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MEMBRANE PROCESSES 13.33

Blending Membrane Permeate and Bypass Water. In cases in which feedwater qual-
ity is such that membrane treatment provides a permeate with significantly better than
needed finished water quality, it may be possible to blend the permeate with source or
pretreated water. The advantages of blending are the

• Reduction of the required membrane capacity for a given finished water flow rate
• Reduced concentrate flow needing disposal for a given finished water flow rate
• Corresponding reduction in plant capital and O&M costs, including concentrate dis-
posal costs
• Reduction in the amount of permeate posttreatment required for corrosion control
The opportunity for blending depends on the relative concentrations of critical con-
stituents in both the membrane bypass and permeate relative to treated water goals and
the composition of the membrane feed (bypass water).
The potential for blending can be determined using mass balance techniques for each
product water quality constituent to determine the allowable blend ratios. Generally, the
designer can easily determine the most critical constituents that need to be evaluated for
a given application. The constituent requiring the permeate flow to be the highest per-
centage of the finished water flow is the limiting constituent and controls the overall al-
lowable blending rate. The potential for blending can be calculated as follows:
Cs ..... - Cgoal
Qp/Qfin = C ...... - Cp

where Qp = membrane permeate flow rate
Qfin = finished water flow rate
Cs .... e = concentration of solute in source (feedwater)
Cgo~ = concentration of finished water (after blending) solute quality goal
Cp = concentration of solute in permeate
The resulting bypass flow rate is
Qbyp = Qfin - Qp
where Qbyp is the membrane bypass flow rate that is blended with the membrane perme-
ate to form the finished water (before subsequent posttreatment processes).
The required source water flow rate can be calculated as follows:
Q ...... =Qfin×(BR +y 1-BR)

where Qsource = source water flow rate
BR = blend ratio = QJQrln
Y = membrane system recovery = Qp/Qf
Qfin = finished water flow rate

Using Membrane Computer Performance Projections for System Design. An RO or
NF system is seldom designed by hand calculations. Most major membrane and system
manufacturers, as well as some consultants, have developed computerized modeling pro-
grams that generate theoretical feed pressures and permeate quality at both initial and fu-
ture operating conditions. However, designers must use caution because many projections
may be accurate for initial operating conditions but can be inaccurate in predicting future
performance, particularly if the effects of site-specific feedwater membrane foulants on