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13.28 CHAPTER THIRTEEN

gree (90% or greater); monovalent solute rejection is much lower (75% or less) and is
strongly dependent on the types of co-ions present in the feed and the feed pH.
Types of Membranes. Membrane composition may affect solute rejection over time.
Composite membranes are generally stable and maintain their rejection properties over
long periods of operation. Cellulosic membranes continuously undergo hydrolysis, which
gradually diminishes their rejection properties. The extent of rejection loss depends on
the rate of hydrolysis, but often the salt passage is assumed to double within 3 years of
operation.
Increased temperature also increases the rate of cellulose acetate membrane hydroly-
sis. Cellulose acetate membrane module manufacturers typically recommend that feed pH
be controlled between 5.0 and 6.2. The useful life of a membrane is also highly depen-
dent on external (nonmembrane) factors associated with the application such as pretreat-
ment operation, type of cleaning chemicals, and frequency of cleaning.
Operating pH. Feedwater pH may affect the rejection properties of composite mem-
branes and the degree of rejection for certain ionic constituents. Composite membranes
generally have an optimum pH at which rejection is maximized, although it is not always
a published parameter. Rejection of certain ions such as fluoride, bicarbonate, and boron
can vary with pH. As pH increases above 5.5, fluoride and bicarbonate ion rejection in-
creases. Boron rejection increases significantly as pH is increased from neutral to high.
Flux. For a system containing more than one stage, the flux value can be expressed as
a system average or as individual values for each stage. The individual module flux rate
within a system generally decreases from beginning to end. Lead modules operate at a
higher flux rate than trailing modules because feed TDS (and osmotic pressure) is lowest
and feed pressure highest at the lead end of the system.
System capacity is determined by the amount of membrane area provided (number of
elements in the system) and the flux rate. The number of membrane elements required
can be calculated, given an average membrane flux and active membrane area per ele-
ment. Most large-capacity NF and RO systems use 8-in.-diameter × 40-in.-long elements
containing 350 to 440 ft 2 (33 to 41 m 2) of membrane area per element and are rated at
8,000 to 15,000 gpd (30 to 57 m3/day) each.
As flux rate increases, the loading rate of potential foulants at the membrane surface
increases. Theoretically, there is an optimum flux rate for every water supply and mem-
brane combination. In general, groundwaters allow higher design flux rates, and surface
water applications are restricted to lower design flux rates.
The water production capacity of a membrane module specified by the manufac-
turer, typically expressed in gallons per day per square foot (gpd/ft2), is based on a
membrane flux achieved during laboratory testing at standard test conditions. Typically,
manufacturer product specifications base performance on fluxes from 20 to 30 gpd/ft 2
(0.034 to 0.051 m/h). Rarely, if ever, can this rate be achieved during system operation
because of fouling constraints and the impact of recovery. Typical average system flux
rates for groundwater are 13 to 18 gpd/ft 2 (0.022 to 0.031 m/h) and for surface waters be-
tween 8 and 12 gpd/ft 2 (0.014 and 0.020 m/h).
Low-pressure RO and NF systems are sometimes designed with flux balancing to bet-
ter balance the flux rates of the various stages. This is an attempt to minimize fouling
rates and extend the intervals between membrane cleanings, or to improve the overall hy-
draulics of the system. Where flux balancing is desired, either permeate backpressuring
the first stage of the system or boosting the pressure of the interstage feeds can be used.
These techniques lower transmembrane pressure of the first stage to limit its production,
shifting a greater proportion of the production to the later stages.
New "low-fouling" RO membranes are now available that have favorable properties:
low membrane surface roughness, little or no surface charge, and less hydrophobicity.