Biological treatment of ironworks wastewater with high-concentration nitrate using a nitrogen gas aerated anaerobic membrane bioreactor

Difficulty in treating ironworks wastewater with high concentrations of nitrate is a major issue in steel manufacturing. We developed an N-2 gas aerated anaerobic membrane bioreactor (AnMBR) and further assessed its nitrate removal capacity at the bench (0.026 m(3)) and pilot (4 m(3)) scales using a comprehensive microbiome survey. Both scale AnMBRs were fed with 5,300-8,000-mg/L nitrate-concentrated ironworks wastewater and methanol. At an early operation phase, nitrate and total organic carbon (TOC) increased in treated wastewater, which coincided with dynamic shift of the sludge microbiome. Thereafter, the accumulated nitrate was degraded in the absence of nitrite, in parallel with the TOC decrease and/or depletion, implying the presence of denitrification with methanol oxidation. At this phase, Hyphomicrobium nitrativorans capable of completely reducing nitrate predominated the microbiome. The increase in hydrolyzing and fermentative bacteria suggested their involvement in degradation of the decayed sludge biomass. H. nitrativorans, collaborating with high-molecular compound metabolizers, contributed importantly to the achievement of the nitrate removal rate of 1.1 +/- 0.1 kg NO3-N/m(3)/day for pilot scale. Compared based on the complete (e.g., >99.9 %) removal, the achieved rate was comparable with the previous records (2.0-2.5 kg NO3-N/m(3)/day). Nevertheless, the supplied nitrate concentrations were remarkably higher than the previous ones (650-1,500 mg/L). These highlight that the N2 gas aerated AnMBR is widely applicable for the effective treatment of the nitrate-concentrated industrial wastewater with the considerably small footprint.