EPS solubilization and waste activated sludge acidification enhanced by alkaline-assisted bi-frequency ultrasonic pretreatment revealed by 3D-EEM fluorescence

Abstract

The effect of alkaline-assisted bi-frequency (28 + 40 kHz) ultrasonic pretreatment on extracellular polymeric substances (EPS) solubilization and waste activated sludge (WAS) acidification was investigated. Experimental results showed that WAS disintegration and (volatile fatty acids) VFAs production were effectively improved by the pretreatment. The optimal ultrasonic treatment time and alkaline dosage were 45 min and 0.04 g NaOH per g TS. Under this optimal condition, a lysis rate of WAS of 36.8% was achieved, and the corresponding increased solute protein and carbohydrate concentrations were 4173 and 853 mg L⁻¹, respectively. Three-dimensional excitation–emission matrix (3D-EEM) fluorescence indicated that the pretreatment applied in this study could effectively decompose the tightly-bound EPS matrix, located in the inner parts of sludge flocks, and further destroy the microbial cell wall. The detachment of EPS and release of intracellular substrates provided sufficient substrates for anaerobic acid-producing microorganisms, which evidently improved the subsequent VFA production from WAS fermentation. The maximum VFA concentration was 10 816 ± 275 mg COD per L (457.1 g COD per g VS) for 72 h fermentation time, which was 5.4-fold higher than that obtained for the un-pretreated WAS.