobic microbes is summarized from the perspective of microbial community. In addition, the market application of this technology was evaluated.

Anaerobic digestion beyond biogas

Anaerobic digestion (AD) is a matured technology for waste (water) remediation/stabilization and bioenergy generation in the form of biogas. AD technology has several inherent benefits ranging from generating renewable energy, remediating waste (water), and reducing greenhouse gas emission to improving health/hygiene and the overall socio-economic status of rural communities in developing nations. In recent years, there has been a paradigm shift in applications of AD technology beyond biogas. This special issue (SI) entitled, "Anaerobic Digestion Beyond Biogas (ADBB-2021)," was conceptualized to incorporate some of the recent advances in AD in which the emphasis is beyond biogas, such as anaerobic biorefinery, chain elongation, treatment of micropollutants, toxicity and system stability, digestate as biofertilizer, bio-electrochemical systems, innovative bioreactors, carbon sequestration, biogas upgrading, microbiomes, waste (water) remediation, residues/waste pre-treatment, promoter addition, and modeling, process control, and automation, among others. This VSI: ADBB-2021 contains 53 manuscripts (14 critical reviews and 39 research). The key findings of each manuscript are briefly summarized here, which can serve as a valuable resource for AD researchers to learn of major advances in AD technology and identify future research directions.