Impact of microalgae species and solution salinity on algal treatment of wastewater reverse osmosis concentrate

Six common microalgal species, including freshwater microalgae Scenedesmus abundans, Chlorella vulgaris, Chlamydomonas reinhardtii and Coelastrum microporum, and marine microalgae Nannochloropsis salina and Dunaliella tertiolecta, were tested in batch treatment to identify the most promising species for remediating a municipal wastewater reverse osmosis concentrate (ROC). Selected species were then studied at different ROC salinity levels (5, 10, and 15 g TDS/L) in semi-continuous treatment to evaluate their potential for nutrient remediation, and biogas production through anaerobic digestion. S. abundans, C. vulgaris, and N. salina showed higher potential for growth and nutrient remediation under salinity stress. Further tests revealed that N. salina adapted well to ROC conditions, and S. abundans could grow better and had higher tolerance to the elevated salinity than C. vulgaris. S. abundans and N. salina performed better for removing nutrients and organic matter (11.5-18 mg/L/d TN, 7.1-8.2 mg/L/d TP, and 8.6-12.4 mg/L/d DOC). Increasing salinity led to growth inhibition and N uptake reduction for freshwater species but had no significant effect on TP removal. Biochemical methane potential tests showed the algal biomass produced a significant amount of methane (e.g., up to 422 mL CH4/g VS for N. salina), suggesting the algae generated from the ROC treatment could produce significant amounts of energy through anaerobic digestion without the need for pretreatment. This study showed the environmental and economic potential of the algal system for future applications.