Ammonia I - AD and digestate management

Posted in: bioenergy

This article explores the process of ammonia emissions from AD digestate, and provides an overview of current mitigation practices in the UK.

Ammonia (NH3) is a chemical compound which has significant importance within the natural world. It is a stable colourless gas which, in nature, is produced by nitrogen fixing microorganisms that convert atmospheric nitrogen (N2) into NH3. Nitrogen is needed by all living organisms for growth and survival, therefore, organic matter contains large amounts of it in the form of proteins, amino acids and DNA. When organisms excrete waste or die, the organic matter is decomposed by microorganisms which convert that “organic nitrogen” into inorganic ammonia through the process of ammonification. There are therefore two biological routes through which ammonia can be released into the environment: nitrogen fixation and ammonification.

Anaerobic digestion (AD) is a process through which organic matter (such as food waste, manure or bakery waste for example) is placed in an oxygen-deprived environment where microorganisms digest and decompose it. This leads to the release of biogas – which can be used for bioenergy purposes – and to the production of digestate, which is the portion of the organic matter which could not be fully digested into gas[iv]. AD-derived biogas is mainly made up of CO2 and methane, however it also contains other trace gases such as NH3. Digestate is also rich in mineralised N in the form of NH4+ and NH3.

Ammonia is not a greenhouse gas (GHG). Once released in the atmosphere, the molecules of ammonia only last a few hours before finding their way back to the surface, either in the form of dry deposition (as a gas) or in the form of wet deposition (as ammonium in precipitation). As the ammonia deposits back onto the surface, in soils or water bodies, it can create a surplus of nutrient in aquatic ecosystems. This phenomenon is referred to as eutrophication. As the habitat’s nutrient content increases due to the high N input, algal growth significantly increases on the surface of the water, leading to light being blocked from reaching below the surface and preventing oxygen from entering the system as well. This leads to hypoxia and eventually to a complete deregulation of the ecosystem and biodiversity loss. Eutrophication also leads to the breakdown of ecosystem services on which human populations rely, such as water quality and pathogen regulation for example. Ammonia molecules in the atmosphere can also bind to other compounds such as sulfuric acid and nitrate to form particulate matter (PM), which are a form of anthropogenic aerosols. These PMs may cause more direct harm to the environment and to human health. In 2016, PM pollution was linked to 374 000 premature deaths in the EU.

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This article was written by Thea Allary, Senior Research Analyst at NNFCC.

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