Co-valorization of carbon-supplemented RO reject water via Rhodopseudomonas palustris for integrated biomolecule and biopolymer production

• Reverse Osmosis Concentrate (ROC) served as a nutrient-rich medium for R. palustris • 50% ROC dilution (RORM50) showed the highest essential amino acid enrichment • 75% ROC dilution (RORM75) enabled balanced biomass, pigment, and biomolecule yields • A maximum PHA content of 58.81% cell dry weight was achieved under RORM75 • ROC valorization enables waste mitigation and sustainable bioproduct generation Reverse osmosis (RO) reject management is increasingly critical amid water scarcity, as domestic RO concentrate (ROC) poses a growing secondary pollutant challenge. This study introduces a practical framework for ROC valorization. Sterile ROC (0, 25, 50, 75, and 100% dilutions) is blended with conventional media, supplemented with glycerol (carbon source) and sodium glutamate (nitrogen source), to create RO reject water medium (RORM) for cultivating Rhodopseudomonas palustris . We evaluated biomass productivity, pigment yield, essential amino acid profile, biomolecular composition (proteins, lipids, carbohydrates), and intracellular polyhydroxyalkanoate (PHA) production to demonstrate waste-to-value transformation. Magnesium and calcium ion levels remained stable across dilutions before and after cultivation, indicating minimal uptake and their role as physiological cofactors rather than bulk nutrients. RORM25 and RORM75 matched control performance; RORM25 delivered high protein (75.0 ± 2.4%), strong biomass (1.65 × 10 3 ± 0.04 mg L⁻¹), and pigment (10.60 ± 0.90 mg L⁻¹) yields. RORM75 offered balanced output with biomass (1.61 × 10 3 ± 0.03 mg L⁻¹), pigment (11.20 ± 0.70 mg L⁻¹), protein (70.0 ± 2.0%), lipid (6.0 ± 0.4%), carbohydrate (24.0 ± 1.6%), and PHA (58.81 ± 0.4% dry cell weight). RORM50 showed the highest essential amino acid enrichment, ideal for animal feed. Unlike traditional disposal methods, this approach diverts ROC from waste streams, curbing pollution while yielding value-added bioproducts and advancing a sustainable circular bioeconomy with reduced freshwater needs.