Transformation and migration of phosphorus in excess sludge reduction pretreatment by alkaline ferrate oxidation combined with anaerobic digestion

Abstract

Recently, more and more attention has been paid to the strong oxidation ability of newly prepared potassium ferrate (NAPF) in sludge reduction process, but less attention has been paid to the change of phosphorus in this process. The feasibility of phosphorus migration and transformation during excess sludge reduction pretreatment using NAPF pre-oxidation combined with anaerobic digestion was investigated. After 70 mg/g suspended solids NAPF pretreatment and 16 days anaerobic digestion, the solid-phase volatile suspended solids decreased by 44.2%, and much organic matter had been released into the liquid-phase and then degraded during digestion by indigenous microorganisms. As the sludge pre-oxidation process was performed, solid-phase organic phosphorus and chemically combined phosphorus also released into the liquid-phase as PO43−, peaking at 100 mg/L. During anaerobic digestion, the Fe3+ in the liquid-phase was gradually reduced to Fe2+, and then formed Fe2+-PO43− compound crystals and re-migrated to the solid-phase. The concentration of PO43− decreased to 17.08±1.1 mg/L in the liquid-phase after anaerobic digestion. Finally, the phosphorus in the Fe2+-PO43− compound accounts for 80% of the total phosphorus in the solid-phase. A large number of vivianite crystals in sludge were observed. Therefore, this technology not only effectively reduces sludge, but also increases the proportion of PO43− in the sludge in the form of Vivianite.