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biogas HANDBOOK of digestate are taken and the content of N, P and K, DM, VM and pH are determined. If the biogas plant co-digests organic wastes, the eventual contamination with heavy metals and persistent organic compounds must be determined also, as their concentration may not exceed the detection limits prescribed by law. Safe recycling as fertiliser requires furthermore that digestate is sanitised, free of prion-transmitted diseases and of physical impurities. 6.7.2 Nutrientmanagementindigestate One of the important aspects regarding recycling of digestate is the load of nutrients on farmland. Nitrate leaching or phosphorus overloading can occur due to inappropriate handling, storage and application of digestate as fertiliser. In Europe, the Nitrate Directive (91/676/EEC) restricts the input of nitrogen on farmland, aiming to protect the ground and surface water from nitrate pollution and allows maximum 170 kg N/ha/year. Nutrient loading on farmland is regulated by national legislation in most European countries (Table 6.3). Table 6.3 Example of national regulations of the nutrient loading on farmland (NORDBERG 1999) Application of digestate as fertiliser must be done on the basis of a fertiliser plan. The fertiliser plan is elaborated for each agricultural field, according to the type of crop, the planned crop yield, the anticipated utilisation percentage of nutrients in digestate, the type of soil (texture, structure, quality, pH ), the existing reserve of macro and micro nutrients in the soil, the pre-crop and the irrigation conditions and the geographic area. Experience from Denmark indicates that the most economic and environmental friendly strategy of application of digestate as fertiliser is by fulfilling the phosphorus requirement of the crops with phosphorus from digestate. Application of digestate to fulfil the phosphorus requirement implies also a partial fulfilment of nitrogen requirement of the crops. The remaining nitrogen requirement can thus be completed by application of mineral fertiliser. 6.7.3 Generalmeasuresforqualitycontrolandsaferecyclingof digestate The experience gathered in Europe with safe recycling of digestate as fertiliser indicates that the aspects listed below should always be considered: • Permanent control AD process stability (temperature, retention time) to obtain a stable end product (digestate) • Sanitation of digestate according to European regulation standards, for effective pathogen reduction • Periodical sampling, analysing and declaration of digestate • Recycling digestate by integration in the fertiliser plan of the farm and by using “good agricultural practice” for application of digestate on farmland Maximum nutrient load Required storage capacity Compulsory season for spreading Austria 170 kg N/ha/year 6 months 28/2-25/10 Denmark 170 kg N/ha /year (cattle) 140 kg N/ha/year ( pig ) 9 months 1/2-harvest Italy 170-500 kg N/ha /year 90-180 days 1/2- 1/12 Sweden Based on livestock units 6-10 months 1/2- 1/12 59PDF Image | biogas HANDBOOK
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