SNOWPURE, LLC, 2005-2018
VERSION 3.5 (XL+EXL)
–FEBRUARY 2018
PAGE 24
A portion of the current is then directly proportional to the ion content of the feed (TDS,
or μS/cm). The other portion of the current, proportional to water splitting, increases
non-
linearly with overvoltage. The “current efficiency” is the fraction of the total current
that is required to move the impurity ions in the EDI feed.
If the module current is higher than expected, it could be because the voltage is set
higher than optimum, and excess water splitting results in the excess current.
Current also depends on the concentrate conductivity, and therefore on the module
water recovery. The nominal concentrate flow is 10% of the feed flow. If the
concentrate flow is lower than recommended, the concentrate will be more conductive
and the current will increase. In this case, lower the voltage to find the optimum.
Steady State Operation
Normally, an EDI module will start up with high quality product water. This is because
the EDI module has excess mixed bed ion-exchange resins in it in the H and OH form, in
the “polishing” section.
However, after operating conditions have changed, a module will take between 8 and 24
hours to reach a new steady state. The true steady state is defined as reaching a mass
balance on the ions entering and leaving the module. At steady state, the kinetics of ion
migration match the ion feed rate. Steady state for trace ions such as silica may take as
long as 2-4 weeks.
If voltage is lowered or ion load is raised, the internal ion-exchange resins will begin to
adsorb the excess ions. In this condition, fewer ions leave the module than enter.
Eventually a new steady state is reached. During this time, the “working ion front”
progresses in the resin bed from near the bottom of the module upward.
If voltage is raised or ion load is lowered, the resins will lose some of their excess ions to
the concentrate stream, and more ions will be exiting the module than enter it. During
this time, the location of the “working ion front” grows closer to the inlet of the module.
This latter is the mechanism of the “regeneration” procedure.
An ion balance done on the module(s) during operation is a valuable tool in determining
if the EDI system is operating at steady state.
@ Steady State:
Total Ions Out = Total Ions In
Module Filling with Ions:
Total Ions Out < Total Ions In
Module Recovering from Overload:
Total Ions Out > Total Ions In
Ionic Species
The ability for EDI to remove ions from a stream depends in part on the properties of the
ionic species. In a standard resin bed, the adsorption strength and kinetics depend on
the ionic size, the degree of hydration, and on the type of resin.
In EDI, the ionic charge is even more important since this is the driving force to move the
ions along the resin surfaces to the membrane, and through it.
Ionic Size