An RO + EDI system is selected by working backward from point-of-use quality and flow, then confirming that RO permeate remains inside the EDI module's feed limits. EDI is a polishing process, not a substitute for raw-water pretreatment and RO.
Define the target as conductivity or resistivity at a named measurement point, provide feed and RO-permeate data, and check hardness, carbon dioxide, silica, temperature, flow profile, sanitization and distribution before selecting EDI capacity.
EDI Feed Water
EDI modules combine ion-exchange media, ion-selective membranes and direct current. They require low-ionic-load RO permeate. Module suppliers specify limits for hardness, conductivity, free chlorine, silica, carbon dioxide, temperature and other feed conditions; the project design must use the limits for the selected module.
Hardness can cause scaling, while dissolved carbon dioxide passes through RO differently from charged ions and increases the polishing load. Silica behavior also depends on pH and form. These values should be reviewed instead of treating RO permeate conductivity as the only EDI feed criterion.
- Pretreatment
- First-pass RO
- Second-pass RO
- EDI polishing
- Storage and loop
Conductivity and Resistivity
Conductivity rises as ionic contamination increases; resistivity is its reciprocal and is often used for high-purity water. State the target, temperature compensation method, instrument location and whether the acceptance point is at the EDI outlet, tank return or point of use.
A high-purity project may discuss values in the 15-18 MOhm.cm range, but that is not a universal equipment promise. Carbon dioxide, silica, organics, temperature, storage contact, microbial control and distribution materials can prevent the complete system from holding an EDI outlet value at the point of use.
Continuous Deionization
EDI uses direct current to move ions through selective membranes and continuously regenerate the resin bed during operation. It avoids routine acid-and-caustic regeneration of a conventional polishing bed, but still needs controlled feed, stable flow, correct concentrate conditions, electrical protection and planned cleaning or sanitization.
The flow profile matters. An EDI module has an operating window; frequent low-flow running, stagnant periods or large peaks may require recirculation, buffering or a different module arrangement. Redundancy is a project decision based on production criticality.
Controls, Sanitization and Distribution
Monitor RO permeate conductivity, EDI product conductivity or resistivity, flow, pressure, voltage and current. Interlocks should protect the module from off-spec feed and low flow. Sanitization method and compatible materials must be defined for the complete generation, tank and distribution system.
See the RO + EDI Ultrapure Water Equipment page for Xinnuo's equipment route and the Ultrapure Water EDI System solution for project boundaries.
Common Design Mistakes
- Feeding EDI directly from inadequately controlled waterEDI must receive feed that stays inside the selected module limits.
- Specifying only an EDI outlet valueStorage, distribution and point-of-use requirements can be more restrictive than the equipment outlet.
- Ignoring carbon dioxide and silicaLow conductivity alone does not prove these loads are acceptable.
- Sizing only for average flowMinimum flow, peak demand, operating hours, recirculation and redundancy affect module selection.
Send both feed and final-use requirements
Provide raw-water analysis, expected RO permeate, target quality and measurement point, net flow, operating hours, sanitization method and distribution information.
Frequently Asked Questions
Can EDI treat untreated raw water?
No. EDI is a polishing process that requires controlled RO permeate within the selected module's feed limits.
What is the difference between conductivity and resistivity?
They are reciprocal measures of ionic conduction. Conductivity is commonly used across RO treatment, while resistivity is often used to express high-purity water quality.
Does EDI always produce 18 MOhm.cm water?
No. Performance depends on module design, feed quality, flow, temperature and operation. Point-of-use quality also depends on storage and distribution.
What project data is needed for an RO + EDI quotation?
Provide raw-water analysis, RO-permeate expectations if available, final quality and measurement point, flow profile, operating hours, utilities, sanitization and distribution scope.