Recovery of phosphorus from public toilet press filtrate using Ca-rich fly ash through the formation of hydroxyapatite (HAP)

Hydroxyapatite (HAP) precipitation is considered to be one of the most promising approaches for phosphorus recovery, though it often requires sophisticated control of local pH, the concentration of Ca²⁺ and PO₄³⁻ to minimize the side effects. In many cases, additional Ca sources are needed to adjust the pH and Ca concentration of the solution, that makes the cost of HAP precipitation process relatively high. Herein, we studied the possibility of using Ca-rich fly ash (CFA) as Ca sources and pH adjuster to provide suitable environment for HAP precipitation from the toilet press filtrate. The main factors that affect the HAP formation were identified and the performance of CFA on P recovery from real toilet press filtrate was examined and the mechanism was proposed. Rsults indicated that the strong buffering property of CFA rendered it workable in a wide pH range from 5 to 12. Products characterization and Visual MINTEQ simulative analysis illustrated that the removal of PO₄³⁻ by CFA was mainly through the formation of HAP. Moreover, the results of PO₄³⁻ removal by CFA in toilet press filtrate proved that when the concentration of CFA was 4 g/L, the removal rate of PO₄-P reached 87.0%.