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

Introduction

Many 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:

• Research institutes
• Laboratories
• Chemical and biochemical facilities
• Pharmacies and pharmaceutical industries
• Hospitals
• Semiconductors
• Microelectronics
• Cafés and restaurants
• Vehicles service centres
• Windows cleaning services
• Solar panels cleaning

The principle of Service DI (SDI) is

• Customer gets a ready-for-use demineralisation column from the SDI supplier
• Once the column is exhausted, it is replaced with a freshly regenerated column

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.

Throughput

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

• Salinity of the feed water
• Volume of resin in the cartridge (throughput is approximately proportional to resin volume)
• Type of resin used
• Quality and efficiency of the off-site regeneration process
• Endpoint (conductivity at which the unit is considered exhausted)

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

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

Example

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:

• High quality demineralised water
• Low investment: in many cases only the service is charged
• No pump required: the system operates with the mains pressure (usually up to 800 kPa / 8 bar)
• No chemicals at the customer site
• No maintenance
• No resin handling: the columns are never opened at the customer site
• Zero waste water on the customer site
• Rapid exchange of cartridges (usually within 1 to 3 days)
• Traceablilty: the SDI operator should make sure that the resins used at each customer site is fully suitable for the intended use