operation and maintenance of a total demineralisation unit

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treatment verification

The verifications that needs to be undertaken on a demineralisation unit mainly include the following measurements :

  • conductivity (or resistivity);
  • silica concentration;
  • sodium concentration if applicable;
  • pH if applicable.

Maximum reliability is achieved through continuous verification, especially in the case of conductivity and silica.

For an accurate interpretation of the conductivity measurement and in order to deduce the ionic leakage value, we need to remember that demineralised water only contains traces of caustic soda (see section characteristics solution constants).

resin disinfection

Operating problems can arise because of the presence of micro-organisms :

  • bed fouling caused by an invasion of bacteria colonies (especially with carboxylic resins);
  • resin pores poisoned internally (especially with anion exchangers).

There are two types of remedies; however, these must never be applied before consulting a specialist :

  • preventive, via preliminary chlorination-dechlorination, continuous or intermittent, of the raw water;
  • remedial, by disinfecting the resin layer using formol or a solution of quaternary ammoniums, or brackish water containing 200 g · L–1 of NaCℓ, and rendered alkaline to pH 12.

resin storage

in its original packaging

protection against dehydration

Resin must be stored in its unopened packaging away from sunlight and at temperatures below 40°C. Check packaging at regular intervals or maintain resin humidity if the packaging has been breached by spraying with water if necessary before reclosing the packaging.

protection against frost

Resins can either be stored in frost-free premises or treated with a saturated brine.

in an installation

protection against dehydration

Keep columns full of water at all times.

protection against frost

Replace water with saturated brine that will provide protection down to a temperature of -17°C. For lower temperatures, use a water/glycol mixture in suitable proportions.

protection against bacteria

Before shutting down, use a prolonged backwash to remove suspended solids. Cationic and anionic resins must be preserved in their saturated state; in the case of anionic resins, this method will also prevent high alkalinity groups from hydrolysing into low alkalinity groups, resulting in a loss of capacity.

Cationic exchanger beds can be filled with a 0.5% formol solution. We recommend checking the concentration at regular intervals and in ensuring that it does not fall below 0.2%.

Anionic exchanger beds can be filled with a 0.1% quaternary ammonium salt solution.

The solution consisting in filling the installation with brine of at least 200 g · L–1 is also effective; this method also provides protection against frost and hydrolysis.