Novel insight on humic acid addition into waste activated sludge to enhance the production of short-chain fatty acids during anaerobic fermentation

Abstract

Anaerobic fermentation for producing short-chain fatty acids (SCFAs) is recognized as a potentially practical method for waste activated sludge (WAS) disposal. However, limited dissolution and hydrolysis processes depressed the yield of SCFAs. In this study, humic acid (HA) was added to the Fe₃O₄-sodium percarbonate (SPC) system for improving SCFA production by anaerobic fermentation, with a particular focus on the underlying mechanisms. The results showed that the maximum accumulation of SCFAs (2448.24 ± 33.19 mg COD/L, acetic acid accounting for 52 %) occurred on the 8th day with the addition of 5 mg HA/g total suspended solids (TSS) to the Fe₃O₄-SPC (20 mg/g TSS Fe₃O₄ and 15 mg/g TSS SPC), which was 2.3 times that of the control group (without any chemicals) and 1.4 times that of the Fe₃O₄-SPC-H0 group (without HA). Free radical assays revealed that ·OH and·O₂^– played a predominant role in enhancing the WAS dissolution. The increasing concentrations of soluble Fe(II) and total Fe in the fermentation liquor indicated that HA could accelerate the Fe(III)/Fe(II) cycle, thereby increasing the oxidizing capacity of the Fe₃O₄-SPC-HA system, and, in turn, improving the dissolution of WAS. Furthermore, the key enzymes analysis indicated that the addition of HA notably boosted the activities of α-glucosidase and acetate kinase, and stimulated the release of NH₄⁺-N, PO₄³⁻-P, and HA from sludge floccule, which was beneficial for the recovery of these nutrients from the fermentation liquor. Therefore, this study presented an effective and sustainable strategy for the production of SCFA and resource recovery through WAS anaerobic fermentation.