(186a) Influence of a Biofouling Layer On Membrane Distillation

Membrane distillation (MD) is a relatively new technology that enables the production of potable water without the need to use the high pressures required for reverse osmosis. MD is a process whereby liquid water on the feed side is vaporized by heat supplied from the hot feed stream; subsequently, the water vapor diffuses through the pores in a hydrophobic membrane and is condensed on the cold permeate side. MD is attractive for several applications. In particular, it provides a means for producing potable water from saltwater while avoiding the high pressures associated with reverse osmosis. In addition, it provides a use for low quality waste heat available from many industrial operations. As such, it is a very attractive Green Technology.

Unfortunately, the optimal performance of MD is compromised owing to concentration polarization and fouling [1]. Models for the MD process developed to date have not incorporated all the effects of a biofouling layer [2]. Moreover, it is not clear how to use existing models for the MD process in order to infer the presence of a biofouling layer. The focus of this presentation is to present a new model for the MD process that incorporates the principal effects of a biofouling layer. The manner in which this model can be used to infer the presence of a biofouling layer will be outlined.

There are two principal effects of a biofouling layer on the MD process: (1) a resistance to heat conduction on the feed side of the membrane that can cause a reduction in the evaporation rate; and (2) a direct reduction in the vapor pressure of the evaporating liquid owing to the Kelvin effect introduced by the small pores in the biofouling layer. The latter effect was recently documented by Goh et al. [2] who used evapoporometry [3] to show that a biofouling layer can have pores ranging from 3.5 to 47.5 nm that can reduce the vapor pressure by as much as 36% and the MD water flux by as much as 50%.

The MD model advanced by Schofield et al. [4] is adapted to include both possible effects of a biofouling layer. This model permits constructing a plot based on the measured values of the MD water flux, feed temperature, and permeate temperature in order to determine if a biofouling layer is influencing the MD process. Moreover, it permits assessing if this biofouling layer offers a significant heat-transfer resistance. It also provides a direct means to determine the influence of the Kelvin effect on lowering the vapor pressure driving force for MD. A useful metric for assessing the effect of a biofouling layer is the temperature polarization coefficient, which is the ratio of the temperature difference across the membrane divided by the difference between the feed and permeate temperatures. A generalized design plot is developed in terms of the temperature polarization coefficient as a function of a dimensionless parameter that provides a measure of the Kelvin effect reduction in the vapor pressure.

[1]   A.G Fane, S. Chang, Techniques to enhance performance of membrane processes, in A.K. Pabby, S.S.H. Rizvi, A.M. Sastre (Eds), Handbook of Membrane Separations: Chemical, Pharmaceutical, Food, and Biotechnological Applications, CRC Press, 2008, 193-232.

[2]   S. Goh, Q. Zhang, J. Zhang, D. McDougald, W.B. Krantz , Y. Liu and A.G. Fane, Impact of a biofouling layer on the vapor pressure driving force and performance of a membrane distillation process, J. Membr. Sci. (2013) http://dx.doi.org/10.1016/j.memsci.2013.03.023.

[3]   W.B. Krantz, A.R. Greenberg, E. Kujundzic, A. Yeo, S.S. Hosseini, Evapoporometry:  A novel technique for determining the pore-size distribution of membranes, J. Membr. Sci. (2013) http://dx.doi.org/10.1016/j.memsci.2013.03.045i.

[4]   R.W. Schofield, A.G. Fane, C.J.D. Fell, Heat and mass-transfer in membrane distillation. J. Membr. Sci. 33(3) (1987) 299-313.