Volatile fatty acids produced by waste activated sludge with low-dose nano-zero-valent iron synchronized with low-intensity ultrasonication pretreatment

Abstract

Recently, using anaerobic hydrolysis of waste activated sludge (WAS) to recover volatile fatty acids (VFAs) as carbon sources for denitrification has been widely studied, and the key is to solve the problem of improving the solubility and bioavailability of WAS organic matter. Considering the energy consumption and chemical cost, this paper used low-dose nano-zero-valent iron (NZVI) coupled with low-intensity ultrasonication (US) to pretreat WAS, and explored the effects of different treatment sequences on the fragmentation of WAS flocs particles, the dissolution of organic matter, and the subsequent biofermentation conversion process. The results showed low-dose NZVI synchronized with low-intensity US (NZVIUS) worked best for WAS anaerobic fermentation. Compared with 2062 mg/L in control group, soluble chemical oxygen demand (SCOD) after NZVIUS treatment reached up to 2786 mg/L during the fermentation cycle, and the initial release rate of SCOD increased by 69 %. The VFAs production was elevated from 1522 to 1940 mg COD/L. NZVIUS pretreatment promoted the solubilization of attached organic matter, manifested by the fragmentation of WAS flocs and cell lysis. At the same time, proteins and carbohydrates in the extracellular polymeric substances (EPS) had different types of migration and transformation, and transfer of EPS can contribute 35.6 % to SCOD. Moreover, adding NZVI promoted PO₄³⁻-P recovery in the fermentation supernatant by 22.3 % more than in day 4 control. This study demonstrated that the NZVIUS pretreatment method can effectively improve the production of VFAs by WAS anaerobic fermentation, meeting the practical needs of wastewater plants.