Membrane Distillation for Treating Hydraulic Fracturing Flow Back Waters

Hydraulic fracturing flow back waters often exhibit very high TDS (in excess of 100,000 ppm).  Treatment of these highly impaired wastewaters is a major challenge due to the presence of high TDS and organic contaminants.  Pressure driven membrane processes such as reverse osmosis are impractical for treating very high salinity wastewaters due to the high osmotic back pressure that must be overcome.  One of the technologies that is being investigated for treating these produced waters is membrane distillation. 

Here we have screened a number of commercially available microporous hydrophobic membranes.  We have characterized membrane surface as well as bulk properties.   Using bulk membrane properties, we calculate a structural parameter that indicates membranes that should display the highest permeate flux.  Permeate fluxes were determined for all membranes using a model feed stream containing a 20,000 ppm (0.34 M) NaCl.  We have compared the observed permeate fluxes with values obtained for the structural parameter.  Next membranes that displayed the highest permeate fluxes were challenged with feed streams containing 100,000 ppm (1.7 M) NaCl.  The feeds stream was concentrated until the permeate flux rose quickly while the conductivity of the permeate increased rapidly above 50 mS cm^-1 indicating the passage of the feed through the membrane pores.  Finally, these membranes were tested with real produced waters.  Our results indicate that a staged process using membranes with different bulk and surface properties is necessary to concentrate the feed to over 350,000 ppm.  Further our results indicate the importance of membrane surface properties in order to prevent breakthrough of the feed.