Volatile and Semi-Volatile Contaminant Transport in Direct Contact Membrane Distillation

Membrane distillation (MD) is a thermally driven separation process based on mass transport of water across a microporous, hydrophobic membrane.  Transport is caused by a vapor pressure differential across the membrane that is generated by a thermal gradient.  Recent advances in new membrane technologies and waste heat capture have made MD systems the subject of renewed interest as an alternative membrane technology for water treatment.  Theory indicates that MD has high rejection rates for non-volatile compounds, such as ions and metals, and high permeability of volatile and semi-volatile compounds.  The general assumption has been that this broad classification of volatile versus non-volatile is sufficient for determining the applicability of MD to a given source water, but little research has been done to examine the mechanics and physical phenomena of volatile contaminant transport in MD systems. Recent experimental data confirms that the earlier assumption was too broad, and that rejection of volatile and semi-volatile compounds vary significantly.    

A novel, gas-tight direct contact membrane distillation system (DCMD) was designed and constructed to evaluate factors influencing transport of volatile and semi-volatile compounds in MD.  The system retains volatile components, allowing for greater contaminant recovery and closure of the mass balance.  Various contaminant classes, including metals, ionic salts, nitrogenous disinfection byproducts, volatile organic compounds (including phthalates, phenols, polyaromatic hydrocarbons, anilines, chlorinated aromatics, and amines), and pharmaceutical and personal care products were tested for transport in the system. These contaminant mixtures include diverse compounds with a very wide range of chemical properties, and spanned 10 orders of magnitude in air-water equilibrium partitioning (K_H).  Rejection of the non-volatiles was generally above 99%, as expected, while the rejection of volatile compounds varied significantly. Membrane passage was compared to the physicochemical characteristics of each compound. Volatility, as assessed by K_H, weakly correlated with rejection, although there exist a number of compounds whose observed transport was inconsistent with their Henry’s law constant.  It was also found that many hydrophobic compounds (e.g., PAHs, large chlorinated aromatics, and phthalates) exhibited extensive sorption and hydrophobic partitioning to system components such as tubing, containers, and the membrane itself, suggesting that DCMD is not an appropriate treatment for these types of compounds. The results of this study are a major step towards our mechanistic understanding of the transport of compounds in DCMD based on their physicochemical properties.