Methods for the Evaluation of Industrial Mechanical Pretreatments before Anaerobic Digesters

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.

Research trends and strategies for the improvement of anaerobic digestion of food waste in psychrophilic temperatures conditions

The organic fraction of municipal solid waste is mainly composed of food waste (FW), and traditional disposal practices for this fraction are generally considered to have negative environmental and economic impacts. However, the organic characteristics of this fraction could also be exploited through the anaerobic digestion of FW (FW-AD), which represents unique advantages, including the reduction of the area required for final disposal and environmental pollution and the same time the generation of renewable energy (mainly methane gas), and a by-product for agricultural use (digestate) due to its high nutrient content. Although approximately 88% of the world's population resides in areas with temperatures below 8 °C, psychrophilic conditions (temperatures below 20 °C) have hardly been studied, while mesophilic (66%) and thermophilic (27%) ranges were found to be more common than psychrophilic FW-AD (7%). The latter condition could decrease microbial activity and organic matter removal, which could affect biogas production and even make AD unfeasible. To improve the efficiency of the psychrophilic FW-AD process, there are strategies such as: measurement of physical properties as particle size, rheological characteristics (viscosity, consistency index and substrate behavior index), density and humidity, bioaugmentation and co-digestion with other substrates, use of inocula with psychrophilic methanogenic communities, reactor heating and modification of reactor configurations. However, these variables have hardly been studied in the context of psychrophilic conditions and future research should focus on evaluating the influence of these variables on FW-AD under psychrophilic conditions. Through a bibliometric analysis, this paper has described and analyzed the FW-AD process, with a focus on the psychrophilic conditions (<20 °C) so as to identify advances and future research trends, as well as determine strategies toward improving the anaerobic process under low temperature conditions.

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Temperature has a great influence on anaerobic digestion of food waste (FW-AD).

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Studies on the psychrophilic condition are limited, warranting further research.

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Physical properties of the substrate and inoculum influence psychrophilic FW-AD.

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The use of inocula adapted to low temperatures could increase biogas production.

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Changes in reactor configurations could improve biogas yield at low temperature.

Pretreatment of Animal Manure Biomass to Improve Biogas Production: A Review

The objective of this research is to present a review of the current technologies and pretreatments used in the fermentation of cow, pig and poultry manure. Pretreatment techniques were classified into physical, chemical, physicochemical, and biological groups. Various aspects of these different pretreatment approaches are discussed in this review. The advantages and disadvantages of its applicability are highlighted since the effects of pretreatments are complex and generally depend on the characteristics of the animal manure and the operational parameters. Biological pretreatments were shown to improve methane production from animal manure by 74%, chemical pretreatments by 45%, heat pretreatments by 41% and physical pretreatments by 30%. In general, pretreatments improve anaerobic digestion of the lignocellulosic content of animal manure and, therefore, increase methane yield.