Finnish Environment Institute | Suomen ympäristökeskus | Finlands miljöcentral

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Storing recycled fertilizers in Biokymppis´ property in Kitee.

Most of the biogas plant digestate can be used as fertilizers and dressing. Biogas production is a closed process and all the nutriens remains in the process. Because of high nutrient content, biogas end-products are typically more suitable for fertilizer use than unprocessed raw materials. Digestate can decrease mineral fertilizer use. It is also possible to achieve nearly closed nutrient recycling.

Digestate included raw materials’ nutrients and minerals. Solids similar to humus are generated including solid matter, water and carbon dioxide. Methane can be used as energy. Depending on the decomposability of raw materials, 40-70 % of the organic matter is decomposed during biogas production. The amount of composting raw material depends on decomposition rates of carbon hydrates, proteins and fat. The composition rates can be determined, but in practice there is some variation involved. Manure is suitable main raw material since it is abundantly available and includes nutrients for microbes and the alkalinity value is high. Green plants have the best methane production potential. Phosphorous, potassium and micronutrients do not change significantly during the process.

A portion of organic nitrogen in manure transforms into ammoniacal nitrogen. Ammoniacal nitrogen is suitable for plants. Depending on the type of manure and processing, 20-30 % of the total nitrogen transform into ammonium, when plants biomass is used the transformation includes 50-85 % of the total nitrogen. In general, 40-60 % of nitrogen in digestate is ammoniacal nitrogen. Biogas processing substantially decreases greenhouse gas emissions of manure and other organic raw materials, but transforming nitrogen into ammoniacal nitrogen can generate ammonia emissions. Most of the greenhouse gases are generated in the storage phase, but these emissions are smaller than emissions of unprocessed manure. Emissions of digestate are mainly generated during spreading (96 % in surface spreading). Therefore, injection spreading should be used to avoid evaporation of nitrogen and leaching. Fertilization effect of solid nitrogen is also maximized. Ammonia emissions of digestate can be decreased by covering storages.

Digestate prodcution decreases the amount of phytotoxins and increases homogenous quality and runniness of manure, making it more suitable for spreading. Odor is decreased up to 80%. At the same time, pathogens and weed seeds are eliminated as wellas organic hazardous substances such as phthalate used as plastic processing and polycyclic aromatic hydrocarbon (PAH), making digestate safer to use.

Published 2018-07-06 at 10:00, updated 2018-07-06 at 10:00
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