Membrane Treatment - Public Water Systems - page 481

TABLE 15-2. Summary of Membrane Operations in Water Treatment
Membrane
Operation

Mechanism of
Separation

Membrane
Structure Driving Force

Microfiltration Pressure Sieve Macropores

Ultrafiltration Pressure Sieve Mesopores

Nanofiltration Pressure Sieve (solution /

diffusion,
exclusion)

Micropores

Reverse osmosis Pressure Solution / diffusion

exclusion

Dense, or thin
film
composite

Electrodialysis Electrical potential

and pressure

Ion exchange,
dialysis

Ion exchange

Source: Adapted from reference 2. (Reprinted from Water Treatment Membrane Processes, by permission.
Copyright

1996, American Water Works Association, AWWA Research Foundation, Lyonnaise des Eaux, and
Water Research Foundation of South Africa.)

where:

Fs is the solute flux, kg / sq m per day or lb / sq ft / day
Ks is the rate coefficient, m / d or ft / d
Cƒ is the feedwater solute concentration, kg / cu m or lb / cu ft
p is the product solute concentration, kg / cu m or lb / cu ft

The recovery rate is equal to the percentage of feedwater that is produced as per-
meate; the solute rejection rate is the percentage of solute in the feedwater that is
retained on the membrane. Theoretically, for a given feedwater and membrane, recov-
ery will depend on pressure, temperature (higher temperatures increase the rate coef-
ficient), and flow rate, and salt rejection will be a constant, depending only on
differential concentration. In practice, recovery and rejection rates are affected by op-
erating conditions of the membrane and by specific characteristics of the feedwater in
addition to solute concentration.

Ion Exchange / Dialysis and Electrodialysis Reversal In the electrodialysis pro-
cess, the membrane consists of a stack of ion-exchange resin material molded into
semipermeable layers. When an electrical potential is applied across the membrane,
charged ions are induced to travel through the membrane to the opposite attractive
charge. Membranes are stacked with alternating cationic and anionic charged mem-
branes. Water to be treated passes through the membrane channels; only the ions and
a small stream of water pass through the membrane—the treated water exits through
the manifolds. To extend time between cleaning and to improve operating efficiency,
the polarity of the system is reversed periodically.

Membrane Characteristics
Cellulose acetate (CA) was one of the first membrane materials and is still widely
used. The characteristics of a membrane can be altered by changing the polymer
content; for example, increasing the acetate content of a CA membrane generally