USING MICROBIAL SULFATE REDUCTION FOR IMPROVING BRACKISH GROUNDWATER DESALINATION

Desalination of brackish groundwater (BGW) with total dissolved solids (TDS) ranging between 1 to 10 g/L, in the US can generate a significant volume of freshwater for potable use. Existing RO-based technologies can recover 75-85% freshwater and generate 15-25% RO-concentrate (brine) to be disposed, which poses economic and environmental challenges, particularly for inland desalination facilities. However, elevated concentrations of dissolved scalants (e.g., Ca²⁺, Mg²⁺, HCO₃^-/CO₃^2-, SO₄^2-, SiO₂) in BGW limits the efficiency of RO-based desalination. Microbial sulfate reduction is natural process that occurs in almost all geochemical systems, known for a long time, and used for remediation of contaminated sites such as acid mine drainage, and aquifers contaminated with metals and metalloids. However, this approach has rarely been used in water treatment processes. In this study, we tested microbial sulfate reduction to reduce the concentration of dissolved SO₄^2- from BGW RO concentrate to decrease the overall TDS and improve desalination. We performed preliminary experiments under anaerobic conditions (O₂ = 0 ppm, H₂ = 4%) at UTSA using synthetic water containing 2,881 mg/L of SO₄^2- and pure strain of Desulfovibrio vulgaris, a well-characterized sulfate reducing bacterium. Upon acclimatization, healthy population of D. vulgaris was transferred to BGW from City of San Antonio H2Oaks Desalination plant RO concentrate. Results showed that the SO₄^2- concentration in the RO concentrate medium decreased from 2,881 mg/L to 1,226 mg/L (57%) over period of 7 days. Acetate as a source of electron donor for D. vulgaris was added externally to initiate this reaction. Ongoing experiments focus on optimization of acetate dose required to accelerate microbial sulfate reduction, as well as to determine the dose of ferrous iron to precipitate microbially formed sulfide in the medium. Geochemical calculations indicated that the overall TDS of BGW RO concentrate would decrease by 34% after successful microbial sulfate reduction, which will significantly improve the desalination. Natural processes like microbial sulfate reduction can provide a viable solution for improving advanced water treatments and efficient use of unconventional water resources such as brackish groundwater.