Chemoautotrophic denitrification based on ferrous iron oxidation: Reactor performance and sludge characteristics

•VLR and VRR for Fe-CAD reactor were 0.26 ± 0.01 kg-N/(m3·d) and 0.09 ± 0.03 kg-N/(m3·d).•Fe-CAD sludge was dominated by Rhodanobacter, Mizugakiibacter, and Sulfuricella.•Maximum removal rate of NO3− by Fe-CAD sludge was 4.68 mg-N/(L·gVSS·h).•Iron-encrustation caused sludge activity dropping and reactor deterioration.

Aiming at treatment of wastewaters with low C/N ratio, a novel ferrous iron-based chemoautotrophic denitrification (Fe-CAD) reactor was developed with inoculum sludge from a municipal sewage plant in Hangzhou, China. The efficiency of the Fe-CAD reactor was remarkable. The volumetric loading rate (VLR) and volumetric removal rate (VRR) of NO3− were 0.26 ± 0.01 kg-N/(m3·d) and 0.09 ± 0.03 kg-N/(m3·d), while the VLR and VRR of Fe2+ were 3.10 ± 0.24 kg-Fe/(m3·d) and 1.69 ± 0.26 kg-Fe/(m3·d), respectively. By means of next generation sequencing, the Fe-CAD sludge was found to be rich in ferrous iron-oxidizing nitrate-reducing bacteria including Rhodanobacter, Mizugakiibacter, Sulfuricella, Comamonas and Gallionella. Reaction dynamics of the Fe-CAD sludge were determined by batch experiments. After fitted by Haldane Model, the maximum specific activity (μmax), the saturation concentration (Ki) and the half inhibition concentration (Ks) of NO3− to the Fe-CAD sludge were calculated as 0.24 mg-N/(h·gVSS), 72.82 mg-N/L and 2722.97 mg-N/L, while the μmax, Ki, Ks of Fe2+ were 2.28 mg-Fe/(h·gVSS), 203.09 mg-Fe/L and 229159.38 mg-Fe/L, respectively. The produced ferric iron formed an brilliant yellow iron-encrustation with irregular shape around the functional microorganisms, and the iron-encrustation resulted in a dropped specific activity of the Fe-CAD sludge. Removal or prevention of the iron-encrustation around microbial cells were suggested to be the key to improve the performance of the Fe-CAD reactor.

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