Assessment of performance and efficiency of membrane distillation for treatment of impaired water and brine with high scaling potential

Abstract

Water recovery is limited in pressure-driven membrane processes such as reverse osmosis (RO) and nanofiltration (NF) due to increase in scaling risk and osmotic pressure of the feed water with concentration. Membrane distillation (MD) is an emerging thermally-driven membrane desalination processes that utilizes a difference in vapor pressure across a microporous, hydrophobic membrane as the driving force. Thus, it is not limited by differences in the osmotic pressure between the feed and permeate and is tolerant of much higher salinity than RO. Nevertheless, MD still suffers from problems associated with membrane fouling, which is one of the major challenges that hinder its commercialization. The overall objective of this dissertation is to elucidate scaling and fouling behavior in MD by various inorganic contaminants relevant to inland brackish desalination, which typically must achieve high water recovery to minimize brine disposal costs. Water flux, thermal efficiency, and rejection were experimentally measured in laboratory experiments using real and synthetic solutions supersaturated with respect to soluble salts, sparingly soluble salts, and silica. Various mitigation and cleaning strategies were tested, and the long-term effects of scaling on MD performance were evaluated by performing repeated experiments on previously-fouled membranes using new solutions. Impacts and control of silica scaling was emphasized because it is ubiquitous in natural water supplies and is of particular concern in brackish desalination. Cleaning of MD membranes scaled by silica was impractical, but several mitigation strategies were effective at preventing silica scale, including modification of feed pH and optimization of feed temperature. Silica scaling propensity in MD was increased by the presence of calcium and magnesium, but the effects were reduced with increased carbonate alkalinity. Desalination of hypersaline brines with high mineral scaling potential were also investigated using water obtained from the Great Salt Lake (GSL). NaCl scaling occurred rapidly at its saturation limit, resulting in immediate loss of performance, and gradual decline in performance was also observed due to both mineral scaling and organic fouling. However, sustainable operation was achieved by operation at low feed temperatures combined with periodically reversing the direction of water flux.