INVESTIGATING BIOLOGICAL METHODS FOR IMPROVING REVERSE OSMOSIS BASED DESALINATION OF BRACKISH GROUNDWATER IN BEXAR COUNTY, TEXAS

Brackish groundwater containing TDS within 1–10 g/L is a vital water resource (8.5×10⁶ km³) in the U.S. that can be used for potable and non-potable applications. Brackish groundwater desalination is mainly achieved by reverse osmosis (RO) treatment. Efficiency is limited by scaling and fouling of membranes. Current RO-based desalination technology can achieve 85% (7.2×10⁶ km³) yield of freshwater recovery. Photobiological silica, calcium, and microbial sulfate removal may allow an additional 10% recovery, increasing the total recovery and decreasing volume of waste by 8.5×10⁵ km³. Sulfate, being one of the most prominent and difficult to remove scalants, has been established as the focus of this experiment in efforts to increase efficiency of RO desalination by this research team. Utilizing sulfate reducing bacteria (SRB) has historically been a successful method of ion removal (Tang et al., 2017, Pi et al., 2017, Miao et al., 2016).

Determination of the brackish water source most suitable for this study was performed by evaluating the aqueous chemistry of 45 brackish groundwater wells in Bexar County, TX, along with six samples from three locations across Texas. Geochemical data was obtained from the USGS national brackish groundwater database (McMahon et al., 2016) and saturation index (SI) of minerals were calculated using PHREEQC. TDS range between 1 - 5 g/L and is composed of mainly: Na⁺ (270±185 mg/L), K⁺ (15±9 mg/L), Ca²⁺(274±183 mg/L), Mg²⁺(82±76 mg/L), total Fe (0.2± 0.3 mg/L), HCO₃^- (261±73 mg/L), Cl^-(597 ± 738 mg/L), SO₄^2-(1,037 ± 765 mg/L), and SiO₂ (16±7 mg/L). The sample site chosen provides three stages of brackish groundwater for experimentation. This will be useful when determining viability of SRB in high TDS water. This location also had one of the highest concentrations of sulfate (446-2618 mg/L). Preliminary experiments show the sulfate reducing bacteria are most productive when provided with an external carbon source (sodium acetate) at the same molar concentration as the sulfate present in the brackish water. Both acetate and sulfate decreased over time with SRB present. Future experimentation involves expanding from low TDS samples used in the preliminary experiments to the two higher brackish samples for comparison in efficiency and survival requirements of SRB.