In a society where the environmental conscience is gaining attention, it is necessary to evaluate the potential valorization options for agricultural biomass to create a change in the perception of ...the waste agricultural biomass from waste to resource. In that sense, the biorefinery approach has been proposed as the roadway to increase profit of the agricultural sector and, at the same time, ensure environmental sustainability. The biorefinery approach integrates biomass conversion processes to produce fuels, power, and chemicals from biomass. The present review is focused on the extraction of value-added compounds, anaerobic digestion, and composting of agricultural waste as the biorefinery approach. This biorefinery approach is, nevertheless, seen as a less innovative configuration compared to other biorefinery configurations, such as bioethanol production or white biotechnology. However, any of these processes has been widely proposed as a single operation unit for agricultural waste valorization, and a thoughtful review on possible single or joint application has not been available in the literature up to now. The aim is to review the previous and current literature about the potential valorization of agricultural waste biomass, focusing on valuable compound extraction, anaerobic digestion, and composting of agricultural waste, whether they are not, partially, or fully integrated.
" This guide is intended for use by industry stakeholders, decision-makers and digester operators in navigating the topic of trace element (TE) supplementation as a management tool for anaerobic ...digester operation. The subject is the application of TE, and supplementation regimes in anaerobic waste-conversion biotechnologies, such as biogas digesters. TE is a term used to include a wide range of micronutrients essential for the microbial community underpinning AD. TE mostly includes elements from the metal groups (e.g. cobalt, nickel, zinc and tungsten) but also other elemental groups, such as metalloids (e.g. selenium). TE are dosed to anaerobic digesters to boost biological activity and to increase biogas production rates. Little is understood about the concentrations and dosing strategies best suited to sustained supplementation and stable performance in anaerobic biotechnologies. A range of companies offer proprietary blends of trace elements for supplementation of anaerobic digesters. Very little joined-up information is available on the concentrations of individual TE best suited to improved digester performance. Moreover, typically no attention whatsoever is paid to the bioavailability of TE dosed to digesters i.e. despite high concentrations, TE may not be available for uptake by the microorganisms underpinning the digestion process. Based on extensive engagement with a range of stakeholders throughout the course of the recent EU COST Action on ‘The ecological roles of trace metals in anaerobic biotechnologies’, and particularly on feedback from industrial partners, it is clear that such a guide is needed by industry stakeholders, decision-makers and operators of anaerobic digesters."
The relationships between extracellular polymeric substances (EPS), soluble microbial product production, metal solubility, and methanogenic activity were investigated. The individual, and joint, ...toxic effects of nickel and cobalt on methanogenic consortia fed with glucose as model substrate were studied using biomethane potential assays. Cobalt was found to be less toxic to methanogens than nickel at each concentration tested, and the combined effects of Ni and Co on methane production in the bimetal experiment was higher than the sum of the effects of each metal alone. The protein content of EPS, and extracellular soluble protein fractions, decreased with increasing concentrations of total metals. Meanwhile, no significant change in response to metal stress was apparent for carbohydrate content of EPS or extracellular soluble carbohydrate. Decreasing protein content of EPS was accompanied by reduced methanogenic activity and an increase in the soluble metal fraction. The strong associations observed between these variables could be due to the critical role of EPS in protecting microbial cells against nickel and cobalt stress, possibly by capturing metal cations through their functional groups, thus reducing metal availability to the microbial cells in the methanogenic consortia underpinning the anaerobic digestion process.
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•Metal toxicity in AD was associated with metal solubility and EPS content.•Solubility and toxicity of Ni was higher than Co at the same total concentrations.•Ni and Co toxicity resulted in lowered EPS and extracellular soluble protein content.•Soluble and EPS carbohydrate contents were not affected by high metal concentrations.
Harnessing microbial capabilities for metal recovery from secondary waste sources is an eco-friendly and sustainable approach for the management of metal-containing wastes. Soluble microbial products ...(SMP) and extracellular polymeric substances (EPS) are the two main groups of extracellular compounds produced by microorganisms in response to metal stress that are of great importance for remediation and recovery of metals. These include various high-, and low, molecular weight components, which serve various functional and structural roles. These compounds often contain functional groups with metal binding potential that can attenuate metal stress by sequestering metal ions, making them less bioavailable. Microorganisms can regulate the content and composition of EPS and SMP in response to metal stress in order to increase the compounds specificity and capacity for metal binding. Thus, EPS and SMP represent ideal candidates for developing technologies for selective metal recovery from complex wastes. To discover highly metal-sorptive compounds with specific metal binding affinity for metal recovery applications, it is necessary to investigate the metal binding affinity of these compounds, especially under metal stressed conditions. In this review we critically reviewed microbial EPS and SMP production as a response to metal stress with a particular emphasis on the metal binding properties of these compounds and their role in altering metal bioavailability. Furthermore, for the first time, we compiled the available data on potential application of these compounds for selective metal recovery from waste streams.
•EPS and SMP have metal-binding affinity and thus can impact metal bioavailability.•Metal stress induce production of microbial EPS and SMP.•Proteins display the highest metal-binding capacity and selectivity.•Extracellular proteins could act as biosorbents for selective metal recovery.
Mechanical pretreatments are commonly used devices at full-scale to reduce the size of feedstock and to increase methane production, improving mixing, heat, and mass transfer into the anaerobic ...digesters of biogas plants. Nevertheless, physical pretreatments require considerable energy. This work aimed to investigate how the energy consumption in different mechanical pretreatments that effectively run under real operational conditions at full-scale level affects physical modifications and anaerobic degradability of agri-based feedstock. Four mechanical pretreatment were investigated: knife milling, hammer milling, extrusion, shredding + hydrodynamic cavitation. After pretreatments, the specific methane yield increased in the range from 1% to 13% and the maximum methane production rate increased in the range from 4% to 48%. Each pretreatment leads to a positive energy balance (0.14–27 kJ kJ−1 used). The increasing of energy consumption indicates a more intense pretreatment with an incremental reduction of particles with diameter bigger than 5 mm. On the basis of the calculated regression analysis, a specific combination of additional specific surface area and additional energy in the day of maximum methane production should be expected with mechanical pretreatment that run effectively at full-scale level.
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•Four mechanical pretreatments in real operational conditions were investigated.•Benefits in methane yield (+1–13%) and methane production rate (+4–48%) were achieved.•10–40 kJ kg−1 leads to incremental reduction of particles ≥ 5 mm.•Methane production rate is related to the increasing of specific surface area.
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•The effect of five mechanical treatments on lignocellulosic biomass was evaluated.•Most effective treatments resulted in a higher number of particles of lower size.•Smaller particles ...were related with a larger solubilization of organic matter.•Anthocyanins were more degraded at the most effective treatments.•A relation between increasing particle number and methane production was achieved.
Mechanical treatments can be simple and feasible methods for enhancing the anaerobic digestion of lignocellulosic substrates. This work aims to relate the direct effect of five different mechanical treatments, i.e., variation in the size and number of particles, with the variations in the chemical composition and, subsequently, the effect over the anaerobic digestion of residual raspberry extrudate, which was used as a model substrate. A high variation in the number of particles and the particle size distribution was achieved depending on the mechanical treatment applied, reaching the highest number of particles for the treatments with knife mills and mortar (around 8000 particles per gram). The higher number of particles was related to higher solubilisation, including phenolic compounds and sugars. The combination of knife mills and mortar pretreatment, which presented the highest number of particles, resulted in a 66% more of polyphenols in comparison to the raw substrate. However, the presence of anthocyanins was higher in mechanical treatments with less effect. The enhancement of the anaerobic digestion was clearly related to the increment in the number of particles of small size after the mechanical treatments. The highest methane yield coefficient (236 ± 11 mL CH4/g volatile solids) was achieved for the raspberry extrudate treated with knife mills.
This work aims to compare the use of olive mill solid waste as substrate in pH-controlled fermentation at acid (pH = 5), neutral (uncontrolled, pH ≈ 7) and alkaline (pH = 9) operating pH levels. The ...results obtained in this study indicate that operating pH strongly affected the anaerobic microorganisms and, hence, different target compounds could be obtained by adjusting the operating pH. Fermentation at neutral pH resulted in the conversion of 93.5% of the fed chemical oxygen demand to methane. However, fermentations at pH 5 and 9 resulted in the inhibition of the methanogenic activity. At pH 9, volatile fatty acids reached a maximum concentration of 3.69 g O2/L, where acetic acid represented up to 79.3% of the total volatile fatty acids. Unlike volatile fatty acid production, an optimal operation of fermentation at pH 5 could allow the recovery of phenols such as vanillin.
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•Olive mill solid waste was fermented at acidic, neutral and alkaline conditions.•More than 90% of the organic matter was biomethanized at neutral pH.•Volatile fatty acids reached a maximum concentration of 3.69 g COD/L at pH 9.•The highest phenolic compounds accumulation occurred at pH 5.
Microalgae are being proposed as excellent substrates for different biorefinery processes. Anaerobic digestion process of microalgae is one of these interesting processes but has some limitations in ...deleting cell walls. For this reason, many studies proposed different types of pre-treatments, entailing energy, operation, and investment costs. This work aims to optimize the anaerobic digestion of the microalgae Chlorella sorokiniana and Chlorella sorokiniana (strain S12/S13/S16) without any pre-treatment by selecting the optimal harvesting time. The greatest influence is seen at 5:00 PM in methane production for both microalgae. For Chlorella sorokiniana, it is the most optimal moment for anaerobic digestion, whereas Chlorella sorokiniana (strain S12/S13/S16) is the least optimal. In the other harvesting times, both microalgae present a similar methane production, i.e. 173 ± 12 mL CH
4
/g of total volatile solids. The highest methane production rate values were obtained during peak sunlight, 1:00 PM and 8:00 AM, respectively, and lower overnight.
A novel analysis addresses the economic viability of biomethane production from small biogas plants in South Spain, as a claim to promote the use of green energy and reduce the consumption of natural ...gas. To this end, the importance of governmental incentives to reach profitability in biomethane plants is illustrated through a case study. To date, no study addressing this problem specifically for South Spain can be found. The study considers the whole process from biogas production to biomethane feeding into the grid, for three different biomethane capacities (50, 100 and 150 m3/h) and includes an exhaustive sensitivity analysis. For the three cases, implementing a biomethane plant is not viable and, therefore, not attractive for investors. Results considering biomethane governmental incentives as feed-in premia show significant improvements on the profitability of the largest plants. For example, supporting 150 m3/h biomethane production capacity plants with a premium price of only 6 €/MWh (6.6 cents/m3) results in 270 k€ NPV. Nevertheless, the smallest biomethane plants are hardly feasible. Concerning governmental support through investment subsidies, 150 m3/h plants are profitable if 10% of the investment is subsidized, whereas the smallest plants do not reach profitability even if 50% of the investment is subsidized.
•A novel analysis for biomethane production in South-Spain are addressed.•The importance of governmental incentives is pointed out to reach profitability.•Incentives as feed-in tariffs showed significant outcomes for high capacity plants.•The smallest biomethane capacity plants were proved to be arduously feasible.
The effect and the response of several trace elements (TE) addition to the anaerobic degradation of key compounds of lignocellulosic biomass were evaluated. Lignin, cellulose and xylose were selected ...as principal compounds of lignocellulosic biomass. Lignin degradation was only improved by the addition of 1000 mg Fe/L, which allowed an improvement on the methane yield coefficient of 28% compared to control. SEM images from an abiotic assay showed that this effect is more likely related with a chemical effect induced by the Fe solution, instead of an enzymatic response. Pre-treatments focused on breaking the recalcitrant structure of the lignin could be more promising than TE addition for rich lignin-content substrates. Unlike to the response observed with lignin, cellulose showed a clear effect of the TE addition on methane production rate, indicating a higher preponderance of the enzymatic activity compared to the lignin biomethanization. Experiments with xylose resulted in a strong accumulation of volatile fatty acids. TE addition should be adapted to the substrate composition given the different response of each lignocellulosic compound to the different TE addition.
•Lignin, cellulose and xylose were selected to represent the lignocellulose biomass.•Trace elements addition affected differently the biomethanization of each compound.•The addition of 1000 mg Fe/L had an abiotic effect on the structure of the lignin.•Biomethanization of cellulose was significantly affected by Mn addition.
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