Deep mechanism of enhanced dewaterability of residual sludge by Na+: Comprehensive analyses of intermolecular forces, hydrophilicity and water-holding capacity of EPS

Extracellular polymeric substance (EPS) is generally considered as the limiting factor affecting sludge dewatering due to its complex components and water-holding capacity. Conventional flocculation conditioning could improve the dewaterability by generating a certain number of channels for water discharge. However, the hydrophilicity and water-holding capacity of EPS still cannot change, resulting in the inability to further consolidate sludge dewaterability. To overcome this challenge, the study explores the application ability of sodium chloride via Na+ conditioning for sludge dewatering and compared with calcium chloride (CaCl2) and ferric chloride (FeCl3) conditioning effects. Results confirmed that the specific resistance to filtration (SRF) and water content (WC) fell dramatically from 14.3 × 1012 m/kg to 8.1 × 1012 m/kg and 80.8 % to 75.4 %, respectively, at the Na+ concentration of 80 mmol/L. The mechanism investigations indicated that addition of Na+ clearly destroyed the structure of EPS and promoted the declines in hydrophilicity and water-holding capability of EPS, resulting in much less bound water, changes in secondary structure and functional groups (e.g. NH, and CO) of EPS proteins. Furthermore, analyses of surface thermodynamic illustrated that the aggregation ability of sludge enhanced after the conditioning of Na+ combined with re-flocculation. Additionally, compared with Ca2+ and Fe3+, applying the combined conditioning method led to stronger hydrophobicity of EPS through the analysis of two-dimension correlation spectroscopy (2D-COS). This work can drive innovation in applying salty water containing sodium for effectively sludge dewatering.