Page 444 -

MEMBRANE PROCESSES 13.41

Temperature and pH. Maximum water temperature limitation for ED/EDR stacks is
generally about 45 ° C. Commonly used membranes can tolerate a pH range from 1 to 10.
Temperature is one of the most important factors affecting ion removal of an ED/EDR
system. Ion removal increases about 2% per degree Celsius temperature rise. A two-stage
ED/EDR system can have TDS removal approximately 25% greater at 30 ° C than at
15°C.
Staging. ED/EDR systems have electrical stages and hydraulic stages, sometimes in a
single stack. Multiple hydraulic stages within one stack are created by using special in-
terstage membranes designed to accommodate elevated pressure differentials. An electri-
cal stage is created by using a pair of electrodes (anode and cathode).
Typically, 40% to 67% salt removal is possible in one hydraulic stage (M38, Ameri-
can Water Works Association, 1995). Greater salt removals are accomplished by using
additional hydraulic stages in series. In an ED/EDR system, salt removal is directly pro-
portional to electric current and inversely to flow rate because of less available detention
time. Because current flow can be regulated independently for each electrical stage, mul-
tiple hydraulic and electrical stages optimize hydraulic and electrical parameters.
Differential Pressure. Demineralized stream pressure is usually controlled to be about
0.5 to 1.0 psi (3 to 7 kPa) greater than concentrate stream pressure to ensure that any
cross-leakage moves toward the concentrate stream and does not lower product water qual-
ity (M38, American Water Works Association, 1995). This differential pressure is main-
tained slightly positive.

Operating Mode. An ED/EDR membrane system can be designed for batch or contin-
uous flow operating mode. Municipal water treatment systems commonly use the con-
tinuous mode when the feed stream is demineralized and passes from the system as dem-
ineralized water. Some of the concentrate is recycled through the stack, and some leaves
as blowdown (feed-and-bleed design). Chemicals are sometimes added to the recycle
stream for scale control.
Electrode Compartments. Electrodes are usually made of platinum-coated titanium.
Chlorine gas, oxygen gas, and hydrogen ions are produced at the anode, creating an acidic
condition. Hydrogen gas and hydroxide ions are generated at the cathode, raising the pH
and increasing the conditions for scaling. Water from electrode compartments is usually
mixed together and transferred to a degasifier before final disposal or possible recycle.

OTHER MEMBRANE PROCESS DESIGN CONSIDERATIONS

Other considerations necessary in membrane process design include the need for post-
treatment, membrane cleaning, disposal of waste residuals, instrumentation and control,
efficient O&M, and building design.

Posttreatment

Membrane product flow streams usually require some form of posttreatment before dis-
tribution. Posttreatment provides disinfection, corrosion control, and removal of dissolved
gases and volatile compounds.