Insights into the Effect of Rhamnolipids on the Anaerobic Fermentation and Microalgae Lipid Production of Waste Activated Sludge: Performance and Mechanisms

In the last few years, the production of waste activated sludge (WAS) has increased dramatically, and the treatment and disposal have attracted widespread attention. In this study, rhamnolipids were used to improve the efficiency of hydrogen production from WAS, and the fermentation solution was used as a substrate by lipid-producing microalgae to recover resources and generate clean energy. Results showed that rhamnolipids had the best effect among several surfactants in promoting hydrogen production, and the hydrogen production of WAS added with 0.4 g/g TS rhamnolipids reached 178.48 ± 6.07 mL/g TS, which improved by 13.53 times compared to the control group. The content of SCOD, soluble protein, and soluble polysaccharide in WAS fermentation solution with the addition of 0.4 g/g TS rhamnolipid increased by 9.19, 11.46, and 42.28 times, respectively, indicating that rhamnolipid could improve the hydrolysis of WAS, which led to an increase in the efficiency of hydrogen production. And three-dimensional excitation–emission matrix fluorescence spectroscopic analysis showed that the intensity of dissolved organic matter increased, suggesting a rise in soluble microbial metabolites. The analysis of the composition of volatile fatty acids in the fermentation solution showed that the type of hydrogen production was mainly butyric acid-type fermentation hydrogen production with high concentrations of n-butyric acid (9330.14 ± 525.33 mg/L) and acetic acid (6324.07 ± 80.67 mg/L). In addition, analysis of the microbial community structure of the WAS fermentation hydrogen production system showed that the dominant genus was Clostridium sensu stricto. Microalgae can utilize acetic acid, ethanol, NH4+-N, PO43–-P, dissolved proteins, and dissolved polysaccharides in the fermentation solution to grow and produce lipids. The biomass and lipid productivity of microalgae cultured by fermentation solution reached 3.16 ± 0.58 g/L and 356.86 ± 4.91 mg/L/day, respectively. The microalgae contained 84.57 ± 4.33% saturated fatty acid content, which was more favorable for biodiesel production. This study not only successfully utilized rhamnolipids to promote hydrogen production from WAS but also coupled microalgae lipid production system, further laying the foundation for realizing the resource utilization of waste resources and clean energy production.