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'13./I.2 CHAPTER THIRTEEN

Posttreatment Disinfection. Most pathogens are removed by RO, NF, UF, and MF mem-
branes, and microbiological quality of the permeate (or filtrate) is usually excellent. How-
ever, it is possible for microbes to pass through during the process when a membrane bar-
rier is compromised (such as a broken fiber) or when other imperfections are present in
the membrane system (such as an O-ring leak or contamination before or during opera-
tion of the permeate side of the membrane module or permeate piping). In ED/EDR sys-
tems, the product water does not pass through a membrane barrier. Consequently, prod-
uct water posttreatment usually includes disinfection, typically with chlorine.
Posttreatment disinfection design is similar to that of conventional water treatment
plants and is described in Chapter 10.
Posttreatmentfor Corrosion Control Corrosion control is not usually required for UF
and MF processes. However, RO, ED/EDR, and to a lesser extent NF product streams are
commonly corrosive because of the lack of calcium and alkalinity and the acidic pH. Post-
treatment operation for corrosion control can include the following activities:
• Releasing carbon dioxide in a degasifier
• Adding caustic (with or without previous CO2 addition), sodium bicarbonate, or so-
dium carbonate to increase bicarbonate alkalinity and pH
• Adding a corrosion inhibitor chemical
• Adding calcium chloride to increase calcium levels
• Adding lime to increase both calcium ion and alkalinity and pH (but this may cause
unacceptable turbidity)
• A combination of these processes
Selection of the most appropriate posttreatment method for corrosion control should
be site-specific and depend heavily on membrane product water quality.

Removal of Gases and Volatile Organic Compounds. Water treatment membrane pro-
cesses do not remove dissolved gases and, in general, provide poor removal of most volatile
organic compounds (VOCs). Therefore these components must be removed during per-
meate posttreatment.
Hydrogen sulfide (if present in the source water) and carbon dioxide are usually the
predominant gases present in membrane product water. Sulfide is often present in ground-
water source waters, and carbon dioxide can be present in groundwaters or generated by
feedwater acidification. Unless the permeate has sufficient alkalinity (which is generally
not the case), carbon dioxide removal is not desirable because the gas can be used as a
source of bicarbonate alkalinity for corrosion control. VOCs are often a problem when
treating groundwaters contaminated by industrial processes.
The most common method of treatment is the use of a degasifier, commonly a packed
tower with a blower. Tower design should be a function of the critical gas or VOC pres-
ent in the product and the degree of removal required. When carbon dioxide and hydro-
gen sulfide are both present, sulfide removal usually controls the degasifier design.
If the pH of the permeate is greater than 6.5, hydrogen sulfde removal may require
that the permeate be acidified before degasification. Acidification ensures that a large pro-
portion of the sulfide is present in the gaseous form, in contrast to sulfide ions. Degasi-
fier off-gases containing sulfides may also require scrubbing to minimize odor and cor-
rosion problems and for safety reasons. Scrubbing is typically achieved by conveying the
gas through chlorine and/or caustic solutions to convert the sulfide to an ionized form.
Degasification for VOC removal may also require off-gas treatment with granular acti-
vated carbon (GAC), depending on state or local air quality regulations.