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13.24 CHAPTER THIRTEEN

If iron or manganese precipitates, or if either metal is present from corrosion products,
ED/EDR, RO, and NF systems can be fouled. Appropriate materials should be used for
construction of feedwater supply or pretreatment systems.
Three pretreatment processes can be used for iron or manganese removal:
• Oxidation using air, chlorine, or another oxidant, followed by granular media filtration,
MF, or UF
• Oxidation using potassium permanganate, followed by a manganese greensand filter
• Cation ion exchange softeners
Residual oxidant levels must be monitored and controlled before entering the mem-
brane system. If lime softening is used for scale control, it has the additional benefit of
lowering iron and manganese levels in feedwater.
Organics Control. Organic matter may be classified as either suspended or dissolved.
Suspended (and colloidal) organic solids and microbial matter (previously discussed) ad-
versely affect membrane systems.
Naturally occurring dissolved organics cause fouling in ED/EDR systems and gen-
erally, to a lesser degree, in RO or NF systems. In fact, some NF and RO systems are
used to remove natural and synthetic dissolved organics and color. Membrane manu-
facturers are aware of many organic compounds that cause problems in their systems
and can be contacted for specific information. There is no definitive correlation between
the quantity of organics present, such as measured by total organic carbon (TOC) analy-
ses, and performance decline of RO or NF membranes. However, if TOC exceeds 10
mg/L in feedwater, the potential effects of fouling usually warrant further investiga-
tions. Pilot testing is often used to determine the effects of dissolved organics on mem-
brane systems.
The organic content of the feedwater to MF and UF systems may control the accept-
able design flux and the backwash and cleaning frequencies. Sometimes, MF/UF mem-
brane systems are designed in a direct filtration arrangement, where metal coagulant ad-
dition and flocculation (often less than 5 min) are provided upstream of the MF/UF unit
to remove organic matter (and solids) and improve performance. In some systems, pow-
dered activated carbon (PAC) is fed prior to the membrane filter to aid in organic removal
and lessen fouling.
Oil, greases, hydrocarbons, and various organic solvents and other chemicals can dam-
age or foul membranes and should not be allowed to enter the feedwater supply.
Conventional coagulation, flocculation, and sedimentation processes and lime soften-
ing can reduce the organic content of feedwater before it enters membrane units. Acti-
vated carbon filters are sometimes used to remove organics or for dechlorination of feed-
water entering RO, NF, and ED systems; but problems caused by carbon fines or biological
microorganisms released from carbon filters have occurred in some systems. Surfactants
and organic polymers, especially the cationic type, can foul membranes and should not
be used without membrane manufacturer approval.
Control ofpH. All membrane types have specified pH ranges in which they should op-
erate. Many types of ED and polyamide RO and NF membranes can tolerate continuous
exposure to feedwater in the pH range of 4 to 10 and intermittent exposure to an even
wider pH range. The various types of cellulose acetate RO and NF membranes normally
require pH between approximately 4 and 6.5 to minimize degradation caused by hydrol-
ysis. Many types of polysulfone, polypropylene, and PVDF MF and UF membranes tol-
erate a pH range of at least 2 to 13. Inorganic MF and UF membranes can generally ac-
cept feedwater with nearly any pH.