Reaching 18 MΩ·cm: RO + EDI Ultrapure Water Process and Troubleshooting

Desalination answers “is there water at all”; industrial pure water answers “is the water pure enough”. Electronics, photovoltaics, pharmaceuticals, boiler make-up and electroplating demand everything from a few µS/cm down to 18.2 MΩ·cm at 25 °C. The process train follows from the purity grade you actually need.

1. Know the purity grades

2. The standard train, stage by stage

Pretreatment (multimedia, activated carbon, softening or antiscalant) protects the RO membranes and controls free chlorine, hardness and SDI.

Reverse osmosis removes 97–99% of dissolved salts, organics and bacteria. Two-pass RO brings product conductivity down to 1–5 µS/cm.

EDI (electrodeionisation) uses an electric field to drive ion migration while ion-exchange resin is continuously regenerated in situ, taking RO permeate to 5–15 MΩ·cm with stable output and no acid or caustic regeneration. This is the technology that replaced the traditional mixed bed.

Polishing mixed bed, UV and final ultrafiltration form the last stage of ultrapure water, pushing resistivity to 18.2 MΩ·cm while controlling TOC, particles and bacteria.

3. EDI or mixed bed?

Choose EDI for continuous production: no acid or caustic, no regeneration wastewater, simple operation, stable running cost. Capital cost is slightly higher, but it removes chemical storage, handling and hazardous-goods management — which is why around 90% of new projects specify EDI.

Choose a mixed bed where capital is tight, where the very highest purity is needed (18 MΩ is achievable), or for small intermittent demand. But acid and caustic regeneration carries real environmental and safety management cost.

A common engineered solution is RO → EDI → polishing mixed bed, combining stability with ultimate purity.

4. When resistivity will not come up

1. CO₂ not removed. RO is almost transparent to dissolved gases; CO₂ passes through and consumes resin capacity in the EDI. With high-alkalinity feed, add a degasser ahead of the RO or adjust pH.
2. RO permeate quality degraded. Falling rejection, aged membranes or a leaking O-ring pushes the EDI beyond its feed specification and directly drags resistivity down.
3. EDI current or voltage set wrongly. Too low and ions are not removed; too high and you electrolyse water and scale the stack. Follow the manufacturer's curve rather than intuition.
4. EDI scaling. Feed hardness above about 1 ppm as CaCO₃ scales the concentrate compartment; acid cleaning or stack replacement follows.
5. Loop and tank contamination. Ultrapure water dissolves everything it touches: use PVDF or polished 316L piping, nitrogen blanket the tank, and take samples from the circulating return.
6. The instrument itself. An uncalibrated resistivity meter or a wrong temperature-compensation setting produces a false alarm. Verify the meter before blaming the plant.

5. Design details that are easy to miss

Recovery and reject reuse. RO concentrate can be reused for washdown or landscaping; raising recovery from 65% to 75% on a large plant saves a meaningful amount of water cost.

Dead legs. An ultrapure loop must keep circulating. Any stagnant branch grows bacteria and degrades quality — design the loop and avoid blind pipe runs.

Materials. No carbon steel or galvanised pipe in the pure water section; iron leachate contaminates the water and keeps resistivity down.

Data logging. Conductivity, differential pressure and flow trends are the only reliable basis for judging membrane and resin condition. Remote monitoring beats manual log sheets.

Shenzhen Duruofei Technology Co., Ltd. supplies both desalination and industrial pure water systems, and can deliver a complete train from seawater intake through desalination, pure water and ultrapure water, or a single-stage retrofit (add EDI, add a second RO pass, add remote monitoring). A feed water report and a water specification are the two inputs we need to design it.