Investigating the biodegradability of iodinated X-ray contrast media in simultaneous nitrification and denitrification system

The present study investigated the biodegradation of three iodinated X-ray contrast media (ICM), namely, iopamidol, iohexol, and iopromide, in simultaneous nitrification-denitrification (SND) system maintained in a sequencing batch reactor (SBR). The results showed that variable aeration patterns (anoxic-aerobic-anoxic) and micro-aerobic condition were most effective in the biotransformation of ICM while achieving organic carbon and nitrogen removal. The highest removal efficiencies of iopamidol, iohexol, and iopromide were 48.24%, 47.75%, and 57.46%, respectively, in micro-aerobic condition. Iopamidol was highly resistant to biodegradation and possessed the lowest Kbio value, followed by iohexol and iopromide, regardless of operating conditions. The removal of iopamidol and iopromide was affected by the inhibition of nitrifiers. The transformation products after hydroxylation, dehydrogenation, and deiodination of ICM were detected in the treated effluent. Due to the addition of ICM, the abundance of denitrifier genera Rhodobacter and Unclassified Comamonadaceae increased, and the abundance of class TM7-3 decreased. The presence of ICM affected the microbial dynamics, and the diversity of microbes in SND resulted in improving the biodegradability of the compounds.