Hybrid bioreactor built-in with fixed bio-carriers for denitrification with low C/N ratio: Hydrodynamic optimization and microbial divergence.

Hydrodynamics played an important role in the design and operation of bioreactors for wastewater treatment. In this work, an up-flow anaerobic hybrid bioreactor built-in with fixed bio-carriers was designed and optimized using computational fluid dynamics (CFD) simulation. The results indicated that the flow regime involving with vortex and dead zone was greatly affected by the positions of water inlet and bio-carrier modules. The ideal hydraulic features were obtained when the water inlet and bio-carrier modules located 9 cm and 60 cm above the bottom of reactor. Using the optimum hybrid system for nitrogen removal from wastewater with low carbon-to-nitrogen ratio (C/N = 3), the denitrification efficiency could reach 80.9 ± 0.4%. Illumina sequencing of 16S rRNA gene amplicons revealed that the microbial community divergence occurred among the biofilm on bio-carrier, the suspended sludge phase and the inoculum. Especially, the relative abundance of denitrifying genera Denitratisoma in the biofilm of bio-carrier reaches 5.73%, 6.2 times higher than that in the suspended sludge, implying the imbedded bio-carrier was conductive to enrich the specific denitrifiers to polish the denitrification performance with low carbon source. This work provided an effective method for the design optimization of bioreactor based on CFD simulation, and developed a hybrid reactor with fixed bio-carrier for nitrogen removal from wastewater with low C/N ratio.