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MEMBRANE PROCESSES 13.39

Most MF and UF membrane systems are equipped for periodic off-line integrity testing
using air, based on the bubble point theory. In the pressure decay test, air is applied to the
membranes at a pressure less than needed to be forced through the pores of intact mem-
branes, and decline in air pressure is measured. If the rate of pressure decay is too large,
the membrane unit is considered to have failed the test. In another test, the diffusive air-
flow test, the volume of water displaced by air passing through the membranes is measured
and evaluated for acceptance. These tests are conducted with the membrane unit out of ser-
vice, although the testing can be automated and scheduled to take place on a timed cycle.
Membrane units not passing the tests can then be diagnosed for the source of the leakage,
such as broken fibers or valves not sealing properly, and remedial measures can be taken.
To meet the requirements of the Long-Term 2 Enhanced Surface Water Treatment Rule
(LT2ESWTR), sufficient air pressure must be used in the test to achieve a resolution of 3
/xm (USEPA Membrane Filtration Guidance Manual, Proposal Draft, June 2003).

Electrodialysis and Electrodialysis Reversal
Electric current through an ED/EDR system can be calculated by using Faraday's law
(Electrodialysis and Electrodialysis Reversal, AWWA M38, 1995):
F X Fd X AN
I=
eXN
where 1 = direct current, A
F = Faraday's constant (96,500 A • s/equivalent/cell pair)
Fa = flow rate of demineralized stream through membrane stack, L/s
AN = change in normality of demineralized stream between inlet and outlet of
the membrane stack
e = current efficiency
N = number of cell pairs in membrane stack
Electrical resistance can be calculated by Ohm's law:
E
R=--
I
where R = resistance, II
E = voltage, V
I = current, A
ED/EDR current efficiency is defined as follows:
FXFaXANX 100
e --
1XN
Recovery. Recovery of an ED/EDR system is often limited by the concentration of spar-
ingly soluble salts, similar to RO and NF units. However, silica scaling is not a concern
because silica is not concentrated (or removed) by ED/EDR. Pretreatment processes, such
as those described previously for RO, can be used to control the concentration of spar-
ingly soluble salts (for example, CaCO3, CaSO4, SrSO4, BaSO4) to allow higher recov-
eries. One major EDR manufacturer allows LSI values up to +2.2 and CaSO4 concen-
trations up to 175% of saturation without chemical addition (Meller, 1984). Higher values
are allowed by the manufacturer with appropriate antiscalant addition.