Pilot-scale production of high-purity struvite fertilizer from livestock waste: Outcomes of the RE-LIVE WASTE project in Cyprus

Abstract

Decarbonizing fertilizer production necessitates the development of innovative practices for producing waste-derived fertilizers, that will mitigate environmental degradation from waste disposal. Struvite crystallization (SC) is a well-known practice for nutrient recovery from waste by producing struvite (MgNH4PO4·6H2O) fertilizers. However, research on SC upgrading from livestock waste is limited. In this study, experiments were conducted in the Cyprus 250-L SC pilot reactor. Ultrafiltration permeates from mixed livestock, dairy, and bio-wastes, Matrix 1, and pig slurry, Matrix 2, were used separately to crystallize struvite with pure magnesium hydroxide (Mg(OH)2). Additionally, Matrix 1 was subjected to SC using technical grade magnesium oxide (T.G. MgO). Matrix 1 was first evaluated as matrix in pilot-scale SC. XRD results indicated that using T.G. MgO and Matrix 1 in SC yielded the highest struvite purity, 99.6 % w/w. However, XRF showed that iron (Fe), manganese (Mn), and nickel (Ni) in the precipitate exceeded limits. Struvite precipitates produced using Mg(OH)2 were of high purity, 89.2 %-98.0 % w/w. SEM-EDX analysis revealed mainly orthorhombic, prismatic, and octahedral struvite crystals up to 200 μm. The precipitates met regulatory standards for pathogens, carcinogens, and heavy metals, and contained beneficial plant macronutrients and micronutrients within limits. Εconomic analysis for a 200 m3 scale-up process using Matrix 1 and Mg(OH)2 showed viability, yielding a net profit of 910,981€ over 10 years, an internal rate of return of 23 %, and a payback period of 7.6 years. Overall, the results indicate that safe-to-use struvite organo-mineral fertilizers can be produced using any matrix and Mg(OH)2.