Insights into size-fractionated anammox granules presented with refractory organics from municipal wastewater-driven partial denitrification to improve the synergy of anammox and denitrification

The recently developed Partial Denitrification/Anammox (PD/A) is an emerging process for simultaneous ammonium and nitrate removal, while the remaining organics from PD may pose an adverse effect on the subsequent anammox process. In this study, a granule-based anammox UASB reactor was developed with receiving municipal wastewater driven PD effluent that contained a certain amount of refractory organic matter, the characterization of different size-fractionated granules on nitrogen removal and microbiological mechanisms were investigated. Anammox activity increased with granule size, while an oversize exceeding 2.5 mm would lead to activity reduction. Significant denitrification activity was detected, it exhibited a contrary relationship with anammox activity. Delightedly, the denitrification was observed to proceed with high nitrite accumulation, especially in smaller granules with nitrate-to-nitrite transformation ratio (NTR) above 84.4%. These results were confirmed by microbial analysis, anammox genus including Candidatus Kuenenia, Brocadia, and Jettenia were identified in all granules, their abundances increased with granule size, and the dominated Kuenenia preferred to grow in larger granule. Various kinds of denitrification bacteria were detected with the relative abundances increased with the decrease in granules size. Particularly, the Thauera that responsible for PD with high nitrite accumulation was revealed to grow in such system, its relative abundance in small-size granules (12.9%,