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In discontinuous filtration, the filtration is periodically interrupted by a backwash or by regeneration. These are necessary in order to rinse the dirt off the medium; a waste water flow is therefore created periodically. Different media are required for different contaminants. For example, a filter kettle can be filled with:

  • Sand
  • Active carbon
  • Manganese dioxide
  • Ion exchanger resin
  • Etc.

Sand filtration

The most common application of the media filters is sand filtration.

A sand filter is a filter filled with sand in order to remove impurities from water. The water to be treated is pumped downwards through a sand bed so that the particles present are captured by the sand layer. In addition, a number of physical and biological processes occur in a sand filter that further remove various substances (Fe3+, Mn2+, NH4+, etc.) from the water. This is called iron removal, demanganisation or denitrification. The contaminants that remain in the sand bed of the filter must be washed out; this is done with a backwash.

Active carbon filtration

With active carbon filtration, contaminants are absorbed. The pores in the active carbon particles create a large active surface, which is positive for the absorption capacity. In case of saturation, active carbon must be replaced. Suspended particles in the water can block the active carbon. Sand filtration is therefore often used as a pre-treatment. Active carbon filtration is used for the removal of mineral oils, BTEX, PAHs, phenols, PFAS, chlorinated hydrocarbons/halogens, heavy metals (partly) and micro contaminants (such as medicine residues). Active carbon can also be used for decolourisation and odour/taste improvements.

Ion exchange

With ion exchange, ions present in the water are bonded by means of adsorption and resin. Ion exchange can have different applications, such as water softening (removal of calcium and magnesium ions), demineralisation, as a polishing step after reverse osmosis and to remove heavy metals.

The ions to be removed from the water can be exchanged with ions with the same charge that are present on a resin (for example, during softening calcium ions are exchanged with any sodium ions that are present), or can be completely removed by the resin through the exchange of H+ and OH- so that it is possible to make demineralised water (H+ + OH- → H2O).

When the resin in the ion exchanger is saturated with exchanged/removed ions, the resin is regenerated with a regenerating agent (acid or base). Then the process can start again.

With ion exchangers, a distinction can be made between cation and anion exchangers. The cation exchangers bind the positive ions and the anion exchangers have the capacity to bind the negative ions. These resins can be either non-specific or specific. For example, specific resins are used for the removal of heavy metals from a water flow. In addition, there can be mixed bed systems in which both cation and anion resins are present; this is often the polishing step after reverse osmosis.

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