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Operation of ion-exchange filters

The complete operation cycle of the ion-exchange filter consists of the following stages:

  • loosening;
  • regeneration (supply of the regeneration solution);
  • backwash;
  • filter run.

The "loosening" stage is required to remove suspended solids accumulated in the ion-exchange resin layer during the water treatment and resin debris built up as a result of ion-exchange resin destruction; presence of such solids and debris results in the increase of pressure difference in the ion-exchange resin layer. Moreover, the loosening performed by the upstream water flow makes the layer less compact and facilitates access of the regeneration solution to the ion-exchange resin beads. During the loosening stage the ion-exchange resin layer shall expand by 30-40%, at the same time its effective size fraction shall not be removed from the filter. Usually the loosening is performed during 20 minutes by wash water of previous regeneration at a supply rate of 2.5-3.0 dm3/s*м2) over the filter cross-section.

Regeneration of each filter is carried out by the solution of the corresponding reagent with the certain concentration. The supply rate of the regeneration solution depends on the intended use of a filter. For regeneration of Na-cation exchange filter 7-8% NaCl regeneration solution is supplied at a rate of to 4-6 m/h, for regeneration of anion-exchange filters 4% NaOH solution is supplied with the same rate. For regeneration of H-cation exchange filters 1.0-1.5% H2SO4 solution is supplied at a rate of not less than 10 m/h to avoid cation-exchange resin "hardening". 

To increase working exchange capacity of H-cation exchange filters the step-by-step regeneration is used, where 1.0-1.5% H2SO4 solution (about 50% of required consumption of commercial H2SO4 acid) is used as the first portion of regeneration solution and the concentration of the following portions is stepwise increased up to 5-6%. To save regents usually the final portions of the regeneration solutions are discharged to the tank for re-use during the following regeneration, so called "complex" regeneration. For the complete demineralization NaOH regeneration solution initially is supplied through the strong base anion-exchange resin and then through the weak base anion-exchange resin that allows to obtain required alkali surplus for A2 filters and significantly save the reagent. Reagent solutions are usually prepared for each group of filters from the own-produced permeate. Depending on the reagent consumption and concentration the duration of its supply amounts to 15-30 minutes.

When supply of the regeneration solution is complete, the backwash of ion-exchange resin layer is performed to remove regeneration products and regeneration solution residuals. This process is performed in two stages:

  • through the regeneration line with its simultaneous washing while the strong reagent is shut off;
  • by incoming water for this filter.

The final portions of wash water are collected into the tank for re-use during the loosening stage or to prepare the regeneration solutions that helps to save in-house water. The backwash of Na-cation exchange filters is finished when the target hardness value is achieved, and backwash of H-cation exchange primary filters is finished when the acid value is archived that exceeds the average acid value per operation cycle by 0.1 mg-eq/dm3. The backwash of anion-exchange primary filters is finished when the target acid value of the permeate is achieved and the chlorides concentration in the permeate does not exceed 1-2 mg/dm3, and backwash of secondary filters is over when the salt content of wash water is less than 2-3 mg/dm3 or silica content is less than 0.02 mg/dm3. More precise parameters of loosening, regeneration and washing processes are determined during the set-up of water treatment plants, then such parameters are put into the parameter chart based on which the equipment is operated.

When the backwash is complete ion-exchange filters may be put in operation or kept in standby mode.

* The text is taken from:
A.S. Kopylov, V.M. Lavygin, V.F. Ochkov, "Water-treatment in power energetics"
(M. MEI Publishing House, 2003)