What is Anaerobic Digestion?

Currently, the majority of biodegradable waste (green waste, food waste, paper/card waste) we generate is sent to landfill for final disposal, which in turn, is having a negative effect on our environment. This is because when sent to landfill, biodegradable waste is broken down by bacteria and releases methane into the atmosphere, a harmful greenhouse gas that contributes to climate change.

Demanding targets have been set by the EU Landfill Directive (1999/31/EC) in order to reduce the amount of biodegradable waste being sent to landfill. These targets are: by 2010, to reduce the amount of landfilled biodegradable municipal waste to 75% of that arising in 1995; by 2013, to reduce the amount to 50% of that arising in 1995; and by 2020, to reduce the amount to 35% of that arising in 1995 (DEFRA).

Anaerobic digestion is a naturally occurring process whereby anaerobic bacteria breakdown organic material, in the absence of oxygen to produce biogas and digestate. It is effectively a controlled and enclosed version of the anaerobic breakdown of biodegradable waste in landfill which releases methane into the atmosphere (FOE). Ultimately, the aim of using anaerobic digestion to treat waste is to reduce the impacts residual waste has on the environment when sent to landfill.  This article will take a closer look at the anaerobic digestion process itself, as well as taking into consideration the process’ potential advantages and disadvantages.

The Anaerobic Digestion Process

Anaerobic Digestion processes last around two to three weeks depending on the ease and degree to which materials are converted into biogas and the technology used. For example, for waste containing a larger amount of woody (high lignin content) material, longer residence times will be required to achieve the desired biogas production (DEFRA). In order to speed up the process, waste materials can be shredded into smaller particle sizes that are easier to be broken down by microbes.

Both wet and dry anaerobic digestion systems can be used to treat waste. Waste with a high moisture content is treated with a wet anaerobic digestion system, whereas waste with a low moisture content is treated with a dry anaerobic digestion system. The digestion process in the wet anaerobic digestion system takes place in air tight containers known as digesters. These digesters stand vertically and provide anaerobic conditions in order for anaerobic digestion to occur. Mixing can be achieved using mechanical stirring devices, or by recirculating biogas or waste through the digestion tank (DEFRA). ‘Dry’ systems use plug flow reactor designs. This approach involves adding fresh waste and/or partially fermented waste into one end of the reactor while fully digested residue is extracted from the other (DEFRA).

Anaerobic digestion produces both biogas and a residue known as digestate. Biogas produced from anaerobic digestion can either be used as a natural gas substitute, or more commonly, it is used to fuel boilers to produce heat (hot water and steam), or to fuel generators in combined heat and power (CHP) applications to generate electricity, as well as heat(DEFRA). Digestate produced from anaerobic digestion can be used to spread on land as a soil fertiliser.

How the Process Works

The initial stage of anaerobic digestion is a chemical reaction known as hydrolysis, where complex organic molecules are broken down into simple sugars, amino acids, and fatty acids (FOE). The next stage of anaerobic digestion is a biological process known as acidogenisis. This process involves the conversion of decomposed organic material into simple organic acids. Following acidogenisis is another biological process known as acetogenisis which involves the conversion of simple organic acids in acetic acid. The final stage of anaerobic digestion is another biological process known as methanogenisis, which involves the conversion of acetic acids into biogas (methane (CH₄) and carbon dioxide (CO₂)).

Potential Advantages

• Anaerobic digestion reduces the biodegradable content of residual waste which in turn reduces the production of the harmful greenhouse gas methane when waste is sent to landfill.
• Anaerobic digestion produces a source of energy (biogas) without using up our limited natural resources.
• Anaerobic digestion is an economically effective treatment of organic waste. It produces biogas, a renewable energy source, and a digestate that can be used as organic fertilizer.

Potential Disadvantages

• Before the anaerobic digestion process can take place, residual waste requires a certain level of mechanical pre-treatment.
• Anaerobic digestion technology for the treatment of mixed waste is somewhat underdeveloped in the UK.
• Anaerobic digestion has high capital and operational costs.

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