Anaerobic digestion (part 2): digestion technologies and applied processes - state-of-the-art

Anaerobic digestion is a biological process of degradation of organic matter, furthering two primary aims: an energetic valorization through the production of biogas, mainly composed of methane (CH₄) and carbon dioxide (CO₂), and an agricultural valorization through the production of digestate (liquid or pasty organic residue of undigested waste), rich in nutrients (nitrogen, phosphorus, potassium) and which can be used as a fertilizer or enrichment (direct spreading or composting). Anaerobic digestion follows a relatively generic global scheme (incoming waste pre-treatment; digestion; produced digestate post-treatment and biogas energy recovery), but existing technologies and associated processes are usually complex and diverse. Some developed technologies are wet ones (infinitely stirred reactors, fixed bed, fluidized bed and sludge bed reactors) and others are dry technologies (vertical reactors with digestate or biogas recirculation, horizontal piston reactors, dry batch-operated percolation reactors, anaerobic digestion cells). Associated processes differentiate by chosen pre-treatment and post-treatment upstream and downstream of anaerobic digestion, with a main constraint: the nature of the waste to be fermented. All variants can be implemented to optimize the effectiveness of the chosen process. (author)