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Nutrient recycling from bio-waste as green fertilizers.

In the transition from a fossil to a bio-based economy, it has become an important challenge to maximally recuperate valuable nutrients coming from waste streams. Nutrient resources are rapidly depleting, significant amounts of fossil energy are used for the production of chemical fertilizers, whereas costs for energy and fertilizers are increasing. In the meantime, biogas production through anaerobic digestion produces nutrient-rich digestates. In high-nutrient regions, these products cannot or only sparingly be returned to agricultural land in its crude unprocessed form. The consequent processing of this digestate requires a variety of technologies producing a lot of different derivatives, which could potentially be re-used as green fertilizers in agriculture. As such, a sustainable alternative for fossil-based mineral fertilizers could be provided. The aim of this study is to characterize the physicochemical properties of digestates and derivatives, in order to identify the fertilizer value and potential bottlenecks for agricultural re-use of these products, in line with European legislative constraints. In addition, the economic and ecological benefits of substituting conventional fertilizers by digestates and derivatives are quantified and evaluated. Waste water from acidic air scrubbers for ammonia removal shows potential as N-S-fertilizer, whereas concentrates resulting from membrane filtrated liquid fraction of digestate show promise as N-K-fertilizer. Substituting artificial fertilizers by air scrubber water or membrane filtration concentrates theoretically always results in significant economic and ecological benefits for the agriculturist. Field research is now on-going in order to evaluate the impact on soil and crop production by application of these new green fertilizers.