Use of biochars in anaerobic digestion

Enhancement of volatile fatty acids degradation and rapid methanogenesis in a biochar-assisted anaerobic membrane bioreactor via enhancing direct interspecies electron transfer

In this investigation, we assessed the efficacy of a biochar-supported anaerobic membrane bioreactor (BC-AnMBR) for continuously treating swine wastewater (SWW) under varying NH4+-N stress levels. Our findings revealed that as the NH4+-N concentration escalated from 440 mg/L to 1400 mg/L, the BC-AnMBR exhibited a notable 14.5 % improvement in NH4+-N removal under heightened ammonia pressure compared to the conventional AnMBR (CG-AnMBR). This enhancement primarily stemmed from ion-exchange interactions between the functional groups (hydroxyl, carboxyl, ester, and aldehyde groups) on the biochar surface and NH4+-N, serving as the primary mechanism of action. Moreover, concerning resource recovery, the BC-AnMBR sustained a standard methane yield of 0.184 LCH4/gCOD, surpassing that of the CG-AnMBR by more than threefold. Microbial community analysis unveiled that the BC-AnMBR fostered the enrichment of ammonia-tolerant electroactive methanogenic archaea, notably from the genera Methanosarcina and Methanolinea. Notably, up-regulation of functional genes associated with key enzymes involved in propionic and butyric acid degradation and the autotrophic methanogenic pathway was observed in the BC-AnMBR, consequently accelerating methane production rates. Ultimately, the incorporation of biochar amplified the activity of the microbial electron transport system by 41.77 % and boosted the concentration of c-type cytochrome by 50.6 %. These enhancements facilitated the establishment of direct interspecies electron transfer, ensuring the stability of the anaerobic digestion process under ammonia-inhibited conditions.

Converting Organic Municipal Solid Waste Into Volatile Fatty Acids and Biogas: Experimental Pilot and Batch Studies With Statistical Analysis

Background

Italy can augment its profit from biorefinery products by altering the operation of digesters or different designs to obtain more precious bioproducts like volatile fatty acids (VFAs) than biogas from organic municipal solid waste. In this context, recognizing the process stability and outputs through operational interventions and its technical and economic feasibility is a critical issue. Hence, this study involves an anaerobic digester in Treviso in northern Italy.

Objective

This research compares a novel line, consisting of pretreatment, acidogenic fermentation, and anaerobic digestion, with single-step anaerobic digestion regarding financial profit and surplus energy. Therefore, a mass flow model was created and refined based on the outputs from the experimental and numerical studies. These studies examine the influence of hydraulic retention time (HRT), pretreatment, biochar addition, and fine-tuned feedstock/inoculum (FS/IN) ratio on bioproducts and operational parameters.

Methods

VFA concentration, VFA weight ratio distribution, and biogas yield were quantified by gas chromatography. A t test was then conducted to analyze the significance of dissimilar HRTs in changing the VFA content. Further, a feasible biochar dosage was identified for an assumed FS/IN ratio with an adequately long HRT using the first-order rate model. Accordingly, the parameters for a mass flow model were adopted for 70,000 population equivalents to determine the payback period and surplus energy for two scenarios. We also explored the effectiveness of amendments in improving the process kinetics.

Results

Both HRTs were identical concerning the ratio of VFA/soluble chemical oxygen demand (0.88 kg/kg) and VFA weight ratio distribution: mainly, acetic acid (40%), butyric acid (24%), and caproic acid (17%). However, a significantly higher mean VFA content was confirmed for an HRT of 4.5 days than the quantity for an HRT of 3 days (30.77, SD 2.82 vs 27.66, SD 2.45 g-soluble chemical oxygen demand/L), using a t test (t8=-2.68; P=.03; CI=95%). In this research, 83% of the fermented volatile solids were converted into biogas to obtain a specific methane (CH4) production of 0.133 CH4-Nm3/kg-volatile solids. While biochar addition improved only the maximum methane content by 20% (86% volumetric basis [v/v]), the FS/IN ratio of 0.3 volatile solid basis with thermal plus fermentative pretreatment improved the hydrolysis rate substantially (0.57 vs 0.07, 1/d). Furthermore, the biochar dosage of 0.12 g-biochar/g-volatile solids with an HRT of 20 days was identified as a feasible solution. Principally, the payback period for our novel line would be almost 2 years with surplus energy of 2251 megajoules [MJ] per day compared to 45 years and 21,567 MJ per day for single-step anaerobic digestion.

Conclusions

This research elaborates on the advantage of the refined novel line over the single-step anaerobic digestion and confirms its financial and technical feasibility. Further, changing the HRT and other amendments significantly raised the VFA concentration and the process kinetics and stability.

Use of aqueous liquor from digested sludge pyrolysis for biogas production: characterization, toxicity assessment, and rate-limiting step determination

This study assessed the characteristics and toxicity of aqueous pyrolytic liquid (APL) derived from digested sewage sludge on anaerobic digestion (AD) and determined its rate-limiting step. Digested sewage sludge was pyrolyzed at multiple temperatures (350-650 °C) and moisture levels (0-40.4 %), resulting in APLs with varying AD toxicities. APL 350 °C-0 % showed the least toxicity, whereas APL 650 °C-40.4 % exhibited the greatest toxicity. Glucose (GL) and sodium acetate (SA) were introduced to elucidate the rate-limiting steps. SA, but not GL, enhanced APL conversion to CH4. And volatile fatty acid lack was observed in treatments without SA addition. This suggested that acidification was the primary rate-limiting step. This finding was confirmed using the modified Gompertz model: SA considerably improved the maximum methane production rate, whereas GL did not. Insights gained from this research clarified the feasibility and potential of AD for APL utilization and conversion.

Biochar confers significant microbial resistance to ammonia toxicity in n-caproic acid production

Microbial chain elongation integrating innovative bioconversion technologies with organic waste utilization can transition current energy-intensive n-caproic acid production to sustainable circular bioeconomy systems. However, ammonia-rich waste streams, despite their suitability, pose inhibitory challenges to these bioconversion processes. Herein, biochar was employed as an additive to enhance the activity of chain elongating microbes under ammonia inhibition conditions, with an objective to detail underlying mechanisms of improvements. Biochar addition significantly improved chain elongation performance even under severe ammonia stress (exceeding 8 g N/L), increasing n-caproic acid yields by 40 % to 158 % and reducing lag times by 51 % to 90 %, compared with the best-performing group without biochar addition. The material contribution to n-caproic production reached up to 94.3 % (at 4 g N/L). These enhancements were mainly attributed to the new electron syntrophy induced by biochar, which improved electron transfer system activity and electrical conductivity of the fermentation system. This is further substantiated by increased relative abundances of the genus Sporanaerobacter, electroactive bacteria, and up-regulated direct electron transfer-related genes including conductive pili and c-type cytochrome. This study demonstrates that biochar can confer robust resilience to ammonia toxicity in functional microbes, paving a way for efficient and sustainable n-caproic acid production.

Integration of Digestate-Derived Biochar into the Anaerobic Digestion Process through Circular Economic and Environmental Approaches-A Review

The growing energy consumption and the need for a circular economy have driven considerable interest in the anaerobic digestion (AD) of organic waste, offering potential solutions through biogas and digestate production. AD processes not only have the capability to reduce greenhouse gas emissions but also contribute to the production of renewable methane. This comprehensive review aims to consolidate prior research on AD involving different feedstocks. The principles of AD are explored and discussed, including both chemical and biological pathways and the microorganisms involved at each stage. Additionally, key variables influencing system performance, such as temperature, pH, and C/N ratio are also discussed. Various pretreatment strategies applied to enhance biogas generation from organic waste in AD are also reviewed. Furthermore, this review examines the conversion of generated digestate into biochar through pyrolysis and its utilization to improve AD performance. The addition of biochar has demonstrated its efficacy in enhancing metabolic processes, microorganisms (activity and community), and buffering capacity, facilitating Direct Interspecies Electron Transfer (DIET), and boosting CH4 production. Biochar also exhibits the ability to capture undesirable components, including CO2, H2S, NH3, and siloxanes. The integration of digestate-derived biochar into the circular economy framework emerges as a vital role in closing the material flow loop. Additionally, the review discusses the environmental benefits derived from coupling AD with pyrolysis processes, drawing on life cycle assessment investigations. Techno-economic assessment (TEA) studies of the integrated processes are also discussed, with an acknowledgment of the need for further TEA to validate the viability of integrating the biochar industry. Furthermore, this survey examines the techno-economic and environmental impacts of biochar production itself and its potential application in AD for biogas generation, aiming to establish a more cost-effective and sustainable integrated system.

Coupling of biogas residue biochar and low-magnitude electric fields promotes anaerobic co-digestion of sewage sludge and food waste

Biochar-assisted anaerobic digestion (AD) remains constrained due to the inefficient decomposition of complex organics, even with the direct interspecies electron transfer (DIET) pathway. The coupling of electrochemistry with the anaerobic biological treatment could shorten lengthy retention time in co-digestion by improving electron transfer rates and inducing functional microbial acclimation. Thus, this work investigated the potential of improving the performance of AD by coupling low-magnitude electric fields with biochar derived from the anaerobically digested biogas residue. Different voltages (0.3, 0.6, and 0.9 V) were applied at various stages to assess the impact on biochar-assisted AD. The results indicate that an external voltage of 0.3 V, coupled with 5 g/L of biochar, elevates CH4 yield by 45.5% compared to biogas residue biochar alone, and the coupled approach increased biogas production by up to 143% within 10 days. This finding may be partly explained by the enhanced utilization of substrates and the increased amounts of specific methanogens such as Methanobacterium and Methanosarcina. The abundance of the former increased from 4.0 to 11.3%, which enhances the DIET between microorganisms. Furthermore, the coupling method shows better potential for enhancing AD compared to preparing iron-based biochar, and these results present potential avenues for its broader applications.

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.

Revealing the Adverse Impact of Additive Carbon Material on Microorganisms and Its Implications for Biogas Yields: A Critical Review

Biochar could be a brilliant additive supporting the anaerobic fermentation process. However, it should be taken into account that in some cases it could also be harmful to microorganisms responsible for biogas production. The negative impact of carbon materials could be a result of an overdose of biochar, high biochar pH, increased arsenic mobility in the methane fermentation solution caused by the carbon material, and low porosity of some carbon materials for microorganisms. Moreover, when biochar is affected by an anaerobic digest solution, it could reduce the biodiversity of microorganisms. The purpose of the article is not to reject the idea of biochar additives to increase the efficiency of biogas production, but to draw attention to the properties and ways of adding these materials that could reduce biogas production. These findings have practical relevance for organizations seeking to implement such systems in industrial or local-scale biogas plants and provide valuable insights for future research. Needless to say, this study will also support the implementation of biogas technologies and waste management in implementing the idea of a circular economy, further emphasizing the significance of the research.

Evaluation of activated pyrochar for boosting anaerobic digestion: Performances and microbial community

In this study, the effects of CO2-activated/non-activated pyrochars (PCs) from cornstalk, cotton straw, and rice straw on anaerobic digestion (AD) performances and microbial characteristics were investigated. The maximum biogas production rate (2.2 L/L/d) with a methane content of 73% was obtained from the AD with CO2-activated cotton straw PC. The activated PC mainly played a strengthening role in the early and middle stages of AD. Specifically, the cornstalk PC could greatly relieve acid inhibition, and cotton straw PC had a significantly positive effect on the regulation of ammonia nitrogen concentration. The rare genera like Verrucomicrobia had obvious differences among groups of AD with PCs. Regarding differential metabolites, cornstalk PC-N2 displayed a positive correlation with isoleucyl-alanine, while exhibiting a negative correlation with deoxyinosine, and the corresponding relative expression levels were + 3.0 and -2.4, respectively. Overall, gas-activated PCs could promote methane production and affect the composition of microbial community.

Impacts of lignite and biochar addition on anaerobic digestion of and fertilizer production from dairy manure

Anaerobic digestion (AD) is one of the most popular technologies to convert organic residues into renewable energy. The use of additives has been shown to increase the process stability and degradation efficiency during AD. However, little attention has been paid to the associated fertilizer production. Thus, this study aims to simultaneously investigate the impact of biochar and lignite on AD and fertilizer production of dairy manure in semi-continuous experiments. Results show that compared to the biogas production rate (BPR) of the reference (394.01 ± 0.39 lN/(kgVS∙d), lignite can increase BPR by 4.98 %, whereas no impact on BPR was detected due to biochar addition. This indicates that higher O/C and H/C ratios attributed to lignite are a dominant factor influencing BPR, while high specific surface is not beneficial. Nitrogen yield in the solid phase was increased by 19.47 % (lignite) and 14.65 % (biochar), significantly promoting the utilization of digestate as solid nitrogen fertilizer.

Influence of Pre-Incubation of Inoculum with Biochar on Anaerobic Digestion Performance

The application of biochar as an additive to enhance the anaerobic digestion (AD) of biomass has been extensively studied from various perspectives. This study reported, for the first time, the influence of biochar incubation in the inoculum on the anaerobic fermentation of glucose in a batch-type reactor over 20 days. Three groups of inoculum with the same characteristics were pre-mixed once with biochar for different durations: 21 days (D21), 10 days (D10), and 0 days (D0). The BC was mixed in the inoculum at a concentration of 8.0 g/L. The proportion of the inoculum and substrate was adjusted to an inoculum-to-substrate ratio of 2.0 based on the volatile solids. The results of the experiment revealed that D21 had the highest cumulative methane yield, of 348.98 mL, compared to 322.66, 290.05, and 25.15 mL obtained from D10, D0, and the control, respectively. Three models-modified Gompertz, first-order, and Autoregressive Integrated Moving Average (ARIMA)-were used to interpret the biomethane production. All models showed promising fitting of the cumulative biomethane production, as indicated by high R2 and low RMSE values. Among these models, the ARIMA model exhibited the closest fit to the actual data. The biomethane production rate, derived from the modified Gompertz Model, increased as the incubation period increased, with D21 yielding the highest rate of 31.13 mL/gVS. This study suggests that the application of biochar in the anaerobic fermentation of glucose, particularly considering the short incubation period, holds significant potential for improving the overall performance of anaerobic digestion.

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.

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.

Influence of nano-Fe3O4 biochar on the methanation pathway during anaerobic digestion of chicken manure

Volatile fatty acids and ammonia nitrogen (AN) accumulate during anaerobic digestion (AD) of high N substrates, such as chicken manure (CM), causing decreases in methane yield. Previous research found that the addition of nano-Fe₃O₄ biochar can alleviate the inhibition caused by acids and ammonia and increase methane production. The mechanism of enhanced methane production in nano-Fe₃O₄ biochar-mediated AD of CM was explored in depth in this study. The results showed the lowest AN concentration in the control and nano-Fe₃O₄ biochar addition groups were 8,229.0 mg/L and 7,701.5 mg/L, respectively. Methane yield of volatile solids increased from 92.0 mL/g to 219.9 mL/g in the nano-Fe₃O₄ biochar treatment, which was attributed to the enrichment of unclassified Clostridiales and Methanosarcina. The mechanism of nano-Fe₃O₄ biochar in AD of CM under high AN level was to improve methane production by promoting syntrophic acetate oxidation and facilitating direct electron transfer between microorganisms.

Anaerobic digestion of sulphate wastewater mediated by biochar

In this paper, the influences of biochar on the anaerobic digestion of sulphate wastewater, including the COD removal rate, methane yield, intermediate products and the change of microbial community structure, were investigated. The results showed that sulphate could promote the anaerobic digestion with the SO₄^2-/COD ratio increasing from 0 to 0.1, while the activity of MPB was inhibited, which led to the decrease of COD removal rate and methane yield with the SO₄^2-/COD ratio increasing from 0.1 to 2. At 1 g biochar loading, 344.97 mL CH₄/gCOD_removal was obtained compared with the control group (220.70 CH₄/gCOD_removal) at 2 of SO₄^2-/COD. Biochar could also reduce the secondary accumulation of NH₄⁺-N and TVFA. Meanwhile, methanogenic microorganisms were selectively enriched especially for methanobacterium, methanosaeta and methanolinea, while the growth of SRB was inhibited with biochar addition.

Life Cycle Assessment (LCA) of Biochar Production from a Circular Economy Perspective

Climate change and environmental sustainability are among the most prominent issues of today. It is increasingly fundamental and urgent to develop a sustainable economy, capable of change the linear paradigm, actively promoting the efficient use of resources, highlighting product, component and material reuse. Among the many approaches to circular economy and zero-waste concepts, biochar is a great example and might be a way to push the economy to neutralize carbon balance. Biochar is a solid material produced during thermochemical decomposition of biomass in an oxygen-limited environment. Several authors have used life cycle assessment (LCA) method to evaluate the environmental impact of biochar production. Based on these studies, this work intends to critically analyze the LCA of biochar production from different sources using different technologies. Although these studies reveal differences in the contexts and characteristics of production, preventing direct comparison of results, a clear trend appears. It was proven, through combining life cycle assessment and circular economy modelling, that the application of biochar is a very promising way of contributing to carbon-efficient resource circulation, mitigation of climate change, and economic sustainability.

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.

Comparison of pyrochar, hydrochar and lignite as additive in anaerobic digestion and NH4+ adsorbent

The impact of pyrochar, hydrochar and lignite addition on anaerobic digestion of food waste was investigated with and without ammonia inhibition under batch conditions. Furthermore, ammonium adsorption capacities of the chars were investigated. To determine anaerobic degradation of char, reference samples containing inoculum and char were analyzed, indicating a significant degradation of hydrochar. Depending on the evaluation method, the increase in methane yield due to hydrochar addition varied between no statistically significant difference and +14 %. No significant impact due to the addition of 5 g/l pyrochar and lignite on AD was found. NH₄⁺ adsorption capacities showed a significantly higher net adsorption capacity of lignite (1.58mg_NH4⁺/g_L), compared to pyrochar (0.63mg_NH4⁺/g_PC). A negative NH₄⁺ adsorption capacity (-0.51 mg_NH4⁺/g_HC) was found for hydrochar. A high H/C-ratio, O/C-ratio and cation exchange capacity of hydrochar and lignite indicate many functional groups and low chemical stability, enabling an increased interaction between NH₄⁺ and char.

Biochar amended microbial conversion of C1 gases to ethanol and butanol: Effects of biochar feedstock type and processing temperature

Biochar feedstock and production method affects its physicochemical properties and subsequent application. This study investigated the effects of biochar from switchgrass (SGB) and poultry litter (PLB) produced at 350 and 700 °C on alcohol formation using CO:CO₂:H₂ (40:30:30) by Clostridium carboxidivorans (P7) and C. ragsdalei (P11). Fermentations were performed in 250 mL bottles with a 50 mL working volume at 37 °C. Strains P7 and P11 produced 1.2- to 2.2-fold more alcohol and consumed 1.2- to 1.9-fold more syngas using biochars made at 700 °C compared to 350 °C. Both strains also produced 1.4- to1.9-fold more alcohol with both biochars made at 700 °C compared to control without biochar. Strain P11 produced 1.1- and 1.6-fold more alcohol and fatty acids, respectively, in medium with PLB made at 700 °C compared to strain P7. These results provide guidance towards the selection of biochar type and production temperature to improve syngas fermentation.

Role of interspecies electron transfer stimulation in enhancing anaerobic digestion under ammonia stress: Mechanisms, advances, and perspectives

Ammonia stress is a commonly encountered issue in anaerobic digestion (AD) process when treating proteinaceous substrates. The enhanced relationship between syntrophic bacteria and methanogens triggered by interspecies electron transfer (IET) stimulation is one of the potential mechanisms for an improved methane yield from the AD plant under ammonia-stressed condition. There is, however, lack of synthesized information on the mechanistic understanding of IET facilitation in the ammonia-stressed AD processes. This review critically discusses recovery of AD system from ammonia-stressed condition, focusing on H₂ transfer, redox compound-mediated IET, and conductive material-induced direct IET. The effects and the associated mechanisms of IET stimulation on mitigating ammonia stress and promoting methanogenesis were elucidated. Finally, prospects and challenges of IET stimulation were critically discussed. This review highlights, for the first time, the critical role of IET stimulation in enhancing AD process under ammonia-stressed condition.

Enhancement of anaerobic fermentation with corn straw by pig bone-derived biochar

Different sources of biochar exhibit different effects on anaerobic fermentation. Here, the effects of activation temperature, activation time, impregnation ratio, and pickling times on the properties of pig bone-derived biochar additives were explored by orthogonal experiments. The pig bone-derived biochar with better performance was optimized to enhance the anaerobic fermentation. The results showed that when the preparation conditions of biochar were as follows: activation temperature of 700 °C, impregnation ratio of 2, activation time of 90 min, and pickling times of 2, the cumulative methane production of corn stalk by anaerobic fermentation exhibited the highest value of 164.54 mL/g VS, which was 68% higher than the control group. The correlation between the characteristics of biochar for promoting anaerobic fermentation and the performance of anaerobic fermentation was established. Interestingly, the pig bone-derived biochar can buffer pH value in straw anaerobic fermentation.

Enhanced Production of Biogas Using Biochar-Sulfur Composite in the Methane Fermentation Process

The methane fermentation of organic waste is one way to minimize organic waste, which accounts for 77% of the global municipal waste stream. The use of biochar as an additive for methane fermentation has been shown to increase the production potential of biogas. Sulfur waste has a potential application to synergistic recycling in a form of composites with other materials including biochar. A composite product in the form of a mixture of biochar and molten sulfur has been proposed. In this experiment, additions of the sulfur−biochar composite (SBC) were tested to improve the fermentation process. The biochar was produced from apple chips under the temperature of 500 °C. The ground biochar and sulfur (<1 mm particle size) were mixed in the proportion of 40% biochar and 60% sulfur and heated to 140 °C for sulfur melting. After cooling, the solidified composite was ground. The SBC was added in the dose rate of 10% by dry mass of prepared artificial kitchen waste. Wet anaerobic digestion was carried out in the batch reactors under a temperature of 37 °C for 21 days. As an inoculum, the digestate from Bio-Wat Sp. z. o. o., Świdnica, Poland, was used. The results showed that released biogas reached 672 mL × gvs−1, and the yield was 4% higher than in the variant without the SBC. Kinetics study indicated that the biogas production constant rate reached 0.214 d−1 and was 4.4% higher than in the variant without the SBC.

Increasing Anaerobic Digestion Efficiency Using Food-Waste-Based Biochar

The efficiency of methane production by anaerobic digestion (AD), during which energy is generated from organic waste, can be increased in various ways. Recent research developments have increased the volume of gas production during AD using biochar. Previous studies have used food waste itself in AD, or, added wood-biochar or sewage sludge charcoal as an accelerant of the AD process. The application of food-waste biochar in AD using activated sludge has not yet been studied and is considered a potential method of utilizing food waste. Therefore, this study investigated the use of biochar prepared by the thermal decomposition of food waste as an additive to AD tanks to increase methane production. The addition of food-waste biochar at 1% of the digestion tank volume increased the production of digestion gas by approximately 10% and methane by 4%. We found that food-waste biochar served as a medium with trace elements that promoted the proliferation of microorganisms and increased the efficiency of AD.

Integration of Hydrothermal Carbonisation and Anaerobic Digestion for the Energy Valorisation of Grass

The integration of hydrothermal carbonisation (HTC) and anaerobic digestion (AD) can overcome some of the disadvantages of thermal or biological processing alone. This study aims to investigate integrated HTC-AD across a range of integration strategies and HTC processing temperatures (150 °C, 200 °C and 250 °C) to improve the energy conversion efficiency (ECE) of grass, compared to AD alone. The separation of hydrochars (HCs) for combustion and process waters (PWs) for digestion appears to be the most energetically feasible HTC-AD integration strategy, compared to HC or HTC-slurry AD. Hydrochars represent the greater energy carrier with between 81–85% of total energy output. The ECE of grass was improved from 51% to 97% (150 °C), 83% (200 °C) and 68% (250 °C) through integrated HTC-AD. Therefore, lower HTC processing temperatures yield more favourable energetics. However, higher HTC temperatures favour more desirable HC properties as a combustion fuel. The hydrochar produced at 250 °C (HC-250) displayed the highest HHV (25.8 MJ/kg) and fixed carbon: volatile matter ratio (0.47), as well as the greatest reduction in slagging and fouling potential (ash flow temperature > 1550 °C). Overall, integrated HTC-AD is an effective energy valorisation strategy for grass. A compromise exists between the quality of hydrochar and the energetic balance. However, at 250 °C the process remains energetically feasible (EROI = 2.63).

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.

The absolute concentration and bioavailability of trace elements: Two vital parameters affecting anaerobic digestion performance of chicken manure leachate

It is currently unclear whether trace elements (TEs) deficiency is due to low bioavailability or low absolute TEs concentrations, especially in high-pH anaerobic digestion (AD) systems. A mixed solution of TEs and EDTA-Na₂ were used separately in mono-AD of chicken manure (CM) leachate to investigate this research gap. The results showed relatively low bioavailability of Fe, Mn, and Zn. The bioavailability of all TEs remained stable along with a gradual increase in total ammonia nitrogen concentration. Both TE and EDTA-Na₂ supplementation improved the bioavailability of TEs, but TEs supplementation also gave a high proportion of soluble TEs. Adding TEs improved methane production efficiency (+38.3%) and decreased the H₂S content. The exchangeable fraction of specific TE (Mo) in H₂/CO₂ pathway was higher in the TEs treatment. TEs bioavailability and absolute concentrations of available TEs are critical aspects that need to be scrutinized to assess the risk of TEs deficiency.

The Influence of Biochar Augmentation and Digestion Conditions on the Anaerobic Digestion of Water Hyacinth

The augmentation of biochar (BC) during anaerobic digestion (AD) has been identified as a potential strategy for improving the AD of complex feedstocks. This study evaluates the influence of oak wood biochar 450 °C and fermentation conditions during the AD of the invasive aquatic plant, water hyacinth (WH). Factorial 22 design of experiments (DOE) allowed the evaluation of the effect of the crucial processing conditions, inoculum-to-substrate ratio (ISR) and biochar load. Further optimisation was performed to identify the best processing conditions for the AD of WH, at an ideal ISR of 1. The contour plots suggested that methane yield is favoured at biochar loads of ≤0.5%, whereas the production rate is favoured by increasing biochar loads. However, biochar addition offered no further improvement or significant effect on the digestion of WH. The subsequent AD of WH samples collected from different locations in India and Uganda exhibited variable biochemical methane potential (BMP) yields. BC addition had little effect on BMP performance, and in some cases, it even reduced the BMP. This study concludes that the amendment potential of biochar is influenced by digestion conditions and the substrate, particularly when working with complex substrates.

Stimulatory effects of biochar addition on dry anaerobic co-digestion of pig manure and food waste under mesophilic conditions

The stimulatory effect of biochar addition on dry anaerobic digestion (AD) has been rarely investigated. In this study, the effects of commonly used biochars (bamboo, rice husk, and pecan shell) on dry co-AD were investigated using mesophilic batch digesters fed with pig manure and food waste as substrates. The results show that the specific methane yield was mildly elevated with the addition of biochars by 7.9%, 9.4%, and 12.0% for bamboo, rice husk, and pecan shell-derived biochar additions, respectively. Biochar did facilitate the degradation of poorly biodegradable organics. In comparison, there was no significant effect on the peak methane production rate by the supplementation of the selected biochars. Among the three mechanisms of enhancing methanogenesis by biochar (buffering, providing supporting surface, and enhancing electron transfer), the first two mechanisms did not function significantly in dry co-AD, while the third mechanism (i.e., enhancing electron transfer) might play an important part in dry AD process. It is recommended that the utilization of biochar for the enhancement of biomethanation in dry AD should be more focused on mono digestion in future studies.

Sustainable additives for the regulation of NH3 concentration and emissions during the production of biomethane and biohydrogen: A review

This study reviews the recent advances and innovations in the application of additives to improve biomethane and biohydrogen production. Biochar, nanostructured materials, novel biopolymers, zeolites, and clays are described in terms of chemical composition, properties and impact on anaerobic digestion, dark fermentation, and photofermentation. These additives can have both a simple physical effect of microbial adhesion and growth, and a more complex biochemical impact on the regulation of key parameters for CH₄ and H₂ production: in this study, these effects in different experimental conditions are reviewed and described. The considered parameters include pH, volatile fatty acids (VFA), C:N ratio, and NH₃; additionally, the global impact on the total production yield of biogas and bioH₂ is reviewed. A special focus is given to NH₃, due to its strong inhibition effect towards methanogens, and its contribution to digestate quality, leaching, and emissions into the atmosphere.

Influence of augmentation of biochar during anaerobic co-digestion of Chlorella vulgaris and cellulose

The anaerobic co-digestion (AcoD) of microalgae is a prospective option for generating biomethane from renewable sources. This study investigates the effects of inoculum-to-substrate ratio (ISR), C/N ratio and biochar (BC) load on the AcoD of Chlorella vulgaris and cellulose. An initial augmentation of BC at ISR 0.5-0.9 and C/N ratio 10-30 offered a pH buffering effect and resulted in biomethane yields of 233-241 mL CH₄/g VS, corresponding to 1.8-4.6 times the controls. BC addition ameliorated significantly AcoD, supporting the digestate stability at less favourable conditions. The effect of the process variables was further studied with a 2³ factorial design and response optimisation. Under the design conditions, the variables had less influence over methane production. Higher ISRs and C/N ratios favoured AcoD, whereas increasing amounts of BC reduced biomethane yield but enhanced production rate. The factorial design highlighted the importance of BC-load on AcoD, establishing an optimum of 0.58 % (w/v).

Bio-based carbon materials with multiple functional groups and graphene structure to boost methane production from ethanol anaerobic digestion

This study evaluated the effects of bio-based carbon materials on methane production by anaerobic digestion. The results showed that biochar and hydrochar can promote cumulative methane yield by 15% to 29%. However, there was no statistical significance (p > 0.05) between hydrochar and biochar produced at different temperature on methane production. 16S rRNA gene sequencing and bioinformatics analysis showed that biochar and hydrochar enriched microorganism that might participate in direct interspecies electron transfer (DIET) such as Pseudomonadaceae, Bacillaceae, and Clostridiaceae. The the surface properties of the modified biochar were characterized with BET, Raman, FTIR and XPS. Bio-based carbon materials with uniform dispersion provided a stable environment for the DIET of microorganisms and electrons are transferred through aromatic functional groups on the surface of materials. This study reveals bio-based carbon materials surface properties on methane production in anaerobic digestion and provides a new approach to recycling spent coffee grounds.

An original Arduino-controlled anaerobic bioreactor packed with biochar as a porous filter media

Bioreactors are commonly used apparatuses generally equipped with several built-in specifications for the investigation of biological treatment studies. Each bioreactor test may require different types of specialty such as heating, agitation, re-circulation and some further technologies like online sensoring. Even thought, there are many ready-to-use fabricated bioreactors available in the market with a cost usually over than 1000 €, it is often not possible to access those advanced (but inflexible) systems for many students, young-researchers or small-scale private R&D companies. In this work, a new low cost (≈100€) packed-bed anaerobic bioreactor was developed, and all methodological details including open-source coding and 3D design files are shared with informative descriptions. Some preliminary tests were conducted to verify the developed bioreactor system's credibility in terms of leak-tightness, accurate gas monitoring, temperature controlling, and mass balance (COD-eq) coverage, which all have shown a very promising performance.•

A consistent model bioreactor that will be called as “tetrapod” was developed for anaerobic treatment of challenging substrates such as pyrolytic liquids.

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Coarse biochar grains were used as an organic packing material to stimulate the microbial bioconversion by increasing the active surface area for the attached-growth anaerobic mixed microbial culture (MMC).

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An open-source Arduino based digital gasometer was developed for online monitoring of biogas change in the lab-scale system. Arduino was also used as a digital controller for maintaining pulse-mode liquid recirculation of the bioreactor.

Valorization of oat husk by hydrothermal carbonization: Optimization of process parameters and anaerobic digestion of spent liquors

The hydrothermal carbonization (HTC) optimization of oat husk was performed using a response surface methodology. Furthermore, anaerobic digestion (AD) of spent liquor and hydrochar addition were evaluated in the biomethane potential (BMP) test. Results found that temperature influences the most in the studied responses (i.e., mass yield (MY) and higher heating value (HHV)). Optimal hydrochar MY (53.8%) and HHV (21.5 MJ/kg) were obtained for 219.2 °C, 30 min, and 0.08 of biomass/water ratio. A successful prediction capability of the optimization approach was observed, archiving an error < 1% between predicted and validated responses. The BMP experiment showed the feasibility of spent liquor as a potential substrate to be treated by AD (144 NmLCH₄/gCOD). Hydrochar boosted the methane production of spent liquor increasing up to 17% compared to digestion with no hydrochar addition. These findings provide new insights regarding oat husk valorization by integrating HTC and AD for energy production.

Sustainable remediation of hazardous environmental pollutants using biochar-based nanohybrid materials

Biochar is a well-known carbon material with diversified functionalities and excellent physicochemical characteristics with high wastewater treatment potential. This review aims to summarize recent advancements in the development of biochar and biochar-based nanohybrid materials as a potential tool for the removal of harmful organic compounds such as synthetic dyes/effluents. The formation of biochar using pyrolysis of renewable feedstocks and their applications in various industries are explained hereafter. The characteristics and construction of biochar-based hybrid materials are explained in detail. Diversity of feedstocks, including municipal wastes, industrial byproducts, agricultural, and forestry residues, endows different biochar types with a wide structural variety. The production of cost-effective biochar drives the interest in manipulating biochars and induces desire functionality using nanoscale reinforcements. Various types of biochars, such as magnetic biochar, layered nanomaterial coated biochar, nanometallic oxide composites, chemically and physically functionalized biochar, have been produced. With the aid of nanomaterial, hybrid biochar exhibits a high potential to remove toxic contaminants. Depending upon biochar type, dyes/effluents can be removed via different mechanisms, including the Fenton process, photocatalytic degradation, π-π interaction, electrostatic interaction, and physical adsorption. In conclusion, desired physicochemical features, and tunable surface properties of biochar present high potential material in removing organic dyes and other effluents. The blended biochar with different materials/nanomaterials endows broader development and multi-functional opportunities for treating dyes/effluents.

Biological conversion of carbon monoxide and hydrogen by anaerobic culture: Prospect of anaerobic digestion and thermochemical processes combination

Waste biomass is considered a promising renewable energy feedstock that can be converted by anaerobic digestion. However, anaerobic digestion application can be challenging due to the structural complexity of several waste biomass kinds. Therefore, coupling anaerobic digestion with thermochemical processes can offset the limitations and convert the hardly biodegradable waste biomass, including digestate residue, into value-added products: syngas and pyrogas (gaseous mixtures consisting mainly of H₂, CO, CO₂), bio-oil, and biochar for further valorisation. In this review, the utilisation boundaries and benefits of the aforementioned products by anaerobic culture are discussed. First, thermochemical process parameters for an enhanced yield of desired products are summarised. Particularly, the microbiology of CO and H₂ mixture biomethanation and fermentation in anaerobic digestion is presented. Finally, the state-of-the-art biological conversion of syngas and pyrogas to CH₄ mediated by anaerobic culture is adequately described. Extensive research shows the successful selective biological conversion of CO and H₂ to CH₄, acetic acid, and alcohols. The main bottleneck is the gas-liquid mass transfer which can be enhanced appropriately by bioreactors' configurations. A few research groups focus on bio-oil and biochar addition into anaerobic digesters. However, according to the literature review, there has been no research for utilising all value-added products at once in anaerobic digestion published so far. Although synergic effects of such can be expected. In summary, the combination of anaerobic digestion and thermochemical processes is a promising alternative for wide-scale waste biomass utilisation in practice.

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.

Improvement of Hydrogen Production during Anaerobic Fermentation of Food Waste Leachate by Enriched Bacterial Culture Using Biochar as an Additive

It has become urgent to develop cost-effective and clean technologies for the rapid and efficient treatment of food waste leachate, caused by the rapid accumulation of food waste volume. Moreover, to face the energy crisis, and to avoid dependence on non-renewable energy sources, the investigation of new sustainable and renewable energy sources from organic waste to energy conversion is an attractive option. Green energy biohydrogen production from food waste leachate, using a microbial pathway, is one of the most efficient technologies, due to its eco-friendly nature and high energy yield. Therefore, the present study aimed to evaluate the ability of an enriched bacterial mixture, isolated from forest soil, to enhance hydrogen production from food waste leachate using biochar. A lab-scale analysis was conducted at 35 °C and at different pH values (4, no adjustment, 6, 6.5, 7, and 7.5) over a period of 15 days. The sample with the enriched bacterial mixture supplemented with an optimum of 10 g/L of biochar showed the highest performance, with a maximum hydrogen yield of 1620 mL/day on day three. The total solid and volatile solid removal rates were 78.5% and 75% after 15 days, respectively. Acetic and butyrate acids were the dominant volatile fatty acids produced during the process, as favorable metabolic pathways for accelerating hydrogen production.

A critical review on the application of biochar in environmental pollution remediation: Role of persistent free radicals (PFRs)

Biochar as an emerging carbonaceous material has exhibited a great potential in environmental application for its perfect adsorption ability. However, there are abundant persistent free radicals (PFRs) in biochar, so the direct and indirect PFRs-mediated removal of organic and inorganic contaminants by biochar was widely reported. In order to comprehend deeply the formation of PFRs in biochar and their interactions with contaminants, this paper reviews the formation mechanisms of PFRs in biochar and the PFRs-mediated environmental applications of biochar in recent years. Finally, future challenges in this field are also proposed. This review provides a more comprehensive understanding on the emerging applications of biochar from the viewpoint of the catalytic role of PFRs.

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

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

Comprehensive evaluation of different strategies to recover methanogenic performance in ammonia-stressed reactors

In this study, strategies for recovery of ammonia-stressed AD reactors were attempted, by addition of preserved bioaugmentation consortium in gel (BioG), fresh consortium in liquid medium (BioL), woodchip biochar (BW), and straw biochar (BS). In comparison to control group with ammonia, effective treatments, i.e., BioG, BioL, BW and BS raised the maximum methane production rate by 77%, 23%, 35%, and 24%, respectively. BW possibly acted as interspecies electrical conduits for Direct Electron Transfer based on conductivity and SEM analysis. BioG facilitated slow release of bioaugmentation inocula from gel into the AD system, which protected them from a direct environmental shock. According to microbial analysis, both BioG, BioL and BW resulted in increased relative abundance of Methanothermobacter thermautotrophicus; and BS induced selective raise of Methanosarcina thermophila. The increase of methanogens via these strategies led to the faster recovery of the AD process.

Improving nutrients removal and energy recovery from wastes using hydrochar

Hydrothermal carbonization (HTC) is an eco-friendly, flexible and efficient way to valorise wet solid wastes, producing a carbon-rich material named as hydrochar. Considerable efforts have been devoted to studying the feasibility of using hydrochar in waste management to achieve the goal of circular economy. However, a comprehensive evaluation of the impacts of hydrochar on energy recovery from anaerobic digestion (AD), nutrient reclamation, and wastewater treatment is currently lacking. To understand the influence of hydrochar type on its application, this review will firstly introduce the mechanisms and biomass treatment for hydrochar preparation. Most recent studies regarding the improvement of methane (CH₄) and volatile fatty acids (VFAs) production after dosing hydrochar in anaerobic digesters are quantitatively summarized and deeply discussed. The potential of using various hydrochar as slow-fertilizer to support the growth of plants are analysed by providing quantitative data. The usage of hydrochar in remediating pollutants from wastewater as effective adsorbent is also evaluated. Based on the review, we also address the challenges and demonstrate the opportunities for the future application of hydrochar in waste management. Conclusively, this review will not only provide a systematic understanding of the up-to-date developments of improving the nutrients removal and energy recovery from wastes by using hydrochar but also several new directions for the application of hydrochar in the future.

Biochar utilisation in the anaerobic digestion of food waste for the creation of a circular economy via biogas upgrading and digestate treatment

A wood waste-derived biochar was applied to food-waste anaerobic digestion to evaluate the feasibility of its utilisation to create a circular economy. This biochar was first purposed for the upgrading of the biogas from the said anaerobic digestion, before treating and recovering the nutrients in the solid fraction of the digestate, which was finally employed as a biofertilizer for the organic cultivation of three green leafy vegetables: kale, lettuce and rocket salad. Whilst the amount of CO₂ the biochar could absorb from the biogas was low (11.17 mg g^-1), it could potentially be increased by modifying through physical and chemical methods. Virgin as well as CO₂-laden biochar were able to remove around 31% of chemical oxygen demand, 8% of the ammonia and almost 90% of the total suspended solids from the digestate wastewater, which was better than a dewatering process via centrifugation but worse than the industry standard of a polytetrafluoroethylene membrane bioreactor. Nutrients were recovered in the solid fraction of the digestate residue filtered by the biochar, and utilised as a biofertilizer that performed similarly to a commercial complete fertilizer in terms of aerial fresh weight growth for all three vegetables cultivated. Contingent on the optimal upgrading of biogas, the concept of a circular economy based on biochar and anaerobic digestion appears to be feasible.

Effect of organic loading rate on the anaerobic digestion of swine waste with biochar addition

The aim of this study was to investigate the impact of biochar addition on the mesophilic semi-continuous anaerobic digestion of swine waste with a focus on the effects of the organic loading rate (OLR) on biogas production, methane yield, total volatile fatty acids (TVFA), alkalinity, ammonium, volatile solids (VS) removal efficiency and process stability. Four reactors, two with amended biochar (R1 and R2) and two without biochar addition as controls (R3 and R4), were operated at OLRs in the range of 2-7 g VS/(L d), which corresponded to hydraulic retention times (HRTs) in the range of 7-2 days, respectively. The addition of biochar initially caused an increase in the generation of biogas and methane when compared to the control reactors when the process operated at OLRs of 2 and 3 g VS/(L d). This behaviour could be attributed to the presence of several trace elements (such as Fe, Co, Ni and Mn) in the biochar, which are involved in the action of acetyl-CoA synthase and methyl coenzyme M reductase to catalyse key metabolic steps, especially the methanogenic stage. The pH, alkalinity, TVFA and TVFA/Alkalinity ratio values for the effluents remained within the optimal ranges for the anaerobic digestion process. It was also found that the increase in OLR in the range of 2-5 g VS/(L d) determined a proportional increase in the VS removal rate. However, when the OLR increased up to 7 g VS/(L d), a drastic decrease in the VS removal rate was found for the control reactors. Biochar amendment contributed to a more balanced state of the anaerobic process, preventing biomass washout.

Beneficial role of biochar addition on the anaerobic digestion of food waste: A systematic and critical review of the operational parameters and mechanisms

The generation of huge amounts of food waste due to the increasing population is a serious global issue. The inadequate management of food waste and lack of proper handling approaches have created adverse negative impacts on the environment and the society. The use of traditional disposal (i.e. landfilling) and treatment (i.e. incineration and composting) methods are not considered to be efficient for managing food waste. Thus, anaerobic digestion (AD) has proven to be promising and cost-effective, as an alternative technology, for digesting and converting food waste into renewable energy and useful chemicals. However, mono-digestion of food waste suffers from process inhibition and instability which limit its efficiency. Adding biochar that has high buffering capacity and ensures optimum nutrient balance was shown to enhance biogas/methane production yields. This review reports on the physicochemical characteristics of food waste, the existing problems of food waste treatment in AD as well as the role of biochar amendments on the optimization of critical process parameters and its action mechanisms in AD, which could be a promising means of improving the AD performance. Also, this review provides insights regarding the selection of the desired/appropriate biochar characteristics, i.e. depending on the source of the feedstock and the pyrolysis temperature, and its role in enhancing biogas production and preventing the problem of process instability in the AD system. Finally, this review paper highlights the economic and environmental challenges as well as the future perspectives concerning the application of biochar amendments in AD.

Food waste and sewage sludge co-digestion amended with different biochars: VFA kinetics, methane yield and digestate quality assessment

This work investigated the impact of the addition of different biochar types on mitigation of volatile fatty acid (VFA) accumulation, methane recovery and digestate quality in mesophilic food waste-sludge co-digestion. Four biochars derived from agricultural and sludge residues under different pyrolysis temperatures were compared. Specific biochar properties such as pH, surface area, chemical properties and presence of surface functional groups likely influenced biochar reactions during digestion, thereby resulting in a varying performance of different biochars. Miscanthus straw biochar addition led to the highest specific methane yield of 307 ± 0.3 mL CH₄/g VS_added versus 241.87 ± 5.9 mL CH₄/g VS_added from control with no biochar addition over 30 days of the co-digestion period. Biochar supplementation led to enhanced process stability which likely resulted from improved syntrophic VFA oxidation facilitated by specific biochar properties. Overall, a 21.4% increase in the overall methane production was obtained with biochar addition as compared to control. The resulting digestate quality was also investigated. Biochar-amended digester generated a digestate rich in macro- and micro-nutrients including K, Mg, Ca, Fe making biochar-amended digestate a potential replacement of agricultural lime fertilizer. This work demonstrated that the addition of specific biochars with desirable properties alleviated VFA accumulation and facilitated enhanced methane recovery, thereby providing a means to achieve process stability even under high organic loading conditions in co-digestions. Moreover, the availability of biochar-enriched digestate with superior characteristics than biochar-free digestate adds further merit to this process.

Liquefaction of lignocellulosic biomass for methane production: A review

Hydrothermal pretreatment (HTP) (Hot water extraction (HWE) and steam pretreatment) and pyrolysis have the potential to liquefy lignocellulosic biomass. HTP produces hydrolysate, consisting mainly of solubilized hemicellulose, while pyrolysis produces aqueous pyrolysis liquid (APL). The liquid products, either as main products or by-product, can be used as anaerobic digestion (AD) feeds, overcoming shortcomings of solid-state AD (SS-AD). This paper reviews HWE, steam pretreatment, and pyrolysis pretreatment methods used to liquefy lignocellulosic biomass, AD of liquefied products, effects of inhibition from intermediate by-products such as furan and phenolic compounds, and pretreatment tuning to increase methane yield. HTP, focusing on methane production, produces less inhibitory compounds when carried out at moderate temperatures. APL is a challenging feed for AD due to its complexity, including various inhibitory substances. Pre-treatment of biomass before pyrolysis, adaptation of microorganism to inhibitors, and additives, such as biochar, may help the AD cultures cope with inhibitors in APL.

Comparative effect of biochar and activated carbon addition on the mesophilic anaerobic digestion of piggery waste in batch mode

A comparative study of the batch mesophilic anaerobic digestion of piggery waste was carried out with the addition of 5% biochar and 5% activated carbon. The results obtained showed that the bioreactors amended with biochar increased cumulative methane production, the kinetic constant for methane production and the COD removal efficiency compared to the control reactors and reactors with activated carbon addition. The maximum methane production and the kinetic constant were 6.9% higher in the reactors with biochar addition compared to the controls; while the COD removal efficiency was 3% higher in the case of biochar addition. In the case of activated carbon, only a slight improvement in anaerobic digestion performance was observed compared to the control.

Increasing 2 -Bio- (H2 and CH4) production from food waste by combining two-stage anaerobic digestion and electrodialysis for continuous volatile fatty acids removal

A novel approach of using two stage anaerobic digestion coupled with electrodialysis technology has been investigated. This approach was used to improving bio hydrogen and methane yields from food waste while simultaneously producing a green chemical feedstock. The first digester was used for hydrogen production and the second digester was used for methane production. The first digester was combined with continuous separation of volatile fatty acids using electrodialysis. The concentrations of carbohydrates, proteins and fats in the prepared food waste were 22.7%, 5.7% and 5.2% respectively. Continuous removal of volatile fatty acids during fermentation in the hydrogen digester not only increased hydrogen yields but also increased the production rate of volatile fatty acids. As a result of continuous VFA separation, hydrogen yields increased from 17.3 mL H₂/g VS _fermenter to 33.68 mL H₂/g VS _fermenter. Methane yields also increased from 28.94 mL CH₄/g VS _fermenter to 43.94 mL CH₄/g VS _fermenter. This represents a total increase in bio-energy yields of 77.1%. COD reduced by 73% after using two stage anaerobic digestion, however, this reduction increased to 86.7% after using electrodialysis technology for separation of volatile fatty acids. Electrodialysis technology coupled with anaerobic digestion improved substrate utilization, increased bioenergy yields and looks to be promising for treating complex wastes such as food waste.