Water management is becoming increasingly challenging and several technologies, including membrane distillation (MD) are emerging. This technology is less affected by salinity compared to reverse osmosis and is able to treat brines up to saturation. The focus of MD research recently shifted from seawater desalination to industrial applications out of the scope of reverse osmosis. In many of these applications, surfactants or oil traces are present in the feed stream, lowering the surface tension and increasing the risk for membrane wetting. In this study, the technological boundaries of MD in the presence of surfactants are investigated using surface tension, contact angle and liquid entry pressure measurements together with lab-scale MD experiments to predict the wetting resistance of different membranes. Synthetic NaCl solutions mixed with sodium dodecyl sulfate (SDS) were used as feed solution. The limiting surfactant concentration was found to be dependent on the surface chemistry of the membrane, and increased with increasing hydrophobicity and oleophobicity. Additionally, a hexadecane/SDS emulsion was prepared with a composition simulating produced water, a waste stream in the oil and gas sector. When hexadecane is present in the emulsion, oleophobic membranes are able to resist wetting, whereas polytetrafluoretheen (PTFE) is gradually wetted by the feed liquid.

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For chemical industries, fresh water availability is a pre-requisite for sustainable operation. However, in many delta areas around the world, fresh water is scarce. Therefore, the E4 Water project (www.e4water.eu) comprises a case study at the Dow site in Terneuzen, The Netherlands, which is designed to develop commercial applications for mild desalination of brackish raw water streams from various origins to enable reuse in industry or agriculture. This study describes an effective two-stage work process, which was used to narrow down a broad spectrum of desalination technologies to a selection of the most promising techniques for a demonstration pilot at 2-4 m³/hour. Through literature study, laboratory experiments and multi-criteria analysis, nanofiltration and electrodialysis reversal were selected, both having the potential to attain the objectives of E4Water at full scale.

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