Process of Phosphorus and Nitrogen Dissolution from Sludge by Anaerobic Digestion and Their Recovery by Modified Red Mud Seeded Crystallization

On purpose of phosphorus (P) and nitrogen (N) recovery from sludge, the P and N release by anaerobic digestion from sludge, and their recovery by modified red mud seeded crystallization of magnesium ammonium phosphate (MAP) were studied. The effects of sludge mixture pH, temperature and anaerobic digestion time on the release of phosphate and ammonia nitrogen were investigated. It showed that under the conditions of 50℃, 5~7 days of anaerobic time and without pH adjustment, the sludge mixture could release around 200.0 mg/L phosphate, 245.0 mg/L ammonia nitrogen, and the N:P molar ratio was 2.5:1. Taking the modified red mud as inducing seed crystal, the phosphate and ammonia nitrogen in the supernatant of the digested sludge were fixed and recovered by MAP crystallization, and the effects of modified red mud dosage, reaction time, stirring speed and precipitating time on the recovery efficiency were investigated. It showed that under the conditions of 10 g/L red mud, 90 minutes reaction time and 200 r/min stirring, the recovery efficiency of P and N was 98.8% and 39.9%, respectively. The electron scanning microscopy and X-ray diffraction were employed to characterize the fixed products of the above process, and it showed that MAP was formed on the surface of the modified red mud.