Civil Engineering Reference
In-Depth Information
States; however, it is used for this purpose in regions where water resources are more
scarce, particularly the Middle East.
NF membranes are sometimes called loose RO membranes or low-pressure RO
membranes, because they exhibit lower removal capabilities and operate at lower pres-
sures than do RO membranes. NF membranes were first developed for softening and
are commonly used on the hard groundwaters found in Florida. Nanofilters generally
do not remove monovalent ions to the extent that RO membranes do, but are capable
of high rejections of divalent ions, such as calcium and magnesium. The lower oper-
ating pressures used in NF make it a more efficient process than RO.
RO and NF are also used for controlling specific ions. Some of the more common
applications are the removal of fluoride, nitrate and nitrite, ammonium, and phosphate.
The anticipation of reduced limits on arsenic in drinking water has spurred interest in
RO and NF for removing this contaminant.
13,14
Both RO and NF are capable of rejecting organic matter from water, and for this
reason are attractive for the removal of DBP precursors and color.
15-17
The extent of
membrane applications for these purposes has been limited by costs, mainly resulting
from pretreatment requirements and membrane fouling. However, more stringent reg-
ulations and improvements in the technology have made the process more feasible.
The Information Collection Rule, promulgated in 1996, has required utilities to test
new processes for DBP precursor control; as a result, 37 utilities are conducting tests
with nanofiltration.
18
Synthetic organic chemicals, and pesticides in particular, can also be removed by
RO and NF processes. European countries and parts of the United States that rely on
polluted surface waters have looked to membranes for pesticide removal.
19,20
The feas-
ibility of the treatment varies greatly with the target pesticide, as well as other con-
ditions specific to the application.
Membrane Characteristics
Several characteristics are important to the performance and longevity of RO and NF
membranes:
Mechanical strength
•
Resistance to chemicals, hydrolysis, and biological attack
•
Temperature stability
•
Selectivity
•
Productivity
•
RO and NF membranes require different characteristics than do UF and MF mem-
branes owing to the different operating conditions and removal mechanisms. RO and
NF membranes operate at much higher transmembrane pressures than do UF and MF
membranes, and for this reason must have a greater mechanical strength. High trans-
membrane pressures can cause membrane compaction, thereby reducing productivity,
or even membrane rupture.
RO and NF membranes can remove particles by physical straining processes, as do
UF and MF membranes, but the key function of RO and NF is the removal of dissolved
contaminants. The separation of solutes (e.g., salts and organic compounds) by RO
and NF membranes is controlled by mechanisms not applicable to UF and MF, mainly
