Electrodeionization (EDI)

An electrodeionisation (EDI) system is a continuous water purification unit that removes dissolved ions from reverse osmosis permeate using a combination of ion exchange resin, selective membranes and an applied DC electrical field. Unlike conventional mixed bed demineralisation, EDI requires no chemical regenerants. The DC field continuously regenerates the resin in-situ by electrochemically splitting water molecules into hydrogen and hydroxyl ions, maintaining ion exchange capacity and producing ultra-pure product water without any service interruption.

Both EDI and mixed bed demineralisation produce ultra-pure water with conductivity below 0.1 microsiemens per centimetre, but they differ significantly in operation. Mixed bed systems operate in batch mode, requiring periodic regeneration with acid and caustic that causes service downtime and variability in product water quality at the start and end of each service cycle. EDI operates continuously with no regeneration cycles, no chemicals and no downtime, making it the preferred technology where uninterrupted ultra-pure water supply and elimination of chemical handling are priorities.

EDI systems require pre-treated feed water from a reverse osmosis system with conductivity typically below 40 microsiemens per centimetre and total dissolved solids below 25 mg/L. Hardness must be below 1 mg/L as calcium carbonate to prevent scaling of the ion exchange membranes, and residual chlorine must be absent to prevent oxidative membrane damage. Carbon dioxide levels should be minimised as CO2 is not efficiently removed by EDI membranes, typically requiring degassing ahead of the EDI stage. IWTE designs the upstream RO and degassing pre-treatment to ensure correct EDI feed quality.

EDI membranes are highly effective at removing ionic silica, achieving product water silica levels below 5 micrograms per litre in normal operation. Carbon dioxide, being a dissolved gas rather than an ion, is not efficiently removed by EDI membranes. Elevated CO2 in the EDI feed reduces product water resistivity and increases the ionic load on the system. Membrane degassing or forced draft decarbonation ahead of the EDI module is recommended where RO permeate contains significant dissolved CO2, and IWTE incorporates degassing stages into the system design where required.

EDI is always positioned as the final polishing stage in a high-purity water treatment train, receiving pre-treated feed water from a reverse osmosis system. A typical train comprises pre-treatment, RO as the primary desalination stage, membrane degassing for CO2 removal, and then the EDI system for final polishing to ultra-pure quality. IWTE designs the complete treatment train as an integrated system with unified PLC and SCADA control, ensuring each stage is matched in flow rate and water quality to the requirements of the EDI module.

Yes. EDI is widely used and regulatory-accepted as a polishing technology in pharmaceutical purified water systems compliant with USP, EP and JP pharmacopoeial requirements. The continuous, chemical-free operation of EDI aligns well with pharmaceutical Good Manufacturing Practice requirements, eliminating the batch variability and chemical contamination risks associated with conventional mixed bed regeneration. IWTE designs pharmaceutical EDI systems to applicable pharmacopoeial specifications, incorporating the monitoring, documentation and validation support required for GMP-regulated environments.

EDI systems require significantly less maintenance than conventional mixed bed demineralisation systems. Routine maintenance consists of periodic cleaning of the concentrate and electrode compartments to remove any scale or fouling accumulation, inspection and replacement of the DC power supply components and electrode assemblies on the manufacturer's recommended schedule, and monitoring of system performance parameters including product conductivity, flow rates and module differential pressures. EDI stack modules typically have service lives of 5 to 10 years with correct pre-treatment and operation.

Yes. IWTE designs and supplies complete high-purity water treatment trains with EDI as the final polishing stage, encompassing pre-treatment, reverse osmosis, membrane degassing and EDI within a unified, automated system. The entire train is engineered, fabricated and commissioned by IWTE as a single integrated scope. EDI systems are also supplied as standalone polishing units for integration into existing RO treatment trains where a chemical-free upgrade from mixed bed demineralisation is required.