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Published on: July 24, 2018
EPS-mediated mineralization drives granule densification and enhances denitratation-anammox coupling under alkaline
Shenbin Cao1, Xingping Liu2, Yucheng Tao2
1College of Architecture and Civil Engineering, Beijing University of Technology, Beijing 100124, China; National Engineering Laboratory for Advanced Municipal Wastewater Treatment and Reuse Technology, Beijing University of Technology, Beijing 100124, China.
Abstract:
The granular-based CANDAN (Complete Ammonium and Nitrate removal via Denitratation-Anammox over Nitrite) process offers a promising low-carbon and high-rate strategy for nitrogen removal; yet the mechanisms by which alkaline conditions regulate granule structure and functional coupling remain insufficiently understood. Here, a 9-L sequencing batch reactor (SBR) was operated for 130 days with stepwise pH elevation from 7.31 ± 0.03 to 8.52 ± 0.08 to elucidate alkaline condition-driven structural and functional adaptations in CANDAN granules. Moderate alkaline conditions significantly improved nitrogen removal, with total nitrogen removal efficiency increasing to 91.4 ± 0.1 %, accompanied by pronounced improvement in sludge settleability (sludge volume index after 30 min of settling, SVI₃₀, decreased from 76.4 to 19.4 mL g⁻¹ SS) and stabilization of dominant granule sizes at 0.5-1 mm, accounting for approximately 69.8 % of the total granules, indicating progressive granule densification. Mineralogical analyses revealed that hydroxyapatite dominated the inorganic matrix, with co-precipitation of calcium carbonate (CaCO₃) and transient magnesium ammonium phosphate formation reinforcing granule structure. Elevated pH also remodeled extracellular polymeric substances (EPS), increasing loosely bound EPS, raising the protein-to-polysaccharide ratio, and enriching tryptophan-like proteins that facilitated EPS-mediated mineral nucleation. Metagenomic analysis revealed streamlined carbon metabolism and enrichment of key nitrogen-cycling genes (napA, nosZ, hzsA), while downregulation of Ca²⁺, Mg²⁺, and phosphate transport genes favored extracellular mineral accumulation. Overall, moderately alkaline conditions drive EPS-mediated mineralization that densifies granules and stabilizes Denitratation-Anammox coupling, providing mechanistic insight for optimizing low-carbon nitrogen removal under alkaline wastewater conditions.
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