Paper title:
Combined fouling of forward osmosis membranes: Synergistic foulant interaction and direct observation of fouling layer formation
Journal:
Journal of Membrane Science Volume 407-408, 15 July 2012, Pages 136-144
Author/s:
Department of Civil and Environmental Engineering, University of Maryland, 1161 Glenn L. Martin Hall, College Park, MD 20742, USA
Summary:
This paper studies the combined effects of organic (alginate) and inorganic (gypsum) foulants in FO. By using microscopic imaging tools, fouling layers were observed and effects of concentration polarization were understood.
Cellulose triacetate (CA) membranes were used for the bench-scale cross-flow FO membrane system. The system was incorporated with a glass window and a microscope-equipped window cell to allow direct observation of foulant deposition on the membrane surface as filtration experiments proceed. NaCl, Na-alginate, and gypsum were used as draw solution, model organic foulant and model inorganic foulant, respectively. The study consisted of baseline, fouling, and cleaning experiments. For direct observation fouling experiments, snapshots were taken every 10 minutes to capture initial deposition of foulants. Afterwards, images were taken on an hourly basis. For cleaning experiments, images were taken at 5-minute intervals to directly observe cleaning efficiency.
FO membrane properties were determined. AFM was used to obtain mean roughness while the Kruss Goniometer was used to obtain contact angle to determine hydrophilicity. Pure water permeability was also measured.
In order to determine any synergistic effects of combined foulants, separate fouling experiments were conducted first for alginate and for gypsum. It was found that organic fouling didn’t experience rapid flux decline (below critical flux) while for gypsum scaling, flux declined steadily at first then quickly dropped after around 1800 minutes. However, results revealed that when both alginate and gypsum are present, rapid flux decline occurs even at the beginning of the experiment, thus, indicating a synergistic effect of both foulants during the combined fouling experiment.
The mechanism that controls the synergistic interactions between alginate and gypsum may be any of these hypothetical mechanisms: (1) presence of gypsum affects alginate deposition and the structure of alginate fouling layer, (2) presence of alginate affects gypsum crystal deposition, (3) deposition of one type of foulant modifies the surface characteristics of the membrane and eventually affects other types of foulant, etc.. The first hypothetical mechanism was found to be false because calcium does not aggravate alginate fouling when the initial membrane flux is below the critical flux. Tests for the second mechanism indicated that the presence of alginate molecules, either in the bulk solution or on the surface of the membrane, significantly increases the size of gypsum crystals (based on SEM results). It was also shown that cake-enhanced concentration polarization affects gypsum scaling of alginate-conditioned membrane. This was exhibited by the growth of gypsum crystals underneath the alginate gel layer. But finally, it was determined that the dominating mechanism for the synergistic effect of combined fouling is adsorption of alginate molecules on membrane surface followed by heterogeneous gypsum crystallization around alginate nuclei on the membrane surface, which results in a combined network of alginate gel and gypsum crystal.
The effects of alginate on the kinetics of the growth of gypsum crystals were understood by using the aforementioned microscope-equipped window cell. This revealed that alginate can shorten nucleation time, increase the rate of gypsum growth, and change the morphology of gypsum crystals.
From the cleaning experiments, it was found that fouling of gypsum on clean membrane is almost fully reversible using physical cleaning with pure water. But when alginate is present, the flux of the membrane can no longer be fully recovered by pure water only. But this doesn’t imply yet that combined fouling is irreversible. Further investigation using chemical cleaning methods should be done.
Contribution and application:
I think that this kind of study may be replicated using molecular dynamics simulations. Same procedure, but without the effort of doing the actual experiment. This study, whether investigated through experiments or MD simulations, is useful not only for fouling investigation but generally for the development of membranes and our knowledge of membrane processes.
By: Hannah Ebro
hannah@gist.ac.kr