Anaerobic Digestion for Producing Renewable Energy-The Evolution of This Technology in a New Uncertain Scenario

Valorization of crop residues and animal wastes: Anaerobic co-digestion technology

To switch the over-reliance on fossil-based resources, curb environmental quality deterioration, and promote the use of renewable fuels, much attention has recently been directed toward the implementation of sustainable and environmentally benign 'waste-to-energy' technology exploiting a clean, inexhaustible, carbon-neutral, and renewable energy source, namely agricultural biomass. From this perspective, anaerobic co-digestion (AcoD) technology emerges as a potent and plausible approach to attain sustainable energy development, foster environmental sustainability, and, most importantly, circumvent the key challenges associated with mono-digestion. This review article provides a comprehensive overview of AcoD as a biochemical valorization pathway of crop residues and livestock manure for biogas production. Furthermore, this manuscript aims to assess the different biotic and abiotic parameters affecting co-digestion efficiency and present recent advancements in pretreatment technologies designed to enhance feedstock biodegradability and conversion rate. It can be concluded that the substantial quantities of crop residues and animal waste generated annually from agricultural practices represent valuable bioenergy resources that can contribute to meeting global targets for affordable renewable energy. Nevertheless, extensive and multidisciplinary research is needed to evolve the industrial-scale implementation of AcoD technology of livestock waste and crop residues, particularly when a pretreatment phase is included, and bridge the gap between small-scale studies and real-world applications.

Current Trends in Biological Valorization of Waste-Derived Biomass: The Critical Role of VFAs to Fuel A Biorefinery

The looming climate and energy crises, exacerbated by increased waste generation, are driving research and development of sustainable resource management systems. Research suggests that organic materials, such as food waste, grass, and manure, have potential for biotransformation into a range of products, including: high-value volatile fatty acids (VFAs); various carboxylic acids; bioenergy; and bioplastics. Valorizing these organic residues would additionally reduce the increasing burden on waste management systems. Here, we review the valorization potential of various sustainably sourced feedstocks, particularly food wastes and agricultural and animal residues. Such feedstocks are often micro-organism-rich and well-suited to mixed culture fermentations. Additionally, we touch on the technologies, mainly biological systems including anaerobic digestion, that are being developed for this purpose. In particular, we provide a synthesis of VFA recovery techniques, which remain a significant technological barrier. Furthermore, we highlight a range of challenges and opportunities which will continue to drive research and discovery within the field. Analysis of the literature reveals growing interest in the development of a circular bioeconomy, built upon a biorefinery framework, which utilizes biogenic VFAs for chemical, material, and energy applications.

Integrated system of anaerobic digestion and pyrolysis for valorization of agricultural and food waste towards circular bioeconomy: Review

Agricultural and food waste have become major issue affecting the environment and climate owing to growing population. However, such wastes have potential to produce renewable fuels which will help to meet energy demands. Numerous valorization pathways like anaerobic digestion, pyrolysis, composting and landfilling have been employed for treating such wastes. However, it requires integrated system that could utilize waste and promote circular bioeconomy. This review explores integration of anaerobic digestion and pyrolysis for treating agricultural and food waste. Proposed system examines the production of biochar and pyro-oil by pyrolysis of digestate. The use of this biochar for stabilizing anaerobic digestion process, biogas purification and soil amendment will promote the circular bioeconomy. Kinetic models and framework of techno-economic analysis of system were discussed and knowledge gaps have been identified for future research. This system will provide sustainable approach and offer carbon capture and storage in form of biochar in soil.

Review of Thermochemical Technologies for Water and Energy Integration Systems: Energy Storage and Recovery

Thermochemical technologies (TCT) enable the promotion of the sustainability and the operation of energy systems, as well as in industrial sites. The thermochemical operations can be applied for energy storage and energy recovery (alternative fuel production from water/wastewater, in particular green hydrogen). TCTs are proven to have a higher energy density and long-term storage compared to standard thermal storage technologies (sensible and latent). Nonetheless, these require further research on their development for the increasing of the technology readiness level (TRL). Since TCTs operate with the same input/outputs streams as other thermal storages (for instance, wastewater and waste heat streams), these may be conceptually analyzed in terms of the integration in Water and Energy Integration System (WEIS). This work is set to review the techno-economic and environmental aspects related to thermochemical energy storage (sorption and reaction-based) and wastewater-to-energy (particular focus on thermochemical water splitting technology), aiming also to assess their potential into WEIS. The exploited technologies are, in general, proved to be suitable to be installed within the conceptualization of WEIS. In the case of TCES technologies, these are proven to be significantly more potential analogues to standard TES technologies on the scope of the conceptualization of WEIS. In the case of energy recovery technologies, although a conceptualization of a pathway to produce usable heat with an input of wastewater, further study has to be performed to fully understand the use of additional fuel in combustion-based processes.

Two-Phase Anaerobic Digestion of Corn Steep Liquor in Pilot Scale Biogas Plant with Automatic Control System with Simultaneous Hydrogen and Methane Production

Experimental studies of two-phase anaerobic digestion of corn steep liquor in semi-continuous automatic and semi-automatic modes of operation of a cascade of two anaerobic bioreactors with monitoring and control systems were performed. Corn steep liquor—a waste product from the process of treating corn grain for starch extraction—was used as a substrate in the process of anaerobic digestion with simultaneous hydrogen and methane production. The daily yields of biohydrogen in bioreactor 1 of the cascade (with a working volume of 8 dm3) are variable. In good operation, they are in the range of 0.7 to 1.0 L of biogas from a 1 dm3 working volume of the bioreactor, and the optimal pH is in the range of 5.0–5.5. The concentration of hydrogen in the biogas from the hydrogen bioreactor 1 is in the range of 14–34.7%. The daily yields of biomethane in bioreactor 2 of the cascade (with a working volume of 80 dm3) vary in the range 0.4 to 0.85 L of biogas from a 1 dm3 working volume of the bioreactor, and the concentration of methane in the biogas from bioreactor 2 is high and remains practically constant (in the range 65–69%). At a dilution rate of 0.4 day−1 and an organic loading rate of 20 gL for bioreactor 1, respectively, and a dilution rate of 0.05 day−1 for bioreactor 2, the best results were obtained. The computer control system is presented. Some energetical considerations were discussed.

Biogas Production and Biofertilizer Estimation from Anaerobic Co-Digestion of Blends of Wastewater and Microalgae

The study aimed to investigate the enhancement of biogas production through anaerobic digestion from blends of wastewater and microalgae. The microalgae functioned as a co-substrate. A series of laboratory-scale batch anaerobic co-digestion of the wastewater and microalgae were carried out under mesophilic conditions for 21 days. Biogas production rates from wastewater (WW) alone and different blends of WW and microalgae (MA) were analysed. In addition, the nutrient values and reduction in the volume of the WW after digestion were determined. The results show that the quantity and quality of biogas produced with different mix ratios of WW to MA (WW only, 3 : 1, and 3 : 2) were 24 mL CH4/g COD, 37 mL CH4/g COD, and 44 mL CH4/g COD, respectively. At the optimum mix ratio, the optimum methane produced was 44 mL CH4/g COD, which is much lower than volumes of gas STP conditions, which is 350 mL CH4/g COD. The values of TS, VS, and COD were also reduced by 43.11%, 40.09%, and 71.99% at the optimum mix ratio, respectively. The optimum mix ratio of 1732.77, 77.14, and 174.26 kg/year of urea, diammonium phosphate, and potash fertilizer, respectively, was obtained. The results indicate that biogas production can be improved through co-digestion of WW and MA as a co-substrate relatively, but to improve the production rate another substrate should be investigated. However, the result can be used as a supplementary investigation for practical application in energy production.

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

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