Insights into Anaerobic Co-Digestion of Lignocellulosic Biomass (Sugar Beet By-Products) and Animal Manure in Long-Term Semi-Continuous Assays

Tree-based machine learning model for visualizing complex relationships between biochar properties and anaerobic digestion

The ideal conditions for anaerobic digestion experiments with biochar addition are challenging to thoroughly study due to different experimental purposes. Therefore, three tree-based machine learning models were developed to depict the intricate connection between biochar properties and anaerobic digestion. For the methane yield and maximum methane production rate, the gradient boosting decision tree produced R² values of 0.84 and 0.69, respectively. According to feature analysis, digestion time and particle size had a substantial impact on the methane yield and production rate, respectively. When particle sizes were in the range of 0.3-0.5 mm and the specific surface area was approximately 290 m²/g, corresponding to a range of O content (>31%) and biochar addition (>20 g/L), the maximum promotion of methane yield and maximum methane production rate were attained. Therefore, this study presents new insights into the effects of biochar on anaerobic digestion through tree-based machine learning.

Co-Fermentation of Microalgae Biomass and Miscanthus × giganteus Silage—Assessment of the Substrate, Biogas Production and Digestate Characteristics

The development of a sustainable bioenergy market is currently largely fueled by energy crops, whose ever-increasing production competes with the global food and feed supply. Consequently, non-food crops need to be considered as alternatives for energy biomass production. Such alternatives include microalgal biomass, as well as energy crops grown on non-agricultural land. The aim of the present study was to evaluate how co-digestion of microalgal biomass with giant miscanthus silage affects feedstock properties, the biogas production process, biogas yields, methane fractions and the digestate profile. Combining giant miscanthus silage with microbial biomass was found to produce better C/N ratios than using either substrate alone. The highest biogas and methane production rates—628.00 ± 20.05 cm3/gVS and 3045.56 ± 274.06 cm3 CH4/d—were obtained with 40% microalgae in the feedstock. In all variants, the bulk of the microbial community consisted of bacteria (EUB338) and archaea (ARC915).

Successful and stable operation of anaerobic thermophilic co-digestion of sun-dried sugar beet pulp and cow manure under short hydraulic retention time

This work consists of a long-term (621 days) experimental study about biogas production from sun dried sugar beet pulp and cow manure. Thermophilic (55 °C) anaerobic co-digestion was performed in semi-continuous reactors, testing ten hydraulic retention times (30-3 days) (HRTs) and organic loading rates (2-24 gVS/L_reactor∙d) (OLRs). Results showed that the best global system performance (regarding stability, biogas production, and organic matter removal) was achieved at an HRT as short as 5 days (OLR of 12.47 gVS/L_reactor∙d) with a biogas yield of 315 mL/gVS_added. The gradual OLR increase allowed system control and time-appropriate intervention, avoiding irreversible process disturbances and maintaining admissible acidity/alkalinity ratios (<0.8) for HRTs ranging from 30 to 4 days. The accumulation of acetic acid was the main cause of the process disturbance observed at short HRTs. It was deduced that for the HRT of 3 days, the methane productivity was mainly owing to the hydrogen-utilizing methanogens pathway. This research clearly shows how an adequate combination of agro-industrial wastes and livestock manure could be processed by anaerobic co-digestion in short HRTs with great efficiency and stability and deepens in the understanding of the start-up, stability and optimization of the co-digestion.

Feasibility of Coupling Anaerobic Digestion and Hydrothermal Carbonization: Analyzing Thermal Demand

Anaerobic digestion is a biological process with wide application for the treatment of high organic-containing streams. The production of biogas and the lack of oxygen requirements are the main energetic advantages of this process. However, the digested stream may not readily find a final disposal outlet under certain circumstances. The present manuscript analyzed the feasibility of valorizing digestate by the hydrothermal carbonization (HTC) process. A hypothetical plant treating cattle manure and cheese whey as co-substrate (25% v/w, wet weight) was studied. The global performance was evaluated using available data reported in the literature. The best configuration was digestion as a first stage with the subsequent treatment of digestate in an HTC unit. The treatment of manure as sole substrate reported a value of 752 m3/d of biogas which could be increased to 1076 m3/d (43% increase) when coupling an HTC unit for digestate post-treatment and the introduction of the co-substrate. However, the high energy demand of the combined configurations indicated, as the best alternative, the valorization of just a fraction (15%) of digestate to provide the benefits of enhancing biogas production. This configuration presented a much better energy performance than the thermal hydrolysis pre-treatment of manure. The increase in biogas production does not compensate for the high energy demand of the pre-treatment unit. However, several technical factors still need further research to make this alternative a reality, as it is the handling and pumping of high solid slurries that significantly affects the energy demand of the thermal treatment units and the possible toxicity of hydrochar when used in a biological process.

Microbial communities and their enzymes facilitate degradation of recalcitrant polymers in anaerobic digestion

Microbial consortia efficiently degrade complex biopolymers found in the organic fraction of municipal solid waste (OFMSW). Through enzyme production and division of labor during anaerobic digestion, microbial communities break down recalcitrant polymers and make fermentation products, including methane. However, microbial communities remain underutilized for waste degradation as it remains difficult to characterize and predict microbial interactions during waste breakdown, especially as cultivation conditions change drastically throughout anaerobic digestion. This review discusses recent progress and opportunities in cultivating natural and engineered consortia for OFMSW hydrolysis, including how recalcitrant substrates are degraded by enzymes as well as the critical factors that govern microbial interactions and culture stability. Methods to measure substrate degradation are also reviewed, and we demonstrate the need for increased standardization to enable comparisons across different environments.

Anaerobic Digestion of Wastewater Sludge and Alkaline-Pretreated Wheat Straw at Semi-Continuous Pilot Scale: Performances and Energy Assessment

During the last decade, the application of pretreatment has been investigated to enhance methane production from lignocellulosic biomass such as wheat straw (WS). Nonetheless, most of these studies were conducted in laboratory batch tests, potentially hiding instability problems or inhibition, which may fail in truly predicting full-scale reactor performance. For this purpose, the effect of an alkaline pretreatment on process performance and methane yields from WS (0.10 g NaOH g−1 WS at 90 °C for 1 h) co-digested with fresh wastewater sludge was evaluated in a pilot-scale reactor (20 L). Results showed that alkaline pretreatment resulted in better delignification (44%) and hemicellulose solubilization (62%) compared to untreated WS. Pilot-scale study showed that the alkaline pretreatment improved the methane production (261 ± 3 Nm3 CH4 t−1 VS) compared to untreated WS (201 ± 6 Nm3 CH4 t−1 VS). Stable process without any inhibition was observed and a high alkalinity was maintained in the reactor due to the NaOH used for pretreatment. The study thus confirms that alkaline pretreatment is a promising technology for full-scale application and could improve the overall economic benefits for biogas plant at 24 EUR t−1 VS treated, improve the energy recovery per unit organic matter, reduce the digestate volume and its disposal costs.

Structure of Microbial Communities When Complementary Effluents Are Anaerobically Digested

Olive oil and pig productions are important industries in Portugal that generate large volumes of wastewater with high organic load and toxicity, raising environmental concerns. The principal objective of this study is to energetically valorize these organic effluents—piggery effluent and olive mill wastewater—through the anaerobic digestion to the biogas/methane production, by means of the effluent complementarity concept. Several mixtures of piggery effluent were tested, with an increasing percentage of olive mill wastewater. The best performance was obtained for samples of piggery effluent alone and in admixture with 30% of OMW, which provided the same volume of biogas (0.8 L, 70% CH4), 63/75% COD removal, and 434/489 L CH4/kg SVin, respectively. The validation of the process was assessed by molecular evaluation through Next Generation Sequencing (NGS) of the 16S rRNA gene. The structure of the microbial communities for both samples, throughout the anaerobic process, was characterized by the predominance of bacterial populations belonging to the phylum Firmicutes, mainly Clostridiales, with Bacteroidetes being the subdominant populations. Archaea populations belonging to the genus Methanosarcina became predominant throughout anaerobic digestion, confirming the formation of methane mainly from acetate, in line with the greatest removal of volatile fatty acids (VFAs) in these samples.

Anaerobic mono and co-digestion of organic fraction of municipal solid waste and landfill leachate at industrial scale: Impact of volatile organic loading rate on reaction kinetics, biogas yield and microbial diversity

The present study was aimed to demonstrate a semi-commercial biomethanation plant based on anaerobic gas lift reactor (AGR) for the mono and co-digestion of organic fraction of municipal solid waste (OFMSW) and landfill leachate (LL) for 47 weeks. The reactors were commissioned at a volatile organic loading rate (VOLR) starting from 0.4 to 6.2 kg VS/(m³·day) to investigate the impact of VOLR on the organic matter removal rates, substrate utilization rate using Stover-Kincannon reaction kinetics, biogas yield and microbial diversity. 16s-metagenomic sequencing of the species present in the inoculum that was acclimatized with OFMSW, LL separately and in combination was also performed to identify the dominant microbial species in the mixed microbial consortia. Results revealed that the VS reduction in AGR 1, AGR 2 and AGR 3 at full load was 46%, 42% and 47% respectively with a corresponding biogas generation of 73.8 m³/day, 42 m³/day and 60.8 m³/day. The biodegradability in AGR 1 was between 73% and 81% whereas in AGR 2 and AGR 3, it was between 57% and 78% and 64% and 86% respectively. The operational strategy of digestate recirculation facilitated in the reduced usage of buffering chemicals which impacted on overall financials of the plant. The techno-economic analysis suggests that these kinds of biomethanation plants are remunerative.