Acid Phase Digestion: the Biological Solution for P Solubilization
An acid phase digester provides a low-oxygen and low pH environment. This facilitates the rapid release of orthophosphates in Bio-P sludge along with the dissolution of crystalized and organically bound phosphorus. Over 60% solubilization of P occurs in the acid phase digesters without using any additional chemicals. In non-Bio-P plants, up to 40% of the phosphorus solubilizes using an acid phase digester.
A CalPrex reactor is placed between the acid phase and gas phase digesters. Sludge from the acid phase digester is dewatered. The centrate is sent to the CalPrex reactor. The dissolved phosphorus in the centrate precipitates with the addition of calcium hydroxide and calcium chloride. By maintaining the pH of the solution at 6.5, phosphorus is recovered as a brushite crystal (CaHPO4· 2H2O). As a fertilizer, Brushite is as effective as other leading phosphate fertilizers on the market today.