Rhamnolipid increases H2S generation from waste activated sludge anaerobic fermentation: An overlooked concern.

• Rhamnolipid significantly increased H 2 S production from sludge fermentation. • Rhamnolipid increased the release of organic sulfurs and inorganic sulfate. • The added and in situ generated rhamnolipid damaged the structure of organic sulfurs. • Rhamnolipid promoted H 2 S production from the hydrolysis of organic sulfurs. • Rhamnolipid facilitated H 2 S production from dissimilatory sulfate reduction. Rhamnolipid (RL), one representative biosurfactant, is widely regarded as an economically feasible and environmentally beneficial additive to improve fermentation efficiency and resource recovery from waste activated sludge (WAS). However, its potentially detrimental impact on WAS fermentation such as H 2 S generation was overlooked previously. This study therefore aims to fill the gap through exploring whether and how the presence of RL affects H 2 S generation from WAS anaerobic fermentation. Experimental results showed that when RL increased from 0 to 40 mg/g total suspended solids (TSS), the cumulative H 2 S yield enhanced from 323.6 × 10−4 to 620.3 × 10−4 mg/g volatile suspended solids (VSS). Mechanism analysis showed that RL reduced WAS surface tension, which benefited transformations of organic sulfurs (e.g., aliphatic-S and sulfoxide) and inorganic sulfate from solid to liquid phase. The presence of RL not only reduced the ratio of α-helix/(β-sheet + random coil) and damaged the hydrogen bonding networks of organic sulfurs but also promoted substrate surface charges and cell membrane permeability. These facilitated the contact between hydrolase and organic sulfurs, thereby increasing sulfide production from organic sulfurs hydrolysis. Further investigations showed that RL promoted the expression of key genes (e.g., aprA/B and dsrA/B) involved in the dissimilatory sulfate reduction, which accelerated the reaction of adenosine 5'-phosphosulfate (APS) → sulfite → sulfide. Meanwhile, RL inhibited the corresponding key genes such as CysH , and Sir , responsible for assimilatory sulfate reduction (APS → 3'-phosphoadenosine-5'phosphosulfate → organosulfur), which reduced substrate competition in favor of H 2 S production from dissimilatory sulfate reduction. Besides, RL decreased the fermentation pH, which benefited the transformation of H S − to H 2 S. [Display omitted] [ABSTRACT FROM AUTHOR]