A novel wastewater-derived cascading algal biorefinery route for complete valorization of the biomass to biodiesel and value-added bioproducts

Abstract

Integration of multiproduct algal biorefineries with wastewater treatment systems could offer a sustainable approach to produce green products in a circular bioeconomy paradigm while keeping the water-energy-environment nexus sustainable. The present study demonstrated a novel biorefinery route employing a newly isolated self-flocculating microalga Chlamydomonas sp. BERC07. Here, the urban wastewater served as a low-cost growth media and improved biomass production (1.24 g/L) by 2-fold with concomitant removal of 100% total nitrogen, 94% total phosphorus, 56% COD, and 41% BOD. To lower the harvesting cost, an inducible sedimentation-based harvesting system was developed by employing the lowest dose of a commercial-grade flocculant, potash alum (0.27 kg/1000L culture) which improved the sedimentation rate of algal flocs by 240-fold with a biomass recovery efficiency of 96–98%. Later, the wastewater-derived biomass was subjected to downstream processing in a cascading fashion for its complete valorization. The biomass yielded 1.83 mg/g of carotenoids and 480 mg/g of lipids. The lipids were transesterified to produce biodiesel which completely met the preset European and American biodiesel quality standards. After carotenoids and lipids extraction, the residual biomass was fermented using Aspergillus niger and Aspergillus oryzae for the very first time. Interestingly, the A. oryzae outcompeted the A. niger and produced 131.6 U/mL of α-amylase along with 375–384 mg/g of mycoproteins utilizing 75–100 g/L of the residual algal biomass as a sole feedstock. All solvents involved in the product recovery were recycled to the primary wastewater, leaving no waste at the end of the processing pipeline. Overall, the data demonstrated a novel and sustainable biorefinery route with the most efficient harvesting and the complete valorization of the biomass, in a zero-waste approach. However, further upscaling, life cycle assessment, and technoeconomic analyses will be required to assess the economic and environmental sustainability of the proposed biorefinery route.