Biological treatment of reverse osmosis concentrate by microalgae cultivation and utilization of the resulting algal biomass

The reverse osmosis system is one of the most advanced membrane-based water treatment technologies that can convert a variety of wastewater into drinking and industrial water. However, it has major drawbacks, including the production of a large amount of reject stream that contains concentrated nutrients. Microalgae are well-known for their rapid growth and ability to consume excessive nutrients very efficiently under a variety of wastewater conditions. In this study, a locally isolated microalga, Chlorella sorokiniana KNUA071, was cultivated in reverse osmosis concentrate (ROC) obtained from a municipal wastewater reclamation facility that contained high levels of nutrients, ion, and metal elements. The aim was to investigate the potential of this microalga to be used in ROC treatments and the related applications of its biomass. Results showed that the C. sorokiniana KNUA071 could successfully remove 91.9% of total nitrogen (TN), 97% of total phosphorus, 98% of Mn, 100% of Al, 64% of Zn, 62% of Co, and 50% of Ni within 8 days, yielding a biomass productivity of 0.9 g dry weight L⁻¹. This study also compared the results obtained from the Desmodesmus sp. KNUA024, which was used in the treatment of primary settled wastewater (PS) in our previously research, and was found to remove only 53.5% of TN in 8 days. C. sorokiniana KNUA071 biomass cultivated in the ROC had ~13.3% of lipid content while its biomass grown in the PS had ~28.2% of lipid content. The lipid contents of Desmodesmus sp. KNUA024 grown in the ROC and PS were ~29.3% and ~27.1%, respectively. Our findings suggest that microalgal cultivation could be employed as an effective strategy to remove nutrients and metals from ROC and at the same time produce biofuel from the lipids accumulated in the algal biomass.