Anaerobic digestion is the breakdown of organic matter by bacteria in the absence of oxygen.

Anaerobic digestion is a natural process, digesters are not a new idea and have been around for some time. They are commonly used for the treatment of sewage and for managing animal waste. Increasing environmental pressures on waste disposal has increased the use of digestion as a process for reducing waste volumes and generating useful byproducts. It is a fairly simple process that can greatly reduce the amount of organic matter which might otherwise end up in landfill or waste incinerators

Almost any organic material, from waste paper to grass clippings, left over food, sewage and animal waste can be processed in this manner. After sorting or screening to remove inorganic or hazardous materials, metals, plastics etc. the material to be processed and are often shredded or minced to achieve a better reaction (even ultrasound has been used in the process to aid in the break up of solids). The material is then fed into a sealed digester together with some water in the case of dry materials.

There are two conventional processes - mesophilic which takes place at ambient temperatures typically between 20^{o and 40oC and thermophilic which takes place at elevated temperatures, typically up to 70oC. The residence time in a digester varies with the feed material and the temperature. In the case of mesophilic digestion residence times may be between 15 and 30 days and from 12 to 14 days with a thermophillic process. Thermophillic digestion results in a faster reaction with a more of the material digested but at a cost of more energy being used to complete the process and at the risk of it being slightly more unstable than the mesophillic process.}

Many continuous digesters have mechanical devices to slowly mix the contents and to allow excess material to be continuously bled off to maintain the volume of the material being treated at a reasonably constant volume.

The digestion of the organic material is done by a range of many different species of different naturally occurring bacteria all doing a different job at a different step in the digestion process. Maintaining suitable conditions in the digester is essential in maitaing a healthy bacterial flora.

Four stages of digestion have been recognised. The first is hydrolysis in which complex organic molecules are brocken down into more simple sugars, amino acids, fatty acids with the addition of hydroxyl groups.

The second stage is acidogenesis where a further breakdown occurs producing ammonia, carbon dioxide and hydrogen sulphide.

The third stage is acetagenesis where the products of acidogenesis are further digested to produce products such as carbon dioxide, hydrogen and acetates.

The fourth stage is methanogenesis where methane, carbon dioxide and water are produced.

There are three principal by-products of anaerobic digester.

• Biogas, a gaseous mixture comprising mostly methane, carbon dioxide and some hydrogen. This can be burnt as a fuel to generate electricity and heat for thermophilic digesters. Excess electricity can be sold to electricty suppliers and heat can also be used for heating other buildings. As the gas is not released directly into the atmosphere and the carbon dioxide comes from an organic source with a short carbon cycle , biogas does not contribute to increasing atmospheric carbon dioxide concentrations it is considered to be an environmentally clean energy source. The production of biogas is not a steady stream. At the beginning of the process it is slow with most of the reaction to come. In the middle it is highest and nearing the end it will slow again with only the hardest to digest material left over. Different

• The second by-product is a liquid that is rich in nutrients and can be an excellent fertiliser dependant on the quality of the material being digested. If the digested materials includes low levels of toxic metals, or peristent sysnthetic organic materials such as pesticides, PCBs, etc. the effect of digestion is to significantly concentrate such materials in the digester liquor and in certain cases this material needs special treatment prior to disposal. In extreme cases the disposal costs and the environmental risks posed by such material can offset any enviromental gains provided by the use of biogas. This is a significant risk when treating sewage from industrialsed catchments.

• The third by-product is a stable organic material comprised largely of lignin and chitin but often with a variety of plastics and mineral components in a matrix of dead bacterial cells. This resembles domestic compost and can be used as a compost or to make low grade building products such as fibre boards.

In almost all plants there are ancillary processes to treat and manage all the by-products. the Gas stream is dried and sometimes sweetened before storage and use.

The sludge liquor mixture has to be separated by one of a variety of ways including pressure filtration, vacuum filtration, settlement etc.

Digestion can be either wet or dry. Dry digestion refers to mixtures which have a solids content of around 30% whereas wet digestion refers to mixtures of 15% or less.

The two main types types of reactors are continuous and batch. Batch is the simplest with the biomass added to the reactor at the beginning and sealed for the duration of the process. In a continuous process ,which is much the most common, organic matter is constantly added to reactor and the end products constantly removed, resulting in a much more constant production of biogas.

Although there will always be a net loss in energy in the whole system (the energy to grow the biomass being more than the output of the reactor), for the processing of waste organic material anaerobic digestion is the preferable environmental solution. The biggest impacts on the environment include the energy and materrials used to build the plant, transport costs and fuel use in transporting material to site and visual and audible impacts of the site operation. Odour can be a severe problem during emptying cycles . This is particularly difficult issue to manage in batch reactors.

To be economically viable there must be a market for the end products. Biogas can be sold or used in almost all parts of the world where it will offset demand on fossil fuel stocks. Where the digester liqour is suitable for use as a fertilser, there are often appropriate markets available although the balance of nutrients often means that supplemental nutrients also need to be applied.

The sludge component, even when dried and available as a soil conditioner, might not be easily disposed of although it can be used in many localities on non agricultural area such as golf courses and cover for landfills. In some localities the sludge is itself used as a fuel in heating systems and the residual ash disposed of in landfill.