Rhamnolipid-assisted anaerobic fermentation of primary sludge to improve the recovery of volatile fatty acids

This work reported for the first time the application of rhamnolipid (RL) in anaerobic fermentation of primary sludge to promote volatile fatty acids (VFAs) recovery. With the addition of RL increasing from 0 to 0.08g/g total suspended solid (TSS), the maximum VFAs yield increased from 26.97 to 278.01mg chemical oxygen demand (COD)/L. Further investigation revealed that adding RL was capable of improving sludge solubilization, more organics (e.g., protein and polysaccharide) were released and then provided for subsequent generation of VFAs. Meanwhile, the release of NH⁺₄-N and PO^3-₄-P was also improved in the presence of RL. Moreover, the cumulative CH₄yield of the fermenters with or without RL ranged from 25.67±3.03-32.10±9.62mL (P>0.05), demonstrating that RL did not significantly suppress next process VFAs consumption. Microbial analysis revealed that 0.08g/g TSS RL respectively increased the cumulative abundance of hydrolytic bacteria and VFAs producer from 9.64% and 3.07-15.16% and 5.09%, favoring the production of VFAs. Furthermore, the presence of RL changed the dominant methanogens from hydrogenotrophic methanogens of the 0g/g TSS to acetoclastic methanogens. Considering that the rich organics and functional bacteria in sludge, RL could be generated in situ, the findings reported in our paper not only expand the application field of RL but also promote the transformation of wastewater treatment plants (WWTPs) operations from the traditional energy & chemical input model to a circular & sustainable model.