Industrial-scale valorization of decolorized soy sauce wastewater into sustainable and cost-effective microalgal biodiesel

The urgent need to reduce carbon emissions and accelerate renewable energy deployment has spotlighted microalgae-based biodiesel as a sustainable alternative. Yet, large-scale production remains hindered by dependence on nutrient-rich media, low photosynthetic efficiency, and limited light availability. Here, this study demonstrates a scalable strategy that repurposes raw soy sauce wastewater a saline, pigmented, nutrient-dense industrial effluent, as a cultivation medium for Chlorella sorokiniana. Following 20 % activated carbon-mediated decolorization and 1:20 dilution, over 95 % of melanoidin pigments were removed while retaining over 89 % of total sugar. This substantially enhanced light transmission and boosted photosystem II efficiency by 3.17-fold, enabling optimized mixotrophic growth. The modified medium led to a 1.53-fold increase in growth rate compared to untreated wastewater and a 4.06-fold improvement over autotrophic cultures. Biomass productivity increased by 2.39-fold, along with effective pollutant removal, including reductions of 96.6 % in total nitrogen, 93.4 % in chemical oxygen demand, 90.6 % in total phosphorus, and 86.6 % in total carbohydrate. Lipid production improved by 2.77-fold, and the resulting biodiesel exhibited favorable fuel properties, including a high cetane number and low iodine value. Economic analysis revealed a 56.6 % reduction in material costs, decreasing unit production cost from $39.63 to $17.19. Over five cultivation cycles, cumulative costs remained substantially lower than with conventional Tris-acetate-phosphate (TAP[sbnd]C) medium. Collectively, this study presents a cost-effective, carbon-neutral approach for transforming high-strength industrial waste into high-quality biodiesel feedstock, advancing circular bioeconomy and sustainable energy transitions.