An Introduction to Anaerobic Digestion of Organic Wastes

Anaerobic digestion is a process which breaks down organic matter in simpler chemicals components without oxygen. This process can be very useful to treat arising organic waste such as:-sewage sludge-organic farm wastes-municipal solid wastes-green/botanical wastes-organic industrial and commercial wastes Before being digested, the feedstock has to und ergo pre-treatment. There are various types of pre-treatment depending on the feedstock. The purpose of such treatment is to mix different feedstock, to add water or to remove undesirable materials such as large items and inert materials (e.g. plastic, glass) to allow a better digestate quality, a more efficient digestion and it will avoid failure in the process. The digestion process itself takes place in a digester, which can be classified in relation to the temperature, the water content of the feedstock and the number of stage (single or multi-stage). Each digester has its characteristics and properties and thus can be more suitable for a specific feedstock. There are at the present more mesophilic (35 o C) than thermophilic digesters (55 o C) but the difference tends to decrease. There used to be more wet digesters than dry digesters but there is no clear trend anymore. Multi-stage processes aim at optimizing digestion and improving control of the process by separating stages of digestion. Only a few of these digesters are used at the present time. Finally, the batch processes are less expensive and less complex but there are also less efficient. The by-products of anaerobic digestion, biogas and digestate, can be used in order to create a source of incomes. Biogas can be upgraded, most of the time by removing the carbon dioxide and the water vapour, and then, used in a CHP unit to produce electricity and heat. The digestate can be used as a fertilizer or further processed into compost to increase its quality. Anaerobic digestion is helped but also limited by legislation. The main UK policies in favour of anaerobic digestion are the Climate Change levy and the new Renewable Obligation. Every premises aiming at treating waste should apply for a waste management license. There are also regulations concerning specific waste such as animal by-products and sewage sludge. The financial aspect of anaerobic digestion includes the capital and the operating costs. Capitol costs are quite high but the source of incomes coming from the sale of electricity, heat and digestate allows important benefits.