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13.32 CHAPTER THIRTEEN

stage permeate flow will be 50 gpm (3.2 L/s) and the concentrate flow will be 25 gpm
(1.6 L/s) from each vessel, or a total of 50 gpm (3.2 L/s). This concentrate flow would
then be fed into a single 6M vessel in the second stage, where 50% recovery for this stage
would yield a concentrate flow and a permeate flow of 25 gpm (1.6 L/s) each. The total
system produces 75 gpm (4.7 L/s) of permeate and 25 gpm (1.6 L/s) of concentrate.
Commonly, two-stage and three-stage design using 6M vessels can yield recoveries of
75% and 85%, respectively. Designs using 7M vessels can achieve a recovery of 65% in
one stage and 85% in two stages. Typically, design involves an iterative process consid-
ering various combinations of pressure vessel staging. There are usually several concen-
trate-staged arrays that meet the treatment objectives and hydraulic criteria, especially if
flux balancing (discussed under Flux) is employed. In many cases, the array selected is
not a "perfect" mathematical taper (i.e., each stage does not have exactly 2 times as many
pressure vessels and membrane elements as the following stage).
For hollow-fiber RO membrane modules (permeators), each pressure vessel contains
a "bundle" of membrane fibers. Modules in each stage are placed in parallel and, for some
systems, with designed pressure loss in each module's concentrate outlet piping or tub-
ing (before concentrate header piping). This pressure drop helps balance flows between
each module. For example, a pressure drop of 35 psi (240 kPa) or more has been used
for many single-stage systems and the final stage of multistage systems. Commonly, max-
imum recoveries up to 50%, 75%, and 90% are used for one-, two-, and three-stage hol-
low-fiber membrane systems, respectively.
Permeate-Staged Design. In applications in which the TDS of the feedwater is too
high to produce a permeate of sufficient quality with a single pass through the membrane,
system design incorporates permeate staging (Figure 13.21). This design is common to RO
systems treating seawater with very high salinities (45,000 mg/L or greater); when treat-
ing typical seawater salinity (35,000 mg/L), where permeate quality requirements are more
stringent than typical drinking water standards; or when an extra safety factor for product
water quality is desired. In this type of design, permeate from the first RO train becomes
feedwater to the second RO train. The first-pass system uses high-pressure seawater RO
membranes, and the second-pass system uses low-pressure brackish water membranes.
Normally, not all first-stage permeate requires treatment in a second pass, and some
can bypass and be blended in finished water. This offers the advantage of reducing the
size of the second pass. Concentrate from the second-pass system is generally recycled
back to serve as feedwater to the first-pass system.

"~---I~---~IP Concentrate
Soorce ; .... -" ....
wa, er " "p

Permeate

I
FIGURE 13.21 Permeate staging (multiple-pass design).