Dear Colleagues,

The number of full-scale biogas plants is continuously increasing worldwide, especially in Germany, with approximately 9800 biogas plants. Biogas technology is not just an energy technology, but an environmental technology as well. It can sanitize and detoxify fecal material such as livestock manure, and the digestates can be used as fertilizer or as a base for humus formation. Digestate, in turn, can be transformed into biochar, but this manufacturing process is not within the scope of this Special Issue. Straw, grass, polluting algae, water hyacinths, wild flowers, municipal biowaste and even restaurant leftovers can also be used as input. In parallel, the question arises as to which waste materials can be best used in a given country without competing with land use for food production. The microbiology in the fermentation reactor also needs attention. Biogas is unique, CO₂ neutral or, in some coupled processes, even CO₂ negative, but sometimes the greenhouse gas balance is challenged. Therefore, articles about the GHG balance are also welcomed. The purified gaseous product, biomethane, is the only one that embodies the three types of energy: electricity, heat and fuel. The biogas can be compressed and used in a decentralized manner. Many countries have a gas grid for conventional sources such as fossil gas, which could serve as a huge energy battery to store biomethane. A country like Denmark, for example, has set itself the goal of becoming completely independent of gas imports in the next decade through biomethane. By producing electricity in a central heating power plant, heat can be used as byproduct. This leads to the highest conversion efficiency of about 90% in renewables, which is feasible. In times of resource dependency, gas is becoming increasingly important as a directly usable product (including the use of CO₂ in pure form for industrial purposes). Anaerobic degradation processes also concentrate salts; the effluent products ammonium and H₂S could represent a severe environmental and technical problem. In some regions, the use of digestates as fertilizers is limited, while in other regions it is highly recommended. Therefore, there are developments in densely populated areas to refine fermentation residues to produce humus formers, transportable inorganic salts as fertilizers and potable water. Therefore, manuscripts on these topics are also welcomed.

Prof. Dr. Paul A. Scherer
Prof. Dr. Richard Arthur
Guest Editors

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• emissions
• greenhouse gases
• effluents
• biogas plants
• digestates
• fertilizers
• biomethane