The transformation of phosphorus fractions in high-solid sludge by anaerobic digestion combined with the high temperature thermal hydrolysis process.

Transforming inactive phosphorus (P) to active P to recover it from waste activated sludge is important. The transformation of P fractions from high-solid sludge by the anaerobic digestion (AD) and acidification phase of AD (AAD) combined with a high temperature thermal hydrolysis process (HTTHP) was investigated. The results showed that the sequence of P release effects by three processes was HTTHP + AAD > AD + HTTHP > HTTHP + AD. The PO43--P release from high-solid sludge was directly affected by the temperature of HTTHP. At 140 °C, each process had more PO43--P release than that at 160 °C. The total amount of PO43--P release in AD + HTTHP was approximately 6 times that of HTTHP + AD. Based on the experimental results, a new process of mesophilic AD - post HTTHP was recommended, in which, enhancement of P release by sulfide ions was also proposed.