a. Product staging. Product staging is true series
The design of reject staging, in order to balance the
utilization of the membrane modules for optimum
operation of two or more reverse osmosis membrane
economical life cycle cost, is a complex activity to be
systems, as shown in figure 7-8. Product staging is
performed by the membrane manufacturer or
used when a single pass through one reverse osmosis
Operations Engineering Manufacturer.
membrane does not bring a constituent concentration
c. Combined product and reject staging. In the
down to specification.
The second stage always
requires its own pressurizing pump, taking suction from
desalination of highly saline waters such as seawater,
the suck-back or storage tank of the first stage reverse
product and reject staging can be effectively combined.
osmosis system. When the water produced from the
The second stage of a product staged system can be
second stage is significantly lower in dissolved
designed as a reject staged subsystem. Any of three
constituents than required, the product water from the
factors may limit reverse osmosis water recovery:
first stage may be blended with the second stage
osmotic pressure; sparingly soluble salts; or turbidity.
product to produce the desired water quality. When
Water from a primary reverse osmosis treatment system
potable water and other waters with lower dissolved
will have three properties pertaining to these limitations:
solids content are required, a product staging system
-A lower osmotic pressure than the raw
can be used to supply the desired quality or qualities
between that of the first and second stage product. Life
-A disaproportionately reduced concentra-
cycle costing should be used to evaluate dual- and
tion of divalent ions.
triple-distribution systems where product staging is
These qualities of primary reverse osmosis product
b. Reject staging. Reject staging, figure 7-9, is
water can allow for greater water recovery from a
secondary product staged reverse osmosis subsystem
used when the low salinity of the raw water permits a
than is allowed by the manufacturer's maximum
high water-recovery ratio. Most membrane module
recommended recovery rate. When the water recovery
manufacturers have a minimum allowable brine reject
of the second stage of a product staged system can be
flow for any given membrane of their manufacture. The
increased by reject staging, the secondary stage shall
manufacturer's recommended maximum feed water flow
be reject staged. When the brine from the secondary
rate and minimum recommended brine reject flow can
stage of a product staged system is less concentrated
be used to calculate a maximum recommended single
than the primary stage feed water by more than 1,000
stage recovery fraction by use of the following equation:
milligrams of total dissolved solids per liter, the use of
dedicated desalination of this lower concentration water
shall be life cycle costed. This life cycle cost for
dedicated secondary stage brine desalination shall be
compared with the life cycle cost of blending the
secondary stage brine into the primary stage feed water.
F = Maximum recommended feed flow per
B = Minimum recommended brine reject flow
Reverse osmosis system design .
process selection does not yield a specific membrane or
R = Maximum recommended recovery rate
even a particular process, then designs must be
prepared for more than one process.
This maximum single stage water recovery is one
a. Identification of work. When a schedule and a
base site have been selected, this information will be
considered for low salinity reverse osmosis desalination.
made available to the design engineer.
When the reject stream is still diluted enough for further
identification of the location and the time schedule will
concentration after the maximum recommended
be considered in the design; this includes the date the
recovery ratio is reached, the brine can be piped directly
system must be online. The minimum number of
into another membrane module for further water
independently operable membrane banks and the
recovery. This is accomplished by combining the brine
minimum capacity of the banks must be determined.
flow from a number of first stage modules onto a fewer
Any restrictions that storage will place on maximum
number of secondary membrane modules.
allowable downtime will also be determined.
occasionally possible to further concentrate the brine on
a third reject stage as shown in figure 7-9.