Micronutrients and COD/N ratio as factors influencing granular size and SND in aerobic granular sequencing batch reactors operated at low organic loading

Abstract

Simultaneous nitrification-denitrification (SND) depends on dissolved oxygen, granule size and COD/N ratio. Slow-growing nitrifying bacteria at low COD/N ratio lead to the formation of small-size granule. Anoxic denitrification is encouraged at higher granule size. In the present study, the effect of different COD/N ratios and addition of micronutrients (Fe, Zn, Mn, Cu, Co, Mo, B) on granule size and SND was studied in a laboratory-scale aerobic SBR at low organic loading (358.4–1075.2 mg COD.(L·day)−1). Synthetic cheese whey wastewater and ammonia was used. Small-size granules (0.28–0.7 mm) were developed at COD/TKN ratio of 8.6–13.7 with micronutrients and long famine phase (83–92% of the cycle). COD/TKN ratio of 13.7 achieved the best removal of inorganic nitrogen Ni (62%). At COD/TKN ratio of 8.6, without micronutrients, the low COD uptake rate caused shortening of the famine phase to 66%. Large-size granules (3.0 mm) with limitation of substrates diffusion showed low Ni removal and dominant slow-growing rhizobia (Bradyrhizobium) as detected by 16S rRNA gene amplicon sequencing. In small granules, without diffusion limitations and higher ammonia exposition, the energetically unfavorable heterotrophic nitrification occurred. Heterotrophic nitrifiers Lysobacter and Dyadobacter were found. Hydrogenophaga was a potential aerobic denitrifier. SND process was probably performed by AOB, heterotrophic nitrifiers, and anoxic and aerobic denitrifiers.