Towards waste valorisation: Phosphorus and potassium resource recovery for fertilisers production

To sustain soil fertility, agriculture needs to return the nutrients removed with harvested crops. Traditionally, this has been achieved by applying livestock manure and sewage sludge to agricultural land. However, inadequate management of these residual streams leads to nutrient losses through volatilisation and leaching, contributing to the contamination of surface and groundwater bodies. At the same time, a large proportion of the mineral fertilisers currently used in agriculture are produced from non-renewable resources through mining activities. In this context, and within the framework of the circular economy, a paradigm shift towards waste valorisation is needed, developing strategies capable of recovering and reusing the nutrients contained in waste streams. This doctoral thesis focuses on the recovery of phosphorus (P), potassium (K), and nitrogen (N) from swine manure and urban wastewater to produce struvite and K-struvite, slow-release fertilisers. The study investigates precipitation as a nutrient recovery method, exploring both chemical precipitation, through the addition of chemical reagents, and electrochemical precipitation, through the electrochemical reduction of water, thereby avoiding the need for external chemical addition. The experimental work, initially conducted at laboratory scale (TRL 3), progressed towards validation in a chemical precipitation pilot plant (TRL 5) and an electrochemical precipitation pilot plant (TRL 5), demonstrating the transferability and operability of the process under real conditions. The main results obtained in this doctoral thesis are: o Demonstration of the feasibility of recovering K and P as K-struvite from pig slurry. From the denitrified effluent, 90% K recovery was achieved through the addition of external magnesium (Mg) and P sources. From the sludge fraction, nutrient recovery was enhanced through an acidification pre-treatment using oxalic acid (OA) (1 mol OA/1 mol Ca2+) to mobilise insoluble nutrients while simultaneously blocking Ca2+ that interferes with struvite recovery. OA acidification to pH 4 achieved phosphate (PO43−) solubilisation efficiencies of 75%. o Development of electrochemical pH control systems capable of promoting nutrient precipitation without the addition of chemical reagents, achieving phosphorus recovery efficiencies above 90%. o Optimisation of the electrochemical design by adjusting reactor configuration parameters and integrating electrochemical pH control and crystallisation into a single tubular electrocrystalliser, improving nutrient recovery efficiency (1.62 gdry matter/Ltreated) while reducing the energy consumption of the process (0.12–0.20 kWh/kg P). o Development of strategies to avoid anodic chloride oxidation during electrochemical precipitation processes in residual streams. These results represent an important step towards the sustainable production of second-generation fertilisers and the valorisation of residual streams. The thesis demonstrates that nutrient recovery through precipitation technologies can contribute to reducing dependence on mineral resources, decreasing the environmental impact associated with waste management, and advancing towards more sustainable agricultural production and consumption models