Anaerobic digestion plants are simply the process facilities in which anaerobic digestion takes place. When people talk of anaerobic digestion plants (AD Plants) though they are usually refering to commercial scale Anaerobic Digestion Facilities, as the smaller “low technology” house and community sewage type digesters are usually referred to as biogas digesters.
A typical commercial AD Plant processes about 10,000 to 30,000 tonnes annually of organic feedstock and comprises:
- A reception facility where re incoming wastes are offloaded checked for the presence of impurities or “contraries” and fed into the plant and AD reactor vessel(s) material processing and supply system
- Pre-treatment facilities as necessary to the feed materials before entering the AD reactor vesssel(s) with pre-treatment being provided for such purposes as removing all recyclables, cutting up and mixing, and optimizing the reactor feed materials for maximum biogas production
- The reaction stage in which the fermentation” takes place in single or multiple tanks, in series or in parallel circuits
- The post-treatment stage where both solid and liquid phase outputs will be further treated to suit the specified provision of a product wherever possible. In most cases there will be a final maturation stage where the solid fibrous “compost” in the output will be allowed to aerobically decompose further
- Ancillary structures such as a biogas storage facility (typically round or oval in shape and distinctive looking), storage tanks to hold liquid digestate etc.
- Equipment for generating electricity from the biogas and this is usually a large reciprocating engine which utilizes the methane gas as its fuel.
- A treatment facility for any excess liquid digestate if this is unsuitable for sale. Where such a facility is needed it will typically be an oxidation stage reactor where air is passed through the water in sequencing batch reactors or a reverse osmosis unit
- Related buidings etc
- Site access roads, drainage and electric connection to the local grid, plus any otrhe services. installations.
The Agri-Food and Biosciences Institute (UK) has a good page abour Anaerobic digestion plants in agriculture, and we quote their statement:
Anaerobic Digestion Plants for Agriculture
Anaerobic digestion (AD) plants can be on-farm units, designed to deal with manures and other organic materials produced at farm level. Alternatively, AD plants can be designed as centralised units to deal with products from a number of farms, along with co-digestion of organic materials from other industries…
The most appropriate way… to realise this energy potential is through centralised anaerobic digestion (CAD) schemes (Frost, 2005). In addition to energy production, there is considerable potential for CAD to assist in centrally managing the distribution of plant nutrients in manures, together with minimising biosecurity risks (pathogen kill) .. . Whilst CAD has potentially a major role… and offers the most appropriate and immediate way forward, there is also significant potential for on-farm AD.
Still on the same page they describe the increasingly popular idea of the centralised Anerobic Digester:
Centralised anaerobic digestion
Typical agriculturally based centralised AD (CAD) plants use farm products (livestock manures and crops) as the main feedstocks, as well as other organic material from, for example, food processing. Co-digestion can provide an additional source of income through gate fees and can improve the yield of biogas per unit of feedstock input. CAD plants can be thermophilic or mesophilic. Compared to typical on-farm plants, CAD plants are larger (0.1-1.0 MWelectricity), give economies of scale and offer better market opportunities for heat (for local industry and/or district heating) and fibre production. CAD schemes can involve a number of farms within a radius of about 10 km from the plant. All agriculturally based CAD schemes distribute digestate back to agricultural land, normally that of the supplying farms. Raw slurry and digestate are rich in plant nutrients (nitrogen, phosphorus and potassium). Digestate must be applied to agricultural land in accordance with crop requirements for plant nutrients. Nutrient management is a major issue for consideration when determining the feasibility of any AD scheme. CAD schemes have major potential to assist in managing and redistributing plant nutrients in slurry. When redistributing digestate to farms it is very important to ensure biosecurity. All CAD schemes should include sterilisation of material prior to redistribution.
So there you have it, and this page is hopefully all you needed in order to understand what an Anaerobic Digestion Plant is.