Efficient Nitrogen Removal from Low-C/N Ratio Wastewater by a New Gel Microsphere with Nitrate-Reducing Fe(II)-Oxidizing Bacteria

Nitrate-reducing Fe(II)-oxidizing (NRFO) bacteria are considered to be ideal microorganisms for denitrification of low-C/N ratio wastewater, but cell encrustation and aggregation limit the application of the microbial NRFO process. Microbial immobilization technology is expected to solve these problems, but the effect of immobilization on the microbial NRFO process remains elusive. Here, Acidovorax sp. BoFeN1, a model NRFO bacterium, was embedded in a calcium alginate hydrogel to form bacterial gel microspheres, and the NRFO process was examined. We found that bacterial gel microspheres can conduct the NRFO process, and the nitrate reduction rate (0.37 mmol L-1 day(-1)) is much higher than that of the bacterial suspension (0.11 mmol L-1 day(-1)). Cryo-scanning electron microscopy analysis confirmed that the three-dimensional network structure of calcium alginate reduced the extent of cell encrustation and aggregation to increase the nitrate reduction rate. A batch culture without the addition of an electron acceptor revealed that alginate in the gel microspheres can be used as a carbon source to support nitrate reduction. In addition, >95% of the nitrate can be removed by bacterial gel microspheres in low-C/N ratio simulated wastewater. Our results provide an efficient denitrification technology for low-C/N ratio wastewater. This will help to promote the development of biological wastewater treatment technology based on NRFO bacteria.