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MEMBRANE PROCESSES 13,23

controlled. Most types of cellulose acetate RO and NF membranes can tolerate up to 1.0
mg/L free chlorine. However, most composite RO and NF membranes, such as polyamide
(PA) membranes, cannot tolerate chlorine or other strong oxidants. If free chlorine is used
prior to membranes intolerant to free chlorine, dechlorination processes, such as sodium
bisulfite or sulfur dioxide addition, can be used upstream of the membranes. Granulated ac-
tivated carbon (GAC) is also sometimes used for dechlorination, but usually only for small
systems. Microbial regrowth downstream of chemical dechlorination systems or microbial
matter released from GAC may also cause problems in the membrane system. The chlori-
nation and dechlorination system, if used, must be designed to be very reliable to prevent
unacceptable oxidant concentrations from reaching the membranes.
Chlorination-dechlorination is also sometimes used for ED systems because most ED
membranes cannot tolerate chlorine. Some EDR membranes may have relatively long life
with a continuous free chlorine exposure of up to 0.3 mg/L.
Limited concentrations of chloramines are also sometimes used for bacterial control
in membrane systems where the membranes can tolerate the combined chlorine concen-
tration. The membrane manufacturer should be contacted to obtain maximum acceptable
oxidant exposure criteria, if any. The chloramination system (feeding ammonia and chlo-
rine) must be designed to very reliably maintain the target combined chlorine residual and
preclude any free chlorine from entering the membrane system.
Biological control through periodic shock treatments with sodium bisulfite or other
nonchlorine compounds is also used for membrane systems. Ultraviolet light disinfection
can be used as pretreatment for membrane systems, but the lack of a residual may result
in regrowth problems. MF or UF membranes in dual-membrane systems also reduce bi-
ological matter prior to the NF or RO system.
Bacterial control should also be considered for membrane units not in service. When
off-line for extended downtime, membranes are commonly placed in preservative and stor-
age solutions of various formulations to retard microbial growth using the cleaning system.
Hydrogen Sulj'ute Control. Where hydrogen sulfide is present, such as in some well
waters, no chlorine or exposure to air can be allowed; colloidal sulfur will form and foul
an RO or NF membrane system. Because chemical cleaning is typically ineffective in re-
moving sulfur from the membrane surface, membranes that become fouled are usually re-
placed. When low levels of hydrogen sulfide exist in feedwater, the system can be de-
signed to be airtight and hydrogen sulfide can be removed in posttreatment degasifiers.
Hydrogen sulfide greater than 0.1 to 0.3 mg/L can cause problems with ED/EDR systems
and must be removed in pretreatment. RO and NF membranes can tolerate relatively high
concentrations of hydrogen sulfide without damage.

Iron and Manganese Control. Iron and manganese may cause problems with RO, NF,
and ED/EDR membrane systems. However, as long as these metals are kept in their reduced
state and dissolved in water, they cause little problem with RO and NF systems. Because
iron and manganese form precipitates in the presence of oxygen or at high pH, acid addi-
tion is often used for metal oxide control (in addition to its other uses, previously discussed).
Sequestrants such as sodium hexametaphosphate are also commonly fed to inhibit iron
and manganese deposition. Some types of polyacrylic acid antiscalants react with iron and
manganese, causing fouling problems in downstream membranes, and should not be used.
Proprietary antifoulants (scale inhibitor/dispersants) are also commonly used to control
fouling and scale in the presence of iron and manganese.
Iron and manganese can foul ED/EDR membranes even when they are in the reduced
state. In addition, manganese can plate out on the electrodes, decreasing their efficiency.
For ED/EDR systems, iron removal is usually recommended if feedwater iron concen-
tration exceeds 0.3 mg/L or if the manganese level exceeds 0.1 mg/L.