Sulfate Reduction in Produced Water via Expanded Granular Sludge-Bed Reactors

Abstract

In 2012, an estimated 890 billion gallons of produced water were generated for all U.S. onshore and offshore oil and gas production sites (Veil, 2015). Reusing produced water seems beneficial for oil companies, but may do more harm than good by inducing scale, such as calcium sulfate scales. This thesis evaluates the potential of using biological sulfate reduction to remove sulfate in a synthetic produced water at 30 °C via two expanded granular sludge bed (EGSB) reactors each with a working volume of 3.3 L, and a reactor pH 7.5. Propionate was supplied as the sole electron donor and sulfate was added to maintain a COD/SO4 ratio between 1.5-2.3. Based on the results from the study, high sulfate reduction efficiencies (90.0%) were achieved at low to moderate salinity levels (10-30 g·L-1 of NaCl). At a salinity level of 30 g·L-1, the average sulfate reduction efficiency was 94.0± 1.2%. Rapidly increasing the salinity content from 15 to 40 g·L-1 of NaCl resulted in poor system performance (sulfate reduction efficiency of 32%), while increasing the salinity in 5.0 g·L-1 increments proved to be an effective method to acclimate the granules to increasing salinity. At higher salinity levels (35-40 g·L-1), the salinity began to affect the system performance, but the effects were reversible. The total dissolved sulfide concentration increased from 90 to 320 mg·L-1 of S2- 10 days after lowering the salinity from 40 to 15 g·L-1 of NaCl. Results from PHREEQC modeling showed that produced waters commonly encountered in Wyoming’s Minnelusa oil formation were susceptible to calcium sulfate scale, and that these systems could greatly benefit from this biological treatment process.