Novel CaO2 beads used in the anaerobic fermentation of iron-rich sludge for simultaneous short-chain fatty acids and phosphorus recovery under ambient conditions.

A novel composite CaO2 bead was prepared to improve total short-chain fatty acids (TSCFAs) production and phosphorus (P) recovery from iron-rich waste activated sludge (WAS) during ambient anaerobic fermentation. Results showed that CaO2 mass percentage of 5% and CaCl2:nylon66 = 1:1 (mass ratio) were the optimal prescription for the preparation of CaO2 beads with porous structure, loose morphology, and sustained-release of CaO2. The highest TSCFAs production (356 mg/g VSS) was observed and about 9% of P in sludge could be recovered on beads. The decrease of Fe-phosphate and Fe-oxides in the sludge were due to different mechanisms. Microbial community analyses showed that CaO2 beads effectively enriched dissimilatory iron-reducing bacteria (DIRB) and promoted iron-reduction related genes. After fermentation, the P-rich beads are easy to separate from sludge for further P recovery, and the supernatant carrying abundant acetate and Fe2+ can be returned to the wastewater treatment line to improve nutrient removal.