Production of Carbohydrate-Rich <i>Chlorella</i> sp. Biomass Using Clarified Aquaponics Effluent for Bioethanol Feedstock Applications

The integration of microalgal cultivation with wastewater streams offers a promising pathway to enhance resource efficiency within circular bioeconomy frameworks. In this study, clarified aquaponics sedimentation effluent was evaluated as a cultivation medium for producing carbohydrate-rich Chlorella sp. biomass targeted for bioethanol feedstock applications. The nutrient composition of the effluent was first characterised, followed by systematic optimisation of wastewater dilution and light intensity under controlled laboratory conditions. Undiluted aquaponics effluent supported robust algal growth without freshwater dilution, achieving a maximum biomass concentration of 2.05 ± 0.03 g L⁻¹ under an optimal light intensity of 185 μmol m⁻² s⁻¹, representing a 78% increase compared with the conventional Bold’s Basal Medium control. Carbohydrate accumulation was significantly enhanced in the wastewater-grown biomass, reaching 40.71 ± 2.14% of dry weight and resulting in a total carbohydrate yield of 0.835 g L⁻¹. Based on stoichiometric conversion, the corresponding theoretical bioethanol potential (0.427 g L⁻¹) was more than three times higher than that of the control. In parallel, the system demonstrated strong bioremediation performance, achieving nitrate and phosphate removal efficiencies of 96.6% and 97.3%, respectively, while maintaining a more stable pH profile than the synthetic medium. These results indicate that clarified aquaponics sedimentation effluent can simultaneously function as an effective growth medium and a wastewater polishing stream.