SNOWPURE, LLC, 2005-2018
VERSION 3.5 (XL+EXL)
–FEBRUARY 2018
PAGE 34
Problems associated with concentrate recirculation include the buildup of all ions which
include scale-forming ions such as Ca
+2
, SO
4
-2
, silica, etc. A circulation system may also
harbor bacteria and require a UV (254 nm) system.
EDI System Protection and Controls
To protect the EDI module(s) and to ensure long lifetime, automatic system protections
are necessary. Some are simply good engineering. The most critical protection is
preventing the application of power to the module without water flow. Non-compliance
with this rule will certainly result in irreversible damage to the EDI module. The critical
measurements and alarms are:
1. Electrode flow above a Minimum
2. Concentrate flow above a Minimum
3. Product flow above a Minimum
4. RO is operational
5. RO conductivity is below a Maximum
6. Temperature is within limits
7.
Pretreatment all working properly (no alarms)
Below is a list of the components typically used in an optimum RO-EDI system, beyond
that a typical P&ID (process and instrumentation diagram) for a single-module system.
For multiple modules, the modules are plumbed and powered in parallel
—the rest of the
concepts are the same.
Component Description of an Optimum EDI System
These descriptions relate to the P&ID following this section.
Activated Carbon: Removes chlorine and some chloramines from feed water to protect
the reverse osmosis membranes, ion exchange resins, and ion selective membranes
from chemical degradation. It also removes many organic solvents and pesticides,
preventing them from passing through to the RO membranes and into the EDI modules.
Softener: Removes hard cations from the feedwater (Ca
+2
, Mg
+2
) in order to prevent
scaling in the RO and the EDI. Softening allows higher recovery in the RO system.
Softening also allows the feedwater pH to be raised in order for CO
2
and silica to be
more effectively removed in the RO and EDI. Will also remove iron (Fe) and other
transition metals that catalyze the oxidation of the PA membranes and bind irreversibly
to the resins in the EDI.
Note that the use of chemical anti-scalant in the RO increases the hardness
passage through the RO membrane, and this is fed to the EDI. To minimize
EDI cleaning frequency, the EDI feed hardness should be minimized.
Sediment Filter: Removes undissolved matter from feed water to prevent the reverse
osmosis membranes from fouling.