ABSTRACT
The membrane processes commonly used for waste minimization are reverse osmosis, ultrafiltration, and microfiltration. Permeation and electrodialysis are also finding increasing use. With the exception of permeation, which is still considered a developing technology, all the above are commercial processes. These processes make use of a thin membrane barrier which can be produced with properties that vary over a wide spectrum. The membrane is essentially solid for reverse osmosis, permeation, and electrodialysis; it is porous with continuous discrete holes for microfiltration. Ultrafiltration membranes can be of either type depending on the application. The processes are somewhat arbitrarily defined by the apparent pore size of the membrane barrier used. This apparent pore size is reported for measured experimental diffusion rates even when it is known that no continuous pores or holes through the membrane exist. This can be done because the form of the diffusion equations is the same for solid and porous membranes. The pore size reported for the solid barrier is that which will give the same flow rate for a given pressure drop through a membrane which does have continuous pores.
