(725c) Impact of Organic Compounds in Produced Water on Desalination By Membrane Distillation

In recent years, hydraulic fracturing has been widely employed by U.S. oil and gas industry for the extraction of oil and gas from unconventional reservoirs. This extraction process is intrinsically associated with production of water (i.e., produced water) that may contain chemicals that were added to the fracturing fluid as well as salts and hydrocarbons from the formation. Global production of this wastewater is increasing and it is currently estimated at 250 million barrels per day. Increasing cost for disposal of produced water in the injection wells emphasises the need for treatment and reused to control the disposal cost and protect the environment. Produced water is characterized by extremely high salinity that can range from 30-300 g/L. It also contains bacteria, sand or mud, oil and grease, and Naturally Occurring Radioactive Material (NORM). As such, it is not amenable to treatment using conventional wastewater treatment processes.

Membrane Distillation (MD) is an emerging potential solution for desalination of produced water. Unlike pressure-driven membrane desalination processes, the main driving force for membrane distillation is the vapor pressure difference across the membrane, which is not significantly affected by salinity. Therefore, MD is uniquely suitable for treatment of high salinity produced water. The presence of high concentration of organic compounds in produced water may be challenging for MD technology because they can cause membrane fouling and wetting and reduce the selectivity of this process. In particular, surfactants can promote membrane wetting by adsorbing onto the membrane surface to alter its hydrophobicity. If that happens, it would be necessary to remove surfactants in a pre-treatment step to achieve high water recovery without membrane fouling to maintain high permeate flux.

In this study, raw produced water from Permian Basin, Texas was obtained and tested in a laboratory-scale membrane distillation system. Increase in permeate salinity observed when treating this produced water was attributed to membrane wetting was observed in this system. It was found that the total organic carbon (TOC) of the produced water can be reduced from 120 mg/L to 20 mg/L by biological treatment and further reduced to 2 mg/L by activated carbon adsorption. However, this reduction in TOC did not eliminate apparent membrane wetting. These preliminary results suggest that membrane wetting by this produced water is primarily due to low surface tension caused by non-adsorbable polar compounds or due to volatilization of organic or inorganic salts present in the produced water.

Several organic compounds and surfactants, including ‘Nonyl phenol ethoxylate’, were detected in both raw water and the permeate. Polymeric form of this surfactant ‘Nonylphenol polyethylene glycol’ was tested in synthetic produced water and membrane wetting was observed. However, membrane wetting due to ‘Nonylphenol polyethylene glycol’ was irreversible while the wetting caused by the actual produced water can be reversed by simple washing with DI water. Several synthetic solutions were tested for their ability to pass through the membrane and it was found that volatilization of organic and inorganic salts is the primary the observed increase in permeate conductivity. Therefore, the usual practice of using the permeate conductivity to monitor membrane wetting may not be appropriate when treating complex wastewaters.