Answers to Recent Questions About Anaerobic Digestion from the Public

Why Would a Farmer Choose Anaerobic Digestion?

Waste biomass from farming and food-industries, such as slurries and the organic waste from food processing can present a considerable waste disposal problem. Release of large quantities of organic wastes into local water courses or seepage into ground water results in pollution of water courses and water supply. To prevent this occurring, the biomass needs to be treated and suitably stored and Anaerobic Digestion is a great way to achieve this.

Biogas is produced by the natural microbial degradation under anaerobic conditions. This technique has been applied to agricultural wastes world-wide on a small scale. Treating the waste by AD gives some environmental benefits, and in addition to AD also results in energy, in the form of biogas. Biogas is an inflammable gas, which can easily be used to generate electricity and heat. The potential benefits are many from on-farm and local use of fossil fuels and provision of energy within rural communities.

Some commentators have said that in addition to energy production, there is considerable potential for Anaerobic Digestion to assist in centrally managing the distribution of plant nutrients in manures, together with minimising biosecurity risks (ie pathogen kill).

Where is Anaerobic Digestion Most Popular?

In countries such as Denmark, Sweden, Germany, Austria and Switzerland there are many examples of co-digestion of farm manure with other suitable organic wastes in prefabricated digestors. There is a large interest in this option in these countries and in other countries both inside and outside the EU.

The most successful digesters utilize all energy produced. The most efficient use of biogas is direct heating. If biogas is burned in an electric generator, the heat produced by the engine must be harnessed and used in order to achieve optimum efficiency. The electricity produced can be used on site or sold into the power grid.

What Happens to the Sludge After the AD Process?

Once the digestion process is complete, the digested manure, or effluent, is released from the digester and stored until it is land applied. The digestion process breaks down many of the solids contained in the manure. As a result, the effluent causes less stress on pumps and agitation equipment than manure does. Also, effluent is homogeneous and may not require agitation prior to land application.

Many agri-food AD systems are located on farms. Farm-based AD systems work well with liquid manure. AD systems provide a valuable manure treatment option, since most other economically effective manure treatment systems (such as composting) require solid materials with dry matter greater than 30%.

What are the Most Popular AD Types of Plants?

Biogas from biomass has historically been used in Asia as a fuel for household uses such as cooking. Denmark and Germany have many modern digesters operating on farms and in central locations using materials such as manure, energy crops, and food-based products and byproducts. These systems typically use biogas to produce electricity and heat.

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.

AD plants can include mechanical separation of fibrous solids from the digestate that, after further processing, can give a value added product, such as compost or pellets (fertiliser or combustible fuel).

How Would I recognise an Anaerobic Digestion Plant if I Saw One?

Anaerobic Digesters themselves are completely sealed vessels which are covered and sealed. Most are insulated and heated, and the digester is an immediatelely recognisable feature at most large biogas plants as a large tall, usually vertical sided, circular tank, with an access ladder and a number of pipes leading to the top where there is also usually a pedestrian access way and handrail.

Biogas plant digesters of this type can usually be spotted very easily when you see them by the characteristic pairing of the digester with the biogas storage tank which acts like a traditional town gas ‘gasometer’, and is usually elegantly curved or “globe” shaped. Many visibly expand and contract to accommodate the volume of biogas present, the result is that most of them become significant landmark features.