When is membrane wetting in membrane distillation accurately diagnosed?

Hydraulic fracturing leads to production of water (i.e., produced water) which contains chemicals that were added to the fracturing fluid as well as high concentration of salts and hydrocarbons from the formation. Membrane Distillation (MD) is an emerging potential solution for desalination of produced water. The main driving force for MD is the vapor pressure difference across the membrane, which is not significantly affected by salinity. The presence of high concentration of organic compounds and surfactants in produced water may be challenging for MD technology because they can cause membrane fouling and wetting. Traditionally, membrane wetting is characterized by an increase in permeate conductivity. However, volatile and semi-volatile compounds can pass through the membrane in the vapor phase and lead to permeate conductivity increase due to dissociation into charged species. Thus, the increase in permeate conductivity can be incorrectly diagnosed as membrane wetting in MD.

In this study, laboratory-scale MD system testing of raw produced water showed increase in permeate conductivity which was attributed to membrane wetting. Attempts to reduce the total organic carbon (TOC) of this water from 120 mg/L to 20 mg/L through biological treatment and to 2 mg/L by activated carbon adsorption did not alleviate the problem of permeate conductivity. These preliminary results suggested that membrane wetting was 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 (NPEG) ' was tested in synthetic produced water and membrane wetting was observed. However, membrane wetting due to NPEG was irreversible while the wetting caused by the actual produced water can be reversed by simple washing with DI water. Several other synthetic solutions were tested and it was found that volatilization of organic and inorganic salts is the primary cause for the observed increase in permeate conductivity. Therefore, the usual practice of using the permeate conductivity to monitor membrane wetting may not be always appropriate.