Sustainable bioremediation of undiluted matured compost leachate and enhanced lipid partitioning in Desmodesmus sp.: a zero-dilution approach for circular bioeconomy.

The economic feasibility of microalgal biodiesel remains constrained by the high cost of synthetic nutrients, driving interest in waste-derived cultivation systems. This study investigates the mixotrophic growth of Desmodesmus sp. using 100% undiluted matured compost leachate as a dual-purpose medium for biomass production and wastewater remediation. To decouple microalgal assimilation from bacterial degradation, a strictly sterile abiotic control was employed, enabling accurate attribution of nutrient removal pathways. The system achieved bioremediation efficiencies of 82.6% chemical oxygen demand (COD) removal, alongside 60.3% nitrate reduction (via biological assimilation) and 65.5% phosphate reduction (via synergistic biological uptake and photosynthesis-induced physicochemical precipitation). Desmodesmus sp. demonstrated acclimatization to the untreated organic effluent, reaching a maximum biomass concentration of 2.69 g L-1-approximately fourfold higher than that obtained in the synthetic control medium. Progressive phosphorus exhaustion, driven by an extreme initial N:P ratio, induced lipid accumulation during the stationary phase, resulting in a lipid yield of 0.715 g L-1 (26.56% dry cell weight). Fatty acid methyl ester (FAME) analysis revealed a predominantly saturated profile (75.6%), dominated by palmitic and lauric acids, indicating a definitive metabolic shift from the highly unsaturated profile of baseline synthetic media. While such composition enhances oxidative stability, it may impose limitations on cold-flow properties. The detection of a non-saponifiable lipid fraction highlights a process bottleneck that must be addressed during downstream transesterification. This study demonstrates a zero-dilution, waste-to-resource platform that integrates high-density biomass production with nutrient recovery, offering a scalable pathway toward circular bioeconomy-driven biodiesel production.