Nutrient recovery from digestate: case study report

Due to the historic presence of intensive livestock production and the limited amount of arable land for manure disposal, nitrate pollution in certain European areas is considerable. The European Nitrates Directive, implemented in 1991, intended to improve water quality in Europe by preventing pollution of ground- and surface water by nitrate leaching from agriculture. This has forced local administrators and government to introduce stringent regulations regarding the use of manure (and later on digestate), resulting in national action plans and the defining of vulnerable zones. The Flemish action plan, for example, consists of an obligation to process manure in such a way that the nitrogen present is not returned on Flemish agricultural soil after treatment, but is either exported, used on non-agricultural land (e.g. in gardens or parks) or converted to nitrogen gas or to a mineral fertiliser. These restrictions, combined with the presence of intensive livestock, imply that anaerobic digestion plants in Flanders (and other nutrient rich areas), may not, or only sparingly, return digestate as a fertilizer in its crude, unprocessed form and have to invest in digestate processing techniques. More recently, the focus in digestate processing techniques has switched from mere processing towards valorization techniques that recover a maximal amount of nutrients (N, P, K). This development is triggered by an increasing worldwide awareness of the depletion of mineral resources (such as phosphorus) and the volatile price of fossil-based mineral fertilizers. Mineral fertilizer use in Europe is high, even in regions with local nutrient surpluses where farmers pay to export or destroy nitrogen in their farmyard slurry. The reason for this is twofold, first of all animal manure spreading is limited to 170 kg of N/ha according to the Nitrates Directive (in vulnerable zones) and second of all nutrient availability and composition in manure can differ from crop requirements. Estimates of the current phosphorus and potassium reserves are highly uncertain, but based on population growth and future nutrient demand, it is predicted that depletion will occur within 93 to 291 years for P and 235 to 510 years for K (Fixen and Johnston, 2012; Van Vuuren et al., 2010; Villalba et al., 2008; Smit et al., 2009). Geopolitical moves can however shift this date forward, making nutrient scarcity an imminent threat which is exemplified by the fact that phosphorus was recently added to th