Influence of Extraction Conditions on Organic Carbon Recovery from Biogas Digestate

Intensive use of mineral fertilizers has reduced natural soil fertility and increased the demand for sustainable nutrient sources. In this study, three digestates obtained from Lithuanian biogas plants were chemically and instrumentally characterized and evaluated as potential sources of organic carbon and plant nutrients. The dried digestates contained 52.9-59.5% organic carbon and appreciable amounts of nitrogen, phosphorus, and potassium, although the measured nutrient concentrations depended on the applied extraction or mineralization procedure. The influence of key extraction parameters, including extractant type (water or 0.5 M KOH), temperature, extraction time, particle size, and extraction method (magnetic stirring or ultrasonication), on organic carbon recovery was systematically investigated. Alkaline extraction markedly increased TOC concentrations in the extracts, yielding values up to approximately 8-29 times higher than those obtained by aqueous extraction after 3 days. The highest TOC concentration (4495 mg/L) was obtained for KP digestate extracted with 0.5 M KOH at 90 °C using magnetic stirring for 6 h. In aqueous extracts, the maximum TOC concentration reached 715 mg/L under ultrasonication at 50 °C. Temperature, extraction time, and particle size had condition- and digestate-dependent effects on TOC recovery. The ultrasonic extraction series yielded the highest TOC concentration among the aqueous extracts, whereas the magnetic-stirring series yielded the highest value under alkaline conditions. In addition, carbon recovery efficiency was calculated relative to the initial organic carbon content of the digestates, providing a quantitative assessment of carbon transfer from the solid material to the liquid phase. The highest carbon recovery efficiency reached approximately 76%. These results identify the extraction conditions that enhance soluble organic carbon recovery from digestate and provide a basis for further chemical characterization and application-oriented evaluation of the obtained extracts.