A more effective method for fouling characterization in a full-scale reverse osmosis process

Membrane fouling is customarily indicated and measured by permeate decline at constant operating conditions. Although this method has been widely used for many years, it is inadequate for characterizing fouling development in modern full-scale RO processes. It has been demonstrated that in some full-scale RO processes with highly permeable membranes, the initial development of membrane fouling may not apparently affect the permeate flux. When the permeate flux is noticeably affected, the membrane is so severely fouled that restoration to its original permeability may become impossible. In this study, the permeate flux of a full-scale RO process and its controlling mechanisms are studied with numerical simulations. Particular attention is given to the relationship between permeate flux and the change of membrane resistance. Results show that the filtration coefficient (a collective parameter related to distribution of the membrane resistance) can be a better indicator or measurement of membrane fouling in a full-scale RO process. With this new fouling indicator, the development of membrane fouling in a full-scale RO process under any conditions can be accurately characterized. The advantages of the new characterization method include early and accurate indication of fouling development and quantitative assessment on the effectiveness of membrane cleaning.

It has been demonstrated that thermodynamic equilibrium can become the controlling mechanism for the permeate flux in the full-scale RO processes under common operating conditions. The flux is insensitive to the change of membrane resistance when the process is operated in or near thermodynamic equilibrium regime. Therefore, membrane fouling in such a membrane system cannot be adequately characterized by the decline in permeate flux. The filtration coefficient and fouling index proposed in this study can be excellent indicators or measurements of membrane fouling in fullscale RO processes because they are intrinsically related to the membrane resistance. The new method is equivalent to the conventional method of flux decline in mass transfer limited regime. Fouling in the thermodynamic equilibrium restricted regime can also be readily characterized with the new method by temporarily reducing the driving pressure to the mass transfer limited regime. The filtration coefficient and fouling index can also be used to assess the effectiveness of membrane cleaning.