A critical review on the techno-economic feasibility of nutrients recovery from anaerobic digestate in the agricultural sector

The unsustainable environmental impact of fossil derived fertilizers and the depletion of raw materials for their production make the research of alternative sources for fertilizers production one of the main priorities of the international agenda. Agricultural digestate, especially if derived from livestock manure, is considered a potential candidate for the recovery of bio-based fertilizers: 180 M tonnes of digestate, which contain high concentrations of nitrogen (2–5 kg/m3) and phosphorous (0.5–1.5 kg/m3), are produced annually in the EU. This paper overviews different technologies, their robustness and yields in terms of nutrients recovery for different biobased fertilizers. Ammonia stripping and struvite precipitation are the technologies more developed at full scale allowing an average removal and recovery yield of 80–90 % for nitrogen and phosphorous, respectively. Due to their high degree of development, costs for ammonia stripping and struvite precipitation are relatively low, ranging from 2 to 7 € per kg of nutrient recovered. However, other technologies are rapidly developing and spreading. Pressure-driven membrane technologies (ultrafiltration and reverse osmosis) are emerging as important processes for nutrient recovery, although not selective. Nitrogen and phosphorous removal efficiencies for these technologies are around 75–95 % and 85–99 %, respectively. The main drawback of membrane processes is represented by the membrane fouling which prevents their large diffusion at full scales because of management difficulties. Moreover, the operational cost of this process is higher compared to other available technologies, ranging from 4 to 12 € per m3 of digestate. Full-scale applications for membrane technologies are available, but still limited, while several studies at laboratory and pilot scale are currently under development. Membrane contactors are receiving attention for the possibility to strip ammonia directly in the membrane with high ammonia recovery (over the 95 %). But this technology is currently developed at low TRL in the EU with several studies at laboratory and pilot scale. Ion exchange, adsorption, evaporation, and electrodialysis are also discussed in this review paper. The difficulty to have solid matrices able to assure a selective nitrogen/phosphorous compound adsorption and the need for regeneration has limited the ion exchange process at full TRL for treatment of agricultural digestate. However, some pilot scale applications of this technology have been investigated for wastewater treatment. Similarly, the high economic and energetic costs for evaporation and electrodialysis have prevented further applications of these techniques for pilot and full scale operation.