Synergistic ultrasound and zero-valent iron pretreatment for enhanced waste activated sludge anaerobic fermentation: Mechanism elucidation, process optimization, and benefit analysis

Abstract

This study employed synergistic ultrasound and zero-valent iron (ZVI) pretreatment to enhance waste activated sludge (WAS) anaerobic fermentation. Using response surface methodology (RSM) to systematically optimize the ultrasonic power, ultrasound time, and ZVI dosage, aiming to determine the optimal synergistic pretreatment conditions for enhancing subsequent anaerobic fermentation. The results indicated that pretreatment reduced the particle size of WAS, increased fractal dimension and zeta potential absolute value, and resulted in a higher specific surface area, promoting electrostatic repulsion, system stability, and microbial growth. Alpha diversity increased (as reflected by the Chao index), and functional microbiota involved in hydrolysis and acidification, such as Actinomycetota and Bacteroidota, were further enriched. Further confirmation through three-dimensional excitation-emission matrix (3D-EEM) spectroscopy and molecular weight distribution analysis of dissolved organic matter revealed that the synergistic pretreatment significantly enhanced sludge hydrolysis efficiency and released substantial biodegradable components, with a higher biological index (BIX) and lower humification index (HIX). Specific analysis demonstrated that a maximum soluble chemical oxygen demand (SCOD) concentration of 1632.99 mg/L in the pretreated supernatant was achieved at an ultrasonic power of 1.86 W/mL, an ultrasound time of 14.10 min, and a ZVI dosage of 0.38 g/L. Furthermore, the maximum volatile fatty acids (VFAs) production (1722.65 mg COD/L) during the fermentation period was attained at an ultrasonic power of 1.78 W/mL, an ultrasound time of 14.48 min, and a ZVI dosage of 0.38 g/L. Benefit analysis suggests that this optimized process demonstrates promising application potential, offering an efficient and cost-effective strategy for sludge resource recovery.