Harnessing sustainable opportunities for nutrient removal and recovery from liquid digestate in a modern municipal food waste biogas power plant

Abstract

This study aims to efficiently remove and recover Nitrogen (N) and Phosphorus (P) from the Liquid Fraction of the Digestate (LFD) of a municipal Food waste (FW) Anaerobic Digestion (AD) biogas power plants. An experimental design was set up to evaluate the influence of pH, temperature, stirring time, stirring speed, and magnesium to phosphorus molar ratio (Mg: P) on nutrient removal efficiency. It was found that the experimental removal efficiency was up to 89% for P and 79% for N at pH 9.5, stirring at 200 rpm, and temperature of 35 °C. The maximum P removal was reached at 40 minutes of stirring time and an Mg:P molar ratio of 2:1, while the highest N removal was at 120 stirring time and Mg:P molar ratio of 4:1. The Mg:P molar ratio and pH significantly influenced N removal, p-value 0.0429 and < 0.0001, respectively; while stirring time greatly affected P removal efficiency, p-value < 0.0111. The analysis of X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) revealed that 92.5% of the precipitate consisted of struvite crystals confirming nutrient recovery. The numerical optimization of the experimental design determined that setting up the experiments at pH 8.8, 27°C, 64 minutes of stirring time at 158 rpm, and an Mg:P molar ratio of 2.35 resulted in 67% N and 81% P removal. This approach led to a cost-effective operation that resulted in a significant 66% reduction in process costs. This research identified a solution to managing effluent's environmental challenges for a FW biogas plants.