ENERGY RECOVERY FROM RESIDUAL WASTE BY MEANS OF ANAEROBIC DIGESTION TECHNOLOGIES

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 mixin