Insight into direct phosphorus release from simulated wastewater ferric sludge: Influence of physiochemical factors

Phosphorous recovery from chemical phosphorus removal (CPR) plants is limited. CPR plants that use iron salts for phosphorous removal generate iron phosphate (Fe-P) rich sludge. This study investigates the influence of pH and competing anions (chloride), on phosphorus and soluble iron release from lab simulated Fe-P sludge. Factors influencing recovery, such as sludge age and Fe/P molar ratio are also investigated. Results showed that alkaline treatment of Fe-P sludge was not effective in releasing Fe (<3%) but effective in releasing P, particularly at controlled pH value of 10 where the %P release was (90 ± 2%). Sludge ageing time did not affect P release from Fe-P sludge at ages less than 5 days. However, a remarkable decrease in %P release (i.e., 50% reduction) was observed at ages of 9 and 11 days for pH values of 9–10. Arsenic extraction of exchangeable surface bound P from Fe-P sludge and surface area determination of hydrous ferric oxide (HFO) under different aging times supported qualitatively the internalization of surface bound P during aging, which is a proposed mechanism responsible for the decrease in P release for older sludge. Chloride effect in releasing P from Fe-P sludge was negligible. The reduction in P release by aging was best described by a zero-order kinetic model. Modeling with the PHREEQC geochemical software did not always agree with the measured release of phosphorus from the Fe-P sludge. PHREEQC assumes all surface sites for phosphorus are exchangeable and has no mechanism to represent phosphorus trapped within HFO particles.