CO2 Treatment of Anaerobic Digestate for Phosphate Dissolution

Phosphate recovery from biogas digestate has gained importance as an alternative to mined mineral fertilizers. Current technologies for phosphate recovery primarily employ acid-induced solubilization to mobilize phosphate from the digestate matrix, subsequently enabling its extraction through solid–liquid separation. Traditionally, sulfuric acid is used for this acidification. In this work, CO 2 was used to acidify liquid digestate to increase the economic and environmental sustainability. Liquid digestate was circulated in batch reactors at 5 and 10 bar in 99.8% CO 2 for 30 min and then filtered. The total phosphorus and phosphate content of the filtrate and filter was analyzed and compared to a N 2 -treated control. Exposure to CO₂ lowered the pH of the system from roughly 8.0 to 6.0. After 5 bar treatment the phosphate concentration of the filtrate increased from 0.34 ± 0.03 mg/l to 0.99 ± 0.10 mg/l, an increase of approximately 200%. After 10 bar treatment, there was an increase of 400% in the phosphate concentration from 0.28 ± 0.05 mg/l to 1.50 ± 0.50 mg/l. Analysis revealed that the effects of temperature and water content are lower than expected. Moreover, the use of CO 2 reduces the leaching of heavy metals, especially aluminum and iron compared to sulfuric acid, hydrochloric acid, and citric acid. The findings demonstrate that recoverable CO₂ represents a promising alternative to inorganic acids for phosphate recovery from manure or digestate.