Reverse osmosis retention limit

Equations (22-86) and (22-89) are the turbulent- and laminar-flow flux equations for the pressure-independent portion of the ultrafiltra-tion operating curve. They assume complete retention of solute. Appropriate values of diffusivity and kinematic viscosity are rarely known, so an a priori solution of the equations isn t usually possible. Interpolation, extrapolation, even precuction of an operating cui ve may be done from limited data. For turbulent flow over an unfouled membrane of a solution containing no particulates, the exponent on Q is usually 0.8. Fouhng reduces the exponent and particulates can increase the exponent to a value as high as 2. These equations also apply to some cases of reverse osmosis and microfiltration. In the former, the constancy of may not be assumed, and in the latter, D is usually enhanced very significantly by the action of materials not in true solution. 

There is no distinct limit between reverse osmosis and ultra-fdtration but the latter employs lower pressures of no more than 10 bar (seldom above 6 bar) and more open membranes for separation of large molecules and ultra-fine, sub-micron solids. Henry tried to make a clear distinction between ultra-filtration and cross-flow filtration by defining the former as retention of only dissolved species from solutions (as opposed to retention of particulate material from suspensions) but this has not caught on in practice, mainly because of the fact that dissolved and undissolved (ultra-fine) soUds are often separated together. Thus, ultra-filtration is used for example for the concentration of proteins from low-cost dairy byproducts, or in the separation of emulsified oil and suspended solids from waste waters. In such processes a cake or a layer of gel would form on the membrane and reduce the filtration rates if it were not for the cross-flow characteristics of most designs, in which the suspension flows at high speed across the membrane surface and prevents cake build-up. 

---