school energy&environment

Anaerobic digestion

Anaerobic digestion is a biochemical conversion process that occurs in the absence of oxygen and consists of the demolition, by micro-organisms, of complex organic substances (lipids, proteins, glucides) contained in vegetal and animal by-products, which produces a gas (biogas) made of methane (50-70%) and the rest is mainly CO₂ and has an average calorific value of 23,000 kilojoules per cubic metre. The resulting biogas is collected, dried, compressed and stored, and can be used as a fuel to feed gas boilers and produce heat (also coupled with turbines for the production of electric energy), or to feed combined-cycle plants, or internal combustion motors (boat engines with a low number of turns are suitable for this). At the end of the effluent fermentation process the main nutritional elements that were already present in the raw material are kept intact (nitrogen, phosphorus, potassium), by favouring the mineralization of organic nitrogen. The effluent results to be an excellent fertilizer. Anaerobic digestion plants can be fed through residues with a high humidity content, like animal faeces, civil waste (waste water), food waste and the organic fraction of urban solid waste.
However, also in those dumps that are suitably equipped for the collection of biogas, only 40% of the gas produced can be collected, while the remaining part is dispersed into the atmosphere. As the methane, that largely composes biogas, is a greenhouse gas with an effect that is twenty times as high as CO₂, emissions of biogas into the atmosphere are not desirable. When the decomposition of organic waste is obtained through anaerobic digestion of (closed) adequate plant digestors, almost all the gas is collected and used as a fuel.
The recovery of the biogas from dumps is a system that has been experimentally adopted in various countries (England has developed an efficient system of biogas recovery from dumps, both for thermal and electric aims). In Sweden, there are biogas refuelling stations that supply methane vehicles.