The microbial electrosynthesis system (MES) offers an attractive platform for effective cathodic reduction of carbon dioxide (CO2) to biomethane (CH4). However, achieving high productivity and energy efficiency remains challenging, often due to sluggish hydrogen evolution reaction (HER) kinetics at the cathode. Here, we showed an efficient CO2 reduction reaction with a stainless steel mesh cathode electrodeposited with magnesium oxide (MgO) and copper oxide (CuO); MgO/CuO-SSM. The electrodes are coupled with an enriched anaerobic culture to facilitate the bioproduction of CH4 from CO2. Material characterization confirmed the successful deposition of MgO and CuO on SSM, while electrochemical analysis revealed superior catalytic performance of the composite electrode compared to bare SSM. Furthermore, optimizing the applied cathode potential from -1 V to -0.9 V vs. Ag/AgCl/KCl (sat'd) for modified SSM improved the methane production while increasing the electrical energy efficiency. At -0.9 V vs. Ag/AgCl/KCl (sat'd), MgO/CuO-SSM recorded an energy efficiency of 21.6% (103.8 +/- 3.8 L(CH4) m-3(catholyte replaced) d-1), surpassing that of bare SSM (14.7%, 117.5 +/- 3.6 L(CH4) m-3(catholyte replaced) d-1) operated at -1 V vs. Ag/AgCl/KCl (sat'd). Overall, our work introduces a promising and facile electrode modification strategy that enhances MES performance, enabling CO2 reduction to CH4 at reduced energy demand towards practical carbon neutrality applications.

Anaerobic digestion (AD) is an established technology for the sustainable valorization of various types of biodegradable waste since it allows biogas production as a clean energy source. Bioplastics gained traction as the ideal alternative to non-biodegradable plastics with long-standing environmental concerns regarding their environmental impact. Given the expansion of their production and use in daily activities, bioplastics found their way into AD facilities along with other organic wastes. While recent studies investigated the impact of bioplastics on AD systems, a holistic understanding of bioplastics’ degradation in AD systems remains limited. This could be attributed to bioplastics’ diverse compositions and complex degradation pathways. This review critically analyzes the factors affecting bioplastics’ degradation in AD systems, including retention time, temperature, pretreatment methods, and the type of bioplastic. Moreover, various approaches, such as co-digestion with organic waste, used to enhance the degradation rate of bioplastics in AD systems, are discussed and assessed. In addition, this review sheds light on the persistent challenges in the AD systems of bioplastics, such as residual bioplastics in digestate, inconsistent degradation rates, and the leaching of toxic elements, all of which pose risks to digestate quality and environmental safety. Finally, a future outlook is provided, identifying key research needs to improve the digestibility and biodegradability of bioplastics when used as sole or co-feedstock in AD systems. Overall, this review presents vital insights to the AD industry, which is currently grappling with maintaining the stability of digesters treating bioplastics.

Dissimilatory nitrate reduction to ammonium is an important nitrogen cycling pathway for nature and wastewater treatment, but its contribution has been underestimated because the produced ammonium can be rapidly assimilated by microorganisms. Here we operated a laboratory-scale sulfide driven continuous flow reactor under different sulfur-to-nitrogen ratios, and combined isotope tracing, batch test, pure culture experiment, metagenomics, and transcriptional analyses were used to investigate nitrogen and sulfur transformation. At a sulfur-to-nitrogen molar ratio of 4, the ammonium conversion efficiency reached to 46.8%, with a potential dissimilatory nitrate reduction to ammonium rate of 30.75 ± 3.81 μmol/(L·h). Electron microscopy and elemental analysis indicated the accumulation of elemental sulfur. Metagenomic and pure-culture analyses with Thiobacillus thioparus ATCC158 suggested that ammonium produced during nitrate reduction was rapidly assimilated into biomass. These findings reveal coupling between sulfur-driven nitrate reduction and ammonium assimilation during nitrogen and sulfur transformation. Ammonium assimilation provides a reliable theoretical support for cell proliferation in sulfur-autotrophic dissimilatory nitrate reduction to ammonium in nature and wastewater treatment plants, which was overlooked earlier, based on metagenomic and incubation experiments.

Anaerobic digestion (AD) is a proven and enduring technology widely employed for sustainable waste management and renewable energy recovery. However, growing evidence indicates that micro/nanoplastics (MPs/NPs), increasingly released into waste and wastewater streams, can disrupt the stability and efficacy of the AD process. While several promising remediation strategies have been proposed in previous studies, thorough mechanistic evaluations of these techniques remain limited. This review comprehensively discusses how MPs/NPs affect AD performance, including reductions in biogas yield, organic degradation efficiency and effluent quality. Key inhibition mechanisms, such as physical damage to microbial cells, enzyme suppression, and the propagation of antibiotic resistance genes, are covered. Importantly, for the first time, the review provides a detailed overview of the emerging remediation strategies that mitigate the impacts of MPs/NPs on AD. Strategies such as thermal hydrolysis, ultrasonication, alkaline conditioning, ozonation, Fenton oxidation, and carbon-based additives are comparatively evaluated. Special attention is paid to how these strategies counteract MPs/NPs-induced stress, restore microbial activity, and enhance AD system resilience. Their practical applicability, advantages, limitations and environmental implications are also addressed. Looking forward, this review spotlights the future research directions needed to enhance the resiliency of the AD process treating MPs/NPs-laden waste streams. This review can serve as a foundational reference for researchers and practitioners seeking to develop efficient strategies to overcome MPs/NPs-caused detrimental impacts on AD process.

During high-solids anaerobic digestion (HSAD) operation, fresh feedstock is typically supplemented with digestate from previous batches to serve as an inoculum. This study investigated the long-term effects of digestate recycling as inoculum in bench-scale HSAD systems with percolate recirculation and fed with source-separated organics (SSO). During the first 4 cycles (phase-1), six reactors (R1-R6) were operated under identical conditions, where the feedstock-to-inoculum (F/I) ratio was maintained at 2. Digestate from the previous cycle served as inoculum for the subsequent ones. In the following two cycles (phase-2), 4 reactors (R1-R4) from phase-1 were operated at different digestate recycling ratios, i.e. 40% (R1/R2) and 50% (R3/R4). Reactors operated with 50% digestate recycling consistently achieved higher methane yield and energy recovery efficiencies, faster reductions in H2S levels in biogas, and better process stability. Microbial community analysis indicated that 40% digestate recycling promoted microbial diversity in Cycle-5; however, it ultimately led to a less diverse community in Cycle-6. Conversely, 50% digestate recycling resulted in a stable methanogenic consortium dominated by Methanosarcina and Methanoculleus in the digester tank, along with strong enrichment of ammonia-tolerant vadinCA11 in the percolate. Overall, these findings provide practical insights into ensuring process stability and energy recovery efficiency in HSAD.

Nanobubble water (NBW) has emerged as a promising technology for enhancing the energy recovery in anaerobic digestion (AD). However, its potential to concurrently recover nutrients has not been reported in the literature. We investigated the impact of nitrogen nanobubble water (N2-NBW) on the AD of waste-activated sludge. In addition to enhancing methane yield by up to 61%, N2-NBW enhanced sludge solubilization, promoting the rapid release of phosphate, ammonium, and magnesium ions into the liquid phase and facilitating high-purity struvite precipitation, as confirmed by XRD, SEM-EDX, and ICP-MS analyses. The results revealed that N2-NBW addition enriched hydrolytic and syntrophic bacteria and known hydrogenotrophic methanogens (Methanobacterium). Moreover, functional gene analysis proved that N2-NBW stimulated active biological ion transport (PiT, citMHS, mgtA) and ammonification (glsA) to create an alkaline microenvironment for struvite crystallization without any addition of exogenous chemicals while upregulating interspecies electron transfer genes (hdrA2, fdoH) to drive efficient methanogenesis. To the best of our knowledge, this is the first study that reports NBW-enabled enhancement of energy and nutrient recovery in AD, highlighting its dual benefits.

Microbial electrochemical cell (MXC) based biosensors enable accurate and real-time detection of wastewater constituents. However, selecting an appropriate biosensing mode is critical for detection and monitoring, particularly for persistent recalcitrant compounds that resist biodegradation and pose detection challenges. This study compares microbial fuel cell (MFC) and microbial electrolysis cell (MEC) based biosensors for detecting naphthenic acids (NAs) in oil sands process-affected water (OSPW). The impact of sensing mode on standing time, sensitivity, and electrochemical losses was examined. MFC biosensor recorded stable current output within 1 h, significantly faster than the 15 h stabilization required by MEC. Over a 24 h operation, MFC exhibited 3.91 times higher sensitivity (response slope 0.43 +/- 0.02) than MEC (0.11 +/- 0.02). A strong linear correlation was observed between the current response and NA concentrations (9.21-1.84 mg/L), with R2 values of 0.986 for the MFC and 0.895 for the MEC biosensor. The MFC biosensor demonstrated a rapid response to lower NA concentrations. These findings suggest that MFC-based biosensors could be a practical and cost-effective solution for real-time monitoring in complex wastewater matrices, such as OSPW.

Naphthenic acid fraction compounds (NAFCs) are primary toxic components in oil sands process-affected water. Chemical treatments in tailings ponds can influence their concentration and composition, which may affect their toxicity. It is therefore crucial to measure the levels and distribution of NAFCs if there is any chemical treatment process being implemented in the tailings ponds. In the literature, some potential chemical treatment processes have shown promising results in terms of methane inhibition, thus reducing fugitive greenhouse gas (GHG) emissions from tailings ponds. However, little has been established to date on how the methanogenesis process along with chemicals used in the treatment affects the concentration and compositional characteristics of the NAFCs in the tailings ponds. In this study, laboratory scale bottle tests simulating oil sands tailings pond environments were used to investigate joint outcomes of chemical treatments on methanogenesis in relation to the concentrations and molecular level characteristics of NAFCs. Among the four chemicals (Na2MoO4·2H2O, Fe2(SO4)3, Na2SO4, and Na3C6H5O7·2H2O) tested, Fe2(SO4)3 reduced both methane production and the concentration of the NAFCs. There was also evidence for oxidative transformation of classical naphthenic acids (i.e., O2-NAFCs) presented in the study. These findings suggest the potential of using Fe2(SO4)3 as a chemical amendment for NAFC attenuation and methane reduction to assist with the cleanup effort for oil sands tailings ponds.

Abstract Adding biochar in anaerobic digestion (AD) has been widely investigated recently in single-stage AD rather than in two-stage AD. Despite a few reports suggesting improved performance of two-stage AD with biochar addition, the existing reports provide limited insights into the role of biochar in microbial dynamics and its mechanisms of enhancement. This study investigated the impact of different biochar dosages (5, 15, and 25 g/L) on the two-stage mesophilic AD of food waste, focusing on biogas production, dynamics of volatile fatty acids (VFAs), organic removal, and microbial community diversity and structure. During the first stage, cumulative hydrogen (H2) production increased by 23−30% in biochar-amended reactors, while in the methanogenesis stage, cumulative methane (CH4) yields increased by 31−41%. However, no statisticallly significant differences (p > 0.05) were obeserved among the different biochar dosages, suggesting that 5 g/L would be the optimum dosage for both stages. Biochar promoted the accumulation of VFAs during the first stage while enhancing VFA conversion to methane in the second stage, with 5 g/L of biochar exhibiting the lowest VFA accumulation. In addition to increased microbial diversity and richness, 5 g/L of biochar also selectively enriched various fast-growing hydrolytic/fermentative taxa in the first stage while significantly stimulating syntrophic fatty-acid degraders (Syntrophomonas) and metabolically versatile methanogens (Methanosarcina) in the second stage, indicating improved syntrophic cooperation and potential facilitation of direct interspecies electron transfer (DIET). These findings highlight biochar as a promising functional additive for improving two-stage AD of food waste.

Long-chain fatty acids (LCFAs) are common in lipid-rich wastewater, yet the dose-dependent effects on anaerobic ammonium oxidation (anammox) remain unclear. In this work, the effect of oleate on anammox was evaluated in short- and long-term experiments at different C/N ratios using multi-omics approaches. Oleate showed a dose-dependent effect. Low oleate dosages, particularly at a C/N ratio of 0.1, improved nitrogen removal performance, whereas high dosages shifted nitrite consumption from anammox toward heterotrophic reduction. Multi-omics analysis showed that appropriate oleate dosages also stimulated central carbon metabolism, while the Wood-Ljungdahl pathway remained comparatively stable. 13C-isotope tracing results demonstrated that oleate-derived carbon entered acetate, central-carbon intermediates, amino acids, peptides, and vitamin-related metabolites, supporting a cross-feeding framework involving LCFA-degrading heterotrophs and anammox bacteria. In contrast, excessive oleate caused incomplete oleate degradation, membrane damage, and community restructuring. These findings provide a basis for controlling LCFA dosage to improve anammox-based treatment of organic-rich side streams.

High-solids anaerobic digestion (HSAD) of source-separated organics (SSO) enables cities to sustainably valorize organic waste, enhance waste diversion from landfills, and promote a circular bioeconomy within urban infrastructure. In cold-climate regions, seasonal variations in SSO composition and their implications for HSAD performance remain poorly understood. Here, we investigated the effects of seasonal changes in SSO characteristics on the performance of percolate-recirculated HSAD systems operated with SSO collected from a green bin program in Edmonton, Canada. Winter/spring SSO was more energy-dense due to the presence of mainly food waste, whereas summer/fall SSO had a significant portion of yard waste with relatively lower energy content. Correspondingly, winter SSO showed the highest methane yield among the samples, while energy efficiency exhibited the opposite trend, peaking in spring and reaching minimum values in winter. Winter SSO favored Methanoculleus dominance, whereas summer and fall shifted toward Methanosarcina and Firmicutes-enriched communities, enabling metabolic diversity and resilience under complex substrates. Across all seasons, the 50% digestate recycling consistently outperformed the 40% recycling, achieving higher-quality biogas (higher methane and lower H2S), better process stability, and higher energy recovery efficiency. Overall, these results provide practical guidance for the seasonally adaptive operational strategy for HSAD facilities in cold-climate regions.

Integration of 15-d HRT mesophilic primary sludge (PS) digesters with hyperthermophilic hydrolysis (HTH) was evaluated in continuously stirred tank reactors (CSTRs) operated with and without enrichment of Caldicellulosiruptor bescii (CBC). Two CSTRs (R1control and R2 CBC) were operated over four phases, including periods without HTH, HTH operation at a 2-day hydraulic retention time (HRT), CBC inoculation, and reinoculation with an increased HRT of 4 days, with the HTH effluent recirculated to the mesophilic digesters. Across all phases, PS biodegradation efficiency in the R1 control was not different from the R2 CBC i.e. the recirculation of the HTH with and without CBC did not improve the methane yield, with average values ranging from 53%–64%. Offline testing of the control and test HTH reactors 2, and 14 days after CBC inoculation with cellobiose and cellulose confirmed that the CBC did not offer any improvement in the acidification rates. To further assess CBC viability and competitive dynamics, confirmatory offline batch tests conducted using PS slurry and dried PS with HTH inoculum, in the presence and absence of CBC demonstrated that CBC fermentation occurred with a more pronounced effect observed for dried PS where competition from native fermenters was reduced. A modified ADM1-based kinetic model calibrated for cellulose removal showed that under the applied operating conditions (HTH HRT 2–4 days; mesophilic HRT 15 days), more than 80% of the CBC inoculum was washed out 7 days after inoculation. Microbial analysis showed that the inoculated species (CBC) failed to achieve long-term dominance. Instead, reactor performance was dictated by native communities selected by the temperature and the substrate used.

Civic development and industrial expansion have created opportunities but also accelerated environmental degradation. Bioremediation has emerged as a sustainable approach alongside biological and physicochemical methods to mitigate this decline. However, controlling the supply of electron donors and acceptors remains a challenge. Microbial electrochemical cells (MXCs) have emerged as a core component in bioelectronic technologies, utilizing electric current as an electron donor or acceptor. At the core of this technology is an electrogenic biofilm that serves as both the sensing and transducing element. This review examines the synergistic role of MXCs in bioremediation and real-time monitoring, highlighting how these biosensors, powered by microbial communities, uniquely combine pollutant detection with environmental bioremediation. It highlights recent advances, relevant studies, and the potential for advancing sustainable MXC-based approaches to contamination management. Emphasis is placed on the progress in biosensing applications in expanding the horizon for MXCs, particularly microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). Lastly, the review deliberates on the barriers hindering the transition of technology and provides an outlook on future opportunities for MXC biosensors.

In recent years, the use of nanobubbles (NBs) in anaerobic digestion (AD) has received significant research attention, with numerous studies reporting notable improvements in AD efficiency when nanobubble water (NBW) is introduced in digesters. The improvements are often attributed to more efficient substrate degradation, higher activity of key enzymes, and the more complex and diverse microbial communities associated with NBW-integrated AD. Despite these promising results, detailed explanations of the mechanisms and modes of action behind NBW, particularly its impact on each step of the AD process—hydrolysis, acidogenesis, acetogenesis, methanogenesis—are still limited. This paper aims to provide a comprehensive review and critical assessment of NBW’s impacts on AD, with a particular focus on how operational conditions can influence NBW-related effects on AD performance. Finally, energy efficiencies, future opportunities and challenges of expanding NB technology to boost AD efficiency are discussed.

Large-scale application of microbial electrosynthesis (MES) demands the extraction of a single product from other by-products. Acetate is one of the most valuable products produced in MES. Therefore, many recent studies investigated several aspects of optimizing acetate production in MES. However, none of the previous studies provided a summary of acetate extraction in MES. This review provides an overview of acetate extraction methods in MES, introducing future improved designs that combine efficient production and extraction processes. Sorption, membrane electrolysis, and electrodialysis were found to be highly efficient technologies for acetate extraction in MES. However, the energy input for these techniques was high. Therefore, an integrated architecture of MES reactors for acetate extraction is crucial for system performance and economic viability. Innovations in in-situ extraction show promise in addressing current limitations. However, further research on adsorbent and membrane fouling, and energy optimization, is needed to make MES sustainable technology.

The efficient and low-cost recovery of phosphorus (P) from wastewater has become one of the main environmental concerns in recent years. This study investigated kinetics, mechanisms, and constraint factors in P recovery from wastewater via contact precipitation using waste eggshells, without any calcination. The kinetic analysis indicated that the dissolution rate of calcium (Ca) from the eggshells is two orders of magnitude faster than the precipitation rate of P at an initial pH of 2.0-7.0. In the fixed-bed column reactor, the removal efficiency of P was 90 % similar to 99 % for real wastewater containing P of 49.5 +/- 1.0 mg/L under the optimal condition that wastewater is pre-acidified to pH 3 with sulfuric acid. In terms of the mechanism, Ca ions dissolve from eggshells, followed by precipitation on the eggshell surface to form hydroxyapatite, progressing from the base to the top along the column height. Concurrently, the freshly generated fine precipitates are intercepted by the eggshell particle filter layer. These processes collectively enable P recovery. The recovered hydroxyapatite can be utilized as a potential biofertilizer in agriculture and as a raw material for the chemical industry. Environmental life cycle assessment reveals that contact precipitation for P recovery outperforms coagulation in reducing greenhouse gas emissions. Additionally, it entails lower chemical costs compared to P removal using iron, aluminum, or Ca reagents.

Microalgae have emerged as a promising feedstock for bioenergy production through anaerobic digestion and fermentation, gaining significant attention due to their rapid growth rate, ability to adapt to diverse environments, and rich biochemical composition. However, the recalcitrant nature of the microalgal cell wall necessitates pretreatment to enhance the accessibility of intracellular components and improve overall bioenergy yields from anaerobic digestion/fermentation. Among the various pretreatment methods, the thermal hydrolysis process has proven to be a promising strategy for enhancing the efficiency of bioenergy recovery from microalgal biomass. The benefits of thermal hydrolysis pretreatment of microalgae include improved organic matter solubilization, enhanced digestibility, and increased product yields in subsequent anaerobic digestion/fermentation processes for biomethane, biohydrogen, and volatile fatty acids production. However, thermal pretreatment poses challenges, such as forming future research by-products like furfural and ammonia, which can adversely affect microbial activities and reduce process efficiency. Thus, addressing its associated challenges is critical for maximizing its effectiveness in bioenergy and resource recovery. This review provides a comprehensive analysis of these challenges and offers recommendations for future research, emphasizing the need for optimized pretreatment strategies for advancing the sustainable and efficient use of microalgae in bioenergy production.

The microbial desalination cell (MDC) has emerged as a fascinating and green desalination process. However, cathode limitation restricts its practical implementation. An efficient catalyst layer for the cathode can improve such deficiencies significantly. The study, for the first time, investigated reduced graphene oxide (rGO) and polymerized aniline (PANI) composite catalysts to improve cathodic performance in MDC. Different rGO layers were achieved on stainless steel mesh (SSM) by varying rGO concentrations, followed by fixed PANI layer deposition. The XRD, FTIR and SEM analysis confirmed the successful formation of PANI on top of rGO on SSM with the nano-fibrous structure having rGO-characteristic XRD peak at 11.7 degrees with O-H, (-C=O), (-C-O), C = C groups. The effect of rGO was optimized by fine-tuning of rGO mass deposition on SSM. The cyclic voltammetry (CV) results have verified that PANI-rGO1.0-SSM (1 mg/mL rGO deposition) was electrochemically the best- performing composite with the highest CV current (2.1 mA). Therefore, three PANI-rGO1.0-SSM composite cathodes were prepared with 15, 30, and 45 deposition cycles to apply in MDC to study the desalination and bioelectrochemical performances. The 30-deposition cycle cathode showed the best improvement among desalination rates (i.e., 40.5 % higher) and internal resistance (i.e., 96 % low) compared to the controlled cathode. Cost estimation revealed that the electro-deposition is a low-cost coating method for PANI-rGO1.0 on SSM (US $6) and is advantageous compared to the platinum coating processes. Future research should optimize rGO and PANI loading rates and coating process to achieve better cathodic improvement for PANI-rGO composite.