Isolation and evaluation of brackish diatoms for the photobiological treatment of reverse osmosis concentrate

Recently, the use of brackish diatoms has been proposed to remove various inorganic constituents, such as dissolved silica, nutrients, calcium, and bicarbonate, to enhance the freshwater recovery in reverse osmosis (RO). In this study, nine strains of brackish diatoms isolated from water and sediment samples from several evaporation ponds in California and Arizona were examined for their ability to assimilate silica and remove other constituents from RO concentrate. In addition to two previously reported strains, namely Gedaniella flavovirens PEWL001 and Nitzschia communis PEWL002, several new isolates including Halamphora sydowii PEWL004, Nitzschia sp. PEWL008, and Halamphora sp. PEWL011 were found to remove more than 95% of silica, 95% of ammonia and orthophosphate, and more than 50% of calcium and carbonate within 6 days. Two additional G. flavovirens strains (Psetr3 and Psetr7) collected from a brackish lake in Aomori, Japan, also showed rapid dissolved silica uptake (32 mg L−1 day−1) comparable to the one isolated from an agricultural drainage water evaporation pond in the Central Valley, California. This study demonstrated that the brackish diatoms isolated from the evaporation ponds could be useful for the treatment of RO concentrate, which would possibly enable more sustainable desalination processes.