Zero valent iron greatly improves sludge destruction and nitrogen removal in aerobic sludge digestion

Abstract

Zero-valent iron (ZVI), a low-cost metallic material, has been previously applied in effectively enhancing sewage sludge anaerobic digestion. However, the potential role of ZVI on aerobic digestion of sludge, a completely different sludge treatment method from anaerobic digestion, is still unknown. Herein, the effects of ZVI on the performance of aerobic sludge digestion were systematically studied, focusing on the sludge degradation, nitrogen removal and sludge dewaterability. Results showed ZVI greatly increased the volatile solids (VS) destruction from 27.0 ± 1.3% to 50.0 ± 1.0% and significantly enhanced the TCOD removal from 26.0 ± 1.2% to 47.9 ± 0.9% in aerobic digesters with different ZVI levels (0–20 g/L). The metabolic intermediate transformation steps of solubilization, hydrolysis and catabolism processes in aerobic digestion were all revealed to be enhanced by ZVI. More importantly, the aerobic digesters with higher ZVI levels achieved higher inorganic nitrogen removal, even with higher sludge degradation for ammonium release, due to the occurrence of both chemical and biological denitrification induced by ZVI. Correspondingly, the microbial compositions in the digesters with ZVI shifted towards the direction that was conducive to sludge degradation and nitrogen removal (e.g., aerobic denitrification) compared to control. Further, the dewaterability of the aerobically digested sludge was also improved with ZVI addition, supported by the reducing capillary suction time (CST) and negative surface potential.