Membrane and Desalination Technologies - TCE Moodle Website

632 P. Kajitvichyanukul et al.
For the performance comparison of RO (and NF) with MF and UF, the term J 25 normalized
flux may not be used as the MF, UF, and MCF systems are typically referenced to a
temperature of 20 C.
To identify changes in productivity that is attributable to fouling of membrane, the flux is
determined as follows:
M25 ¼ J25
(12Þ
NPD
Here, M25 = temperature- and pressure-normalized flux (gfd/psi), J25 = normalized flux at
25 C (gfd), and NDP = net driving pressure (psi).
Rejection rate (Re) is a fraction of the total contaminant mass/concentration rejected by the
membrane. It is also determined by the following Eq. (13):
Re ¼ 1 ðCp=CfÞ; (13Þ
where Re = rejection rate (%), Cf = concentration of solid in feed (mg/L), and CP =
concentration of solid in permeate (mg/L).
In determining the rejection rate (Re), the conductivity measurement can be performed.
Conductivity in the RO testing procedure could be replaced by hardness, alkalinity, nitrate,
phosphate, silica, etc. (measured in ppm or mg/L). Rejection rates less than 95% may indicate
that the membrane should be replaced.
Volume recovery, %: Volume recovery (Rv) is the fraction of the total feed volume of the
test water that can be recovered as clean permeate:
Rv ¼ Vp=Vf; (14Þ
where R v = volume recovery, V F = volume of feed, L, and V p = volume of filtrate, L.
4.2. Calculations
Example 1
Find the rejection percentage of RO units after the consumer had run the RO system for 20
min and found that the hardness of the RO water is 5 ppm. Let the tap water hardness be 80
ppm.
Solution
Find the rejection percentage from
Re ¼ 1 ðCp=CfÞ
Here, Cf = feed average concentration = 80 ppm and CP = filtrate concentration = 5 ppm.
Therefore; Re ¼ 1 ð5=80Þ
Re ¼ 1 0:0625
Re ¼ 0:9375 ¼ 93:75%
Thus, membrane hardness rejection is 93.75%. Rejection rates less than 95% may indicate
that the membrane should be replaced.