The direct use of agricultural residues as organic fertilizer (e.g. manure and anaerobic digestate from biogas plants) to close nutrient cycles faces several challenges. The plants can only absorb the nutrients to a limited extend and the nutrients are easily washed out by heavy rain events. Some of them are washed into the adjacent surface waters and cause eutrophication. The german fertilizer ordinance (Düngemittelverordnung) therefore limited the amount of organic fertilizer that can be applied to the fields. As a result of the regulation there is frequently a strong regional oversupply of manure and digestate and large amounts of wet residues are already being transported over long distances.
Hydrothermal carbonisation (HTC) is an alternative treatment for these residues by which phosphorus is immobilized on the hydrochar under optimal conditions. Leaching of the hydrochar with acid dissolves the phosphorus which can then be precipitated with nitrogen. Magnesium ammonium phosphate (MAP) is obtained as a ready-to-use fertilizer and a hydrochar for energy or material recycling. Hydrochar can also be used as structural material in culture substrates, e.g. as a peat substitute.
|The topic of phosphate recovery using HTC is currently being investigated. The HTC process is to be carried out in such ways that phosphate is not bound to the resulting hydrochar but transferred to the process water. The phosphate should then be simultaneously separated by membrane filtration during the reaction|