23 Jun 2011


Resin bead
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Service de-ionisation
Water demineralisation with cartridges
and off-site regeneration


SDI cartridgesMany industries require relatively small amounts of good quality demineralised water. Furthermore, they do not want to use regeneration chemicals — acid and caustic soda. Instead of investing in a small size regenerable ion exchange plant, the solution they have found is to rent demineralisation columns that are regenerated off-site. Examples of users are:

The principle of Service DI (SDI) is

Size and flow rate

The column size is usually between about 5 litres and 50 litres of mixed bed resin. The corresponding flow rate would be 50 to 150 L/h for the small unit and 500 to 1500 L/h for the large unit. The columns can be used intermittently, and the throughput produced depends on the salinity of the feed water.

In many cases, several columns are installed in parallel for a higher and more flexible production.

Treated water quality

In good SDI systems, the treated water quality is monitored continuously with a conductivity meter. With a single column, a conductivity of 1 µS/cm (which is a resistivity of 1 MΩ·cm) is usually achieved. With a lead-lag configuration, or with a polishing cartridge, ultrapure water with a resistivity of 18.2 MΩ·cm can be obtained, suitable for demanding applications in the semiconductor industry. In such cases, special "semiconductor grade" resins are used, which also deliver a treated water with very low TOC.
With most waters, the silica leakage is 10 to 20 µg/L as SiO2.


The throughput (volume produced until the cartridge has to be replaced) depends on several criteria:

With good resins and good regeneration, the throughput can be approximately calculated as:

Throughput [L] = 500 × (Resin volume [L]) / (Salinity [meq/L])

Area of application Paris region
Water salinity 5.5 meq/L
Resin volume in column 25 L
Endpoint 20 µS/cm
Throughput 500 × 25 / 5.5 = 2770 L

A higher throughput is obtained with lower salinity, and vice-versa.

Advantages of SDI

Service de-ionisation offered by reputable companies in many countries offer the following advantages:

© François de Dardel