Page 378 - Water and wastewater engineering

REVERSE OSMOSIS AND NANOFILTRATION 9-17

Solution:
a. The saturation value of dissolved CO 2 from the atmosphere can be determined using
Henry’s law with K H 0.033363 mole/L · atm.
.
]
.
[CO KP (0033363 mole/L )(370 10 6 atm ) 1123 10 5 moles/L
2 Hg
b. Using an expression derived from the equilibrium equations, determine the hydrogen ion
concentration (Masters, 1998).
] K
[H 2 [CO (aq )] 10 14
1 a 2
(447 10 7 )(1 23 100 5 moles/L) 10 14
.
.
553 10 12
.

6

and [H ] 2.35 10 moles/L.
c. The pH is
.
pH log (2 35 10 6 moles/L 5 63
.
)
Comment. This pH is the theoretical limit, not a practical limit, that can be achieved by air
stripping.
In addition to air stripping, some of the CO 2 can be converted to bicarbonate/carbonate al-
kalinity by raising the pH of the permeate before stripping using a strong base such as NaOH or
Ca(OH) 2 . This step must precede air stripping to make use of the CO 2 in the permeate.
A third alternative, and one most likely to be practiced in softening, is to bypass a fraction of the
raw water and blend it with the permeate. The raw water alkalinity will help stabilize the water.

Operation and Maintenance
Routine monitoring of the flow rate and pressure provides information on potential scale build
up. A 10 to 15 percent decline in temperature- and pressure-normalized flux or about 50 percent
increase in differential pressure may indicate fouling. Fouling requires chemical cleaning. Only
agents approved by the membrane manufacturer in writing should be used. NF/RO systems are
normally designed to operate for three months to one year between chemical cleanings (U.S.
EPA, 2005). Pressure checks or mini-challenge tests with a surrogate particle are used to assure
membrane integrity. The membrane must be free of breaks 3 m in diameter.
Because RO/NF systems are pressure driven, special safety precautions should be empha-
sized. These include (AWWA, 1999):
• Do not overpressurize equipment.
• Assure that pressure vessels are suitably anchored.

• Be sure pressurized vessels are depressurized before working on them.
• Inspect pressure relief and shutdown devices regularly.
• Minimize equipment and piping vibrations and water hammer.