Adapted Hedley fractionation for the analysis of inorganic phosphate in biogas digestate

The major share of phosphate in biogas digestate is inorganic. For optimized nutrient recovery, inorganic phosphate must be analyzed adequately. Therefore, the photometric spectra of extracts from Hedley fractionation were measured and analyzed for their peaks using the molybdenum blue method. The ideal wavelength was 709 nm. The lower calibration limit needed to be raised from 15 ?g L-1 to 50 ?g L-1 to avoid underestimation of phosphate concentration. Drying digestate before extraction increased H2O-P by 78.4% and NaHCO3-P by 44.9% compared to undried digestate. The filter paper of the filtration between extractions was added to the next extraction to avoid phosphate losses. This made it necessary to rinse the samples with 30 mL deionized H2O after the H2O extraction, with 60 mL NaHCO3 after NaHCO3 extraction and 60 mL NaOH after the NaOH extraction. Ultimately, the results showed that the phosphate concentration in extracts was independent of extraction time.