Optimized hydrodynamic cavitation-based Pretreatment Strategies to enhance residual Biogas Yield from Solid Digestate and Struvite Recovery from Liquid Fraction

Digestate valorisation is essential for improving the sustainability and efficiency of anaerobic digestion systems. This study explores an integrated approach combining hydrodynamic cavitation (HC) pretreatment of the solid fraction with struvite precipitation from the liquid fraction to enhance energy and nutrient recovery. Solid and liquid fractions of digestate were separated and processed independently. HC pretreatment of the solid fraction significantly improved biochemical methane potential (BMP), increasing methane yield by over 95% and reducing the lag phase during anaerobic digestion. Kinetic modeling indicated accelerated biodegradation rates, and the energy output from additional methane production exceeded the input required for cavitation, confirming the process as energy positive. In parallel, the liquid fraction was subjected to struvite precipitation by adjusting pH and dosing magnesium chloride. The precipitate was characterized using X-ray diffraction and scanning electron microscopy, confirming the formation of crystalline struvite with high purity. Nutrient recovery efficiencies exceeded 80% for both ammonium and phosphate. A mass and energy balance revealed that the integrated HC and struvite processes achieved efficient conversion of residual organics into biogas and dissolved nutrients into fertilizer. The results demonstrate a feasible and scalable strategy for digestate valorization, offering a dual benefit of enhanced biomethane recovery and nutrient recycling. This approach contributes to the circular economy and improves the sustainability of biogas plants.