Page 377 - Water and wastewater engineering

9-16 WATER AND WASTEWATER ENGINEERING

The design process is iterative in that a number of combinations are investigated to examine
the most economical arrangement that yields the desired water quality objectives. In RO/NF
softening systems, blending with bypassed water is common and should be considered in select-
ing the array arrangement.

Applying Manufacturer’s Standard Conditions. The manufacturer’s standard conditions for
estimating the permeate flow rate ( Q P ) can be corrected to local conditions for design estimates
using the following equation (AWWA, 1999):

Q (PCF )(TCF )(MFRC/FF )( Q ) (9-14)
i
p

where Q p product water flow at operating conditions
PCF pressure correction factor
TCF temperature correction factor
MFRC membrane flux retention coefficient
FF fouling factor
Q i initial product water flow at standard conditions

PCF is further defined as
PCF P 0 5.( h ) P (9-15)
F L P FC P
where P F feed pressure, Pa
h L headloss through feed-concentrate channel, Pa

P P permeate pressure, Pa
FC average feed concentrate osmotic pressure, Pa
P permeate osmotic pressure, Pa
TCF is specific to a given membrane product and should be obtained from the manufacturer.
MFRC is taken to be about 0.65 to 0.85 over a 3 to 5 year operating period. FF is generally about
0.8 to 0.9 over 3 years (AWWA, 1999).

Stabilization Design for NF/RO Softened Water. Because the NF/RO membranes do not
remove dissolved gases, the CO 2 in groundwater passes through the membrane. Acid pretreat-
ment to prevent scaling results in the conversion of bicarbonate ion to CO 2 . Thus, the permeate
has a low pH as a result of the formation of H 2 CO 3 . It is very corrosive. In a typical NF/RO soft-
ened water, the raw water pH is decreased to between 5.5 and 7.0 (AWWA, 1999).
A common post-treatment process is air stripping to remove the CO 2 . The CO 2 concentration
to achieve a design pH can be estimated using the carbonate equilibria. As demonstrated in the
following example, the pH that can be achieved by air stripping the CO 2 is not high enough to
make the permeate noncorrosive.

Example 9-3. Assuming that an air stripper can reduce the CO 2 in the permeate from an NF/
RO unit to the theoretical limit of equilibrium with the CO 2 in the atmosphere, what will the pH
of the permeate be? The atmospheric concentration of CO 2 in 2005, as measured at Mauna Loa,
Hawaii, was 370 ppm.