Dual-loop involving microbial single-cell protein production from soybean-processing wastewater and effluent-based refinement for circular bioeconomy applications

Nutrient-rich food-processing wastewaters represent valuable yet under-utilised side streams for sustainable protein production in the form of microbial biomass. Here we present an integrated dual-loop bioprocess that converts soybean-processing wastewater into microbial single-cell protein (SCP) while achieving substantial nutrient removal and product refinement. In the first loop, previously enriched microbial consortia were inoculated and cultivated in four parallel sequencing batch reactors (SBRs) for 44Ldays at a hydraulic retention time (HRT) of 3Ldays. This bioprocess configuration demonstrated features that support future scale-up while maintaining process stability, achieving a protein content of 33.3L±L3.2%, doubling the protein yield (15.32L±L3.49Lg dry weight per g soluble TKN) and quadrupling the production rate (0.29L±L0.06Lg dry weight LL¹ dL¹) compared to operating reactors without inoculation (HRT: 7.2Ldays). Effluent treatment was stable, with 84% carbon and 78% nitrogen removal efficiencies, demonstrating efficient nutrient recovery. The SCP biomass was enriched in functional taxa, including Acidipropionibacterium , Lactococcus , Megasphaera, and Azospirillum , suggesting that reactor conditions and inoculum selection promoted a stable, protein-productive microbial community with potential probiotic benefits. In the second loop, bioreactor effluent was reused as aqueous matrix for heat treatment (60°C) of the SCP biomass, reducing the RNA content from 8.6% to 2.6%, with a 39% biomass loss accompanied by a 30% increase in total amino acid concentration. Hence, our valorisation approach integrates microbial biomass production, effluent reuse, and product refinement within a circular framework. The system provides a resource-efficient pathway for converting food-sector side streams into high-quality microbial community-based SCP, highlighting its potential scalability for sustainable nutrient and water management.