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Anaerobic Digestion

Anaerobic digestion is the treatment of industrial or agricultural waste in an oxygen free environment to reduce organic strength and produce biogas. The biogas is a methane rich gas which can be used to produce electricity and/or heat for an industrial process or to be exported.

An anaerobic digestion system typically comprises of the following components:

• A solids removal process to remove suspended solids and other indigestible or dangerous materials in the waste which is usually achieved using one or more of the following technologies: a screw press, belt press, centrifuge, lamella separator or dissolved air flotation system.
• The pre-treatment tank which corrects the pH of the waste and acts as a buffer tank to continuous process reactor tanks.
• The reactor tank which digests the organic material in the waste in four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis which converts the complex organic molecules to methane, carbon dioxide and water.
• A post treatment stage to remove sludge produced in the anaerobic tank or to further polish the organic strength of the waste.
• The biogas produced is then stored is a suitable vessel and consumed in either a gas engine, turbine, boiler or flare.

There are a wide range of anaerobic technologies available to digest waste, from simple lagoons to upflow anaerobic sludge blanket and expanded bed reactor systems. Simple technologies tend to require higher hydraulic retention times, which require more space for larger reactor tanks. However the advantages include that they are more robust and less expensive.

The most suitable technology is dependent on the composition and volume of the waste to be treated and space restriction on site. The volume and methane content of the gas produced is dependent on the digestibility of the waste and the reactor used to digest the waste. The digestibility and levels of methane produced can be determined through laboratory testing with various suitable technologies and typically takes a few weeks. A feasibility survey will determine the most suitable technology and the gas production rates for a particular application.