Direct Contact Membrane Distillation for Solar Water Needs

For solar energy facilities (SEF) in remote desert locations, typical water sources available include groundwater, brackish groundwater, and reclaimed on-site water. However, conventional water treatment methods are not always suitable for treating such water sources at the small scale needed for SEFs. Direct contact membrane distillation (DCMD) is a potential water treatment method that can be used to meet the water needs of SEF, which include cooling and cleaning solar surfaces, such as panels and mirrors. DCMD utilizes a microporous, hydrophobic membrane that allows the passage of water vapor while rejecting non-volatile contaminants present in groundwater and brackish groundwater, such as salts and minerals. The driving force in DCMD is the difference in partial vapor pressure due to a temperature variance; the membrane separates warm, impaired feed water from cool, distilled product water, and water vapor passes through the membrane and condenses to create treated water. DCMD pairs well with solar energy and can have low energy demands, especially when the feed water contains latent heat or is pre-heated in a solar collector. DCMD is well suited for remote locations due to its small footprint, relative simplicity, and modular configurability. This study evaluated DCMD for SEF water needs; specifically treating on-site groundwater and solar panel wash water. Surfactants are typically present in solar wash waters, with previous studies showing that their presence can negatively impact the DCMD membrane, resulting in an inferior water quality. In this work, multiple commercially available membranes were used to treat water containing different surfactants. Surfactant effects on DCMD, in terms of membrane hydrophobicity, flux, and treated water salinity, were then analyzed. Quantification of a surfactant concentration threshold that allows DCMD to remain viable, was investigated. The surfactant concentration at which DCMD membranes began to lose their hydrophobicity was determined using a novel method developed for this work that utilizes real-time measurements of water flux and distillate conductivity.This thesis demonstrates the feasibility of reusing on-site wash water to meet the water needs of SEF, and defines the surfactant loading levels that various commercial membranes can tolerate. These data, as well as additional results on the performance of commercial membranes for treating groundwater, may assist the SEF industry in developing protocols for the use of non-potable on-site and reuse waters. This information will, accordingly, expand the potential applications of DCMD for other waters containing surfactants. Results of this work can also assist DCMD in becoming an additional solution to the water-energy nexus, especially in areas with scarce freshwater sources and abundant solar irradiation