The aim of this study was to investigate the effect of nanoscale zero-valent iron (nZVI) on the formation of aerobic granules, nutrient removal and bacterial growth during the treatment process of the municipal wastewater. For this purpose, two sequencing batch reactors (SBR) were simultaneously and automatically operated in a cyclic batch mode with four phases per cycle: feed, react, settle and decant. The sequancial operation of the reactors consisted four cycles per day and lasted for sixty days in which 10 mg/L of nZVI particles were added to the infflent of reactor 2. The reactors were fed with synthetic wastewater (3 liters per cycle) and acclimated with seed sludge collected from a full-scale municipal wastewater treatment plant in Istanbul. The effluent of the reactors was regularly analyzed for nitrate, nitrite, ammonia, phosphate and COD concentrations. In addtion to that, their removal pathways including the direct adsorption to nZVI, utilization by microorganisms and adsorption within the generated granules were discussed. The removal efficiency of COD, ammonia and phosphate kept increasing, and almost a complete removal was observed after the formation of aerobic granules on day 50. Furthermore, after the addition of nZVI to R2 on day 24th, the removal efficiency of ammonia, COD and phosphate slightly improved. The addition of nZVI stimulated the production of Extracellular Polymeric Substances (EPS) in R2 including protein and carbohydrate generation. NGS analysis showed that the addition of nZVI into R2 increased the growth rate of some bacterial species such as Rhizobiales and Xanthomonadales and decreased others such as Clostridiales, confirming that the effect of nZVI on the bacterial growth was genera dependent. The aerobic granules were successfully formed in the reactors in less than 50 days and the addition of nZVI improved to some extent the size and settling rate of the formed granules in R2.
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