Enhanced short-chain fatty acids production from anaerobic fermentation of secondary sludge by lignosulfonate addition: Towards circular economy

Abstract

This study is the first to unveil the potential of lignosulfonate as an additive in the production of short-chain fatty acids (SCFAs) from anaerobic fermentation of secondary sludge. Batch-scale tests were performed to assess SCFAs production and distribution with lignosulfonate addition of 20–80 mg/g volatile solids (VS), along with potential mechanisms and phytotoxicity. The results showed that lignosulfonate addition improved the SCFAs production and the ratio of acetic acid, while shortening the fermentation time required. The maximum SCFAs production (185.0 ± 1.0 mg COD/g VS) and acetic acid ratio (51.4 ± 0.6%) were achieved with 60 mg lignosulfonate/g VS at 4 d of fermentation time, outperforming the control (93.1 ± 2.7 mg COD/g VS, 44.4 ± 1.8%). Lignosulfonate addition decreased the zeta potential between sludge particles by 19.9%–45.1%, and increased the extractable extracellular polymeric substances matrix by 1.45–2.33 times, compared to the control group. Furthermore, lignosulfonate addition effectively promoted the solubilization, hydrolysis, and acidification processes by up to 116.0%, 58.8%, and 47.6%, respectively. The seed germination index of Chinese cabbage and lettuce using fermented sludge residues increased by up to 486 ± 29% and 158 ± 13%, respectively. These findings suggest that lignosulfonate addition is a sustainable method for SCFAs recovery from sludge, benefiting sludge agricultural reuse, and contributing to the circular economy.