Abstract 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. Graphic Abstract
Purwing et al. (Mon,) studied this question.