Identifying Pore Wetting Thresholds of Surfactants in Direct Contact Membrane Distillation

Solar energy facilities (SEF) utilize water for cooling and cleaning of panels and mirrors to improve solar efficiency, however water resources and treatment infrastructure are typically lacking in areas where SEF are located. For panel cleaning applications, high-quality water is necessary as particles or dissolved salts can leave residuals on the panels, counteracting the benefits of cleaning. Thus, an on-site treatment method that can simultaneously provide a high-quality water and while removing heat from the panel surfaces is highly desirable.

Direct contact membrane distillation (DCMD) is a potential water treatment option for the SEF. DCMD uses a hydrophobic, microporous membrane to separate a warm, dirty feed water from a cool, clean distillate water; the temperature difference across the membrane leads to a vapor pressure difference that drives transport of water vapor through the membrane while rejecting molecules less volatile than the water, such as salts and metals. In the case of SEF, the ‘dirty’ water can be used for panel cooling, which serves as a pre-heating mechanism for DCMD, and the high-quality distillate water from DCMD will then be used for panel washing. However, as part of the panel wash, surfactants are often used to improve cleaning efficiencies, and due to limited water resources at many SEF, the panel wash water is captured for on-site reuse. The resulting low-levels of surfactants may be a major challenge for DCMD treatment, as surfactants may accumulate at the membrane surface and decrease hydrophobicity, which ultimately can result in pore wetting and decreased membrane performance.

In this work, we evaluate the effects of both anionic and non-surfactants on range of commercially available membranes from made from various materials and with varied pore sizes. The surfactant concentration at which the onset of pore wetting occurred was determined using a novel graphical method developed for this work, and verified using a spectrophotometric method. Membrane hydrophobicity before and after surfactant-induced pore wetting was determined using an optical tensiometer. Overall, the membrane material and surfactant type had the biggest impact on DCMD performance. PTFE membranes were generally able to tolerate a higher concentration of the surfactants, and all membranes were more sensitive to anionic surfactants than to non-ionic surfactants.