Improved volatile fatty acid production during waste activated sludge anaerobic fermentation by different bio-surfactants

[Display omitted] •Evaluated the effects of three bio-surfactants on VFA production.•VFAs was increased from 75 to about 300gCOD/gVSS using the bio-surfactants.•EEM fluorescence spectroscopy was used to characterize dissolved organic matters.•Sludge solubilization and hydrolysis were improved in various degrees.•Methanogenesis inhibition by rhamnolipid and degradation of saponin both led to VFAs. The application of chemical surfactants in the production of volatile fatty acids (VFAs) during sludge anaerobic fermentation has drawn wide attentions. Compared with chemosynthetic surfactants, bio-surfactants are readily biodegradable and thus have a low negative impact on the environment. This paper studied the effects of three types of bio-surfactants (surfactin, rhamnolipid and saponin) on VFA production during sludge anaerobic fermentation. The release of nitrogen and phosphorus was also observed. Dissolution of organic matters, fluorescence spectra of filtrates, key enzyme activities, surface activity and biodegradation of the bio-surfactants were analyzed to reveal the mechanisms. The results indicated that VFA production was increased to approximately 4-fold versus the blank (75mgCOD/gVSS) using surfactin, rhamnolipid and saponin at the dosage of 0.05, 0.05, 0.10g/g DS, respectively. However, some differences existed in their mechanisms of improved acid production. Surfactin mainly increased the dissolution of organic matters to reach a high VFA accumulation. Rhamnolipid also enhanced the dissolution but inhibited the methanogenesis. In addition, its effects on microbial activities limited the further improvement of VFA production. Despite a lower surface activity than the other two bio-surfactants, saponin showed a similar improvement of VFA production through its self-degradation.