Elucidating biochemical transformations of Fe and S in an innovative Fe(II)-dosed anaerobic wastewater treatment process using spectroscopic and phylogenetic analyses

An innovative process consisting of Fe(II)-dosed anaerobic bioreactors and an oxidizing basin was used to continuously treat a synthetic wastewater (COD/sulfate mass ratio 2:1 and Fe/S molar ratio 1:1). Sludge recycling effects were evaluated on ten occasions, in which anaerobic sludge was collected, biochemically oxidized with mechanical aeration in the oxidizing basin before being mixed with the wastewater influent. The sludge recycling resulted in better effluent quality compared to the baseline operation without recycling. More Fe and S were retained as sludge in the bioreactors with sludge recycling (Fe 94%, S 91%) than those when the bioreactors were operated without sludge recycling (Fe 76%, S 86%). Scanning electron microscopic analysis showed that bacterial cells and iron sulfide intermixed in the anaerobic sludge and the presence of microbial exopolymeric substances. X-ray spectroscopic analyses showed amorphous FeS formed from the dosed ferrous iron and biogenic bisulfide, and suggested long-term conversion of the amorphous FeS to more stable crystalline FeS and FeS2 in the anaerobic bioreactors. In the oxidizing basin, oxidation of iron sulfides was of both chemical and biological nature, and their oxidized forms including amorphous FeO/Fe2O3 mixture and partially/fully oxidized sulfurs. Experimental results also indicated amorphous FeS was more readily oxidized than FeS2. Phylogenetic analysis revealed microorganisms related to Desulfomonile tiedjei (sulfur reducing) and Alkaliphilus metalliredigens (iron reducing) in the anaerobic bioreactors, and Thiobacter subterraneus (sulfur oxidizing) and Rubrivivax gelatinosus (iron oxidizing) related microorganisms in the oxidizing basin.