Rhamnolipid pretreatment enhances methane production from two-phase anaerobic digestion of waste activated sludge.

In this work, a rhamnolipid (RL) pretreatment technology was proposed to promote methane production from two-phase anaerobic digestion of waste activated sludge. In the first phase (i.e., acidogenic phase), the WAS hydrolysis and acidogenesis were significantly enhanced after RL pretreatment for 4 day, under which the concentration of soluble protein and the short-chain fatty acids (SCFA) in the presence of RL at 0.04 g/g TSS was respectively 2.50 and 5.02 times higher than that without RL pretreatment. However, methane production was inhibited in the presence of RL. In the second phase (i.e., methanogenic phase), batch biochemical methane potential tests suggested that the addition of RL is effective in promoting anaerobic methane production. With an increase of RL dosage from 0 to 0.04 g/g TSS, the cumulative methane yield increased from 100.42 ± 3.01 to 168.90 ± 5.42 mL. Although the added RL could be utilized to produce methane, it was not the major contributor to the enhancement of methane yield. Further analysis revealed that total cumulative yield from the entire two-phase anaerobic digestion (sum of the yield of the acidogenic phase and methanogenic phase) increased from 113.42 ± 3.56 to 164.18 ± 5.20 mL when RL dosage increased from 0 to 0.03 g/g TSS, indicating that the addition of RL induced positive effect on the methane production of the entire two-phase anaerobic digestion. The enzyme activity analysis showed that although higher dosages of RL still inhibited the microorganisms related to methanogenesis to some extends in the methanogenic phase, the inhibitory effect was significantly weakened compared to the acidogenic phase. Microbial analysis revealed that RL reduced the abundance of Candidatus_Methanofastidiosum sp. while increased the abundance of Methanosaeta sp., causing the major methanogenesis pathway to change from hydrogenotrophic to aceticlastic. Moreover, the community of hydrolytic microbes and acidogens was shifted in the direction that is conducive to hydrolysis-acidogenesis. The findings reported not only expand the application field of RL, but also may provide supports for sustainable operation of wastewater treatment plants (WWTPs).