Mechanism of Activated Sludge Floc Disintegration Induced by Excess Addition of NaCl

Abrupt salt intrusion in municipal wastewater treatment plants has triggered serious disturbances and deteriorated the biological treatment process from steady state into unsteady state. The effects of sodium (Na+) concentration on the structure and properties of activated sludge during the shock period were investigated in this paper. A floc disintegration characterized as the decreased floc size and increased porosity in the floc matrix was found when excess Na+ was added to the bioreactor. Increased Na+ resulted in a slight decrease in the normalized capillary suction time. About 53-57% of the total extracellular polymeric substances (EPS) derived from activated sludge was protein, at Na+ concentrations <150.74 mmol/L, and this value even reached 68% at 286.04 mmol/L Na+. Over 50% of Na+ in EPS was found in the loosely bound EPS (LB-EPS). The Na+ sorbed in LB-EPS followed Langmuir model with a maxium sorption capacity of 2.086 mmol/g VSS. With the increase of Na+ concentration in LB-EPS, the Mg2+ concentration in LB-EPS decreased until reaching equilibrium while there was no significant change of Ca2+ concentration. A concept model is proposed for the first time targeting to improve the understanding of sodium ion effects on activated sludge.