COMPARISON OF CHEMICAL AND PHOTOBIOLOGICAL METHODS FOR THE REMOVAL OF DISSOLVED SILICA AND CALCIUM HARDNESS FROM BRACKISH GROUNDWATER REVERSE OSMOSIS CONCENTRATE

Brackish water is an important alternative water supply in many southwestern and southeastern states such as Texas, California, and Florida. More than 1 billion gallons per day of brackish water is being desalinated and used for various purposes including drinking water, industrial processes, and power generation in the US. Reverse osmosis (RO) is the key process for removing salinity and other inorganic constituents from the brackish water. Typical brackish water desalination facilities (BWDFs) recover 75% to 85% of fresh water from the brackish water sources, including lakes and aquifers. As the RO process removes dissolved constituents, they are concentrated in the waste stream (i.e., concentrate), which is the remainder of 15% to 25% of the feed flow. One of the challenges associated with the RO-based BWDFs is the concentrate management and disposal. In fact, water in the RO concentrate is still recoverable based on the salinity (5,000 to 18,000 mg/L as total dissolved solids) and osmotic pressure. However, certain inorganic constituents, including dissolved silica and calcium hardness, can be at near saturation which often prevents BWDFs from recovering more fresh water. Calcium hardness can be readily removed as calcium carbonate by chemical softening by adjusting the pH to >10. It is also known that dissolved silica co-precipitates with calcium carbonate. In addition, a novel photobiological method using brackish diatom has been proven effective in both dissolved silica and calcium hardness removal. In this research, we compared the photobiological and chemical methods for the removal of dissolved silica (130 mg/L) and calcium hardness (700 mg/L as CaCO₃) from a brackish groundwater RO concentrate. While the photobiological method using a brackish diatom Gedaniella flavovirens Psetr3 could remove >90% and 75% of dissolved silica and calcium hardness, respectively, the chemical precipitation with sodium hydroxide could remove 40% and 90% of these constituents, respectively. The photobiological method appears to have an advantage of less chemical and energy requirements over the chemical method. Detailed analysis of biomass and precipitate using X-ray diffraction and scanning electron microscopy and energy dispersive X-ray spectroscopy is currently underway.