What is (Continuous) Electrodeionization (CEDI)?
Continuous Electrodeionization CEDI is a chemical-free water purification technology that removes dissolved ions by applying an electrical field. Ions migrate from the dilute compartments into the concentrate compartments, producing high-purity water. Unlike conventional ion exchange systems, the electrical current continuously regenerates the resin, eliminating the need for acid or caustic chemicals. CEDI is typically used to polish reverse osmosis (RO) permeate.
Why choose CEDI instead of a conventional mixed-bed deionization system?
CEDI operates continuously and only requires electricity to regenerate the ion exchange resin. In contrast, conventional mixed-bed systems work in batches and require regeneration chemicals, storage tanks, dosing equipment, neutralization systems, and additional controls. As a result, CEDI systems generally reduce chemical handling, save space, and simplify operations.
Why is the feed water quality so important for CEDI performance?
A CEDI system's efficiency and lifespan depend heavily on incoming water quality. Certain contaminants, such as hardness, iron, manganese, chlorine, and carbon dioxide (CO₂), can reduce performance or cause damage if they exceed recommended limits.
Why are hardness limits in CEDI feed water so strict?
Even relatively low hardness concentrations can lead to scaling inside the concentrate compartments of a CEDI module. This is due to high recovery rates and localized pH changes within the module, which increase the likelihood of scale formation.
Why are iron and manganese levels restricted in CEDI feed water?
Although ion exchange resins can remove iron and manganese, these elements may accumulate within the CEDI resin bed. Over time, they can oxidize and precipitate, potentially fouling the resin and reducing module performance.
Can CEDI systems tolerate small amounts of free chlorine?
No. Both RO membranes and CEDI modules are highly sensitive to free chlorine. Damage has been observed at concentrations as low as 0.05 ppm. In many cases, CEDI modules may show signs of chlorine damage before RO membranes are affected.
How does CEDI handle dissolved carbon dioxide (CO₂)?
CEDI systems can remove CO₂ similarly to dissolved salts. However, CO₂ receives special attention because it is only weakly ionized and therefore is not detected through conductivity measurements. In addition, RO systems do not effectively remove dissolved CO₂, making it a significant load for CEDI.
Can caustic soda be used to increase feed water pH and reduce CO₂ levels?
Adjusting the pH can improve CO₂ removal, but it must occur before the RO system, not before the CEDI unit. Increasing the pH upstream of the RO converts CO₂ into bicarbonate, allowing the RO membrane to remove it. Raising the pH after RO treatment does not improve CEDI product water quality.
Why must the CEDI feed water temperature remain above 5°C (41°F)?
As water temperature decreases, electrical resistance within the CEDI module increases. CEDI power supplies are designed to compensate for this effect down to 5°C (41°F). Below this temperature, performance may decline because the available DC voltage may no longer be sufficient.
How much back pressure can a CEDI module withstand?
The allowable back pressure depends on maintaining an inlet pressure of 100 psi or less. For example, if the operating pressure drop across the CEDI system is 20 psi, the maximum permissible back pressure would be 80 psi.
Can a CEDI system be installed in a distribution loop?
Yes. CEDI systems can successfully operate within a distribution loop. However, the reject stream continuously removes approximately 5% of the loop water. In most applications, this water can be redirected upstream for reuse.
Is outdoor installation of a CEDI system possible?
You can install CEDI systems outdoors, provided they are adequately protected. The equipment should be shielded from direct sunlight and should not be exposed to temperatures above 35°C (95°F) or below 5°C (41°F) on a regular basis. Outdoor installations also require suitable protective enclosures to guard against moisture, temperature fluctuations, rain, snow, and other environmental conditions.
How quickly does a CEDI system return to specification after standby mode?
Following a standby period, a CEDI system typically reaches the required water quality within a few seconds after startup. Recovery after cleaning or sanitization takes much longer and may require several hours, or even overnight operation, before full performance returns.
How often should a CEDI system be sanitized?
Sanitization frequency depends on process requirements, system design, and microbiological quality standards. Some installations never require sanitization, while systems producing purified water are commonly sanitized anywhere from weekly to quarterly.
What is the expected service life of CEDI resin and membrane cell packs?
A typical service life for CEDI cell packs ranges from three to five years. Actual lifespan depends on factors such as feed water quality, pretreatment effectiveness, operating conditions, and the quality of system maintenance and monitoring.
How long do CEDI modules typically last?
Under normal operating conditions and with proper maintenance, CEDI cell packs generally perform reliably for three to five years. Consistent monitoring, effective pretreatment, and good operational practices can help maximize module lifespan.