FLUX DECLINE DUE TO GYPSUM PRECIPITATION ON RO MEMBRANES

This paper extends our initial study [3] of flux decline caused by precipitation fouling of RO membranes. A systematic series of experiments covering the Reynolds number range of 4,700-11,600 was carried out with a saturated solution of CaSO4 fed to a parallel plate osmotic cell. Flux decline data were found to be well correlated on the basis of a simulation model describing surface blockage of the membrane by radial spread of crystals from nucleation sites. The model is based on a single adjustable parameter k(m)(N)1/2 embodying a crystal growth rate coefficient (k(m)) and a nucleation sites density magnitude (N). The data obtained indicated that gypsum crystal growth followed first-order kinetics. Salinity measurements showed a clear trend for permeate salt concentration to increase with the decrease in permeate flux. The salinity data were successfully correlated by the simulation model. A few experiments were also carried out measuring flux decline due to CaCO3 precipitation on the membrane surface at the Reynolds number range of 4,900-9,100. Permeate flow decline data were again successfully correlated by the simulation model using a well-known kinetic expression for CaCO3 precipitation.