Nitrogen removal performance and sludge characteristics of wastewater from industrial recirculating aquaculture systems via anammox coupled with denitrification

Wastewater from industrial recirculating aquaculture systems is highly susceptible to eutrophication and may harm aquatic organisms. To address this problem, a simultaneous anammox and denitrification (SAD) process was applied to treat industrial recirculating aquaculture systems wastewater, and static research test and continuous flow test lasting 132 days were conducted herein. Based on the results, the optimal working parameters of the SAD coupling process are as follows: temperature = 30 ?, pH = 8, dissolved oxygen = 0-0.3 mg/L, and solid-liquid ratio = 1:1. At 132 d of operation, the NH4+-N, NO2--N, and total nitrogen (TN) removal rates were 93.13 %, 93.54 %, and 89.42 %, respectively. The average particle size of the anammox granular sludge increased from 1 mm to 1 cm; the extracellular polymeric substances content increased from 39.09 mg/g VSS to 137.31 mg/g VSS; and the PN/PS value increased from 1.97 to 5.73, which enhanced the aggregation performance of granular sludge. A very strong correlation was observed between heme concentration and nitrogen removal load, and the coefficient of determination of linear regression was close to 0.99. The 16S rRNA gene sequencing showed that the Planctomycetes became the dominant phylum, with the abundance increasing from 18.62 % to 41.21 %. Moreover, the abundance of Candidatus Kuenenia increased from 1.22 % to 15.52 %, and the abundance of denitrifying bacteria also increased. It indicated that the synergistic effect of AnAOB and denitrifying bacteria in nitrogen removal was excellent. Finally, the SAD coupling process was successfully used to treat actual aquaculture wastewater with a TN removal rate of 86.51 %.