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As a part of industrial or commercial discharge, the influx of nanoplastics (NPs) to the wastewater treatment plants is inevitable. Consequently, it has become a must to understand the effects of these NPs on different unit processes. This study aimed to investigate the impact of three different concentrations of polystyrene nanoplastics (PsNPs) on the fermentation of primary sludge (PrS), implemented in batch anaerobic bioreactors, at pH 5 and 10, considering the pH-dependent nature of the fermentation process. The results showed that PsNPs stimulated hydrogen gas production at a lower dose (50 μg/L), while a significant gas suppression was denoted at higher concentrations (150 μg/L, 250 μg/L). In both acidic and alkaline conditions, propionic and acetic acid predominated, respectively, followed by n-butyric acid. Under both acidic and alkaline conditions, exposure to PsNPs boosted the propagation of various antibiotic resistance genes (ARGs), including tetracycline, macrolide, β-lactam and sulfonamide resistance genes, and integrons. Notably, under alkaline condition, the abundance of sul 2 gene in the 250 μg PsNPs/L batch exhibited a 2.4-fold decrease compared to the control batch. The response of the microbial community to PsNPs exposure exhibited variations at different pH values. Bacteroidetes prevailed at both pH conditions, with their relative abundance increasing after PsNPs exposure, indicating a positive impact of PsNPs on PrS solubilization. Adverse impacts, however, were detected in Firmicutes , Chloroflexi and Actinobacteria. The observed variations in the survival rates of various microbes stipulate that they do not have the same tolerance levels under different pH conditions. [Display omitted] • PsNPs exposure up to 250 μg/L did not hinder VFA production in sludge fermentation. • Composition of VFAs varied by different pH levels rather than PsNPs levels. • PsNPs enhanced the proliferation of most antibiotic resistance genes (ARGs). • Dominant ARGs varied by different pH levels. [ABSTRACT FROM AUTHOR]

The imperative shift to a circular economy, driven by population growth, urbanization, and economic development, highlights the importance of recovering resources from waste streams. Anaerobic digestion (AD) is promising for stabilizing organic wastes and producing renewable methane-rich biogas. However, AD faces challenges, including operational instability, fatty acids accumulation, ammonium inhibition, and low yields. This study investigates the applicability of a novel Ydro Process® bioaugmentation on municipal sewage sludge anaerobic digestion. The process led to pre-hydrolysis of sewage sludge, as evidenced by an increase in volatile fatty acids (VFA), from 134 mg HAc/L in raw TWAS to 997 mg HAc/L in augmented sludge and improved soluble chemical oxygen demand (SCOD) yields. Additionally, the viscosity decreased by 41 % in the Ydro Process® bioaugmented sludge. Biogas production significantly increased from 732 mL/day to 1338 mL/day during semi-continuous digestion, with methane yield rising from 116 mL/g TCOD added to 218 mL/g TCOD added. Microbial analysis revealed an enhanced population of hydrogenotrophic methanogens. [ABSTRACT FROM AUTHOR]

In this study, mesophilic acidogenic fermentation, thermophilic acidogenic fermentation, and thermal hydrolysis process (THP) were compared to generate sludge liquors for bioenergy recovery with microbial electrolysis cells (MECs). The results showed that THP at 170 °C was the most effective for hydrolysis of particulate organics in sewage sludge, while fermentation under thermophilic temperature led to the highest accumulation of volatile fatty acids (VFAs) in sludge liquor. However, THP yielded the highest percentage of acetate in VFAs, which resulted in superior MEC performance compared to fermented sludge liquors in terms of current density (2.7 vs. ~1.3 A/m2), coulombic efficiency (50% vs. 31–34%), bio-H 2 potential (1114 vs. 839–881 mL), and H 2 production rate (50.3 mL/d vs. 28–32 mL/d). The utilization sequence of the VFAs was found to be acetate > butyrate > propionate. Overall, our results show that generating sludge liquors through THP could provide a feasible solution to produce bio-H 2 from sewage sludge; however, coulombic efficiencies should be further improved before practical application. • MECs were operated with sludge liquors produced through three different processes. • Thermal hydrolysis process (170 °C) led to superior MEC performance. • Sludge liquors from fermentations (38 °C and 55 °C) showed comparable performance. • VFAs degradation profiles followed the order of acetate > butyrate > propionate. [ABSTRACT FROM AUTHOR]

This study, for the first time, documented microbial community shifts in response to the changes in anode potential in a microbial electrolysis cell (MEC) operated with primary sludge. At an anode potential of −0.4 V vs. Ag/AgCl, the MEC showed COD and VSS removal efficiencies of 73 ± 1% and 75 ± 2%, respectively. The volumetric current density and specific hydrogen production rate were 23 ± 1.2 A/m3, and 145 ± 4.1 L/m3-d, respectively. The anodic microbial community was consisted of various fermentative/hydrolytic bacteria (e.g., Bacteroides and Dysgonomonas) and anode-respiring bacteria (Geobacter), while different hydrolytic/fermentative bacteria were abundant in suspension. The MEC showed substantially inferior performance along with a higher accumulation of various volatile fatty acids when the anode potential was switched to more positive values (0 V and +0.4 V). Both biofilms and suspended communities were also shifted when the anode potential was changed. Notably, at +0.4 V, Geobacter genus entirely disappeared from the biofilms, while Paludibacter species (known fermentative bacteria) were selectively enriched in biofilms. Also, the relative abundance of genus Bacteroides (known hydrolytic bacteria) substantially decreased in both biofilms and suspension, which was correlated with the inferior hydrolysis of VSS. Quantitative comparison of biofilms and suspended microbial communities at different anode potentials revealed a sharp decrease in bacterial cell numbers in anode biofilms after changing anode potential from −0.4 V to +0.4 V. By contrast, bacterial cell numbers in suspension were slightly decreased. Collectively, these results provide new insights into the role of anode potential in shaping key microbial players associated with hydrolysis/fermentation and anodic respiration processes when MECs are operated with real biowastes. Unlabelled Image • Impact of changing anode potentials on primary sludge fed MEC was studied. • Changes in biofilms and suspended bacterial communities were evaluated. • Superior MEC performance was achieved only at −0.4 V vs. Ag/AgCl. • Higher anode potentials adversely affected ARB and hydrolytic bacteria. • Key fermentative bacteria were also influenced by anode potentials. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbon cloth amendment increased methane production by 22 % in a pilot HSAD system. • Pseudomonas , Methanobacterium , and Methanosarcina possibly facilitated DIET. • Microbial richness, diversity, and evenness were enhanced. • Carbon cloth reduced the abundance of antibiotic resistance genes (ARGs) by 44.6 %. This study examined the effectiveness of introducing conductive carbon cloth into a pilot-scale high-solids anaerobic digestion (HSAD) system. Adding carbon cloth increased methane production by 22 % and improved the maximum methane production rate by 39 %. Microbial community characterization indicated a possible direct interspecies electron transfer–based syntrophic association among microbes. Using carbon cloth also enhanced microbial richness, diversity, and evenness. Carbon cloth effectively reduced the total abundance of antibiotic resistance genes (ARGs) by 44.6 %, mainly by inhibiting horizontal gene transfer, as shown by the significant decrease in the relative abundance of integron genes (particularly intl 1). The multivariate analysis further demonstrated strong correlations of intl 1 with most of the targeted ARGs. These findings suggest that carbon cloth amendment can promote efficient methane production and attenuate the spread of ARGs in HSAD systems. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the addition of conductive materials can promote direct interspecies electron transfer (DIET) between bacteria and methanoarchaea. This study demonstrated that carbon fibers could significantly stimulate methanogenic conversion of propionate and butyrate as co-substrate, while only butyrate was completely degraded in the unamended control bioreactor. In the carbon fibers-amended bioreactor, specific methane production (mL-CH 4 /g COD Initial ) and methanogenesis rate (d −1 ) increased by around 2.4 and 6.7 times, respectively. Various electroactive bacteria were abundant in the carbon fibers-amended bioreactor, whereas different known fermentative bacteria were abundant in the control. Moreover, carbon fibers substantially increased the abundance of Methanosaeta species. These results suggest that electroactive bacteria could be involved in DIET with Methanosaeta species enabling co-degradation of propionate and butyrate. Additionally, electrical conductivities of the biomass were comparable in both configurations, indicating that carbon fibers were the primary route for DIET. [ABSTRACT FROM AUTHOR]

[Display omitted] • 150 µg/L of polystyrene nanoplastics (PsNPs) decreased methane yield in AD by 32.3%. • Granular activated carbon (GAC) was examined as a remediation method. • GAC could offset the negative impact of PsNPs and further boost methane production. • GAC decreased ARGs propagation which was increased by PsNPs exposure. Nano/microplastics (NPs/MPs) in sewage sludge can induce oxidative stress to the anaerobic digestion (AD) and also proliferate antibiotic resistance genes (ARGs). Recently, granular activated carbon (GAC) has been used as an additive to enhance methane production in AD via direct interspecies electron transfer (DIET); however, its impact on AD exposed to NPs/MPs is yet to be studied. This study examined the effect of GAC (5 and 15 g/L) on sludge AD exposed to 150 µg/L of polystyrene nanoplastics (PsNPs). PsNPs decreased methane yield by 32.3% due to elevated levels of reactive oxygen species. However, GAC addition counteracted this adverse effect and improved methane production, attributed to the potential enrichment of DIET-active microbes and the adsorption of PsNPs by GAC. Moreover, GAC reduced the total abundance of ARGs, which was increased by PsNPs exposure. Thus, GAC can provide dual benefits in mitigating methanogenic inhibition caused by PsNPs and ARG spread. [ABSTRACT FROM AUTHOR]

[Display omitted] • Impact of PLA and PHB on ARGs in AD was studied. • PHB showed a higher biodegradation rate and methane generation than PLA. • PLA showed a higher proliferation of ARGs and their potential host microbes. • A correlation was found between MGEs and ARGs in PLA and PHB reactors. • Both PLA and PHB stimulated the secretion of extracellular polymeric substances. As the global concern over plastic pollution grows, efforts are underway to find environmentally friendly alternatives to traditional plastics. Bioplastics are being extensively researched and developed as a possible solution. This study compared the impact of two bioplastics, polylactic acid (PLA) and polyhydroxy butyrate (PHB), on the proliferation of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) during anaerobic digestion (AD). Both bioplastics (250–500 particles) could be degraded to a certain extent over 79 days, as indicated by higher methane production than the control without bioplastic particles. The PHB 500 reactor showed the highest methane yield along with the highest biodegradation efficiency (91 %) than other reactors amended with PHB and PLA particles. The highest ARG and MGE abundances were also observed in PLA 500, and the lowest ARG abundance was in PLA 250. Conversely, PHB reactors showed a relatively lower ARG abundance than the control. The correlation analysis suggested that most ARGs were positively correlated with PLA and negatively correlated with PHB (except for tet A, tet B, and tet X). Moreover, a correlation between MGEs and ARGs in PLA and PHB reactors was revealed by correlation analysis. These results show that AD responds differently to the different types/levels of bioplastics, which can ultimately influence the behavior of ARG proliferation. Thus, bioplastics may also pose a potential risk for spreading antibiotic resistance. These findings can be used as a basis for setting environmental standards for bioplastics and creating monitoring and control measures to prevent potential negative impacts on public health. [ABSTRACT FROM AUTHOR]

Engineering strategies to reduce greenhouse gases (GHGs) emissions by inhibiting methanogenesis in oil sands tailings have rarely been examined. In this study, we explored the potential impact of chemical treatment (lime) and biological treatment using enzymes (lysozyme and protease) on inhibiting methane emissions from tailings. Overall, treatment with protease 3%, lysozyme 3%, and lime 5000 ppm reduced CH 4 production (by 52%, 28%, and 25%, respectively) and were weakly associated with the archaeal abundance. Enzymes treatment resulted in a higher reduction in CH 4 production compared with lime treatment. A 3% lysozyme treatment suppressed CH 4 production (the change in methane was 0.48 mmol) and reduced the degradation of hexane throughout the experiment. Similarly, 3% protease suppressed CH 4 production throughout the experiment (the change in methane was 0.78 mmol), which could be attributed to the pH reduction to pH 4.9 at week 23 resulting from the formation of volatile fatty acids. Another possible mechanism could be the formation of toxic compounds, such as high nitrogen content, after protease treatment that inhibited the microbial community. The toxicity effect to Vibrio fischeri was greater with lysozyme 3% and protease 3% treatment than with lime treatment (124 TU and 76 TU, respectively). Lime treatment resulted in the highest reduction in 16S rRNA gene copies from 5.7 × 106 cells g−1 (control) to 2.7 × 105, 1.71 × 105, and 1.4 × 105 cells g−1 for 1600, 3500, and 5000 ppm treatments, respectively. This study supports further work to examine and determine the optimum conditions (e.g., enzyme and lime dosages) for CH 4 inhibition. [Display omitted] • The impact of enzymatic and lime treatment on methanogenic inhibition was studied. • Enzymatic treatment provided higher methane reduction than lime treatment. • Enzymatic treatment had a more toxic effect towards Vibrio fischeri than lime. • 3% Protease provided the highest reduction in methane production from tailings. • Lime treatment led to the highest reduction in 16S rRNA and archaeal gene copies. [ABSTRACT FROM AUTHOR]

• HPr/HAc ratios of 2.5 and 5 were detrimental to methanogenesis in MEC-AD. • Geobacter and Methanobacterium species were abundant at lower HPr/HAc ratios. • High HPr/HAc ratios prompted competition between acetogens and methanogens. • EPS protein and expression of EET-related genes reduced under high HPr/HAc ratios. • Numerical MEC-AD model could simulate the effects of different HPr/HAc ratios. This study examined the significance of propionate to acetate (HPr/HAc) ratios on microbial syntrophy and competition in microbial electrolysis cell-assisted anaerobic digestion (MEC-AD). In addition to molecular biology and phylogenetic analysis, a numerical MEC-AD model was developed by modifying Anaerobic Digestion Model No.1 to predict the effects of different HPr/HAc ratios (0.5, 1.5, 2.5, and 5). The HPr/HAc ratios of 0.5 and 1.5 maintained efficient syntrophy among electroactive bacteria, hydrogenotrophic methanogens, and homoacetogens, leading to higher methane yields. In contrast, higher HPr/HAc ratios of 2.5 and 5 were detrimental to methanogenesis. Both microbial community analysis and numerical modeling results suggested that higher propionate levels could promote the enrichment of H 2 -utilizing acetogens, thereby triggering their competition with hydrogenotrophic methanogens. Moreover, protein fraction in extracellular polymeric substances and the relative expression of genes associated with extracellular electron transfer in both anode and cathode biofilms were markedly decreased with increasing HPr/HAc ratios, indicating partial inhibition of microbial electroactivity. Overall, these results illuminate deep insight into anaerobic syntrophy, contributing to the process kinetics and methane yields in MEC-AD systems. Furthermore, from a practical viewpoint, the results can also be helpful in effective control of MEC-AD operation without propionate accumulation. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Biochar was synthesized from biosolids at 300 °C, 425 °C, and 550 °C. • Biochar was examined for the first time as an additive for septic tanks. • Biochar reduced start-up time and improved system stability at OLRs > 2 g COD/L/d. • Control reactor showed process instability at higher OLRs due to VFAs accumulation. • Biochar addition promoted enrichment of robust and diverse microbial communities. Septic tanks have been widely used for blackwater treatment in developing countries, while high-rate septic tanks with improved methane recovery are yet to be achieved. This study investigated biosolids-derived biochar (synthesized at 300℃, 425℃, and 550℃) as an additive for developing high-rate septic tanks. The experiments were conducted with anaerobic bioreactors operated with synthetic blackwater under septic tank conditions. All biochar amended reactors demonstrated a steady increase in daily methane production for increasing OLR from 0.08 to 3 g COD/L/d. The control reactor showed significant process disturbances at OLRs ≥ 2 g COD/L/d with an accumulation of volatile fatty acids followed by pH drop. At OLR of 3 g COD/L/d, the daily methane production from biochar amended reactors was ~ 4.3 times higher than the control (300 vs. 70 mL per day). Biochar addition established a robust microbiome consisted of a higher abundance of hydrogenotrophic and acetoclastic methanogens and hydrogen-producing fermentative bacteria. [ABSTRACT FROM AUTHOR]

• Impact of intermittent applied potential on electro-methanogenesis was assessed. • Turning off the applied potential for 6 h/day did not alter methane production. • Performance deteriorated when applied potential switched off for 12 h/day. • Microbial communities considerably altered by intermittent applied potential. • Applying external potential for 18 h/day led to the highest net energy income. From the perspective of energy saving in the operation of microbial electrolysis cell assisted anaerobic digester (MEC-AD), this study focused on developing an intermittent power supply scheme. The applied potential was switched off for 12 and 6 hours/day during the operation of a laboratory-scale MEC-AD system fed with glucose. The results from the operation under continuous applied potential served as the control. The overall biomethane generation and net energy income from the process were unaffected when the applied potential turned off for 6 hours/day. Both quantitative and qualitative analyses of microbial communities suggested that a balanced microbiome could be maintained under short-term switching-off the applied potential. However, performance substantially deteriorated when the applied potential turned off for 12 hours/day. Overall, the results of this study suggest that MEC-AD operation does not need a continuous power supply, and higher energy efficiency can be effectively achieved by intermittently powering the reactor. [ABSTRACT FROM AUTHOR]

Understanding the toxic effect of silver nanoparticles (AgNPs) on various biological wastewater treatment systems is of significant interest to researchers. In recent years, microbial electrochemical technologies have opened up new opportunities for bioenergy and chemicals production from organic wastewater. However, the effects of AgNPs on microbial electrochemical systems are yet to be understood fully. Notably, no studies have investigated the impact of AgNPs on a microbial electrochemical system fed with a complex fermentable substrate. Here, we investigated the impact of AgNPs (50 mg/L) exposure to a biofilm anode in a microbial electrolysis cell (MEC) fed with glucose. The volumetric current density was 29 ± 2.0 A/m3 before the AgNPs exposure, which decreased to 20 ± 2.2 A/m3 after AgNPs exposure. The biofilms produced more extracellular polymeric substances (EPS) to cope with the AgNPs exposure, while carbohydrate to protein ratio in EPS considerably increased from 0.4 to 0.7. Scanning electron microscope (SEM) imaging also confirmed the marked excretion of EPS, forming a thick layer covering the anode biofilms after AgNPs injection. Transmission electron microscope (TEM) imaging showed that AgNPs still penetrated some microbial cells, which could explain the deterioration of MEC performance after AgNPs exposure. The relative expression level of the quorum signalling gene (Lux R) increased by 30%. Microbial community analyses suggested that various fermentative bacterial species (e.g., Bacteroides , Synergistaceae_ vadin CA02, Dysgonomonas , etc.) were susceptible to AgNPs toxicity, which led to the disruption of their syntrophic partnership with electroactive bacteria. The abundance of some specific electroactive bacteria (e.g., Geobacter species) also decreased. Moreover, decreased relative expressions of various extracellular electron transfer associated genes (omc B, omc C, omc E, omc Z, omc S, and pil A) were observed. However, the members of family Enterobacteriaceae, known to perform a dual function of fermentation and anodic respiration, became dominant after biofilm anode exposed to AgNPs. Thus, EPS extraction provided partial protection against AgNPs exposure. Unlabelled Image • Impact of AgNPs was studied for a biofilm anode fed with a fermentable substrate. • AgNPs exposure disturbed syntrophy between fermentative and electroactive bacteria. • Fermentation efficiency reduced, followed by a decrease in current density. • AgNPs exposure induced EPS excretion and altered EPS composition. • AgNPs exposure influenced expressions of EET and QS genes. [ABSTRACT FROM AUTHOR]

• Recent developments in electro-methanogenesis were critically reviewed. • Key bacterial and archaeal members and their functional roles were summarized. • Extrinsic parameters for process optimization and development were reviewed. • Research gaps were highlighted to provide directions for future development. Electro-methanogenesis represents an emerging bio-methane production pathway that can be achieved through integrating microbial electrolysis cell (MEC) with conventional anaerobic digester (AD). Since 2009, a significant number of publications have reported superior methane productivity and kinetics from MEC-AD integrated systems. The overall objective of this review is to communicate the recent advances towards promoting electro-methanogenesis in the anaerobic digestion process. Firstly, the electro-methanogenesis pathways and functional roles of key microbial members are summarized. Secondly, various extrinsic process parameters, such as applied voltage/potential, pH, and temperature are discussed with emphasis on process optimization. Moreover, available methods for the inoculation and start-up of MEC-AD process are critically reviewed. Finally, system design and scale-up considerations, such as the selection of electrode materials, surface area and surface chemistry of electrode materials, and electrode spacing are summarized. [ABSTRACT FROM AUTHOR]

[Display omitted] • Process parameters influence ARG removal during AD of organic waste. • Heavy metals and nano/micro-scale pollutants may promote ARG dissemination in AD. • Additives and pre-treatment can enhance both ARG removal and biogas production. • Further research is needed to understand interactions among different parameters. The overuse and inappropriate disposal of antibiotics raised severe public health risks worldwide. Specifically, the incomplete antibiotics metabolism in human and animal bodies contributes to the significant release of antibiotics into the natural ecosystems and the proliferation of antibiotic-resistant bacteria carrying antibiotic-resistant genes. Moreover, the organic feedstocks used for anaerobic digestion are often highly-rich in residual antibiotics and antibiotic-resistant genes. Hence, understanding their fate during anaerobic digestion has become a significant research focus recently. Previous studies demonstrated that various process parameters could considerably influence the propagation of the antibiotic-resistant genes during anaerobic digestion and their transmission via land application of digestate. This review article scrutinizes the influences of process parameters on antibiotic-resistant genes propagation in anaerobic digestion and the inherent fundamentals behind their effects. Based on the literature review, critical research gaps and challenges are summarized to guide the prospects for future studies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Impact of various mixing conditions was studied for GAC and PAC amended digesters. • GAC amended digester attained the highest methane production without any mixing. • Intermittent mixing in GAC and control resulted in comparable methane production. • Continuous mixing in GAC amended digester provided the lowest methane production. • Mixing affected the enrichment/retention of bacterial and archaeal populations. Conductive materials amendment in anaerobic digestion (AD) is a promising strategy for boosting the methanogenesis process. Despite mixing is a critical parameter, the behavior of digesters amended with conductive additives upon different mixing conditions has rarely been investigated. This study investigated continuous mixing, intermittent mixing (10 min in every 12 h), and non-mixing conditions for digesters amended with granular activated carbon (GAC) and powdered activated carbon (PAC). The non-mixed GAC digester provided the highest methane yield (318 ± 28 mL/g COD) from synthetic blackwater, while intermittently mixed GAC and control exhibited similar methane yields (290–294 mL/g COD). For non-mixed systems, microbial richness and diversity increased with GAC and PAC amendment. In contrast, continuous and intermittent mixing increased microbial diversity and richness in control reactors while reduced the same in GAC and PAC amended reactors. Overall, various mixing conditions distinctly changed the degree of enrichment/retention of microbes and consequently influenced methane recovery. [ABSTRACT FROM AUTHOR]

The addition of conductive materials in anaerobic digestion (AD) is a promising method for boosting biomethane recovery from organic waste. However, conductive additives have rarely been investigated for the high-solids anaerobic digestion (HSAD). Here, the impact of adding carbon cloth in the solid phase of an HSAD system with percolate recirculation was investigated. Furthermore, spatial patterns of microbial communities in suspended biomass, percolate, and carbon cloth attached biofilm were assessed. Carbon cloth increased biomethane yield from source-separated organics (SSO) by 20% more than the unamended control by shortening the lag phase (by 15%) and marginally improving the methanogenesis rate constant (by ∼8%) under a batch operation for 50 days. Microbial community analysis demonstrated higher relative abundances of the archaeal population in the carbon cloth amended reactor than in unamended control (12%–21% vs. 5%–15%). Compared to percolate and suspension, carbon cloth attached microbial community showed higher enrichment of known electroactive Pseudomonas species along with Methanosarcina and Methanobacterium species, indicating the possibility of DIET-based syntrophy among these species. [Display omitted] • Carbon cloth led to a 20% higher methane yield in high-solids anaerobic digestion. • Relative abundance of the archaeal population increased by carbon cloth addition. • Spatial variations in microbial communities suggested DIET-based syntrophy. • Known DIET-active microorganisms were highly enriched on the carbon cloth surface. [ABSTRACT FROM AUTHOR]