Evaluation of Biochemical Methane Potential and Kinetics on the Anaerobic Digestion of Vegetable Crop Residues

Tomato Residue Management from a Biorefinery Perspective and towards a Circular Economy

The tomato industry is a relevant socio-economic activity in the European Union, while it generates a large variety of residues. Tomatoes unfit for consumption, tomato peels, seeds, industrial pomace, and plants are examples of residues of this industry. Commonly, some of the residues can be left in the field, composted, used for animal feeding, or valorized through anaerobic digestion. However, more economic value can be attributed to these residues if a biorefinery approach is applied. Indeed, many value-added compounds can be obtained by the integration of different processes while closing the carbon and nutrient loops. The extraction of bioactive compounds followed by anaerobic digestion and composting seems to be a viable proposal for a biorefinery approach. Thus, this study aims to review the biorefinery strategies for valorizing tomato residues, highlighting the main processes proposed. The recovery of lycopene, β-carotene, and phenolic compounds has been widely studied at the lab scale, while energy recovery has already been applied at the industrial scale. Although techno-economic analysis is scarce for tomato residue valorization processes, positive net present values (NPV) and low payback times (PBT) have been reported in the literature. Thus, more work comparing multiple extraction technologies and biorefinery strategies coupled with economic and environmental assessment should be performed to select the most promising management route for tomato residues.

Advances in methane emissions from agricultural sources: Part I. Accounting and mitigation

Methane is one of the major greenhouse gases (GHGs) and agriculture is recognized as its primary emitter. Methane accounting is a prerequisite for developing effective agriculture mitigation strategies. In this review, methane accounting methods and research status for various agricultural emission source including rice fields, animal enteric fermentation and livestock and poultry manure management were overview, and the influencing factors of each emission source were analyzed and discussed. At the same time, it analyzes the different research efforts involving agricultural methane accounting and makes recommendations based on the actual situation. Finally, mitigation strategies based on accounting results and actual situation are proposed. This review aims to provide basic data and reference for agriculture-oriented countries and regions to actively participate in climate action and carry out effective methane emission mitigation.

Bioaugmentation of anaerobic digesters with the enriched lignin-degrading microbial consortia through a metagenomic approach

The recalcitrance of lignin impedes the efficient utilization of lignocellulosic biomass, hindering the efficient production of biogas and value-added materials. Despite the emergence of anaerobic digestion as a superior alternative to the aerobic method for lignin processing, achieving its feasibility requires thorough characterization of lignin-degrading anaerobic microorganisms, assessment of their biomethane production potential, and a comprehensive understanding of the degradation pathway. This study aimed to address the aforementioned necessities by bioaugmenting seed sludge with three distinct enriched lignin-degrading microbial consortia at both 25 °C and 37 °C. Enhanced biomethane yields was detected in the bioaugmented digesters, while the highest production was observed as 188 mLN CH4 gVS-1 in digesters operated at 37 °C. Moreover, methane yield showed a significant improvement in the samples at 37 °C ranging from 110% to 141% compared to the control, demonstrating the efficiency of the enriched lignin-degrading microbial community. Temperature and substrate were identified as key factors influencing microbial community dynamics. The observation that microbial communities tended to revert to the initial state after lignin depletion, indicating the stability of the overall microbiota composition in the digesters, is a promising finding for large-scale studies. Noteworthy candidates for lignin degradation, including Sporosarcina psychrophila, Comamonas aquatica, Shewanella baltica, Pseudomonas sp. C27, and Brevefilum fermentans were identified in the bioaugmented samples. PICRUSt2 predictions suggest that the pathway and specific proteins involved in anaerobic lignin degradation might share similarities with those engaged in the degradation of aromatic compounds.

Assessment of biochemical methane potential of dairy wastewater with different co-substrates and evaluation of different kinetic models

Dairy industry wastewater can be considered as an important source of pollution due to its high amounts and pollutant concentrations. Anaerobic treatment is seen as a suitable alternative over aerobic treatment which requires huge aeration systems. Biochemical methane potential (BMP) testing is a widely applied technique for estimating the performance of anaerobic digesters and still has no clear alternative. In the study, the biochemical methane potential change was investigated by mixing dairy wastewater with different co-substrates (cattle manure, chicken manure and slaughterhouse wastewater) at different rates. The highest biogas potential per gram of chemical oxygen demand added (CODadded) was determined as 574 mLbiogas in a mixture of 74% dairy wastewater + 2% chicken manure + 24% slaughterhouse wastewater inoculated with granular sludge. The highest methane potential was determined as 340 mLCH4 in the same co-substrate mixture inoculated with anaerobic sludge. In recent years, mathematical modeling offers an alternative to BMP tests and many different models are used for this purpose. In the study, six different mathematical models were used to simulate the BMP results, and the highest correlation coefficient in almost all mixtures ranged from 0.900 to 0.997 with the Modified Gompertz equation and Fitzhugh models.

Co-anaerobic digestion of sawdust and chicken manure with plant herbs: Biogas generation and kinetic study

Plant herbs specifically serai wangi (SW) and peppermint (PPM) are selected for its insect repellent properties as the use of chicken manure (CM) in anaerobic digestion (AD) potentially attract flies due to the digestate produced. Hence, the addition of SW and PPM in the AD system of CM could deter flies' infestation while producing biogas. Previous work has shown that AD of sawdust (SD) and CM with these plant herbs were able to produce biogas and reduce the flies attraction towards the digestate. However, the combination of SW and PPM for AD of CM has yet to be investigated. This work describes the effect of mixing SW and PPM on the co-AD of SDCM with respect to biogas production, methane yield and kinetic analysis. The mixture of SW and PPM was varied at different concentrations. The composition of methane in biogas was characterized every 10 days by using gas chromatography (GC) equipped with a thermal conductivity detector (TCD). The results suggest that co-AD of 10SW10PPM exhibited the highest biogas production (52.28 mL/g_vs) and methane yield (30.89 mL/g_vs), which the purity of methane increased by 18.52% as compared to SDCM. However, increasing the concentration of SW and PPM does not significantly improve the overall process. High R² (0.927-0.999), low RMSE (0.08-0.61) and low prediction error (<10.00%) were displayed by the modified Gompertz, logistic and Cone models. In contrast, Monod and Fitzhugh model is not preferred for the co-AD of SDCM with a mixture of SW and PM, as a high prediction error is obtained throughout the study. Increasing the dosage of PPM decreases the maximum cumulative methane yield, ranging from 31.76 to 7.01 mL/g_vs for modified Gompertz and 89.56 to 19.31 mL/g_vs for logistic model. The Modified Gompertz obtained a lag phase of 10.01-28.28 days while the logistic model obtained a lag phase of 37.29-52.48 days.

The current status and challenges of biomass biorefineries in Africa: A critical review and future perspectives for bioeconomy development

Africa benefits from diverse biomasses that are rich in high-added value materials and precursors for energy, food, agricultural, cosmetic and medicinal applications. Many African countries are interested in valorizing biomasses to develop efficient and integrated biorefinery processes and their use for local and regional economic development. Thus, this report critically reviews the current status of African biomass richness, its diversity, and potential applications. Moreover, particular attention is given to bioenergy production, mainly by biological and thermochemical conversion processes. This also includes biomass valorization in agriculture, particularly for the production of plant-based biostimulants, which are a potential emerging agri-input sector worldwide. This study points out that even though several processes for biofuel, biogas, biofertilizer and biostimulant production have already been established in Africa, their development on a larger scale remains limited. This study also reports the different socioeconomic and political aspects of biomass applications, along with their challenges, opportunities, and future research perspectives, to promote concrete technologies transferable into an industrial level.

Methane production of banana plant: Yield, kinetics and prediction models influenced by morphological parts, cultivars and ripening stages

Banana trees and fruits with three ripening stages, including green, ripe, and overripe, of two cultivars, namely Nam wa and Hom were separated into different morphological parts for biogas yield determination. Specific methane yields (SMY) were significant different among banana parts (p ≤ 0.05). High non-structural carbohydrates and high non-lignocellulosic residual in substrates promoted high SMY. Pseudostem showed the highest share of energy yields among farm wastes which Nam wa cultivar provided higher energy potential than Hom. Peel presented the major energy source from fruit wastes which ripening stages did not have a significant effect on its SMY. Modified Gompertz model presented the best fit for methane production of most substrates. The SMY prediction models based on chemical constituents were developed to obtain conveniently used methane estimating tool which showed that a combination of lignin, hemicellulose, non-lignocellulosic residual, and crude fiber contents presented the highest performance for banana substrates.

Dry Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste: Biogas Production Optimization by Reducing Ammonia Inhibition

The aim of this work is to optimize biogas production from thermophilic dry anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) by comparing various operational strategies to reduce ammonia inhibition. A pilot-scale plug flow reactor (PFR) operated semi-continuously for 170 days. Three scenarios with different feedstock, namely solely OFMSW, OFMSW supplemented with structural material, and OFMSW altered to have an optimal carbon-to-nitrogen (C/N) ratio, were tested. Specific biogas production (SGP), specific methane production (SMP), the biogas production rate (GPR), and bioenergy recovery were evaluated to assess the process performance. In addition, process stability was monitored to highlight process problems, and digestate was characterized for utilization as fertilizer. The OFMSW and the structural material revealed an unbalanced content of C and N. The ammonia concentration decreased when the optimal C/N ratio was tested and was reduced by 72% if compared with feeding solely OFMSW. In such conditions, optimal biogas production was obtained, operating with an organic loading rate (OLR) equal to 12.7 gVS/(L d). In particular, the SGP result was 361.27 ± 30.52 NLbiogas/kgVS, the GPR was 5.11 NLbiogas/(Lr d), and the potential energy recovery was 8.21 ± 0.9 MJ/kgVS. Nevertheless, the digestate showed an accumulation of heavy metals and low aerobic stability.

Biochemical characterization and anaerobic degradability of flower wastes: Preliminary assessment and statistical interpretation towards energy recovery

The use of ornamental flowers and plants is widespread in several regions of the world, but the management of flower (or floral) waste (FW), classified as herbaceous biomasses, is scarcely addressed in the literature. However, climate change, population growth and the depletion of resources are expected to push towards the development of FW management strategies, according to principles of flexibility and integration of technologies. This study focuses on the characterization of ten different varieties of flowering plants, of which the wastes are of concern in the Pistoia Province (Italy). The possibility of recovering energy by means of anaerobic digestion is also preliminarily investigated. The interpretation of data through Principal Component Analyses proved to be effective to orientate the selection of technological solutions. The three main parts of each plant variety were analysed separately, showing that the biochemical composition of stems is statistically different from that of leaves and flowers, thus suggesting the viability of adopting different strategies to optimize material (value-added products) recovery from FW. Conversely, regarding biogas generation and energy recovery, the methane yield (in the range 82-330 NmLCH₄^.gVS^-1) is not significantly affected by the type of FW part, nor by the variety of flowering plant or by the use of pesticides during cultivation, whereas lower kinetics were observed for stems compared to leaves and flowers. In view of full-scale application, and depending on FW amounts locally produced, a careful evaluation is required, encompassing aspects of technical feasibility and economic expenses associated with FW parts separation.

Effects of pretreatment methods on biomethane production kinetics and microbial community by solid state anaerobic digestion of sugarcane trash

Solid state anaerobic digestion (SS-AD) of lignocellulose is effective in improving biomethane productivity but is limited by low biomass digestibility and lack of substrate-specific working microorganisms. In this study, the effects of different pretreatment methods on biomethane production by SS-AD of sugarcane trash were studied. The biomethane production, fitted to a modified Gompertz's model, predicted a maximum methane yield of 214.2 L/kg volatile solids (VS) and productivity of 6.9 L/kg VS/day from KOH-pretreated trash, respectively. Microbial community analysis showed that bacterial community was significantly associated with volatile acids and pretreatment types while archaeal community was significantly associated with methane yield. Microbial community dynamics was revealed in SS-AD. Main genera related to pretreatment method were identified and discussed. This study generated important information on SS-AD of lignocellulosic biomass pretreated by different methods, which is useful for developing bioaugmentation strategies to improve biomethane production by SS-AD.

Multilinear Regression Model for Biogas Production Prediction from Dry Anaerobic Digestion of OFMSW

The aim of this study was to develop a multiple linear regression (MLR) model to predict the specific methane production (SMP) from dry anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW). A data set from an experimental test on a pilot-scale plug-flow reactor (PFR) including 332 observations was used to build the model. Pearson′s correlation matrix and principal component analysis (PCA) examined the relationships between variables. Six parameters, namely total volatile solid (TVSin), organic loading rate (OLR), hydraulic retention time (HRT), C/N ratio, lignin content and total volatile fatty acids (VFAs), had a significant correlation with SMP. Based on these outcomes, a simple and three multiple linear regression models (MLRs) were developed and validated. The simple linear regression model did not properly describe the data (R2 = 0.3). In turn, the MLR including all factors showed the optimal fitting ability (R2 = 0.91). Finally, the MLR including four uncorrelated explanatory variables of feedstock characteristics and operating parameters (e.g., TVSin, OLR, C/N ratio, and lignin content), resulted in the best compromise in terms of number of explanatory variables, model fitting and predictive ability (R2 = 0.87).

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.

Evaluation of the anaerobic degradation of food waste collection bags made of paper or bioplastic

The amount of compostable bioplastics collected with the food waste is constantly growing, particularly due to the bags used for collection. According to the Italian legislation, compostable bioplastics must be accepted by all biological treatment plants, including aerobic and anaerobic facilities. Anyway, the compostability standard requires only the assessment of the aerobic degradability, while it is generally not required to test the behaviour under anaerobic conditions. This aspect is evaluated in the paper, where the anaerobic degradability of bioplastic bags used for the food waste collection is assessed. First, Biochemical Methane Potential (BMP) tests were performed on four commercial types of bioplastic bags, including those designed only for the collection of food waste and the shoppers, that can be reused for the same purpose. Subsequently, an innovative approach for this kind of substrate was applied, subjecting two bags to semi-continuous co-digestion tests together with the food waste. Both tests were performed by comparing the behaviour of bioplastic bags with that of an alternative collection paper bag. Finally, tests to evaluate the influence of physical phenomena on the degradation of bioplastics were performed to better understand the results of biological tests. BMP tests indicated a good degradability (>71%) of bioplastic bags, while semi-continuous tests showed a much lower degradability (<27%), confirmed by the observation of the undigested bag pieces. On the contrary, the paper bag presents interesting characteristics, because its degradability in the semi-continuous tests (82%) resulted even higher than that observed in the BMP tests (74%). These results highlight an important difference between the bags mono-digestion by means of BMP tests and the semi-continuous co-digestion tests with food waste, which better simulate the full-scale operational conditions.

Agricultural Biogas Production—Climate and Environmental Impacts

Livestock manure is a major source of the greenhouse gases (GHGs) methane (CH4) and nitrous oxide (N2O). The emissions can be mitigated by production of biogas through anaerobic digestion (AD) of manure, mostly together with other biowastes, which can substitute fossil energy and thereby reduce CO2 emissions and postdigestion GHG emissions. This paper presents GHG balances for manure and biowaste management as affected by AD for five Danish biogas scenarios in which pig and cattle slurry were codigested with one or more of the following biomasses: deep litter, straw, energy crops, slaughterhouse waste, grass–clover green manure, and household waste. The calculated effects of AD on the GHG balance of each scenario included fossil fuel substitution, energy use for transport, leakage of CH4 from biogas production plants, CH4 emissions during storage of animal manure and biowaste, N2O emissions from stored and field applied biomass, N2O emissions related to nitrate (NO3−) leaching and ammonia (NH3) losses, N2O emissions from cultivation of energy crops, and soil C sequestration. All scenarios caused significant reductions in GHG emissions. Most of the reductions resulted from fossil fuel substitution and reduced emissions of CH4 during storage of codigestates. The total reductions in GHG emissions ranged from 65 to 105 kg CO2-eq ton−1 biomass. This wide range showed the importance of biomass composition. Reductions were highest when straw and grass–clover were used as codigestates, whereas reductions per unit energy produced were highest when deep litter or deep litter plus energy crops were used. Potential effects of iLUC were ignored but may have a negative impact on the GHG balance when using energy crops, and this may potentially exceed the calculated positive climate impacts of biogas production. The ammonia emission potential of digestate applied in the field is higher than that from cattle slurry and pig slurry because of the higher pH of the digestate. This effect, and the higher content of TAN in digestate, resulted in increasing ammonia emissions at 0.14 to 0.3 kg NH3-N ton−1 biomass. Nitrate leaching was reduced in all scenarios and ranged from 0.04 to 0.45 kg NO3-N ton−1 biomass. In the scenario in which maize silage was introduced, the maize production increased leaching and almost negated the effect of AD. Methane leakage caused a 7% reduction in the positive climate impact for each percentage point of leakage in a manure-based biogas scenario.

Nano-Biochar as a Sustainable Catalyst for Anaerobic Digestion: A Synergetic Closed-Loop Approach

Nowadays, the valorization of organic wastes using various carbon-capturing technologies is a prime research area. The anaerobic digestion (AD) technology is gaining much consideration in this regard that simultaneously deals with waste valorization and bioenergy production sustainably. Biochar, a well-recognized carbonaceous pyrogenic material and possessing a broad range of inherent physical and chemical properties, has diverse applications in the fields of agriculture, health-care, sensing, catalysis, carbon capture, the environment and energy. The nano-biochar-amended anaerobic digestion approach has intensively been explored for the past few years. However, an inclusive study of multi-functional roles of biochar and the mechanism involved for enhancing the biogas production via the AD process still need to be evaluated. The present review inspects the significant role of biochar addition and the kinetics involved, further focusing on the limitations, perspectives, and challenges of the technology. Additionally, the techno-economic analysis and life-cycle assessment of biochar-aided AD process for the closed-loop integration of biochar and AD and possible improvement practices are discussed.

Specific Methane Yield of Wetland Biomass in Dry and Wet Fermentation Technologies

Our study evaluated the specific methane yield (SMY) of selected wetland species subjected to wet and dry anaerobic digestion: Carex elata All. (CE), a mixture (~50/50) of Carex elata All. and Carex acutiformis L. (CA), Phragmites australis (Cav.) Trin. ex Steud. (PA), Typha latifolia L. (TL) and Phalaris arundinacea L. (PAr). Plants were harvested in late September, and therefore, the study material was characterised by high lignin content. The highest lignin content (36.40 ± 1.04% TS) was observed in TL, while the lowest (16.03 ± 1.54% TS) was found in CA. PAr was characterised by the highest hemicellulose content (37.55 ± 1.04% TS), while the lowest (19.22 ± 1.22% TS) was observed in TL. Cellulose content was comparable in almost all plant species studied and ranged from 25.32 ± 1.48% TS to 29.37 ± 0.87% TS, except in PAr (16.90 ± 1.29% TS). The methane production potential differed significantly among species and anaerobic digestion (AD) technologies. The lowest SMY was observed for CE (121 ± 28 NL kgVS−1) with dry fermentation (D–F) technology, while the SMY of CA was the highest for both technologies, 275 ± 3 NL kgVS−1 with wet fermentation (W–F) technology and 228 ± 1 NL kgVS−1 with D–F technology. The results revealed that paludi-biomass could be used as a substrate in both AD technologies; however, biogas production was more effective for W–F. Nonetheless, the higher methane content in the biogas and the lower energy consumption of technological processes for D–F suggest that the final amount of energy remains similar for both technologies. The yield is critical in energy production by the AD of wetland plants; therefore, a promising source of feedstock for biogas production could be biomass from rewetted and previously drained areas, which are usually more productive than natural habitats.

Impact of microalgae species and solution salinity on algal treatment of wastewater reverse osmosis concentrate

Six common microalgal species, including freshwater microalgae Scenedesmus abundans, Chlorella vulgaris, Chlamydomonas reinhardtii and Coelastrum microporum, and marine microalgae Nannochloropsis salina and Dunaliella tertiolecta, were tested in batch treatment to identify the most promising species for remediating a municipal wastewater reverse osmosis concentrate (ROC). Selected species were then studied at different ROC salinity levels (5, 10, and 15 g TDS/L) in semi-continuous treatment to evaluate their potential for nutrient remediation, and biogas production through anaerobic digestion. S. abundans, C. vulgaris, and N. salina showed higher potential for growth and nutrient remediation under salinity stress. Further tests revealed that N. salina adapted well to ROC conditions, and S. abundans could grow better and had higher tolerance to the elevated salinity than C. vulgaris. S. abundans and N. salina performed better for removing nutrients and organic matter (11.5-18 mg/L/d TN, 7.1-8.2 mg/L/d TP, and 8.6-12.4 mg/L/d DOC). Increasing salinity led to growth inhibition and N uptake reduction for freshwater species but had no significant effect on TP removal. Biochemical methane potential tests showed the algal biomass produced a significant amount of methane (e.g., up to 422 mL CH₄/g VS for N. salina), suggesting the algae generated from the ROC treatment could produce significant amounts of energy through anaerobic digestion without the need for pretreatment. This study showed the environmental and economic potential of the algal system for future applications.

Integrated management of residues from tomato production: Recovery of value-added compounds and biogas production in the biorefinery context

The biorefinery approach must be boosted in the management of agro-residues in the future. The present study aims to investigate the valorization of tomato production residues, namely rotten tomato (unfit for consumption - RT), green tomato (GT), and tomato branches (TB). The assessment involves the recovery of value-added compounds through the extraction process followed by biogas production through anaerobic digestion. A thorough characterization of the three residues (RT, GT, and TB) was carried out, including the identification of volatile compounds by solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS). The volatiles analysis revealed the presence of flavor enhancer compounds and molecules with insecticidal properties. A solid-liquid extraction with ethanol allowed the recovery of value-added compounds in the extracts, in particular phenolic compounds, β-carotene, and lycopene, which contributed to the antioxidant activity. RT and TB extracts were found to be richer in total phenolic compounds (~27 mg GAE/g_db dry basis) and exhibited higher antioxidant activity (IC₅₀ = 0.911 and 0.745 mg/mL). The tomato branches extract had the highest concentration of carotenoids with 37.23 and 3.08 mg/kg_db of β-carotene and lycopene, respectively. The biochemical methane potential (BMP) was assessed in sealed reactors operating in anaerobic conditions for all the raw (RT, GT, and TB) and extracted substrates waste (RTe, GTe, and TBe). While the BMP of RT and GT was in the range of 232-285 mL CH₄/g VS, a lower value of 141 mL CH₄/g VS was obtained for TB. The methane production for each pair of raw and extracted substrates (RT/RTe, GT/GTe, and TB/TBe) was considered statistically similar at a 95 % confidence level. Overall, the value-added compounds recovery through ethanolic extraction did not compromise the methane production of the materials.

Bioenergy Production through Mono and Co-Digestion of Tomato Residues

The agro-industry of tomato generates three types of residues: ripe rotten tomato (unfit for consumption) (RT), green (unripe) tomato (GT), and tomato branches including leaves and stems (TB). These materials are commonly wasted or used as feed for livestock. Energy production through anaerobic digestion is an alternative way to manage and simultaneously valorise these materials. Initially, the operating conditions of mono anaerobic digestion were investigated using RT. Thus, a design of experiments based on a two-level fractional factorial design with resolution V was performed to determine the factors that affect biochemical methane potential (BMP). The substrate to inoculum ratio (SIR), total volatile solids concentration (VSt), working volume (WV), presence of nutrients (Nu), and the pre-incubation of the inoculum (Inc) were investigated. The results showed that SIR is the most important factor. The maximum BMP for RT was 297 NmLCH4/gVS with SIR = 0.5; tVS = 20 g/L; WV = 20%; no pre-incubation and the presence of nutrients. Using these optimum operating conditions, co-digestion was investigated through a mixture design approach. The substrates RT and GT presented similar BMP values, whereas TB led to a significantly lower BMP. Indeed, when high concentrations of TB were used, a significant decrease in methane production was observed. Nonetheless, the highest BMP was achieved with a mixture of 63% RT + 20% GT + 17% TB, with a production of 324 NmLCH4/gVS, corresponding to a synergetic co-digestion performance index of about 1.20. In general, although the substrate RT generates the highest BMP, the mixture with GT did not impair the methane yield. Overall, the co-digestion of tomato residues must be conducted with SIR close to 0.5 and the content of tomato branches in the reaction mixture should be kept low (up to 20%).

Biodegradation of Functionalized Nanocellulose

Nanocellulose has attracted widespread interest for applications in materials science and biomedical engineering due to its natural abundance, desirable physicochemical properties, and high intrinsic mineralizability (i.e., complete biodegradability). A common strategy to increase dispersibility in polymer matrices is to modify the hydroxyl groups on nanocellulose through covalent functionalization, but such modification strategies may affect the desirable biodegradation properties exhibited by pristine nanocellulose. In this study, cellulose nanofibrils (CNFs) functionalized with a range of esters, carboxylic acids, or ethers exhibited decreased rates and extents of mineralization by anaerobic and aerobic microbial communities compared to unmodified CNFs, with etherified CNFs exhibiting the highest level of recalcitrance. The decreased biodegradability of functionalized CNFs depended primarily on the degree of substitution at the surface of the material rather than within the bulk. This dependence on surface chemistry was attributed not only to the large surface area-to-volume ratio of nanocellulose but also to the prerequisite surface interaction by microorganisms necessary to achieve biodegradation. Results from this study highlight the need to quantify the type and coverage of surface substituents in order to anticipate their effects on the environmental persistence of functionalized nanocellulose.

Evaluation of lignin inhibition in anaerobic digestion from the perspective of reducing the hydrolysis rate of holocellulose

In this study, Anaerobic Digestion Model No. 1 (ADM1) were modified to simulate anaerobic digestion (AD) process of microcrystalline cellulose (MCC) and five lignocellulosic substrates, with the goal of predicting the hydrolysis rates of holocellulose fractions in environments with and without lignin inhibition. After model verification, the hydrolysis rate constant of MCC, i.e., the hydrolyzability of cellulose without lignin inhibition, was 3.227 d^-1, while those of the holocellulose fractions of five lignocellulosic substrates (I_k_hyd) were in the range of 1.270 d^-1 to 3.364 d^-1 (average of 2.242 d^-1), which demonstrated remarkable suppression of holocellulose hydrolysis by lignin. Lignin inhibition index (LII) was proposed as an indicator to intuitively quantify and characterize the lignin inhibitory strength in a specific substrate. A series of factors with the potential to affect the LII were analyzed sequentially. This study provides an advanced understanding of the participation and behavior of lignin in the AD process.

Recycling anaerobic digestate enhances the co-digestion potential of agro-industrial residues: influence of different digestates as sources of microbial inoculum

Anaerobic codigestion (AcD) of agroindustrial residues was investigated. Granular sludge from bench-scale bioreactors digesting different manure were acclimated and recycled as microbial seed sludge to demonstrate inoculum-type influence on digestion performance. The biomethane potential (BMP) assay was operated for 30 days at 40 ± 2 °C in batch-type laboratory-scale reactors (100 mL). In inoculum amended reactors, codigestion showed significant, yet distinctive, biomethanation than monodigestion with a 5-fold increase (p < 0.05) in average biogas (248.3 ± 5.30 mL gVS-1) and CH4 yield (207.5 ± 4.15 mL gVS-1). The pH, soluble chemical oxygen demand (sCOD) and volatile fatty acids (VFAs) concentrations were within limits for stable AcD process with elevated total solids (TS) and volatile solids (VS) removal efficiencies. This study reinforces advancements in the recycling of digestate in biodigesters and suggests the appropriate selection of inoculum, preferably cow manure, to essentially boost methane production from these wastes.

Pretreatment, Anaerobic Codigestion, or Both? Which Is More Suitable for the Enhancement of Methane Production from Agricultural Waste?

Pretreatment and codigestion are proven to be effective strategies for the enhancement of the anaerobic digestion of lignocellulosic residues. The purpose of this study is to evaluate the effects of pretreatment and codigestion on methane production and the hydrolysis rate in the anaerobic digestion of agricultural wastes (AWs). Thermal and different thermochemical pretreatments were applied on AWs. Sewage sludge (SS) was selected as a cosubstrate. Biochemical methane potential tests were performed by mixing SS with raw and pretreated AWs at different mixing ratios. Hydrolysis rates were estimated by the best fit obtained with the first-order kinetic model. As a result of the experimental and kinetic studies, the best strategy was determined to be thermochemical pretreatment with sodium hydroxide (NaOH). This strategy resulted in a maximum enhancement in the anaerobic digestion of AWs, a 56% increase in methane production, an 81.90% increase in the hydrolysis rate and a 79.63% decrease in the technical digestion time compared to raw AWs. On the other hand, anaerobic codigestion (AcoD) with SS was determined to be ineffective when it came to the enhancement of methane production and the hydrolysis rate. The most suitable mixing ratio was determined to be 80:20 (Aws/SS) for the AcoD of the studied AWs with SS in order to obtain the highest possible methane production without any antagonistic effect.

Enhanced anaerobic digestion of waste-activated sludge via bioaugmentation strategy-Phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt2) analysis through hydrolytic enzymes and possible linkage to system performance

In this study, anaerobic digestion of waste-activated sludge was bioaugmented with hydrolytic bacteria, Bacteroidetes uniformis (Bacteroidetes, B) and Clostridium sp. (Firmicutes, F) at various dosages. Bioaugmentation resulted in enhanced methane conversion of waste-activated sludge. The highest methane yield of 298.1 mL CH₄/g-COD, 85.2% COD conversion efficiency was obtained when Bacteroidetes uniformis and Clostridium sp. were augmented at 100 and 900 CFU/mL, respectively. The microbial community analysis demonstrated that bioaugmentation increased the proportion of Bacteroidetes, Firmicutes, and Proteobacteria. Furthermore, at the highest methane yield, the principal methanogenic pathway was altered from acetoclastic to a mixture of hydrogenotrophic and acetoclastic; the major species shifted from Methanosaeta concilii to Methanobacterium subterraneum. Predicted gene analysis revealed that increased expression of hydrolases resulted in enhanced methane conversion through bioaugmentation.

A comparative analysis of biogas production from tomato bio-waste in mesophilic batch and continuous anaerobic digestion systems

Annually, agricultural activity produces an enormous amount of plant biomass by-product. Many studies have reported the biomethane potential of agro-industrial wastes, but only a few studies have investigated applying the substrates in both batch and continuous mode. Tomato is one of the most popular vegetables globally; its processing releases a substantial amount of by-product, such as stems and leaves. This study examined the BMP of tomato plant (Solanum lycopersicum Mill. L. cv. Alfred) waste. A comparative test revealed that the BMPs of corn stover, tomato waste,and their combination were approximately the same, around 280 mL methane/g Volatile Solid. In contrast, the relative biogas production decreased in the presence of tomato waste in a continuous mesophilic anaerobic digestion system; the daily biogas productions were 860 ± 80, 290 ± 50, and 570 ± 70 mL biogas/gVolatile Solid/day in the case of corn stover, tomato waste, and their mixture, respectively. The methane content of biogas was around 46–48%. The fermentation parameters of the continuous AD experiments were optimal in all cases; thus, TW might have an inhibitory effect on the microbial community. Tomato plant materials contain e.g. flavonoids, glycoalkaloids (such as tomatine and tomatidine), etc. known as antimicrobial and antifungal agents. The negative effect of tomatine on the biogas yield was confirmed in batch fermentation experiments. Metagenomic analysis revealed that the tomato plant waste caused significant rearrangements in the microbial communities in the continuously operated reactors. The results demonstrated that tomato waste could be a good mono-substrate in batch fermentations or a co-substrate with corn stover in a proper ratio in continuous anaerobic fermentations for biogas production. These results also point to the importance of running long-term continuous fermentations to test the suitability of a novel biomass substrate for industrial biogas production.

Rapid Biochemical Methane Potential Evaluation of Anaerobic Co-Digestion Feedstocks Based on Near Infrared Spectroscopy and Chemometrics

Biochemical methane potential (BMP) of anaerobic co-digestion (co-AD) feedstocks is an essential basis for optimizing ratios of materials. Given the time-consuming shortage of conventional BMP tests, a rapid estimated method was proposed for BMP of co-AD—with straw and feces as feedstocks—based on near infrared spectroscopy (NIRS) combined with chemometrics. Partial least squares with several variable selection algorithms were used for establishing calibration models. Variable selection methods were constructed by the genetic simulated annealing algorithm (GSA) combined with interval partial least squares (iPLS), synergy iPLS, backward iPLS, and competitive adaptive reweighted sampling (CARS), respectively. By comparing the modeling performances of characteristic wavelengths selected by different algorithms, it was found that the model constructed using 57 characteristic wavelengths selected by CARS-GSA had the best prediction accuracy. For the validation set, the determination coefficient, root mean square error and relative root mean square error of the CARS-GSA model were 0.984, 6.293 and 2.600, respectively. The result shows that the NIRS regression model—constructed with characteristic wavelengths, selected by CARS-GSA—can meet actual detection requirements. Based on a large number of samples collected, the method proposed in this study can realize the rapid and accurate determination of the BMP for co-AD raw materials in biogas engineering.

A systematic evaluation of biomethane production from sugarcane trash pretreated by different methods

Biomethane production was systematically evaluated with sugarcane trash pretreated by liquid hot water (LHW), dilute acid (DA) and KOH solutions. Multiple linear regression analysis identified glucan in pretreated solid residue as well as C5 sugars and acetic acid in pretreatment hydrolysate as the key parameters affecting biomethane potentials. Moreover, biomethane production was best simulated using Chen & Hashimoto model with a predicted highest methane yield of 187 mL/g initial total solids (TS) based on LHW (130 °C for 15 min) and KOH (10% on trash, 150 °C for 60 min) pretreatments. KOH pretreatment led to a biomethane yield of 167 mL/g initial TS at day 25, 82%, 34% and 33% higher than those achieved with untreated and pretreated trash samples with optimal LHW and DA conditions, respectively. This study led to the identification of best kinetic model and pretreatment condition for biomethane production from sugarcane trash through a systematic evaluation.

Comparative analysis of prediction models for methane potential based on spent edible fungus substrate

In this study, ten spent edible fungus (SEF) with different compositional features were used for the maximum methanogenic potential (P₀) evaluation, and the prediction models including regression and kinetics based on this were developed separately. The results showed that the regression model with more chemical components had a good correlation with the P₀, and at least three chemical compositions could reach the threshold of sensitivity. The Cone model showed the best fitting effect on P₀ in all kinetic models, which had higher R-square (>0.994) and lower error (1.004-5.672%). Meanwhile, the minimum digestive testing time (14 days) was determined by the evaluation of sensitivity via statistical indicators. It is concluded that the determination of the prediction model of P₀ should be evaluated with the combination of statistical indicators and specific requirements.

Synergistic Co-Digestion of Microalgae and Primary Sludge to Enhance Methane Yield from Temperature-Phased Anaerobic Digestion

A two-stage temperature-phased mesophilic anaerobic digestion assay was carried out to study the interaction between various biological pretreatment conditions and the possible synergistic co-digestion of microalgae and primary sludge. The study of growth kinetics of the biochemical methane potential test revealed that a maximum of 36% increase in methane yield was observed from co-digestion of a substrate pretreated by thermophilic aerobic conditions (55 °C and HRT = 2 days) and an 8.3% increase was obtained from the anaerobic pretreated substrate (55 °C and HRT = 3 days). Moreover, no synergistic effects on methane yields were observed in co-digesting the substrate pretreated with high temperature (85 °C). The study also identified specific conditions in which interaction between biological pretreatment and co-digestion might substantially reduce methane yield. Careful optimization of operating conditions, both aerobic and anaerobic pretreatment at moderate thermophilic conditions, can be used as a biological pretreatment to enhance methane yield from the co-digestion of microalgae and primary sludge.

Existing Empirical Kinetic Models in Biochemical Methane Potential (BMP) Testing, Their Selection and Numerical Solution

Biochemical Methane Potential (BMP) tests are a crucial part of feasibility studies to estimate energy recovery opportunities from organic wastes and wastewater. Despite the large number of publications dedicated to BMP testing and numerous attempts to standardize procedures, there is no “one size fits all” mathematical model to describe biomethane formation kinetic precisely. Importantly, the kinetics models are utilized for treatability estimation and modeling processes for the purpose of scale-up. A numerical computation approach is a widely used method to determine model coefficients, as a replacement for the previously used linearization approach. However, it requires more information for each model and some range of coefficients to iterate through. This study considers existing empirical models used to describe biomethane formation process in BMP testing, clarifies model nomenclature, presents equations usable for numerical computation of kinetic parameters as piece-wise defined functions, defines the limits for model coefficients, and collects and analyzes criteria to evaluate and compare model goodness of fit.

Kitchen waste valorization through a mild-temperature pretreatment to enhance biogas production and fermentability: Kinetics study in mesophilic and thermophilic regimen

Biowaste valorization through anaerobic digestion is an attractive option to achieve both climate protection goals and renewable energy production. In this paper, a complete set of batch trials was carried out on kitchen waste to investigate the effects of mild thermal pretreatment, temperature regimen and substrate/inoculum ratio. Thermal pretreatment was effective in the solubilisation of macromolecular fractions, particularly carbohydrates. The ability of the theoretical methodologies in estimating hydrogen and methane yields of complex substrates was evaluated by comparing the experimental results with the theoretical values. Despite the single batch configuration, a significant initial hydrogen production was observed, prior to methane yield. Main pretreatment effect was the gain in hydrogen production; the extent was highly variable according to the other parameters values. High hydrogen yields, up to 113 mL H2/g VSfed, were related to the prompt transformation of soluble sugars. Thermophilic regimen resulted, as expected, in faster digestions (up to 78 mL CH₄/gVS/day) and sorted out pH inhibition. The relatively low methane yields (342-398 mL CH₄/g VS_fed) were the result of the consistent lignocellulosic content and low lipid content. Thermal pretreatment proved to be a promising option for the enhancement of hydrogen production in food waste dark fermentation.

Biogas Production from Oil Palm Empty Fruit Bunches and Palm Oil Decanter Cake using Solid-State Anaerobic co-Digestion

Oil palm empty fruit bunches (EFB) and palm oil decanter cake (DC) were used to investigate biogas production by using solid-state anaerobic co-digestion (SS-AcoD) with 15% total solid (TS) content. Solid state anaerobic digestion (SS-AD) using substrate to inoculum (S:I) ratio of 3:1, methane yields of 353.0 mL-CH4/g-VS and 101.5 mL-CH4/g-VS were respectively achieved from mono-digestion of EFB without oil palm ash (OPA) addition and of DC with 10% OPA addition under mesophilic conditions 35 °C. By adding 5% OPA to SS-AD using 3:1 S:I ratio under thermophilic conditions (55 °C), mono-digestion of EFB and DC provided methane yields of 365.0 and 160.3 mL-CH4/g-VS, respectively. Furthermore, SS-AcoD of EFB:DC at 1:1 mixing ratio (volatile solid, VS basis), corresponding to carbon to nitrogen (C:N) ratio of 32, gathering with S:I ratio of 3:1 and 5% ash addition, synergistic effect is observed together with similar methane yields of 414.4 and 399.3 mL-CH4/g-VS, achieved under 35 °C and 55 °C, respectively. According to first order kinetic analysis under synergistic condition, methane production rate from thermophilic operation is 5 times higher than that from mesophilic operation. Therefore, SS-AcoD could be potentially beneficial to generate biogas from EFB and DC.

Development of a Modified Plug-Flow Anaerobic Digester for Biogas Production from Animal Manures

Traditional plug-flow anaerobic reactors (PFRs) are characterized by lacking a mixing system and operating at high total solid concentrations, which limits their applicability for several kinds of manures. This paper studies the performance of a novel modified PFR for the treatment of pig manure, characterized by having an internal sludge mixing system by biogas recirculation in the range of 0.270–0.336 m3 m−3 h−1. The influence on the methane yield of four operating parameters (recirculation rate, hydraulic retention time, organic loading rate, and total solids) was evaluated by running four modified PFRs at the pilot scale in mesophilic conditions. While the previous biodegradability of organic matter by biochemical methane potential tests were between 31% and 47% with a methane yield between 125 and 184 LCH4 kgVS−1, the PFRs showed a suitable performance with organic matter degradation between 25% and 51% and a methane yield of up to 374 LCH4 kgVS−1. Operational problems such as solid stratification, foaming, or scum generation were avoided.

A Preliminary Study of the Effect of Bioavailable Fe and Co on the Anaerobic Digestion of Rice Straw

Rice straw is an abundant and sustainable substrate for anaerobic digestion (AD), but it is often deficient in essential trace elements (TEs) for proper microbial growth and metabolism. A lack of TEs leads to AD imbalances and suboptimal biogas yields. However, the total TE concentration is not a sufficient indicator of the amount of TEs available to the microorganisms. Therefore, this study investigated the degree of bioavailability of iron (Fe) and cobalt (Co) during the AD of rice straw, and correlated it to the biomethane yields and volatile fatty acids (VFAs) produced. When the two TEs were dosed at 205 µg Fe/g TS and 18 µg Co/g TS of rice straw, the biomethane production was approximately 260 mL CH4/g VS, i.e., similar to that obtained when Fe and Co were not added. Despite an increased bioavailable fraction of 23 and 48% for Fe and Co, respectively, after TEs addition, the AD performance was not enhanced. Moreover, VFAs did not exceed 250 mg HAc/L both in the presence and absence of added TEs, confirming no enhancement of the methanogenesis step. Therefore, the bioavailability of Fe and Co was not a limiting factor for the biomethane production at low total VFAs concentration.