Formation of Anaerobic/Anoxic Denitrifying Granular Sludge in an Upflow Sludge Blanket Reactor for Treating Municipal Sewage

Anaerobic/anoxic denitrifying reactors, such as an upflow sludge blanket (USB) reactor, can retain high concentration of biomass inside of the reactor as granular sludge, which allows high nitrogen removal performance from sewage and several kinds of industrial wastewater . However, settleability of granular sludge is critical to maintain high nitrogen removal performance. In this study, granular sludge was developed in a denitrifying USB reactor supplemented sodium nitrate, and nitrogen removal performance, settleability, and microbial community structure of granular sludge were investigated. A 11.4 L USB reactor was operated at an upflow velocity of 50 cm?h-1, and hydraulic retention time of 2.9 h under ambient temperature. Granular sludge with up to 3.8 mm of diameter was formed within 15 days, and the rate of nitrate removal increased to 22.5 mg-N?gMLVSS-1?h-1. 16S rRNA amplicon sequencing indicated that Cloacibacterium sp. was the (most?) abundant in the granular sludge, suggesting this bacterium was responsible for granulation. Also, propionate-producing bacterium Paludibacter sp., acetate-utilizing denitrifying bacterium Acidovorax sp. and Dechloromonas sp. were also abundant in the granular sludge. Therefore, denitrifying granular sludge was successfully formed in the USB reactor for treating actual municipal sewage.