From waste to high-value fertilisers: Harvesting nutrients from liquid anaerobic digestate for a circular bioeconomy

Climate change and population growth are driving a shift towards a circular economy to combat the escalating challenges of resource depletion, waste accumulation and pollution. The investigation of sustainable avenues to replace dwindling fossil feedstocks in the agricultural sector is pivotal to developing resource and energyefficient farming practices. Sustainable fertilisers can be developed by harnessing valuable nutrients from abundant, readily available waste streams such as liquid anaerobic digestate (LAD). This review explores the implementation of LAD as renewable feedstock for the recovery of nutrients, with a specific emphasis on closing the loops for nitrogen and phosphorous nutrients. A comprehensive review has been conducted of the state-ofthe-art technologies for nutrient extraction from LAD, considering technical feasibility, energy consumption, economic viability, and scalability. Specifically, this review aims to unravel key correlations between process design, energy footprint and recovery efficiencies to highlight key technological and economical challenges that hinder the scalability of nutrient recovery technologies. Technologies at TRL 9 can serve as a roadmap for upscaling those promising nutrient recovery processes such as microalgae cultivation and electro-driven membrane processes, unlocking their full potential to contribute to the circular economy of fertiliser production.