Volatile fatty acids accumulation and rhamnolipid generation in situ from waste activated sludge fermentation stimulated by external rhamnolipid addition

The solubilization and acidification of waste activated sludge (WAS) were apparently enhanced by external rhamnolipid (RL) addition. The maximum solute carbohydrate concentrations increased linearly from 48 ± 5 mg COD L−1 in the un-pretreated WAS (blank) to 566 ± 19 mg COD L−1, and protein increased from 1050 ± 8 to 3493 ± 16 mg COD L−1 at RL dosage of 0.10 g g−1 TSS. The highest VFAs concentration peaked at 3840 mg COD L−1 at RL dosage of 0.04 g g−1 TSS, which was 4.24-fold higher than the blank test. RL was generated in situ during WAS fermentation when external RL was added. It was detected that RL concentration was increased from initial 880 ± 92 mg L−1 to 1312 ± 7 mg L−1 at the end of 96 h with RL dosage of 0.04 g g−1 TSS, which was increased to 1.49-fold. Meanwhile, methane production was notably reduced to a quite low level of 2.0 mL CH4 g−1 VSS, showing effective inhibition of methanogens by RL (58.8 mL CH4 g−1 VSS in the blank). In addition, the activity of hydrolytic enzymes (protease and α-glucosidase) was enhanced accordingly. VFAs accumulation and RL generation in situ demonstrated that the additional RL substantially performed enhanced biological effects for waste activated sludge fermentation.