A combined light regime and carbon supply regulation strategy for microalgae-based sugar industry wastewater treatment and low-carbon biofuel production to realise a circular economy

The replacement of fossil fuels with clean and renewable biofuels is of both research and market interest for realising a circular economy. However, microalgae-based biofuels have shown promise as alternative low-carbon biofuels to other crop-based biofuels, some key obstacles in their production remain to be addressed, such as high costs and low lipid productivity. In this study, a Chlorella sp. CSH4 was cultivated using a combined light regime and carbon supply regulation strategy to enhance sugar industrial wastewater bioremediation, biomass accumulation and lipid production. Blue light irradiance of 200 μmol photons m⁻² s⁻¹ together with 10 g/L glucose and 9.2 g/L glycerol supply was found to effectively enhance the biomass accumulation and pollutant-removal capacity of Chlorella sp. during the growth phase and its lipid production during the stationary phase. Furthermore, the biodiesel properties of the lipid retrieved from Chlorella sp., as demonstrated by its fatty acid profile, were found to be suitable for commercial application. Possible mechanisms were explored to explain how this combined strategy caused this microalga to exhibit highly efficient biomass and lipid production together with efficient pollutant removal. Moreover, upscaled semi-continuous treatment using both sugar industry wastewater and negligible carbon sources (e.g., food waste hydrolysate and crude glycerol) with a mass balance analysis was conducted to initially validate the feasibility of applying our combined strategy for microalgae-based wastewater treatment. In sum, this study demonstrated the feasibility of cultivating a microalga using a combined strategy comprising a light regime and carbon supply regulation to achieve both wastewater treatment and low-carbon biofuel production.