(379e) Predicting the Conductivity Increase By Volatile Salts Penetration in Direct Contact Membrane Distillation (DCMD) Permeate Water

Global freshwater crisis in recent years has forced worldwide demands for alternative water resources. As the major proportion of water body on the earth is salty water, desalination techniques have drawn significant attention. Accordingly, conventional pressure-driven desalination methods have been developed and optimized to provide solutions to salty water reuse. However, pressure-driven desalination methods have to get over the osmotic pressure contributed by water salinity, which limited its capability when treating high salinity water.

Direct Contact Membrane Distillation (DCMD) is an emerging thermal desalination process with high salt rejection. The only driving force of DCMD is the vapor pressure difference between feed and permeate caused by temperature difference, which grants the capability of treating high salinity water such as seawater and shale gas produced water. The low capital and operational cost of DCMD when integrated with waste heat and solar energy also leads to a promising commercialization prospect. In DCMD system, the hydrophobic membrane inhibits the permeation of liquid feed due to the high surface tension between hydrophobic membrane and aqueous phase, while allows the transport of water vapor through the membrane. This performance of DCMD will be limited by the volatile salts in the feed when volatile molecules pass the membrane in gas phase and contaminate the permeate water. The dissociation of volatile salts will then increase the permeate conductivity, deteriorating this significant evaluation index for permeate quality.

In this study, a conductivity model describing the transport of volatile salt for DCMD systems was developed according to data from bench-scale DCMD experiments for synthetic shale gas produced water with volatile contaminants (i.e. ammonium chloride, ammonium acetate, sodium acetate and phenol). It is shown that the existence of volatile salts in feed will cause the permeate conductivity increase immediately after the desalination process starts and the increase rate is determined by multiple factors of the contaminants (i.e. partial pressure, molecular weight, dissociation constant, etc.) as well as configuration parameters of DCMD system (i.e. temperature, flow rate, membrane pore size, etc.). Lab-scale DCMD experiments were also employed to verify the prediction, it is indicated that the volatile salts can penetrate the membrane and increase permeate conductivity without fouling the membrane, the permeate quality is compromised due to the invasion of these impurities and may fail the discharge standards. This study shows that DCMD treatment with certain polluted water may require pre-treatment to eliminate the impact of volatile salts and fulfill the discharge requirements.