NAMS 2002 Workshop - ICOM 2008

Membrane Fouling IV - RO & Desalination – 6
Friday July 18, 12:15 PM-12:45 PM, Moloka’i
Effect of Foulant-Foulant Interaction on the Limiting Flux for RO and NF
Membranes during Organic Fouling - Model Development and AFM
Adhesion Force Measurement
C. Tang (Speaker), Nanyang Technological University, Thailand - cytang@ntu.edu.sg
Y. Kwon, Stanford University, Palo Alto, California, USA
J. Leckie, Stanford University, Palo Alto, California, USA
A limiting flux model has been recently developed to predict the fouling behavior
of reverse osmosis and nanofiltration membranes by organic macromolecules
(Tang and Leckie, 2007). Several interesting results have been observed: a)
there was a maximum pseudo stable flux (the limiting flux) beyond which further
increase in applied pressure did not translate to a greater stable flux; b) all
membrane samples attained the limiting flux under constant pressure conditions
as long as their initial flux was greater than the limiting flux; c) the limiting flux did
not depend on the properties of membranes; d) the limiting flux had strong
dependence on the feedwater composition, such as pH, ionic strength, and
divalent ion concentration. The current study investigates the dependence of
limiting flux on intermolecular interaction between foulant molecules. It was
observed that the limiting flux was directly proportional to the intermolecular
electrostatic repulsive force and that conditions enhancing foulant-depositedfoulant
repulsion resulted in greater limiting flux values. Such observations agree
well with a theoretical model capturing both hydrodynamic and DLVO
interactions. Adhesion force measurements by atomic force microscopy (AFM)
were also performed. The limiting flux correlated well with AFM adhesion force
between the model foulant and the fouled membrane surface. Finally, membrane
fouling was primarily controlled by the initial-flux-over-limiting-flux ratio - a greater
ratio inevitably resulted in more severe flux reduction, greater foulant
accumulation, and greater density of the foulant layer.
Reference:
C. Y. Tang and J. O. Leckie, "Membrane independent limiting flux for RO and NF membranes
fouled by humic acid," Environ. Sci. Technol., vol. 41, pp. 4767-4773, 2007.