Dear Colleagues,

This Special Issue mainly deals with the current aspects of anaerobic digestion (AD). The AD treatment process is considered among the main technologies aiming at (but not limited to) the recovery of energy from high-loaded organic wastes, such as sludge biosolids, and leading to their stabilization and subsequent reuse, e.g., for agricultural purposes (soil conditioner). 

For this reason, AD is usually included among renewable energy sources as the main product is the produced biogas (a mixture mainly consisting of methane and carbon dioxide used as an alternative fuel source), as well as the digestate (semi-solid stabilized organic residues), which could subsequently be reused as raw material for the production of several valuables (biofuels, chemicals, etc.)

This SI aims to summarize the current state of the art, indicating the recent trends of relevant research, such as the biogas upgrade aspects (separating most of the carbon dioxide content and producing high-grade biomethane), the further treatment of digestate (for the recovery of water, nutrients, etc.), as well as other alternative modern options. Improved biogas production can be achieved through, e.g., the application of appropriate sludge pre-treatment methods and the further utilization of gaseous streams (e.g., methane as fuel, carbon dioxide for industrial purposes).

Additional topics of interest which may be also included in this issue are life cycle assessment applications in anaerobic digestion, carbon footprint issues, zero carbon balance, the energy–water–waste nexus, the limiting of secondary pollution aspects created by AD operation, and other environmental and operational problems.

Prof. Dr. Anastasios Zouboulis
Dr. Panagiotis G. Kougias
Dr. Konstantinos N. Kontogiannopoulos
Guest Editors

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• biogas valorization
• digestate treatment and stabilization
• resource recovery from organic wastes