The performance of thiosulfate-driven denitrification (TDD) granule reactor and the mechanism of granule sludge bulking were investigated in this study. The results showed that TDD granule bulking occurred under 12 kgNm-3d-1 of nitrogen loading rate (NLR). The higher NLR promoted accumulation of intermediates in the carbon fixation pathway, including citrate, oxaloacetate, oxoglutarate and fumarate. The carbon fixation improved amino acids biosynthesis, which increased proteins (PN) in extracellular polymers (EPS) to 134.6 ± 11.8 mg/gVSS. The excessive PN altered the content, components and chemical groups of EPS, leading to change of granule structure and decline in settling property, permeability and nitrogen removal. By adopting the strategy of intermittently reducing NLR, excess amino acids in sulfur-oxidizing bacteria was consumed through microbial growth-related metabolism instead of EPS synthesis. Therefore, the nitrogen removal rate increased to 10.23 kg-Nm-3d-1 and maintained stable in the long term. The EPS contents decreased from 168.8 ± 13.5 mg/gVSS to 93 ± 11.5 mg/gVSS and the SVI5 decreased from 66 ± 3.5 ml/g to 25 ± 1.5 ml/g. These findings provide an effective strategy to prevent granule bulking and guide practical application of TDD process.
Keywords: Granule sludge bulking; Metabolomics; Sulfide-oxidizing bacteria; Thiosulfate-driven denitrification.
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