Comparing activated carbon of different particle sizes on enhancing methane generation in upflow anaerobic digester

Operational mode and powdered activated carbon promoting syntrophic propionate oxidation during anaerobic digestion of complex organic substances

Operational mode and powdered activated carbon (PAC) are key factors facilitating microbial syntrophy and interspecies electron transfer during anaerobic digestion, consequently benefiting process stability and efficient methanogenesis. In this study, continuous-flow reactor (CFR) and sequencing batch reactor (SBR), with and without the addition of PAC, respectively, were operated to examine their effects on system performance and methanogenic activity. Based on the cycle-test result, the PAC-amended CFR (CFRPAC) recorded both the highest methane yield (690.1 mL/L) and the maximum CH4 production rate (28.8 mL/(L·h)), while SBRs exhibited slow methanogenic rates. However, activity assays indicated that SBRs were beneficial for organics removal in batch experiments fed with peptone. Taxonomic and functional analysis confirmed that CFRs were optimal for proliferating oligotrophs (e.g., Geobacter) and SBRs were more suitable for copiotrophs (e.g., Desulfobulbus). Metagenomic analysis revealed that CFRs had efficient acetate metabolic pathways from propionate and ethanol, whereas SBRs did not, resulting in the buildup of propionate. Furthermore, Methanobacterium and Methanothrix were acclimated to the different operational conditions, while acetoclastic Methanosarcina and hydrogenotrophic Methanolinea were acclimated in SBRs (5.1-13.4%) and CFRs (0.3-1.7%), respectively. This study confirmed the enhancement of microbial syntrophy by the addition of PAC as well as the acclimation of electroactive bacteria (e.g., Geobacter) with complex organic substances.

Deciphering anaerobic ethanol metabolic pathways shaped by operational modes

Efficient anaerobic digestion requires the syntrophic cooperation among diverse microorganisms with various metabolic pathways. In this study, two operational modes, i.e., the sequencing batch reactor (SBR) and the continuous-flow reactor (CFR), were adopted in ethanol-fed systems with or without the supplement of powdered activated carbon (PAC) to examine their effects on ethanol metabolic pathways. Notably, the operational mode of SBR and the presence of CO2 facilitated ethanol metabolism towards propionate production. This was further evidenced by the dominance of Desulfobulbus, and the increased relative abundances of enzymes (EC: 1.2.7.1 and 1.2.7.11) involved in CO2 metabolism in SBRs. Moreover, SBRs exhibited superior biomass-based rates of ethanol degradation and methanogenesis, surpassing those in CFRs by 53.1% and 22.3%, respectively. Remarkably, CFRs with the extended solids retention time enriched high relative abundances of Geobacter of 71.7% and 70.4% under conditions with and without the addition of PAC, respectively. Although both long-term and short-term PAC additions led to the increased sludge conductivity and a reduced methanogenic lag phase, only the long-term PAC addition resulted in enhanced rates of ethanol degradation and propionate production/degradation. The strategies by adjusting operational mode and PAC addition could be adopted for modulating the anaerobic ethanol metabolic pathway and enriching Geobacter.

New insights into syntrophic ethanol oxidation: Effects of operational modes and solids retention times

Anaerobic ethanol oxidation relies on syntrophic interactions among functional microorganisms to become thermodynamically feasible. Different operational modes (sequencing batch reactors, SBRs, and continuous flow reactors, CFRs) and solids retention times (SRT, 25 days and 10 days) were employed in four ethanol-fed reactors, named as SBR25d, SBR10d, CFR25d, and CFR10d, respectively. System performance, syntrophic relationships, microbial communities, and metabolic pathways were examined. During the long-term operation, 2002.7 ± 56.0 mg COD/L acetate was accumulated in CFR10d due to the washout of acetotrophic methanogens. Microorganisms with high half-saturation constants were enriched in reactors of 25-day SRT. Moreover, ethanol oxidizing bacteria and acetotrophic methanogens with high half-saturation constants could be acclimated in SBRs. In SBRs, Syner-01 and Methanothrix dominated, and the low SRT of 10 days increased the relative abundance of Geobacter to 38.0%. In CFRs, the low SRT of 10 days resulted in an increase of Desulfovibrio among syntrophic bacteria, and CFR10d could be employed in enriching hydrogenotrophic methanogens like Methanobrevibacter.

Enhancing methane production and organic loading capacity from high solid-content wastewater in modified granular activated carbon (GAC)-amended up-flow anaerobic sludge blanket (UASB)

Anaerobic digestion of high solid-content wastewater is hindered by high organic loading rates (OLRs). Granular activated carbon (GAC) was reported to promote direct interspecies electron transfer (DIET) and enhance reactor performance. In this study, three up-flow anaerobic sludge blanket (UASB) reactors were supplied with GAC in different locations: bottom (R1), top (R2), and bottom+top (R3). The performances of three reactors at different OLRs treating high solid-content wastewater were evaluated. At a low OLR, the highest methane yield (74 ± 4 %, g CH4-COD/g TCOD) was detected when GAC was supplied at top of the UASB (R2). When a high OLR was applied, the UASB supplemented with GAC at both bottom and top (R3) achieved the highest methane yield (66 ± 2 %, g CH4-COD/g TCOD), whereas the UASB supplemented with GAC at the top (R2) failed. Further studies on spatial distributions of sludge stability, specific methanogenic activities (SMAs), and microbial communities demonstrated the different impacts of GAC location on reactor performance and sludge characteristics under different OLRs. This study highlights the significance of considering organic loading capacity treating high solid-content wastewater when choosing GAC-based UASB systems.

Boosting thermophilic anaerobic digestion with conductive materials: Current outlook and future prospects

Thermophilic anaerobic digestion (TAD) can provide superior process kinetics, higher methane yields, and more pathogen destruction than mesophilic anaerobic digestion (MAD). However, the broader application of TAD is still very limited, mainly due to process instabilities such as the accumulation of volatile fatty acids and ammonia inhibition in the digesters. An emerging technique to overcome the process disturbances in TAD and enhance the methane production rate is to add conductive materials (CMs) to the digester. Recent studies have revealed that CMs can promote direct interspecies electron transfer (DIET) among the microbial community, increasing the TAD performance. CMs exhibited a high potential for alleviating the accumulation of volatile fatty acids and inhibition caused by high ammonia levels. However, the types, properties, sources, and dosage of CMs can influence the process outcomes significantly, along with other process parameters such as the organic loading rates and the type of feedstocks. Therefore, it is imperative to critically review the recent research to understand the impacts of using different CMs in TAD. This review paper discusses the types and properties of CMs applied in TAD and the mechanisms of how they influence methanogenesis, digester start-up time, process disturbances, microbial community, and biogas desulfurization. The engineering challenges for industrial-scale applications and environmental risks were also discussed. Finally, critical research gaps have been identified to provide a framework for future research.

Petroleum coke supplementation for enhanced biogas production and phosphate removal under mesophilic conditions

The use of carbon-based conductive materials has been shown to lead to an increase in biogas and methane yields during anaerobic digestion (AD). The effect of these additives on AD using synthetic substrates has been extensively studied, yet their significance for wastewater sludge digestion has not been adequately investigated. Therefore, the aim of this research was to optimize the concentration of petroleum coke (PC) that is a waste by-product of oil refineries, for the anaerobic digestion of wastewater sludge and investigation of phosphate removal in the AD process in the mesophilic temperature range. According to the results of the experiments, supplementing reactors with PC could significantly improve biogas and methane production. Supplementation of reactors with 1.5 g/L PC led to 23.40 ± 0.26% and 42.55 ± 3.97% increase in biogas production and methane generation, respectively. Moreover, the average volatile solids (VS), phosphate, and chemical oxygen demand (COD) removals were 43.43 ± 0.73, 46.74 ± 0.77%, and 60.40 ± 0.38%, respectively.

Direct interspecies electron transfer mechanisms of a biochar-amended anaerobic digestion: a review

This paper explores the mechanisms of biochar that facilitate direct interspecies electron transfer (DIET) among syntrophic microorganisms leading to improved anaerobic digestion. Properties such as specific surface area (SSA), cation exchange capacity (CEC), presence of functional groups (FG), and electrical conductivity (EC) were found favorable for increased methane production, reduction of lag phase, and adsorption of inhibitors. It is revealed that these properties can be modified and are greatly affected by the synthesizing temperature, biomass types, and residence time. Additionally, suitable biochar concentration has to be observed since dosage beyond the optimal range can create inhibitions. High organic loading rate (OLR), pH shocks, quick accumulation and relatively low degradation of VFAs, and the presence of heavy metals and toxins are the major inhibitors identified. Summaries of microbial community analysis show fermentative bacteria and methanogens that are known to participate in DIET. These are Methanosaeta, Methanobacterium, Methanospirillum, and Methanosarcina for the archaeal community; whereas, Firmicutes, Proteobacteria, Synergistetes, Spirochetes, and Bacteroidetes are relatively for bacterial analyses. However, the number of defined cocultures promoting DIET is very limited, and there is still a large percentage of unknown bacteria that are believed to support DIET. Moreover, the instantaneous growth of participating microorganisms has to be validated throughout the process.

Methane production by Methanothrix thermoacetophila via direct interspecies electron transfer with Geobacter metallireducens

Methanothrix is widely distributed in natural and artificial anoxic environments and plays a major role in global methane emissions. It is one of only two genera that can form methane from acetate dismutation and through participation in direct interspecies electron transfer (DIET) with exoelectrogens. Although Methanothrix is a significant member of many methanogenic communities, little is known about its physiology. In this study, transcriptomics helped to identify potential routes of electron transfer during DIET between Geobacter metallireducens and Methanothrix thermoacetophila. Additions of magnetite to cultures significantly enhanced growth by acetoclastic methanogenesis and by DIET, while granular activated carbon (GAC) amendments impaired growth. Transcriptomics suggested that the OmaF-OmbF-OmcF porin complex and the octaheme outer membrane c-type cytochrome encoded by Gmet_0930, were important for electron transport across the outer membrane of G. metallireducens during DIET with Mx. thermoacetophila. Clear differences in the metabolism of Mx. thermoacetophila when grown via DIET or acetate dismutation were not apparent. However, genes coding for proteins involved in carbon fixation, the sheath fiber protein MspA, and a surface-associated quinoprotein, SqpA, were highly expressed in all conditions. Expression of gas vesicle genes was significantly lower in DIET- than acetate-grown cells, possibly to facilitate better contact between membrane-associated redox proteins during DIET. These studies reveal potential electron transfer mechanisms utilized by both Geobacter and Methanothrix during DIET and provide important insights into the physiology of Methanothrix in anoxic environments. IMPORTANCE Methanothrix is a significant methane producer in a variety of methanogenic environments including soils and sediments as well as anaerobic digesters. Its abundance in these anoxic environments has mostly been attributed to its high affinity for acetate and its ability to grow by acetoclastic methanogenesis. However, Methanothrix species can also generate methane by directly accepting electrons from exoelectrogenic bacteria through direct interspecies electron transfer (DIET). Methane production through DIET is likely to further increase their contribution to methane production in natural and artificial environments. Therefore, acquiring a better understanding of DIET with Methanothrix will help shed light on ways to (i) minimize microbial methane production in natural terrestrial environments and (ii) maximize biogas formation by anaerobic digesters treating waste.

Graphene oxide addition to anaerobic digestion of waste activated sludge: Impact on methane production and removal of emerging contaminants

The effect of graphene oxide on the anaerobic digestion of waste activated sludge was investigated at two graphene oxide concentrations (0.025 and 0.075 g graphene oxide per g volatile solids) using biochemical methane potential tests. The occurrence of 36 pharmaceuticals was monitored in the solid and liquid phases before and after the anaerobic treatment. The addition of graphene oxide improved the removal of most pharmaceuticals detected, even those that are considered persistent to biological degradation, such as azithromycin, carbamazepine, and diclofenac. No significant differences were observed in the final specific methane production without graphene oxide and with the lowest graphene oxide concentration, yet the highest graphene oxide concentration partially inhibited methane production. The relative abundance of antibiotic resistance genes was not affected by the graphene oxide addition. Finally, significant changes in the microbial community including bacteria and archaea were detected with graphene oxide addition.

Role of pine needle biochar in operation and stability of anaerobic processes

Utility of biochar addition in anaerobic processes for promoting direct interspecies electron transfer (DIET) is demonstrated in this research. Biochar produced from pyrolysis of pine needle forest residue was used as conductive material for DIET. Three CSTRs were operated in parallel with and without biochar addition in fed-batch mode. Reactor without biochar which represented indirect interspecies electron transfer (IIET) exhibited wide variation in pH and VFA and took longer period during startup. All the rectors were operated at steady state with an OLR ranging from 0.5 to 1.75 kg-COD/m³.d. As OLR increased, performance of reactor without biochar resulted in rapid pH drop and increase in VFA, leading to its eventual failure at OLR of 1.75 kg-COD/m³.d. As against to this, performance of reactors with biochar remained robust and relatively unaffected at higher OLR values. Daily VFA accumulation from fed-batch mode always remained highest in reactor without biochar.

Granular activated carbon and exogenous hydrogen enhanced anaerobic digestion of hypersaline phenolic wastewater via syntrophic acetate oxidation and hydrogenotrophic methanogenesis

High salinity in phenolic wastewater inhibited anaerobes' metabolic activity, thereby affecting the anaerobic biotransformation of phenol. In this study, granular activated carbon (GAC) coupled with exogenous hydrogen (H₂) was used to enhance the anaerobic digestion of phenol. The GAC/H₂ group's accumulative methane production, coenzyme F₄₂₀ concentration, and interspecies electron transfer system activity increased by 24 %, 53 %, and 16 %, respectively, compared with the control group. In the floc sludge of the GAC/H₂ group, the relative abundance of syntrophic acetate-oxidizing bacteria such as Syntrophus and Syntrophorhabdus were 18.7 % and 1.1 % at genus level, respectively, which were around 93.5 and 7.5 times of that of the controlgroup. Moreover, Acinetobacter (77.6 %), Methanobacterium (44.0 %), and Methanosarcina (34.2 %) were significantly enriched on the GAC surface in GAC/H₂ group. Therefore, the coupling of GAC and H₂ provided a novel attempt at anaerobic digestion of hypersaline phenolic wastewater via syntrophic acetate oxidation and hydrogenotrophic methanogenesis pathway.

Facilitation of interspecies electron transfer in anaerobic processes through pine needle biochar

Role of biochar in promoting methanogenesis during anaerobic processes was investigated in this research. Biochar produced from Himalayan pine needles was used as medium for conductive material mediated interspecies electron transfer (CM-IET) amongst the electron producing microorganisms and electron consuming methanogenic archaea. Three anaerobic continuous stirrer tank reactors (CSTRs) with 0, 5 and 10 g/L pine needle biochar (PNB) were operated at steady state organic loading rate (OLR) of 2.0-2.5 kgCOD/(m³.d). R0 (0 g/L PNB), representing indirect interspecies electron transfer (IIET), failed at an OLR of 2.0 kgCOD/(m³.d) due to the highest volatile fatty acid (VFA) concentration of 6,300 mg/L among the three CSTRs. On the other hand, at an OLR of 2.5 kgCOD/(m³.d), R2 (10 g/L PNB) showed the most superior performance with chemical oxygen demand (COD) removal of 55% and volatile fatty acid (VFA) concentration of 3,500 mg/L, while R1 (5 g/L PNB) recorded COD removal of 45% and VFA concentration of 4,400 mg/L. In comparison, fixed biofilm reactor (FBR) with 80 g/L of PNB as support material operated satisfactorily at OLR of 13.8 kgCOD/(m³.d) with 70% COD removal and VFA concentration of 1,400 mg/L. These investigations confirmed the beneficial role of biochar in anaerobic processes by promoting CM-IET amongst VFA degrading bacteria and methane producing archaea.

Influence of Animal/Plant Activated Biochar Properties on Methane Production from Corn Stalk by Anaerobic Fermentation

Activated biochar (ABC) was prepared from typical plant/animal biochar (pig bone biochar/corn stalk biochar) by optimizing the gas production characteristics of anaerobic fermentation. The effects of the physical and chemical properties (specific surface area, surface functional group and conductivity) of ABC on the gas production characteristics of anaerobic fermentation were investigated. The results showed that the effect of pig-bone activated biochar (PABC) on anaerobic fermentation gas production characteristics was better than that of corn-stalk activated biochar (CABC). The peak period of gas production or methane production was up to 4 days earlier than that of the control group, and the cumulative methane production was up to 68% higher; this can shorten the fermentation period for up to 7 days, and the effect of stabilizing pH is better. In addition, the surface functional groups are not the dominant factors affecting the gas production characteristics, but the effects of conductivity and specific surface area cannot be neglected. For most experimental groups, when the specific surface area of PABC is more than 90 m2/g and the specific surface area of CABC is more than 100 m2/g. Methane production increases with the specific surface area increases and the controllable range of CBAC is relatively wider than that of PBAC. When the conductivity of CABC is more than 650 μS/cm and the conductivity of PABC is more than 1000 μS/cm, the conductivity has a positive correlation with methane production.

Optimized production conditions and activation of biochar for effective promotion of long-chain fatty acid degradation in anaerobic digestion

Engineered biochar production and utilization in anaerobic digestion (AD) potentially overcome its limited application to the treatment of slowly degradable or inhibitory substrates. Here an attempt was made to develop an optimized biochar production procedure for use in AD to stimulate palmitic acid biodegradation via direct interspecies electron transfer (DIET). The electrical conductivity of biochar was greatly increased with an elevated pyrolysis temperature and K₂CO₃ activation, and the conductivity reached a comparable level (0.6-1.4 S/cm) to that of carbon black at 800 °C. In addition, the K₂CO₃ activation greatly improved biochar wettability. When using K₂CO₃-activated biochar produced at 800 °C, the maximum methane production rate from palmitic acid was 1.3 times that of a control without biochar addition.

Synergetic effects of biochar addition on mesophilic and high total solids anaerobic digestion of chicken manure

High solids anaerobic digestion (AD) of chicken manure (CM) is often challenging due to ammonia-N inhibition and accumulation of volatile fatty acids (VFAs). This study evaluated the effect of adding biochars from different feedstock to ameliorate semi-dry AD of fresh CM during batch fermentation. Experiments were performed in 300 mL at two total solid (TS) levels (12% and 15%) under mesophilic (36 ±1ᵒC) conditions for 55 d, using activated sludge as inoculum. Treatments included: fresh CM (at 12% or 15% TS) mixed separately with rice husks char (RB), wood char (WB) and bamboo char (BB) at biochar dosages of 2.5%, 5% and 10% of TS in the CM, inoculum only and inoculum plus CM without addition of char as the control. Results indicated that addition of biochar reduced the lag phases to 4-5.4 d and AD performances were significantly improved with total volatile solids removal of 53-67% and 62-71%, and cumulative methane of 277-380 mL/gVS (CH₄ content ≈ 51-63%) and 297-438 mL/gVS (CH₄ content ≈ 49-67%) at 12% and 15% TS, respectively. Biochar buffered over acidification and stabilized pH in the range of 6.5-7.8 but mild ammonia inhibition still occurred in all biochar treatments due to the high residual total ammonia-N (4.3 g-5.6 g/L). For all the investigated parameters, WB amended digesters exhibited the best results owing to its high specific surface area, porosity, cationic exchange capacity, and elemental composition which were superior to those of RB and BB. At 10% dosage of all tested biochars, the AD process was more stable and methane content neared optimal of >65% CH₄. Therefore, addition of biochar from lignocellulosic materials at a given threshold dosage could promote semi-dry and dry biogas production from chicken manure and thus add value to this waste which in most cases is improperly managed.

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.

Enhanced anaerobic treatment of synthetic protein-rich wastewater promoted by organic xerogels

Carbon-based materials have been shown to enhance anaerobic digestion processes by promoting direct interspecies electron transfer in methanogenic consortia. However, little is known on their effects during the treatment of complex substrates, such as those derived from protein-rich wastewaters. Here, organic xerogels (OX) are tested, for the first time, as accelerators of the methanogenic activity of an anaerobic consortium treating a synthetic protein-rich wastewater. Three OX with distinct pore size distribution (10 and 1000 nm for OX-10 and OX-1000, respectively) and structural conformation (graphene oxide integration into OX-10-GO polymeric matrix) were synthesized. OX-1000 promoted the highest methane production rate (5.21 mL/g*h, 13.5% increase with respect to the control incubated without OX) among the synthesized OX. Additionally, batch bioreactors amended with OX achieved higher chemical oxygen demand (COD) removal (up to 88%) as compared to the control, which only showed 50% of COD removal. Interestingly, amendment of bioreactors with OX also triggered the production of medium-chain fatty acids, including caprylate and caproate. Moreover, OX decreased the accumulation of ammonium, derived from proteins hydrolysis, partly explained by their adsorption capacities, and probably involving their electron-accepting capacity promoting anaerobic ammonium oxidation. This is the first time that OX were successfully applied as methanogenic accelerators for the anaerobic treatment of synthetic protein-rich wastewater, increasing the methane production rate and COD removal as well as triggering the production of medium chain fatty acids and attenuating the accumulation of ammonium. Therefore, OX are proposed as suitable materials to boost the efficiency of anaerobic systems to treat complex industrial wastewaters.

Improved methanogenesis in anaerobic wastewater treatment by magnetite@polyaniline (Fe3O4@PANI) composites

The magnetite@polyaniline (Fe₃O₄@PANI) composites with different Fe₃O₄ loadings were prepared, and their effect on methane production in anaerobic systems was investigated. The Fe₃O₄@PANI composite with a 40% loading of Fe₃O₄ showed a better performance on accelerating methane production rate than other composites. The methane production rate was increased by 26.98% at the Fe₃O₄@PANI dosage of 0.6 g L^-1. The results of the contact angle and CLSM revealed that Fe₃O₄@PANI had a good bio-affinity and contact directly with bacteria and archaea. Then the mechanisms related to the enhancement of methane production by the composites were explored by the species annotation and enzyme activity. It showed that Fe₃O₄@PANI promoted the enrichment of DIET-related functional bacteria and archaea and improved the enzyme activity related to the acetoclastic methanogenic pathway.

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.

Carbon-based conductive materials enhance biomethane recovery from organic wastes: A review of the impacts on anaerobic treatment

Amongst the most important sustainable waste management strategies, anaerobic biotechnology has had a central role over the past century in the management of high-pollution load sources, such as food, agricultural and municipal wastes. During anaerobic digestion (AD), valuable by-products such as digestate and biogas are produced. Biogas (mainly composed of methane) is generated through a series of reactions between bacteria and archaea. Enhancement of AD process with higher methane yield, accelerated methane production rate, and shorter start-up time is possible via tapping into a novel methanogenic pathway discovered a decade ago. This fundamentally new concept that is a substitute to interspecies hydrogen transfer is called direct interspecies electron transfer (DIET). DIET, a thermodynamically more feasible way of electron transfer, has been proven to occur between bacteria and methanogens. It is well-documented that amendment of carbon-based conductive materials (CCMs) can stimulate DIET via serving as an electrical conduit between microorganisms. Therefore, different types of CCMs such as biochar and activated carbon have been amended to a variety of AD reactors and enhancement of process performance was reported. In this review, a comparative analysis is presented for enhancement of AD performance in relation to major CCM related factors; electrical conductivity, redox properties, particle size and dosage. Additionally, the impacts of AD operational conditions such as organic loading rate and temperature on CCM amended reactors were discussed. Further, the changes in microbial communities of CCM amended reactors were reviewed and future perspectives along with challenges for CCM application in AD have been provided.

Enhancing anaerobic digestion of pharmaceutical industries wastewater with the composite addition of zero valent iron (ZVI) and granular activated carbon (GAC)

Anaerobic digestion of pharmaceutical wastewater is challenged by its contained toxic compounds which limits the stability and efficiency of methane production and organic degradation. In this study, zero valent iron (ZVI) and granular activated carbon (GAC) were added with different strategies to improve anaerobic digestion of pharmaceutical wastewater. The results confirmed synergy effects of ZVI + GAC for both COD removal (increased by 13.4%) and methane production (increased by 11.0%). Furthermore, ZVI + GAC improved the removal of pharmaceutical intermediates, in particular, the residues (%) of dehydroepiandrosterone (DHEA) and 2,2'-methylenebis(6-tert-butyl-4-methylphenol) were only 30.48 ± 6.53 and 39.92 ± 4.50, and effectively reduced biotoxicity. The promoted results were attributed to the establishment of direct interspecies electron transfer (DIET). Microbial community analysis revealed that ZVI + GAC decreased species evenness and richness in bacterial whereas increased in archaeal. The relative abundance of acetotrophic methanogens decreased but hydrogenotrophic and methylotrophic methanogens increased, which broadening the pathway of methane production.

Coupling direct voltage and granular activated carbon modified nanoscale zero valent iron for enhancing anaerobic methane production

Anaerobic digestion technology has been widely used because it has a unique advantage of producing biogas as a renewable energy source. Therefore, several methods were studied to facilitate anaerobic methane production process. Coupling direct voltage and single conductive particles was an effective method to improve anaerobic wastewater treatment efficiency and methane production. However, the enhancement method was limited in this process due to the current of direct voltage or the toxicity of nanoparticles. Therefore, the granular activated carbon loaded with nanoscale zero valent iron (GAC-NZVI) particles prepared by co-precipitation method were added to the anaerobic synthetic wastewater system with direct voltage (0.10 V) to improve the treatment efficiency in this study. GAC-NZVI particles were added into anaerobic system with 0.10 V direct voltage to enhance CH₄ production process. The COD removal and total CH₄ production were enhanced by 4.22 % and 10.83 % with GAC-NZVI particles. The measurement results of EPS and Fe concentration showed that GAC-NZVI particles promoted the secretion of EPS by microorganisms, which could improve the floc strength of granular sludge. The measurements of conductivity and cyclic voltammetry (CV) showed that particles accelerated the metabolism of microorganism and promoted the electron transfer process. The increasing of Methanothrix and Methanobacterium could strengthen the methanogenesis. The abundances of bacteria and archaea using indirect interspecies electron exchange form (such as H₂ or formate transfer microorganisms) were decreased after adding the particles. The results indicated that anaerobic treatment efficiency could be enhanced under the combined action of direct voltage and particles.

Biochar amendment rapidly shifts microbial community structure with enhanced thermophilic digestion activity

Effects of powdered (<0.075 mm) biochar on thermophilic anaerobic digestion were investigated with biochemical methane potential (BMP) assays. The assays had substrate to inoculum ratios (SIR) of 2.2 and 4.4 g-volatile solids (VS)/g-VS and biochar dosing of 6 g/g-total solids (TS)_inoculum. Compared to control, biochar amendment enhanced methane production rates by 94%, 75%, and 20% in assays utilizing substrates of acidified sludge at 70 °C, 55 °C and non-acidified mixed sludge, respectively. All controls experienced acute inhibition with lag phases from 12 - 52 days at SIR of 4.4 g-VS/g-VS, while assays with biochar generated methane from day 4. Biochar addition resulted in a rapid shift in microbial community structure associated with an increase in Methanothermobacteraeae (hydrogenotrophic) and Methanosarcinaceae archaea, as well as various volatile fatty acid (VFA)-degrading and hydrogen-producing bacteria. Biochar presents great potential to tackle VFA accumulation, abbreviate lag phase and increase methane rate, particularly at high organic loadings.

Significance of different mixing conditions on performance and microbial communities in anaerobic digester amended with granular and powdered activated carbon

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.

Anaerobic bacterial responses to carbonaceous materials and implications for contaminant transformation: Cellular, metabolic, and community level findings

Carbonaceous materials (CM) enhance the abundance and activity of bacteria capable of persistent organic (micro)pollutant (POP) degradation. This review synthesizes anaerobic bacterial responses to minimally modified CM in non-fuel cell bioremediation applications at three stages: attachment, metabolism, and biofilm genetic composition. Established relationships between biological behavior and CM surface properties are identified, but temporal relationships are not well understood, making it difficult to connect substratum properties and "pioneer" bacteria with mature microorganism-CM systems. Stark differences in laboratory methodology at each temporal stage results in observational, but not causative, linkages as system complexity increases. This review is the first to critically examine relationships between material and cellular properties with respect to time. The work highlights critical knowledge gaps that must be addressed to accurately predict microorganism-CM behavior and to tailor CM properties for optimized microbial activity, critical frontiers in establishing this approach as an effective bioremediation strategy.

Promotion of direct interspecies electron transfer and potential impact of conductive materials in anaerobic digestion and its downstream processing - a critical review

Addition of conductive materials (CMs) has been reported to facilitate direct interspecies electron transfer (DIET) and improved anaerobic digestion (AD) performance. This review summarises the benefits and outlines remaining research challenges of the addition of CMs with a focus on the downstream processing of AD. CM addition may alter biogas quality, digestate dewaterability, biosolids volume, and centrate quality. Better biogas quality has been observed due to the adsorption of H₂S to metallic CMs. The addition of CMs results in an increase in solid content of the digestate and thus an additional requirement for sludge dewatering and handling and the final biosolids volume for disposal. This review highlights the need for more research at pilot scale to validate the benefits of CM addition and to evaluate CM selection, doses, material costs, and the impact on downstream processes. The lack of research on the impact of CMs on the downstream process of AD is highlighted.

Improved hydrogen production from pharmaceutical intermediate wastewater in an anaerobic maifanite-immobilized sludge reactor

A novel anaerobic maifanite-immobilized sludge reactor (AMSR) was employed to investigate the feasibility and performance of continuous hydrogen production for the treatment of pharmaceutical intermediate wastewater (PIW) at different organic loading rates (OLR) (from 12 to 96 g COD L-1 d-1) according to changes in the hydraulic retention time (HRT). A reactor without maifanite was also employed as a control. The results indicate that maifanite accelerates granular sludge formation and the AMSR presents more efficient and stable performance than the control in terms of the hydrogen production rate. In the AMSR, the highest hydrogen production rate of 11.2 ± 0.4 mmol L-1 h-1 was achieved at an optimum OLR of 72 g COD L-1 d-1. The main metabolic route for hydrogen production was ethanol-type fermentation, which was reflected in the relative abundance of E. harbinense, which was dominant for all of the OLRs. The maximum energy conversion efficiency in the dual production of hydrogen and ethanol was determined to be 24.5 kJ L-1 h-1 at an OLR of 72 g COD L-1 d-1.

Nickel Foam Promotes Syntrophic Metabolism of Propionate and Butyrate in Anaerobic Digestion

Enhanced interspecies electron transfer (IET) among symbiotic microorganisms is an effective method to increase the rate of methane (CH₄) production in anaerobic digestion. Direct interspecies electron transfer (DIET), which does not involve dissolved redox media, is considered an alternative and superior method to enhance methane production by interspecific hydrogen (H₂) transfer (IHT). In this study, nickel foam was built into a semicontinuous anaerobic reactor to investigate its effect on the metabolism of propionate and butyrate. Both increased the average yield of CH₄ in anaerobic digestion by 18.1 and 15.9%, respectively. Analysis of bacterial and archaeal communities showed that the addition of nickel foam could increase the relative abundance of microbial communities involved in DIET and could increase the diversity of microorganisms in the reactor. Moreover, the anaerobic digestion performance of the nickel foam reactor was good at high hydrogen partial pressure.

On-site CO2 bio-sequestration in anaerobic digestion: Current status and prospects

The advantages of anaerobic digestion (AD) technology in organic solid waste treatment for bioenergy recovery are evidenced in worldwide. Recently, more attention has been paid to on-site biogas research, as well as biogenic CO₂ sequestration from AD plant, to promote "carbon neutral". Single-phase and two-phase AD system can be incorporated with various CO₂ bioconversion technologies through H₂ mediated CO₂ bioconversion (in-situ and ex-situ biogas upgrading), or other emerging strategies for CO₂ fixation without exogenous H₂ injection; these include in-situ direct interspecies electron transfer reinforcement, electromethanogenesis, and off-gas reutilization. The existing and potential scenarios for on-site CO₂ bio-sequestration within the AD framework are reviewed from the perspectives of metabolic pathways, functional microorganisms, the limitations on reaction kinetics. This review concluded that on-site CO₂ bio-sequestration is a promising solution to reduce greenhouse gas emissions and increase renewable energy recovery.

Coupling granular activated carbon and exogenous hydrogen to enhance anaerobic digestion of phenol via predominant syntrophic acetate oxidation and hydrogenotrophic methanogenesis pathway

Anaerobic digestion is a promising biological method for treating phenol-containing wastewater. However, the low methane yield of phenol due to the biological toxicity limits its potential application. This study presents a novel method to enhance the conversion rate of phenol to methane by coupling of granular activated carbon and exogenous hydrogen (GAC/H₂). The cumulative methane production in the GAC/H₂, H₂, GAC, and control groups were 408.2 ± 16.2, 336.5 ± 5.7, 287.2 ± 26. 2, and 258.1 ± 8.6 mL‬ CH₄/g COD, respectively. Compared with the control group, the hydrogenotrophic methanogenic activity and electron transfer activity of GAC/H₂ group were increased by 403.9 and 367.4%, respectively. The results of the 16SrRNA analysis indicated GAC enhanced the relative abundances of Syntrophus and Syntrophorhabdus, and hydrogen promoted the relative abundances of Cryptanaerobacter, Aminicenantes, and Methanobacterium. Therefore, the coupling of GAC and exogenous hydrogen promoted a dominate SAO-HM pathway to convert phenol to methane.

Electron transfer and mechanism of energy production among syntrophic bacteria during acidogenic fermentation: A review

Volatile fatty acids (VFAs) production plays an important role in the process of anaerobic digestion (AD), which is often the critical factor determining the metabolic pathways and energy recovery efficiency. Fermenting bacteria and acetogenic bacteria are in syntrophic relations during AD. Thus, clear elucidation of the interspecies electron transfer and energetic mechanisms among syntrophic bacteria is essential for optimization of acidogenic. This review aims to discuss the electron transfer and energetic mechanism in syntrophic processes between fermenting bacteria and acetogenic bacteria during VFAs production. Homoacetogenesis also plays a role in the syntrophic system by converting H₂ and CO₂ to acetate. Potential applications of these syntrophic activities in bioelectrochemical system and value-added product recovery from AD of organic wastes are also discussed. The study of acidogenic syntrophic relations is in its early stages, and additional investigation is required to better understand the mechanism of syntrophic relations.

Robustness of granular activated carbon-synergized anaerobic membrane bioreactor for pilot-scale application over a wide seasonal temperature change

A novel granular activated carbon-synergized anaerobic membrane bioreactor (GAC-AnMBR), consisted of four expanded bed anaerobic bioreactors with GAC carriers and a membrane tank, was established in pilot scale (10 m³/d) to treat real municipal wastewater (MWW) at ambient temperature seasonally fluctuating from 35 to 5 °C. It showed sound organic removal over 86% with the permeate COD less than 50 mg/L even at extremely low temperatures below 10 °C. COD mass balance analysis revealed that membrane rejection (with a contribution rate of 10%-20%) guaranteed the stable organic removal, particularly at psychrophilic temperature. The methane yield was over 0.24 L CH_{4 (STP)}/g COD _removed at mesophilic temperature and 0.21 L CH_{4 (STP)}/g COD _removed at 5-15 °C. Pyrosequencing of microbial communities suggested that lower temperature reduced the abundance of the methane producing bacteria, but the methane production was enhanced by selectively enriched Methanosaeta, syntrophic Syntrophobacter and Smithella and exoelectrogenic Geobacter for direct interspecies electron transfer (DIET) on the additive GAC. Compared with previously reported pilot-scale AnMBRs, the GAC-AnMBR in this study showed better overall performance and higher stability in a wide temperature range of 5-35 °C. The synergistic effect of GAC on AnMBR guaranteed the robustness of GAC-AnMBR against temperature, which highlighted the applicational potential of GAC-AnMBR, especially in cold and temperate climate regions.

Recent advances in methanogenesis through direct interspecies electron transfer via conductive materials: A molecular microbiological perspective

Conductive materials can serve as biocatalysts during direct interspecies electron transfer for methanogenesis in anaerobic reactors. However, the mechanism promoting direct interspecies electron transfer in anaerobic reactors, particularly under environments in which diverse substrates and microorganisms coexist, remains to be elucidated from a scientific or an engineering point of view. Currently, many molecular microbiological approaches are employed to understand the fundamentals of this phenomenon. Here, the direct interspecies electron transfer mechanisms and relevant microorganisms identified to date using molecular microbiological methods were critically reviewed. Moreover, molecular microbiological methods for direct interspecies electron transfer used in previous studies and important findings thus revealed were analyzed. This review will help us better understand the phenomena of direct interspecies electron transfer using conductive materials and offer a framework for future molecular microbiological studies.

Biohydrogen production from traditional Chinese medicine wastewater in anaerobic packed bed reactor system

Three anaerobic packed bed reactors (APBR) packed with activated carbon, maifanite and tourmaline as support material were continuously operated for 165 days to generate hydrogen from traditional Chinese medicine wastewater at different organic loading rates (OLR) from 15.2 to 91.3 g COD L⁻¹ d⁻¹ by changes of hydraulic retention time (HRT) varying from 24 to 6 h. The best performance with hydrogen production rate (HPR) of 7.92 ± 0.27 mmol L⁻¹ h⁻¹ and hydrogen yield (HY) of 3.50 ± 0.09 mmol g⁻¹ COD was achieved for the reactor with tourmaline at OLR of 60.8 g COD L⁻¹ d⁻¹ (HRT = 6 h), followed by activated carbon and maifanite. The main metabolic products for each reactor were found to be acetate and butyrate in the effluent with pH range of 5.6–6.4 and microbial analysis revealed that the dominant communities in all cultures were C. carboxidivoran and C. butyricum, responsible for acetate and butyrate production respectively.

Three anaerobic bed reactors packed with activated carbon, maifanite and tourmaline were continuously operated for 165 days to generate hydrogen from traditional Chinese medicine wastewater at different loading rates (15.2 to 91.3 g COD L⁻¹ d⁻¹).

Sparking Anaerobic Digestion: Promoting Direct Interspecies Electron Transfer to Enhance Methane Production

Anaerobic digestion was one of the first bioenergy strategies developed, yet the interactions of the microbial community that is responsible for the production of methane are still poorly understood. For example, it has only recently been recognized that the bacteria that oxidize organic waste components can forge electrical connections with methane-producing microbes through biologically produced, protein-based, conductive circuits. This direct interspecies electron transfer (DIET) is faster than interspecies electron exchange via diffusive electron carriers, such as H2. DIET is also more resilient to perturbations such as increases in organic load inputs or toxic compounds. However, with current digester practices DIET rarely predominates. Improvements in anaerobic digestion associated with the addition of electrically conductive materials have been attributed to increased DIET, but experimental verification has been lacking. This deficiency may soon be overcome with improved understanding of the diversity of microbes capable of DIET, which is leading to molecular tools for determining the extent of DIET. Here we review the microbiology of DIET, suggest molecular strategies for monitoring DIET in anaerobic digesters, and propose approaches for re-engineering digester design and practices to encourage DIET.

Multi-Walled Carbon Nanotubes Enhance Methanogenesis from Diverse Organic Compounds in Anaerobic Sludge and River Sediments

Conductive nanomaterials affect anaerobic digestion (AD) processes usually by improving methane production. Nevertheless, their effect on anaerobic communities, and particularly on specific trophic groups such as syntrophic bacteria or methanogens, is not extensively reported. In this work, we evaluate the effect of multi-walled carbon nanotubes (MWCNT) on the activity of two different anaerobic microbial communities: an anaerobic sludge and a river sediment. Methane production by anaerobic sludge was assessed in the presence of different MWCNT concentrations, with direct methanogenic substrates (acetate, hydrogen) and with typical syntrophic substrates (ethanol, butyrate). MWCNT accelerated the initial specific methane production rate (SMPR) from all compounds, with a more pronounced effect on the assays with acetate and butyrate, i.e., 2.1 and 2.6 times, respectively. In the incubations with hydrogen and ethanol, SMPR increased 1.1 and 1.2 times. Experiments with the river sediment were performed in the presence of MWCNT and MWCNT impregnated with 2% iron (MWCNT-Fe). Cumulative methane production was 10.2 and 4.5 times higher in the assays with MWCNT-Fe and MWCNT, respectively, than in the assays without MWCNT. This shows the high potential of MWCNT toward bioenergy production, in waste/wastewater treatment or ex situ bioremediation in anaerobic digesters.

Co-Digestion Biomethane Production and the Effect of Nanoparticle: Kinetics Modeling and Microcalorimetry Studies

To improve the production rate of methane, powder-activated carbon (PAC), granule activated carbon (GAC), titanium dioxide-anatase (TiO₂), and synthesized zeolite (permutit) were added in the co-digestion process. The co-substrates were corn stover (CS) and pig manure (PM) mixed in the ratio of 1:2 (w/w). The kinetic analysis model and ADM1da model were applied to obtain the kinetic parameters of the process. Besides, the heat flow analysis of the co-digestion process was determined using isothermal microcalorimetry. The addition of the PAC, GAC, TiO₂, and synthesized zeolite improved the methane cumulative yield by 40.12, 31.25, 31.17, and 43.74% respectively, as compared with the control reactor. The kinetic analysis and ADM1da model results indicated that the overall rate constant of the co-digestion process increased by 1.5 times averagely because of the effect of these materials. It was also observed that much higher heat energy released from the experimental sample compared with the control reactor, which indicated that the improvement of the metabolic process of the AcoD system. The addition of TiO₂-anatase improved methane production by 31.17%, which could be a promising method to improve the biomethane in a large-scale due to its availability and accessibility.

Role of magnetite in methanogenic degradation of different substances

For better understanding the role of magnetite in methanation of different substrates, two up-flow anaerobic sludge bed reactors (RM with magnetite; RS with silica) were built using acetate (stage I), propionate + butyrate (stage II), and sucrose (stage III) as the substrates, respectively. RM reactor showed better COD removal efficiency and adaptability to different substrate impacts. More extracellular polymeric substances (EPS) were produced for anaerobic sludge granulation, and the sludge in RM had better intensity, hydrophobicity and electroactivity compared with those in RS. Interestingly, magnetite had different promoting effects on methanogenic degradation of different substrates, and magnetite facilitated different syntrophic partners, like Desulfovibrio, Smithella, unidentified Clostridiates and Methanosaeta in different stages. The strengthening factor of biogas production from sucrose was the highest (1.23 ± 0.03), and analysis of key enzyme activities indicated that the potential magnetite-induced direct interspecies electron transfer (DIET) improved the process between the glycolysis, oxidation of pyruvate and CO₂-dependent methanogenesis.

Improved anaerobic digestion efficiency of high-solid sewage sludge by enhanced direct interspecies electron transfer with activated carbon mediator

High-solid anaerobic digestion (AD) faces the problems of easy acidification and low methane production efficiency. In this study, activated carbon (AC)-enhanced direct interspecies electron transfer (DIET) was investigated to overcome such problems. Results showed the conversion of volatile fatty acids (VFAs) into methane rate was increased with AC addition, which led improved methane production efficiency. The methane yields from the early AD stage improved by 124.0-146.3% with AC addition. The T₈₀ shortened by 8-9 days with AC addition. The relative abundances of Geobacter, Syntrophomonas and Methanosaeta that associated with DIET improved for 63.65%, 256.3% and 4.35% by AC addition, which reflected the enhanced DIET with AC addition. The redox activity of AC might be responsible for the enhanced DIET. This study would advance the understanding of DIET and provide a potential solution to the problems existed in high-solid AD.

Powdered activated carbon facilitates methane productivity of anaerobic co-digestion via acidification alleviating: Microbial and metabolic insights

Low methanogenic efficiency caused by excess acidification is a challenge during anaerobic digestion. This study indicated that both granular activated carbon (GAC) and powdered activated carbon (PAC) promoted the start-up of methanogenesis and methane output in anaerobic co-digestion of food waste and fruit-vegetable waste. Moreover, PAC performed better than GAC. Specifically, the highest cumulative methane yield and shortest lag phase were observed in 5 g/L PAC and 10 g/L PAC group, 22.0% higher and 62.5% shorter than that without activated carbon supplementation, respectively. PAC facilitated the methane productivity by effectively accelerating volatile fatty acids (VFAs) consumption and thereby alleviating acidification. Syntrophic VFAs oxidizing bacteria (Gelria and Syntrophomonas) and direct interspecies electron transfer related microorganisms (Geobacter and Methanosarcina) were remarkably enriched by PAC. Furthermore, metagenomic analysis showed that both PAC and GAC might facilitate the electron transfer between microbes by acting as the electrical bridge and enhance both hydrogenotrophic and aceticlastic pathways.

Effects of different conductive nanomaterials on anaerobic digestion process and microbial community of sludge

The effects of four conductive nanomaterials (nano-carbon powder, nano-Al₂O₃, nano-ZnO, nano-CuO) on sludge anaerobic digestion (AD) performance and microbial community were investigated through a 36-day fermentation experiment. Results showed that biogas production enhanced by 16.9% and 23.4% with nano-carbon powder and nano-Al₂O₃ added but decreased by 90.2% and 17.3% with nano-ZnO and nano-CuO. Total solids (TS) removal efficiency was increased by 38.73% and 27.11% with nano-carbon powder and nano-Al₂O₃ added but decreased by 70.67% and 43.70% with nano-ZnO and nano-CuO. Kinetic analysis indicated four conductive nanomaterials could shorten the lag phase of AD sludge with an average rate of 51.75%. 16S rRNA amplicon sequencing results demonstrated microbes such as Syntrophomonas and Methanosaeta were enriched in nano-carbon powder and nano-Al₂O₃ reactors. However, microbial community diversity and richness were both inhibited by adding nano-ZnO and nano-CuO. Redundancy analysis (RDA) revealed that genera belong to Firmicutes and Chloroflexi could conduce to methanogenesis process.

TiO2 nanoparticles accelerate methanogenesis in mangrove wetlands sediment

In this study, the response of methane (CH₄) production to the addition of titanium dioxide nanoparticles (TiO₂ NPs) with three types of short-chain fatty acids (sodium acetate, sodium propionate and sodium butyrate) as carbon sources in mangrove sediment was investigated. The results showed that the maximum CH₄ formation rate increased by 45.2%, 32.7% and 48.6% and the maximum cumulative CH₄ production increased by 25.2%, 7.7% and 6.3% with the addition of TiO₂ NPs in the sodium acetate, sodium propionate and sodium butyrate systems, respectively. The microbial community analysis revealed that the electrogenic bacteria Proteiniclasticum and Pseudomonas, butyrate oxidizing bacteria Syntrophomonas and methanogens Methanobacterium and Methanosarcina were significantly enriched in the presence of TiO₂ NPs, indicating that TiO₂ NPs can enhance CH₄ production by stimulating the growth of different species of methanogens and butyrate oxidizing bacteria. The enlarged distance between microbes, the enhanced conductivity of the sediment and the typical microorganisms for direct interspecies electron transfer (DIET) with the addition of TiO₂ NPs suggest that the promoted DIET between distinct microorganisms could be another possible explanation for the improvement in CH₄ production. It can be speculated that a weaker effect on methanogenesis increases under the natural concentration of TiO₂ NPs compared with the experimental conditions; however, the amounts of TiO₂ NPs are increasing enriched in wetland environments. Therefore, the findings of this study increase current knowledge about the effect of nanomaterials on global CH₄ emissions and suggest that the discharge of wastewater containing TiO₂ NPs from the synthesis and incorporation of TiO₂ NPs in customer products needs to be monitored.

Enhanced Anaerobic Digestion by Stimulating DIET Reaction

Since the observation of direct interspecies electron transfer (DIET) in anaerobic mixed cultures in 2010s, the topic “DIET-stimulation” has been the main route to enhance the performance of anaerobic digestion (AD) under harsh conditions, such as high organic loading rate (OLR) and the toxicants’ presence. In this review article, we tried to answer three main questions: (i) What are the merits and strategies for DIET stimulation? (ii) What are the consequences of stimulation? (iii) What is the mechanism of action behind the impact of this stimulation? Therefore, we introduced DIET history and recent relevant findings with a focus on the theoretical advantages. Then, we reviewed the most recent articles by categorizing how DIET reaction was stimulated by adding conductive material (CM) and/or applying external voltage (EV). The emphasis was made on the enhanced performance (yield and/or production rate), CM type, applied EV, and mechanism of action for each stimulation strategy. In addition, we explained DIET-caused changes in microbial community structure. Finally, future perspectives and practical limitations/chances were explored in detail. We expect this review article will provide a better understanding for DIET pathway in AD and encourage further research development in a right direction.

Immobilizing Microcystis aeruginosa and powdered activated carbon for the anaerobic digestate effluent treatment

The environment pollution caused by livestock anaerobic digestate effluent (ADE) is becoming increasingly severe recently. In this study, immobilized technology, embedding Microcystis aeruginosa (MA) and powdered activated carbon (PAC) with sodium alginate (SA), was employed to investigate the removal performance of nitrogen (N), phosphorus (P) and dissolved organic matter (DOM) in the treatment of ADE solution. Initially, orthogonal experiment was carried out to achieve the optimal conditions of the beads fabrication with the concentration of imbedding agents (PAC-SA) of 5% (w/w) and the ratio of microalgae and imbedding agents was 1:1 (v/v). The results indicated that the total nitrogen (TN), total phosphorus (TP) and total organic carbon (TOC) can be efficiently removed under the optimal operation conditions, with average removals of 91.88 ± 2.91% in TN, 98.24 ± 0.12 in TP and 78.31 ± 1.57% in TOC, respectively. Moreover, the fluorescence excitation-mission matrix (EEM) results illustrated that IMA-PAC beads joined system can efficiently diminish the concentrations of protein-like compounds and humic substances. Therefore, the organic contaminants and nutrients (i.e. N and P) can be efficiently removed in IMA-PAC beads joined system, which would contribute to developing new strategies for the treatment of ADE solution and nutrient recycle.

Improving performance and phosphorus content of anaerobic co-digestion of dairy manure with aloe peel waste using vermiculite

Phosphorus content of the digestate is crucial for evaluating its fertilizer utilization in anaerobic digestion system. The vermiculite containing rich-phosphorus is firstly used as an accelerant in anaerobic batch co-digestion system of aloe peel waste and dairy manure. After introducing vermiculite, the cumulative biogas production (295.14-353.96 mL/g VS), chemical oxygen demand removal rate (45.53%-71.03%), and volatile solid removal rate (50.70%-52.76%) are remarkably higher than those of reference reactor (234.08 mL/g VS, 39.38%, 45.10%). The thermal and fertility analyses manifest the digestates with vermiculite possess superior stability, admirable fertilizer values (5.97%-6.81%), and excellent total phosphorus content (11.44-13.29 g/kg). The improved co-digestion performance can be attributed to the addition of vermiculite. This work introduces a novel approach for improving the performance of anaerobic co-digestion and the fertilizer utilization of digestate in the co-digestion systems.

Effects of graphene oxide and reduced graphene oxide on acetoclastic, hydrogenotrophic and methylotrophic methanogenesis

This study describes the effects of graphene oxide (GO) and reduced graphene oxide (rGO) on the acetoclastic, hydrogenotrophic and methylotrophic pathways of methanogenesis by an anaerobic consortium. The results showed that GO negatively affected the hydrogenotrophic and acetoclastic pathways at a concentration of 300 mg/L, causing a decrease of ~ 38% on the maximum specific methanogenic activity (MMA) with respect to the controls lacking GO. However, the presence of rGO (300 mg/L) promoted an improvement of the MMA (> 45%) achieved with all substrates, except for the hydrogenotrophic pathway, which was relatively insensitive to rGO. The presence of either rGO or GO enhanced the methylotrophic pathway and resulted in an increase of the MMA of up to 55%. X-ray photoelectron spectroscopy (XPS) analysis revealed that GO underwent microbial reduction during the incubation period. Electrons derived from substrates oxidation were deviated from methanogenesis towards the reduction of GO, which may explain the MMA decreased observed in the presence of GO. Furthermore, XPS evidence indicated that the extent of GO reduction depended on the metabolic pathway triggered by a given substrate.

Conductive materials in anaerobic digestion: From mechanism to application

Anaerobic digestion (AD) is an effective strategy combined advantages of maintaining the global carbon flux and efficient energy conversion. Various conductive materials (CMs) have been applied in anaerobic digesters to improve the performance of anaerobic fermentation and methanogenesis, including carbon-based CMs and metal-based CMs. Generally, CMs facilitated the AD thermodynamically and kinetically because they triggered more efficient syntrophic metabolism to increase electron capture capability and accelerate reaction rate as well as enhance the performance of AD stages (hydrolysis-acidification, methanogenesis). Besides, adding CMs into anaerobic digester is benefit to dealing with the deteriorating AD, which induced from temperature variation, acidified working condition, higher H₂ partial pressure, etc. However, few CMs exhibited inhibition on AD, including ferrihydrite, magnesium oxide, silver nanoparticles and carbon black. Inhibition comes from a series of complex factors, such as substrate competition, direct inhibition from Fe(III), Fe(III) reduction of methanogens, toxic effects to microorganisms and mass transfer limitation.

RNA-based spatial community analysis revealed intra-reactor variation and expanded collection of direct interspecies electron transfer microorganisms in anaerobic digestion

Granular activated carbon (GAC) has been shown to mediate direct interspecies electron transfer (DIET) in anaerobic digestion. Adding GAC to up-flow anaerobic sludge bed reactor increased the total biomass slightly from 20.0 to 26.6 gVSS/reactor, and maximum organic removal capacity remarkably from 285 to 1660 mgCOD/L/d. Since GAC occupied 7% of reactor volume (denser than suspended sludge, settled to the reactor bottom), we used a spatial sampling strategy (sludge bed top/mid/bottom layers, and tightly attached GAC-biofilm) and DNA- and RNA-based community analyses. RNA-based analysis demonstrated significant community differences between the non-GAC and GAC-amended reactors (p < 0.05) based on ANOSIM statistical analysis. In comparison, DNA-based analysis showed little community difference between these reactors (p > 0.05). RNA-based analysis revealed active enrichments in GAC-biofilm, including bacteria Geobacter, Syntrophus, Desulfovibrio and Blvii28, and archaea Methanosaeta and Methanospirillum. These are potential electro-active syntrophic microorganisms related with DIET, which expand the previously defined list of DIET microorganisms.

Effects of different thermal pretreatments on the biodegradability and bioaccessibility of sewage sludge

Thermal hydrolysis has proven to be a successful approach to make sewage sludge more amenable to anaerobic digestion. Three heat pretreatment scenarios were compared in this study, i.e. thermal alkaline treatment (LAT, 0.1 M NaOH, 80 °C), low temperature thermal treatment (LT, 80 °C) and high temperature thermal treatment (HT, 170 °C). The biodegradability of pretreated sludge was testified by using biochemical methane potential (BMP) test, meanwhile, the repartition and complexities of organic matters in sludge subjected to various pretreatments were characterized by a revised chemical extraction protocol combined with 3D fluorescence spectroscopy. The cumulative methane yield of sewage sludge was significantly increased by LAT (+135%), LT (+95%) and HT (+112%) as compared to the control. Nevertheless, results show that the solubilization degree of sludge was insignificantly correlated to BMP values, meanwhile high correlation values were observed for the soluble polysaccharide concentration in hydrolysate. The degradation rates of bioaccessible fraction of soluble particulate organic matter (SPOM) and readily extractible organic matter (REOM) were improved after thermal pretreatments at varied levels, which indicates that the chemical accessibility is positively correlated with the bioaccessibility. Furthermore, the biodegradable index F_digestion was proposed to evaluate the biodegradability of organic matter, which is helpful for the optimization of various pretreatment strategies.