Biochar enhanced high-solid mesophilic anaerobic digestion of food waste: Cell viability and methanogenic pathways

Effect of biochar addition on the anaerobic digestion of food waste: microbial community structure and methanogenic pathways

This study assessed the effects of the addition of biochar prepared at 700 °C with different dosages on the anaerobic digestion of food waste. The biochar addition at a concentration of 10.0 g/L increased the cumulative methane yield by 128%, and daily methane production was also significantly promoted. The addition of biochar derived from poplar sawdust significantly increased the relative abundance of dominant bacteria for anaerobic digestion by 85.54-2530% and promoted the degradation of refractory organic matter and the transfer of materials between the hydrolysis and acid production stages. Further analysis has demonstrated that Bathyarchaeia and hydrogenotrophic methanogens were enriched by the biochar addition. Meanwhile, the relative abundances of functional genes, including C5-branched dibasic acid metabolism, and pyruvate metabolism, were increased by 11.38-26.27%. The relative abundances of genes related to major amino acid metabolism, including histidine metabolism, lysine biosynthesis, and phenylalanine, tyrosine, and tryptophan biosynthesis, were increased by 11.96-15.71%. Furthermore, the relative abundances of genes involved in major replication and repair were increased by 14.76-22.76%, and the major folding, sorting, degradation, and translation were increased by 14.47-19.95%, respectively. The relative abundances of genes related to major membrane transport and cell motility were increased by 10.02 and 83.09%, respectively.

Biochar modulates intracellular electron transfer for nitrate reduction in denitrifying anaerobic methane oxidizing archaea

Denitrifying anaerobic methane oxidizing (DAMO) archaea plays a significant role in simultaneously nitrogen removal and methane mitigation, yet its limited metabolic activity hinders engineering applications. This study employed biochar to explore its potential for enhancing the metabolic activity and nitrate reduction capacity of DAMO microorganisms. Sawdust biochar (7 g/L) was found to increase the nitrate reduction rate by 2.85 times, although it did not affect the nitrite reduction rate individually. Scanning electron microscopy (SEM) and fluorescence excitation-emission matrix (EEM) analyses revealed that biochar promoted microbial aggregation, and stimulated the secretion of extracellular polymeric substances (EPS). Moreover, biochar bolstered the redox capacity and conductivity of the biofilm, notably enhancing the activity of the electron transfer system by 1.65 times. Key genes involved in intracellular electron transport (Hdr, MHC, Rnf) and membrane transport proteins (BBP, ABC, NDH) of archaea were significantly up-regulated. These findings suggest that biochar regulates electrons generated by reverse methanogenesis to the membrane for nitrate reduction.

Metagenomic analysis reveals the mechanisms of biochar supported nano zero-valent iron in two-phase anaerobic digestion of food waste: microbial community, CAZmey, functional genes and antibiotic resistance genes

The mechanisms of biochar supported nano zero-valent iron (BC/nZVI) on two-phase anaerobic digestion of food waste were investigated. Results indicated that the performance of both acidogenic phase and methanogenic phase was effectively facilitated. BC/nZVI with the amount of 120 mg/L increased methane production by 32.21%. In addition, BC/nZVI facilitated direct interspecies electron transfer (DIET) between Geobacter and methanogens. Further analysis showed that BC/nZVI increased the abundance of most CAZymes in acidogenic phase. The study also found that BC/nZVI had positive effects on metabolic pathways and related functional genes. The abundances of acdA and ackA in acidogenic phase were increased by 151.75% and 36.26%, respectively, and the abundances of pilA and TorZ associated with DIET were also increased. Furthermore, BC/nZVI mainly removed IMP-12, CAU-1, cmeB, ErmR, MexW, ErmG, Bla2, vgaD, MuxA, and cpxA from this system, and reduced the antibiotic resistance genes for antibiotic inactivation resistance mechanisms.

Enhanced thermophilic high-solids anaerobic digestion of organic fraction of municipal solid waste with spatial separation from conductive materials in a single reactor volume

Despite benefits such as lower water and working volume requirements, thermophilic high solids anaerobic digestion (THSAD) often fails due to the rapid build-up of volatile fatty acids (VFAs) and the associated drop in pH. Use of conductive materials (CM) can promote THSAD through stimulation of direct interspecies electron transfer (DIET), while the need for their constant dosing due to poor separation from effluent impairs economic feasibility. This study used an approach of spatially separating magnetite and granular activated carbon (GAC) from the organic fraction of municipal solid waste (OFMSW) in a single reactor for THSAD. GAC and magnetite addition could both mitigate the severe inhibition of methanogenesis after VFAs build-up to ∼28-30 g/L, while negligible methane production was observed in the control group. The highest methane yield (286 mL CH4/g volatile solids (VS)) was achieved in magnetite-added reactors, while the highest maximum CH4 production rates (26.38 mL CH4/g VS/d) and lowest lag-phase (2.83 days) were obtained in GAC-added reactors. The enrichment of GAC and magnetite biofilms with various syntrophic and potentially electroactive microbial groups (Ruminiclostridium 1, Clostridia MBA03, Defluviitoga, Lentimicrobiaceae) in different relative abundances indicates the existence of specific preferences of these groups for the nature of CM. According to predicted basic metabolic functions, CM can enhance cellular processes and signals, lipid transport and metabolism, and methane metabolism, resulting in improved methane production. Rearrangement of metabolic pathways, formation of pili-like structures, enrichment of biofilms with electroactive groups and a significant improvement in THSAD performance was attributed to the enhancement of the DIET pathway. Promising results obtained in this work due to the spatial separation of the bulk OFMSW and CM can be useful for modeling larger-scale THSAD systems with better recovery of CM and cost-effectiveness.

Metagenome reveals the possible mechanism that microbial strains promote methanogenesis during anaerobic digestion of food waste

For better understanding the mechanism of microbial strains promoting methane production, four strains Hungatella xylanolytica A5, Bacillus licheniformis B1, Paraclostridium benzoelyticum C2 and Advenella faeciporci E1 were inoculated into anaerobic digestion systems. After bioaugmentation, the cumulative methane production of A5, B1, C2 and E1 groups elevated by 11.68%, 8.20%, 18.21% and 15.67% compared to CK group, respectively. The metagenomic analysis revealed that the species diversity and uniformity of the experimental groups was improved, and hydrolytic acidifying bacteria, represented by Clostridiaceae, Anaerolineaceae and Oscillospiraceae, and methanogens, such as Methanotrichaceae and Methanobacteriaceae, were enriched. Meanwhile, the abundance of key genes in carbohydrate, pyruvate and methane metabolism was increased in the inoculated groups, providing reasonable reasons for more methane production. The strengthening mechanism of microbial strains in this study offered a theoretical foundation for selecting a suitable bioaugmentation strategy to solve the problems of slow start-up and low methane production in anaerobic digestion.

Review in anaerobic digestion of food waste

Due to the special property of food waste (FW), anaerobic digestion of food waste is facing many challenges like foaming, acidification, ammonia nitrogen and (NH4+-N) inhibition which resulted in a low biogas yield. A better understanding on the problems exiting in the FW anaerobic digestion would enhance the bio-energy recovery and increase the stable operation. Meanwhile, to overcome the bottle necks, pretreatment, co-digestion and additives is proposed as well as the solutions to improve biogas yield in FW digestion system. At last, future research directions regarding FW anaerobic digestion were proposed.

Advances, trends and challenges in the use of biochar as an improvement strategy in the anaerobic digestion of organic waste: a systematic analysis

A recently strategy applied to anaerobic digestion (AD) is the use of biochar (BC) obtained from the pyrolysis of different organic waste. The PRISMA protocol-based review of the most recent literature data from 2011-2022 was used in this study. The review focuses on research papers from Scopus® and Web of Knowledge®. The review protocol used permits to identify 169 articles. The review indicated a need for further research in the following challenges on the application of BC in AD: i) to increase the use of BC in developing countries, which produce large and diverse amounts of waste that are the source of production of this additive; ii) to determine the effect of BC on the AD of organic waste under psychrophilic conditions; iii) to apply tools of machine learning or robust models that allow the process optimization; iv) to perform studies that include life cycle and technical-economic analysis that allow identifying the potential of applying BC in AD in large-scale systems; v) to study the effects of BC on the agronomic characteristics of the digestate once it is applied to the soil and vi) finally, it is necessary to deepen in the effect of BC on the dynamics of nitrogen and microbial consortia that affect AD, considering the type of BC used. In the future, it is necessary to search for new solutions in terms of the transport phenomena that occurs in AD with the use of BC using robust and precise mathematical models at full-scale conditions.

Biomethane is produced by acetate cleavage, not direct interspecies electron transfer: genome-centric view and carbon isotope

Understanding the source of methane (CH4) is of great significance for improving the anaerobic fermentation efficiency in bioengineering, and for mitigating the emission potential of natural ecosystems. Microbes involved in the process named direct interspecies electron transfer coupling with CO2 reduction, i.e., electrons released from electroactive bacteria to reduce CO2 into CH4, have attracted considerable attention for wastewater treatment in the past decade. However, how the synergistic effect of microbiota contributes to this anaerobic carbon metabolism accompanied by CH4 production still remains poorly understood, especial for wastewater with antibiotic exposure. Results show that enhancing lower-abundant acetoclastic methanogens and acetogenic bacteria, rather than electroactive bacteria, contributed to CH4 production, based on a metagenome-assembled genomes network analysis. Natural and artificial isotope tracing of CH4 further confirmed that CH4 mainly originated from acetoclastic methanogenesis. These findings reveal the contribution of direct acetate cleavage (acetoclastic methanogenesis) and provide insightsfor further regulation of methanogenic strategies.

A Review on Performance Improvement of Anaerobic Digestion Using Co-Digestion of Food Waste and Sewage Sludge

Anaerobic co-digestion (AcoD) is a vital technology in the decarburization of the economy because of its ability to process organic waste, recover nutrients, and create biogas as a sustainable biofuel all at the same time. This attribute also makes this technology a viable partner in pursuing a circular economic model. However, the poor biogas output of typical substrates like sewage sludge and animal manure and the hefty installation costs limit its viability. This review paper with literature analysis provides a good grasp of the anaerobic co-digesting process with diverse food digestion methods. In this survey, we have analyzed the Anaerobic Digestion of water waste, food waste, and animal manure and the anaerobic co-digestion of animal waste with water waste and food waste with water waste. This analysis demonstrates that anaerobic co-digestion produces more methane biogas than anaerobic digestion. Also, it has been shown that by adjusting the ratio of food and animal waste to water waste, we can produce more methane. In the future, we would like to supplement anaerobic co-digestion by altering the proportion of different wastes that are mixed with water waste in order to increase methane production.

Microbial response behavior to powdered activated carbon in high-solids anaerobic digestion of kitchen waste: Metabolism and functional prediction analysis

Anaerobic digestion (AD) can not only treat organic waste, but also recycle energy. However, high-solids AD of kitchen waste usually failed due to excessive acidification. In this study, the effect of activated carbon (AC) on kitchen waste AD performance was investigated under high-solids conditions (total solids contents = 15%). The results showed that efficiencies of acidogenesis and methanogenesis were promoted in presence of moderate concentration (50 g/L > AC >5 g/L), but high concentration (AC >70 g/L) weakened AD performance. Moreover, AC addition enhanced the methane production rate from 66.0 mL/g VS to 231.50 mL/g VS, i.e., up to 250.7%. High-throughput sequencing results demonstrated that the abundance of electroactive DMER64 increased from less than 1%-29.7% (20 g/L AC). As AC gradually increased，aceticlastic methanogenesis changed to hydrogenotrophic pathway. Predicted functional analysis indicated that AC can enhance abundances of energy and inorganic ion metabolism, resulting in high methane production.

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

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

Use of iron-loaded biochar to alleviate anammox performance inhibition under PFOA stress conditions: Integrated analysis of sludge characteristics and metagenomics

The negative effects of perfluorooctanoic acid (PFOA) on biological nitrogen removal performance in wastewater treatment plants, are receiving increasing attention due to the widespread reporting of this issue. In this study, pomelo peel iron-loaded biochar (Fe-PBC) was added to an anammox bioreactor to alleviate the negative effects of PFOA. Results showed that the addition of Fe-PBC increased the ammonia and nitrite removal efficiencies from 77.7 ± 9.6 % and 79.5 ± 5.6 % to 94.45 ± 5.1 % and 95.9 ± 5.0 %, respectively. In addition, Fe-PBC promoted the removal of PFOA from wastewater, increasing the PFOA removal efficiency from 5.2 % to 29.2 ± 4.3 % from 100 to 200 days. The introduction of iron-loaded biochar into the anammox bioreactor increased the CO ratio by 13.64 % by 150 days. In addition, a CO fitting peak was detected in the Fe-PBC, indicating that the Fe-PBC was loaded with microorganisms. Microbial community analysis showed a decrease in the relative abundances of Proteobacteria and Nitrospirae from 31 % and 3.4 % to 16.8 % and 0.9 %, respectively, while the relative abundance of Planctomycetes increased from 26.8 % to 44.1 %. Metagenomic analysis found that the functional genes hzsB and hdh increased from 98,666 ± 11,400 and 3190 ± 460 to 119,333 ± 15,534 and 138,650 ± 11,233 copy numbers/MLSS. The increase in anammox biomass may be attributed to the presence of iron, an essential element for the synthesis of key anammox enzyme. Furthermore, iron was also associated with the enhanced extracellular electron transfer in the anammox system induced by Fe-PBC.

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

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

Evaluation of digestate-derived biochar to alleviate ammonia inhibition during long-term anaerobic digestion of food waste

The feasibility of using food waste anaerobic digestate-derived biochar (FWDB) to mitigate ammonia toxicity in an anaerobic digester was evaluated. The optimal conditions for preparing and adding the activated FWDB were explored using response surface experiments, and the long-term effects of adding activated FWDB on digester performance under optimum conditions were verified in semi-continuous experiments. The results showed that the optimal preparation and addition conditions for activated FWDB were pyrolysis temperature of 565 °C, particle size of 0-0.30 mm, and dosage of 15.52 g·L-1. During the long-term operation of the digesters, when the total ammonia nitrogen (TAN) concentration was higher than 2000 mg·L-1, the control and experimental digesters showed deteriorated reactor performance. Volatile fatty acids in the control digester accumulated to 20,306 mg·L-1 after the TAN concentration increased to 3391 mg·L-1, the methane yield decreased to 31 mL·g VS-1, and the digester experienced process failure. In contrast, the experimental digester with added activated FWDB only suffered a slight short-term accumulation of acetate and a slight decline in methane yield. This may be attributed to the adsorption of NH4+/NH3 by activated FWDB, which reduced the TAN concentration in the anaerobic digestion (AD) system and mitigated ammonia toxicity. Microbial analysis and metagenome predictions demonstrated that the community richness, diversity, and evenness, as well as the abundance of acetogens and related key genes (ACSM1, paaF, and acdA) were higher in the experimental digester than in the control digester. This study provides a closed-loop AD enhancement strategy by pyrolysis of digestate and in-situ supplementation into the digester.

Engineered Nanoparticles, Natural Nanoclay and Biochar, as Carriers of Plant-Growth Promoting Bacteria

The potential of biochar and nanoparticles to serve as effective delivery agents for beneficial bacteria to crops was investigated. Application of nanoparticles and biochar as carriers for beneficial bacteria improved not only the amount of nitrogen-fixing and phosphorus-solubilizing bacteria in soil, but also improved chlorophyll content (1.2-1.3 times), cell viability (1.1-1.5 times), and antioxidative properties (1.1-1.4 times) compared to control plants. Treatments also improved content of phosphorus (P) (1.1-1.6 times) and nitrogen (N) (1.1-1.4 times higher) in both tomato and watermelon plants. However, the effect of biochars and nanoparticles were species-specific. For example, chitosan-coated mesoporous silica nanoparticles with adsorbed bacteria increased the phosphorus content in tomato by 1.2 times compared to a 1.1-fold increase when nanoclay with adsorbed bacteria was applied. In watermelon, the situation was reversed: 1.1-fold increase in the case of chitosan-coated mesoporous silica nanoparticles and 1.2 times in case of nanoclay with adsorbed bacteria. Our findings demonstrate that use of nanoparticles and biochar as carriers for beneficial bacteria significantly improved plant growth and health. These findings are useful for design and synthesis of novel and sustainable biofertilizer formulations.

Effects of modified biochars on the shifts of short-chain fatty acid profile, iron reduction, and bacterial community in paddy soil

Biochar is well known as an effective means for soil amendment, and modification on biochar with different methods could improve the benefits for environmental remediation. In this study, two modified biochars were generated with nitric acid (NBC) and hydrogen peroxide (OBC) pretreatment, and a control biochar was produced after washing with deionized water (WBC). The dynamics of short-chain fatty acids (SCFAs), iron concentration and bacterial community in rice paddy soil amended with different biochars or without adding biochar (CK) were studied during 70 days of anaerobic incubation. Compared to CK treatment, the accumulation of SCFAs was largely inhibited by the amendment of biochars. Besides, OBC and WBC increased the accumulation of Fe(II) at the initial stage of incubation. Via 16S rRNA gene sequencing, modified biochars caused significant response of bacterial community in comparison to WBC at Day 0-1, and three biochars favored bacterial α-diversity in the paddy soil at the end of the incubation. Interestingly, positive and negative correlations between NBC and several bacteria taxa (e.g. Geobacter, Fonticella and Clostridium) were observed. The study revealed that modified biochars had significant effects on the shifts of SCFAs, Fe(III) reduction and bacterial diversity, which provides fundamental information for future application of modified biochars in rice cropping ecosystem.

Production of biochar from crop residues and its application for anaerobic digestion

Anaerobic digestion (AD) is a viable and cost-effective method for converting organic waste into usable renewable energy. The efficiency of organic waste digestion, nonetheless, is limited due to inhibition and instability. Accordingly, biochar is an effective method for improving the efficiency of AD by adsorbing inhibitors, promoting biogas generation and methane concentration, maintaining process stability, colonizing microorganisms selectively, and mitigating the inhibition of volatile fatty acids and ammonia. This paper reviews the features of crop waste-derived biochar and its application in AD systems. Four critical roles of biochar in AD systems were identified: maintaining pH stability, promoting hydrolysis, enhancing the direct interspecies electron transfer pathway, and supporting microbial development. This work also highlights that the interaction between biochar dose, amount of organic component in the substrate, and inoculum-to-substrate ratio should be the focus of future research before deploying commercial applications.

Impact of total solids content on biochar amended co-digestion of food waste and sludge: Microbial community dynamics, methane production and digestate quality assessment

This study evaluates the impact of biochar addition on the performance of anaerobic co-digestion of food waste (FW) and sewage sludge at different total solids (TS) contents (2.5 %, 5.0 %, and 7.5 %). Biochar co-digestion improved hydrolysis and acidogenesis by neutralizing volatile fatty acids (VFAs) reducing its inhibitions (2.6-fold removal), which elevated the soluble chemical oxygen demand (sCOD) degradation by 2.5 folds leading to a higher cumulative methane production compared to the control. This increase corresponded to an improvement of methane yields by ∼21 %-33 % (242-340 mL/gVS_add) at different TS contents. The biochar surface area offered substantial support for direct interspecies electron transfer (DIET) activity, and biofilm-mediated growth of methanogens i.e., Methanosarcina, Methanosata, and Methanobrevibacter. The biochar-enriched digestate improved the seed germination index, and bioavailability of plant nutrients such as N, P, K, and NH₄^+-N. This study reports an improved biochar-mediated anaerobic co-digestion for efficient and sustainable FW valorization.

Improvement of anaerobic digestion of food waste by addition of synthesized allophane

Anaerobic digestion (AD) of food waste (FW) always confronts the challenges of over-acidification in application. This work evaluated the effectiveness of synthesized allophane, a mineral with desirable physicochemical properties (e.g., high pH buffer and organic matter adsorption capacity, and high porosity and specific surface area), in increasing biogas yield during AD of FW as an additive. Results showed that allophane addition (0 to 10 g total solid (TS)) increased the cumulative biogas yield from 409.69 ± 20.77 mL/g TS to 624.06 ± 6.63 mL/g TS, and methane production from 224.12 ± 9.26 mL/g TS to 391.52 ± 0.87 mL/g TS. Improved AD performance was mainly attributed to mitigating over-acidification during the start-up period, and favoring microbial growth, particularly the acetotrophic methanogen of Methanosarcina, indicating an intensified acetoclastic methanogenic pathway. The findings provided a mechanistic insight into the improved AD performance with allophane addition, and offered a potential strategy to stabilize AD of FW in application.

Size-dependent effects of polystyrene microplastics on anaerobic digestion performance of food waste: Focusing on oxidative stress, microbial community, key metabolic functions

Polystyrene (PS) microplastics (MPs) are widely existed in food waste (FW) due to the usage of plastic food-packaging. However, the effects and mechanisms of PS MPs with different sizes on anaerobic digestion (AD) performance of FW have not been comprehensively studied yet. Herein, the impacts of different PS MPs sizes (1 mm, 100 µm and 1 µm) with 20, 200 particles/g-TS were investigated. Results showed that 20 particles/g-TS PS MPs decreased cumulative methane production by 1.46-18.11 %, while the higher levels (200 particles/g-TS) significantly inhibited by 9.14-33.08 % (p < 0.05) compared with control group. The inhibiting effects were enhanced as particle size smaller. Physicochemical analysis indicated that MPs prolonged organic matter hydrolysis, weakened the volatile fatty acids metabolism and inhibited methanogenesis-related microorganisms (Synergistetes, Proteiniphilum and Methanosarcina). Small-sized MPs could induce more reactive oxygen species causing cell toxicity and suppressed key enzymes (α-glucoside, protease, acetate kinases and F₄₂₀) activities, thereby restraining methane production. The analyses of acetyl-CoA synthase and methyl-coenzyme M reductase functional genes illustrated that small-sized MPs negatively affected acetoclastic methanogenesis pathways. Overall, these results provide new insights into the size-dependent effects on AD performance induced by PS MPs.

Biochar induced inhibitory effects on intracellular and extracellular antibiotic resistance genes in anaerobic digestion of swine manure

Distribution of intracellular (iARGs) and extracellular ARGs (eARGs) in manure anaerobic digestion (AD) process coupled with two types of biochar (BC and BP) were investigated. And the effects of biochar on the conjugation transfer of ARGs were explored by deciphering the interaction of biochar with bacterial stress responses, physiological metabolism and antibiotic resistances. Results showed that AD process could effectively remove all the detected eARGs with efficiency of 47.4-98.2%. The modified biochar (BP) with larger specific surface area (SSA) was propitious to decrease the absolute copy number of extracellular resistance genes. AD process could effectively remove iARGs by inhibiting the growth of host bacteria. The results of structural equation models (SEM) indicated that biochar put indirect influences on the fate of ARGs (λ = -0.23, P > 0.05). Analysis on oxidative stress levels, antioxidant capacity, DNA damage-induced response (SOS) response and energy generation process demonstrated that biochar induced the oxidative stress response of microorganisms and enhanced the antioxidant capacity of bacteria. The elevated antioxidant capacity negatively affected SOS response, amplified cell membrane damage and further weakened the energy generation process, resulted in the inhibition of horizontal transfer of ARGs.

Polycyclic aromatic hydrocarbons stimulate acidogenesis, acetogenesis and methanogenesis during anaerobic co-digestion of waste activated sludge and food waste

Polycyclic aromatic hydrocarbons (PAHs) have been reported to influence acetic acid production during anaerobic treatment. However, investigations of the impacts of PAHs on the anaerobic co-digestion of waste activated sludge and food waste are limited. Therefore, the effects of PAHs on anaerobic co-digestion were explored in this study. Four kinds of PAHs all exhibited positive contributions to methane production, especially phenanthrene. Mechanism exploration revealed that acidogenesis, acetogenesis, and methanogenesis were improved in the presence of phenanthrene, and acetotrophic methanogenesis had the greatest improvement with 69.4%. Dominant bacteria and archaea related to acetic acid and methane accumulation were changed by phenanthrene. Moreover, extracellular polymeric substances, coenzyme F₄₂₀, and McrA gene copy number were promoted by phenanthrene, which was beneficial for the generation of acetic acid and methane. Overall, this study provides new insights into the role of organic pollutants in the anaerobic co-digestion of solid wastes.

Effects of Iron-Loaded Biochar on the Anaerobic Co-Digestion of Food Waste and Sewage Sludge and Elucidating the Mechanism Thereof

The inhibition of volatile fatty acid (VFA) production is an important factor affecting biogas (methane) production in the anaerobic co-digestion systems comprising food waste and sewage sludge. In this study, batch experiments were conducted at medium temperature (36 ± 0.5 °C), during which the biogas production index and material–liquid characteristic parameters of the anaerobic digestion systems containing different concentrations of iron-loaded biochar (Fe-BC) were monitored. The cumulative biogas production data were analyzed using a modified Gompertz kinetic model to determine the effect of the Fe-BC on biogas production in the anaerobic co-digestion system. Studies have shown that addition of Fe-BC does not significantly influence the hydrolysis and acidification stages of anaerobic co-digestion, but does have a significant effect on promoting methanogenesis by alleviating the accumulation of VFAs and improving both the buffer capacity of the system and the efficiency of substrate-to-biogas conversion. When the Fe-BC concentration was 16 g·L−1, the cumulative biogas production reached 329.42 mL·g-VS−1, which was 49.7% higher than the blank group, and the lag period was 3.55 d, which was 42% shorter than the blank group. Mechanistic studies have shown that Fe-BC increased the concentration of coenzyme F420 and the conductivity of the digestate in the co-digestion system, which increased the activity of methanogens in the anaerobic digestion system, thereby promoting methanogenesis.

Electrical stimulation enhancing anaerobic digestion under ammonia inhibition: A comprehensive investigation including proteomic analysis

Microbial electrolysis cell (MEC) coupled anaerobic digestion (AD), named as MEC-AD system, can effectively promote methane production under ammonia inhibition, but the inherent mechanism is still poorly understood. This study comprehensively explored the MEC-AD performance and mechanism under high-concentration ammonia stress including using proteomic analysis. It was found that the methane generation rates in MEC-AD systems were 2.0-2.7 times that of AD ones under 5.0 g/L ammonia stress. Additionally, the experimental conditions for methane generation in MEC-AD systems were optimized using response surface methodology. Further analysis indicates that the activities of acetate kinase and F₄₂₀ were improved, and particularly the direct interspecies electron transfer (DIET) was promoted in MEC-AD systems, as indicated by increased electroactive extracellular polymeric substance, decreased charge transfer resistance, and enrichment of electroactive microbes such as Geobacter on the bioelectrodes. Moreover, proteomic analysis reveals that the DIET associated proteins such as Cytochrome C was up-regulated, and ammonia transfer-related proteins were down-regulated and ammonium detoxification-related proteins were up-regulated in MEC-AD systems. This work provides us a better understanding on the MEC-AD performance especially for the treatment of wastewater containing high-concentration ammonia.

Effect of biochar derived from biogas residue on methane production during dry anaerobic fermentation of kitchen waste

Kitchen wastes (KW) dramatically increasing with population and economy enhancing, and dry anaerobic fermentation was used to treat it. However, the large amount of biogas residue severely restricted the application of dry anaerobic fermentation, because the high total solid might lead to the system failure. Therefore, it is urgent to find appropriate way to improve the efficiency of dry anaerobic fermentation and reduce the great amount of biogas residue. In this study, a tentative experiment was conducted to investigate the effect of biochar prepared from biogas residue on the performance of dry anaerobic fermentation system. The results showed that almost half of the biogas residue was reduced and converted into biochar. At the presence of biochar, methane yield was 308.6 mL/gVS, which was 10.5% higher than that of control. Compared to the system without biochar, the highest volatile fatty acid (VFA) concentration was 19.3% higher and the percentage of acetate and valerate was 25.3％ and 12.8％, while it was 16.3% and 22.0% in the control, suggesting that biochar accelerated the degradation of VFA. Bacteria community diversity increased, Fastidiosipila and Proteiniphilum enriched at the presence of biochar, which might accelerate the hydrolysis and acidification of KW. Hydrogenotrophic methanogens was dominated and syntrophic acetate oxidation was the primary pathway to produce methane. This study developed a new recycle route for improving the efficiency of dry anaerobic fermentation while reducing the large amount of biogas residue generated from dry anaerobic fermentation.

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

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

The role of biochar on alleviating ammonia toxicity in anaerobic digestion of nitrogen-rich wastes: A review

This paper reviewed the mechanisms of biochar in relieving ammonia inhibition. Biochar affects nitrogen-rich waste's anaerobic digestion (AD) performance through four ways: promotion of direct interspecies electron transfer (DIET) and microbial growth, adsorption, pH buffering, and provision of nutrients. Biochar enhances the DIET pathway by acting as an electron carrier. The role of DIET in relieving ammonia nitrogen may be exaggerated because many related studies don't provide definite evidence. Therefore, some bioinformatics technology should be used to assist in investigating DIET. Biochar absorbs ammonia nitrogen by chemical adsorption (electrostatic attraction, ion exchange, and complexation) and physical adsorption. The absorption efficiency, mainly affected by the properties of biochar, pH and temperature of AD, can reach 50 mg g^-1 on average. The biochar addition can buffer pH by reducing the concentrations of VFAs, alleviating ammonia inhibition. In addition, biochar can release trace elements and increase the bioavailability of trace elements.

Effect of hydrothermal pretreatment on the degrease performance and liquid substances transformation of kitchen waste

Hydrothermal treatment (HT) is a pragmatic approach for pretreatment of kitchen waste (KW). This work investigated the effect of hydrothermal pretreatment (HTP) on the deoiling, desalting and liquid substances transformation of KW. The orthogonal test method was used to study the effects of three factors at five levels, including solid to liquid ratio (A_1-5), heating time (B_1-5) and hydrothermal temperature (C_1-5). The results indicated that the floatable oil content was improved significantly after HTP. The highest floatable oil content was 84.54 mL/kg at the hydrothermal condition of 1/1.5, 20 min and 100 °C, which was 2.42 times higher than the control. The maximum desalination ratio (92.66%) was at A₅B₁C₅ (1/2.5, 5 min, 100 °C), which was 4.48 times higher than control group (No.0) (20.67%). The VFAs concentration was the highest (11441.05 mg/kg) at 1/2.5, 5 min and 100 °C, which increased by 711.03% compared to the No.0 (1410.78 mg/kg). In addition, the maximum TOC value was obtained at 53530.84 mg/kg. After HTP, the acetic acid and butyric acid concentrations of the liquid phase increased, while the ethanol concentration decreased. The contents of T,NH₄⁺-N and organic nitrogen in the liquid phase of the HTP system increased, while NO₃^--N remained at a low level (4.96-20.48 mg/kg). The range and variance analysis showed that the temperature had the greatest effect on the deoiling and the liquid substances transformation of KW among these three factors, followed by solid to liquid ratio and heating time. Based on the orthogonal experiment, the optimal parameters for KW deoiling were A₃ (1/1.5), B₄ (25 min) and C₅ (100 °C). This work provided a reference for the KW deoiling and hence improve the efficient utilization of KW.

Carbon based conductive materials mediated recalcitrant toxicity mitigation during anaerobic digestion of thermo-chemically pre-treated organic fraction of municipal solid waste

High-temperature thermal pretreatment alone or in conjugation with chemical pretreatment (highly acidic or alkaline) produced recalcitrant compounds, which inhibits the anaerobic digestion (AD) process performance. This study aims to develop a strategy to use carbon-based conductive materials to mitigate the recalcitrant toxicity and enhance the methane generation in the downstream AD. The formation of recalcitrant compounds, mainly the furan derivatives, i.e., furfural and 5-HydroxyMethyl furfurals (5-HMF) during thermo-chemical pretreatment of OFMSW at 150 °C, 175 °C, 200 °C with 3 g/L-NaOH dose, and the alleviation of their inhibitory effects by adding 25 g/L of each of granular activated carbon (GAC) and granular biochar (GBC) during mesophilic AD were studied. The addition of conductive materials resulted in the highest biogas yield of 462 mL/gVS_added (GAC) and 449 mL/gVS_added (GBC) for 175°C-3g/L-NaOH pretreatment, which was >45% higher over control. The highest improvement of >65% in biogas yield was observed for 200°C-3g/L-NaOH pretreatment despite the lower biogas yield. The conductive materials amended digester shows a significant decrease in the 5-HMF and furfurals concertation. The highest reduction in 5-HMF (44%) and furfural (51%) concentrations were observed for 200°C-3g/L-NaOH pretreatment, and 25 g/L GBC amended tests. The score plots from the principal component analysis (PCA) of the characterization of the digestate showed that the data were significant, whereas the loading plots depicted the correlation of different experimental parameters studied (like fate of recalcitrant, biogas yield and other parameters post AD of OFMSW when aided with conductive materials). Application of regression models in all the batch assays depicted that a lag phase of 2-4 days was observed in Modified Gompertz Model (MGM), 4-5 days in Logistic Model (LM) and a rapid hydrolysis was proven with the value of hydrolysis coefficient being between 0.003 and 0.029 from the first-order (FO) model.

Comparison of cobalt ferrate-based nanoparticles for promoting biomethane evolution from lactic acid anaerobic digestion

Some inhibition of biomethane (bioCH₄) production system can be observed, which is due to the propionic acid generation from lactic acid degradation. In this work, the three cobalt ferrate-based nanoparticles (NPs) such as CoFe₂O₄, CoAl_0.2Fe_1.8O₄ and CoCu_0.2Fe_1.8O₄ were synthesized to promote the bioCH₄ evolution from lactic acid. The CH₄ yields from the CoAl_0.2Fe_1.8O₄, CoCu_0.2Fe_1.8O₄ and CoFe₂O₄ groups at 300 mg/L of NPs were 431.52, 392.12 and 396.6 mL/g lactic acid, respectively. Moreover, the highest CH₄ yield was 34.15% higher than that of the control reactor (321.67 mL/g lactic acid) without NPs. The three NPs accelerated lactic acid biodegradation and propionic acid conversion, thus obtaining more CH₄. Surprisingly, microbial structure revealed that CoAl_0.2Fe_1.8O₄ increased the abundance of Bacteroidetes_vadinHA17 to 16.6%, promoting the conversion from propionic acid to acetic acid. Meanwhile, the abundance of Methanobacterium in archaeal community from CoAl_0.2Fe_1.8O₄ group rose from 45.81% to 68.45%, which facilitated bioCH₄ production.

Evaluations of biochar amendment on anaerobic co-digestion of pig manure and sewage sludge: waste-to-methane conversion, microbial community, and antibiotic resistance genes

Effects of biochar on co-digestion of pig manure and dewatered sewage sludge under different total solids (TS) were investigated. Biochar could accelerate the start-up of methanogenesis and shorten the adaptation phase. At TS5%, the methane daily production in biochar group was 60.6% higher than the control; nevertheless, when TS increased, the gap between two groups gradually narrowed. Additionally, the change on antibiotics resistance genes (ARGs) was also affected by TS and the biochar addition. Moreover, biochar was beneficial to reduce ARGs in liquid phase. At TS14%, the total ARGs abundance in the liquid phase of biochar group was 41.4% lower than the control, among which the reduction rates of etB(P), sul1, rpoB2, macA, mupA and mupB were more prominent. These findings could provide useful guidance for developing ARGs elimination strategy before their release into the environment.

Biochar amendment to advance contaminant removal in anaerobic digestion of organic solid wastes: A review

Anaerobic digestion (AD) has been widely applied to convert organic solid wastes into biogas, a renewable energy, and digestate, a bio-fertilizer, to sustain waste management. Nevertheless, several vexing contaminants in OSWs restrict digestate application in agriculture. Biochar has been evidenced to effectively improve AD by promoting organic biodegradation and alleviating the accumulation of inhibitory substances (e.g. ammonia and volatile fatty acids). Furthermore, biochar could advance contaminant removal in AD given its highly porous, conductive and alkaline features. Thus, this review aims to highlight the role of biochar amendment to advance contaminant removal in AD of OSWs. Key contaminants, such as antibiotics, heavy metals, microplastics, polycyclic aromatic hydrocarbons, furfural and 5-hydroxy methyl furfural (5-HMF) that ubiquitously present in OSWs were demonstrated. The underlying mechanisms of biochar to amend the removal of these contaminants by AD were discussed. Furthermore, future perspectives to the development of biochar-assisted AD for OSWs treatment were provided.

Multicomponent nanoparticles as means to improve anaerobic digestion performance

Sufficient quantity of trace metals is essential for a well performing anaerobic digestion (AD) process. Among the essential trace elements in active sites of multiple important enzymes for AD are iron and nickel ions. In the present study, iron and nickel in the form of Fe₂O₃ and NiO were coated on TiO₂ nanoparticles to be used in batch and continuous operation mode. The effect of TiO₂, Fe₂O₃-TiO₂, and NiO-TiO₂ nanoparticles on each step of AD process was assessed utilizing simple substrates (i.e. cellulose, glucose, acetic acid, and mixture of H₂-CO₂) as well as complex ones (i.e. municipal biopulp). The hydrolysis rate of cellulose substrate increased with higher dosages of the coated TiO₂ with both metals. For instance, the hydrolysis rate was increased up to 54% at Fe₂O₃-TiO₂ and at a concentration of 23.5 mg/L for NiO-TiO₂ it was increased up to 58%, while higher dosage suppressed the hydrolytic activity. Experimental results revealed that low dosages of NiO-TiO₂ increased the accumulated methane production up to 24% probably by increasing the enzymatic activity of acetoclastic methanogenesis. NiO-TiO₂ showed positive effect on batch and continuous AD of biopulp and improved methane yield up to 8%.

High-Solid Anaerobic Digestion: Reviewing Strategies for Increasing Reactor Performance

High-solid and solid-state anaerobic digestion are technologies capable of achieving high reactor productivity. The high organic load admissible for this type of configuration makes these technologies an ideal ally in the conversion of waste into bioenergy. However, there are still several factors associated with these technologies that result in low performance. The economic model based on a linear approach is unsustainable, and changes leading to the development of a low-carbon model with a high degree of circularity are necessary. Digestion technology may represent a key driver leading these changes but it is undeniable that the profitability of these plants needs to be increased. In the present review, the digestion process under high-solid-content configurations is analyzed and the different strategies for increasing reactor productivity that have been studied in recent years are described. Percolating reactor configurations and the use of low-cost adsorbents, nanoparticles and micro-aeration seem the most suitable approaches to increase volumetric production and reduce initial capital investment costs.

Thermophilic co-digestion of blackwater and organic kitchen waste: Impacts of granular activated carbon and different mixing ratios

Biogas (methane) as a source of renewable energy, was produced in the anaerobic co-digestion of blackwater (BW, municipal toilet wastewater) and organic kitchen waste (KW). The impact on methane production of various BW to KW mixing ratios, with and without the addition of granular activated carbon (GAC), were studied under thermophilic (55 °C) temperatures. GAC is reported to enhance methane production in such digestions through direct interspecies electron transfer. The results showed that the co-digestion of BW and KW under the 1:2 VS ratio significantly improved the biomethane potential (BMP). In the absence of GAC, an optimal BW:KW ratio was found to be 1:2, achieving a BMP of 0.76 g CH₄-COD/g feed-COD. With GAC addition, the BMP increased to 0.81 g CH₄-COD/g feed-COD, the lag phase in the digestion was significantly reduced, and the methane production rate increased. Microbial communities in the BW-KW anaerobic digestion were analyzed with and without the addition of GAC. Methanothermobacter and Methanosarcina were predominant archaea in BW-KW digests, with and without GAC amendment, while a third methanogen, Methanomassiliicoccus, was enriched with the addition of GAC to the digest. Further, through SEM image, the enrichment of pili-like stucture was observed in GAC surface.

Food waste treating by biochar-assisted high-solid anaerobic digestion coupled with steam gasification: Enhanced bioenergy generation and porous biochar production

A food waste treating system was proposed in this study by combining biochar-assisted high-solid anaerobic digestion and subsequent steam gasification of the digestate. The effect of solid level, biochar dosage in anaerobic digestion on the properties of biogas, syngas, and final biochar products were investigated. Results showed that at a high total solid level and biochar dosage of 25 g/L and 50 g/L, the accumulative methane yield reached 110.3 mL CH₄/g VS and 126.7 mL CH₄/g VS, respectively. From steam gasification of different digestates under 850 °C for 15 min, a maximum of 34.92 mmol/g for the hydrogen yield and 11.44 MJ/m³ for the higher heating value could be obtained for the syngas. Furthermore, the by-product produced from steam gasification was a nutrient-enriched porous biochar, which was suitable to be used as compost. This study demonstrated a pathway for food waste treating to produce methane-enriched biogas, hydrogen-enriched syngas, and nutrient-enriched biochar.