Desalination of produced water with electrodialysis: ion exchange membrane fouling, system performance, and fate of organic constituents

Abstract

Treatment of oil and gas (O&G) wastewater is a growing practice that increases water resources available for beneficial reuse. Electrodialysis (ED) is an established desalination process with the potential to remove ions from saline produced water (PW), a byproduct of O&G production. However, the current ED literature lacks studies that investigate the fate of organic contaminants during ED treatment using real PW. This work examines ion exchange membrane (IEM) fouling, system performance, and the transport of organic constituents through IEMs during ED treatment of real PW. A bench-scale ED apparatus was operated in batch mode at constant voltage to desalinate pre-treated PW using two types of IEMs. Pretreatment methods included combinations of biologically active filtration, ultrafiltration, and microfiltration with the intention of progressively increasing IEM exposure to organic matter. Impacts of membrane selection were investigated through measuring stack resistance, current utilization, and loss of water from the diluate due to electro-osmosis. Pristine and fouled IEMs were characterized using environmental scanning electron microscopy (ESEM) and ion exchange capacity measurements. Fouling layers were not observed using ESEM, though both IEM types experienced decreases in ion exchange capacity. Neosepta IEMs retained a larger proportion of organic matter in the diluate and demonstrated a lower rate of electro-osmosis compared to GE IEMs. Salinity removal was not affected by initial TOC loading in the feed. The mechanism for TOC transport using Neosepta IEMs may be convection, while diffusion is likely the driving force for TOC transport through the GE IEMs. A mass balance of TOC shows that some organic matter was unaccounted for—because excessive membrane fouling was not immediately apparent, this may be evidence that organic matter undergoes oxidation at the anode during ED treatment.