Rapid Denitrifying Granular Sludge Formation Methods for Closed-recirculating Seawater Aquaculture System

Denitrification is an important process for water quality control in recirculating aquacultural systems. Denitrifying granular sludge reactors, such as upflow sludge blanket (USB) reactors, can achieve a high NO3−-N removal efficiency under a short hydraulic retention time. These biological reactors require the seeding of anaerobic granular sludge and an extended start-up period, thereby limiting their use in denitrifying granular sludge reactors. Herein, a rapid granular sludge formation method was developed by increasing nitrate and total organic carbon concentrations up to 500 mg-N·L−1 and 500 mg·L−1, respectively, in the influent substrate of the USB reactor. High sludge concentration (31.8 g-mixed liquor suspended solids (MLSS)·L−1) and denitrification activity (2.6 mg-N·g-MLSS−1·h−1) were observed in the denitrifying granular sludge on Day 20. Analysis of 16S rRNA amplicon sequencing indicated that Colwellia sp., a type of bacteria renowned for synthesizing extracellular polymeric substances, was the most abundant in granular sludge. Therefore, increasing the NO3−-N and organic concentrations is effective for forming denitrifying granular sludge.