Anaerobic digestion (AD) is a natural process, spontaneously taking place in natural surroundings, such as marshes,bogs, paddies, or in cesspits, landfills, and dedicated digesters, used for the conversion of organic waste into a rich gas. Typical AD substrates are: manure, sewage sludge, high BOD wastewater, and – more recently - the organic fraction of Municipal Solid Waste (MSW), which is at the heart of this survey. Anaerobic digestion thus converts moist materials that would not sustain combustion into flammable biogas, either used as a boiler or motor fuel, or upgraded to pipeline quality.
Anaerobic digestion is largely based on a sequence of biochemical steps (hydrolysis, formation of volatile fatty acids and acetic acid, conversion into methane and carbon dioxide) and is hence a relatively slow process that can be seriously retarded by inhibitors and toxicants. Moreover, part of the organics (plastics, rubber, wood, lignin in board & paper…) do not degrade and, depending on process conditions, the bacteria responsible for various stages in the process show quite distinct growing rates, yet must still cohabitate harmoniously. Large molecules and hydrophobic substrates either require special pre-treatment for rendering digestion possible, or remain almost unchanged in a moist residue, the digestate.
Treatment technology is subdivided into batch and continuous processes, continuous single step and double or multiple step (staged) digestion, vertical and horizontal treatment units, with various flow arrangements and mixing methods, meso- and thermophilic processes, and also into 'dry' (high solids) and wet' (low solids concentration) digestion . Changes in feed or operating conditions can either disturb equilibrium or result in intermediaries that may inhibit the overall process or shut it down altogether. It is crucial to use adequate designs, enhancing stability, as well as control technologies to continually monitor and adjust the environment to prevent such occurrence.
Anaerobic digestion has been promoted for three decades by National as well as E.U. subsidies, destined for financing R&D and demonstration plants, resulting sometimes in successful process demonstration. However, both technical difficulties and poor economics have hampered a more widespread application. The promise of Green Energy subsidies will somewhat influence upon decision making. Hence, the more promising fields of application of energy recovery by AD are to be identified, as well as subsisting economic challenges. Several processes successfully demonstrated AD of organics from MSW. Unfortunately, only part of fuel values available can be converted by AD and the moist residue still leaves most of the original waste for final disposal by either landfill or thermal techniques. Thus, technical opportunities are limited to those cases in which the digestate can be recycled, i.e. when the fractions to be treated are sufficiently pure and exempt of heavy metals, plastics, and other undesirable contaminants!