Co-liquefaction of sewage water grown Chlorella sorokiniana and Scenedesmus abundans for sustainable energy production

Chlorella sorokiniana (CS) and Scenedesmus abundans (SA) are green microalgae widely considered for biofuel production. The viability of combining two sewage-grown microalgae species for generating bioenergy via hydrothermal liquefaction (HTL) is explored in this work. A large-scale 10,000 L open raceway pond was used to culture both species separately in sewage water. The effect of feedstock ratio on bio-oil composition and yield was investigated. Three ratios of feedstock, namely, CS:SA = 1:1, 1:2, and 2:1, were studied. The HTL reaction was carried out at 250 °C, 60 bar, with 2 h holding time in a 50 L HTL reactor. The maximum bio-oil yield reached 44.43% with a higher heating value (HHV) of 45.5 MJ kg⁻¹ by co-liquefaction (co-HTL) of CS:SA = 2:1. The highest biochar yield of 4.53% was obtained by co-HTL of CS:SA = 1:1. The bio-oil was characterized to reveal the presence of both light and heavy oils whereas the biochar disclosed lower crystallinity with medium volatility. Thermal efficiency values of 45.3%, 53.23%, and 58.2% and energy return on investment (EROI) of 0.78, 0.9, and 1.02 were obtained from the co-HTL of CS:SA = 1:1, 1:2, and 2:1, respectively. Analysis of the energy parameters of the bio-oil indicated that co-HTL of CS and SA in 2:1 ratio is energetically feasible for achieving a sustainable bioenergy generation system. The strategies proposed serve as a roadmap to reduce carbon emissions and manage waste effectively.