Advanced Dairy Wastewater Treatment by Tetradesmus obliquus and Saccharomyces cerivisiae Co-Cultivation: Insights into Nutrient Recovery Applying Batch and Semicontinuous Processes

Cheese whey, a byproduct of the dairy industry, has a high organic load and nutrient availability, associated with parameters such as chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP), representing an environmental problem when improperly disposed, and even considering the traditional biological wastewater treatment (secondary treatment), a polishing step (tertiary treatment) could be required in order to meet legislation parameters of discharge in water bodies. This study evaluated the efficiency of co-cultivation between the microalga Tetradesmus obliquus and the yeast Saccharomyces cerevisiae during the tertiary (advanced) treatment of dairy effluent. The process was operated in batch mode to optimize the COD:N ratio and, subsequently, in semicontinuous mode applying the volumetric replacement rates (VRRs) of 40% and 60%. In the batch stage, the COD:N ratio of 20 stood out as the most balanced in terms of nutritional requirement, achieving removal rates of 85.49% for COD, 96.23% for total Kjeldahl nitrogen (TKN), and 100% for TP. In the semicontinuous system, a VRR of 40% optimized nitrogen (91.67%) and phosphorus (95.93%) recovery while COD was also removed (71.68%). The pH remained stable within the range of 7.0 to 7.5 at the end of the process, indicating self-buffering of the consortium. Biomass production reached 915 mg·L−1 (dry cell weight) in batch operation mode and 720 mg·L−1 in semicontinuous mode (VRR of 40%). The results confirmed that the T. obliquus and S. cerevisiae co-cultivation constitutes a stable and sustainable strategy for nutrient recovery during dairy wastewater treatment, aligning with the principles of circular bioeconomy.