Nutrient Removal and Recovery from Dairy Wastewater via Co-Cultivation of Scenedesmus obliquus and Lemna minor

Dairy wastewater is an agro-industrial effluent characterized by elevated concentrations of organic matter, nutrients and suspended solids, posing environmental risks when inadequately treated. This study assessed the feasibility of a simultaneous Scenedesmus obliquus–Lemna minor co-cultivation system within a single, shared bioreactor for the treatment of modified dairy wastewater, comparing its performance against the two individual monocultures. Raw dairy wastewater was subjected to sequential heat treatment, ultrasound and filtration, which significantly altered its physicochemical composition (p < 0.05) prior to cultivation. Experiments were conducted in parallel laboratory-scale open reactors over 14 days with periodic sampling. The mixed system achieved the highest removal efficiencies (95.3% COD, 91.3% TOC, 97.1% NO3-N and 94.6% PO4-P), with rapid pollutant reduction during the first 3–6 days followed by a slower stabilization phase. Despite these high removal efficiencies, the final COD concentration remained relatively high in absolute terms, indicating that the system is best suited as an on-site pretreatment step at the dairy facility, reducing the organic and nutrient load prior to discharge into the municipal sewerage network for further treatment at a municipal WWTP. The recovered microalgal biomass was nutrient-rich and exhibited preliminary biofertilizer potential, as reflected in the positive growth trends observed in two of the three tested Lactuca sativa L. cultivars, although these did not reach statistical significance relative to controls. Overall, the integrated system represents an effective, low-impact approach combining pollutant removal, biomass valorization and circular bioeconomy principles.