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Centralised softening of drinking water

Good water

In the last years centralised softening of drinking water is increasingly being discussed and realised - in particular in areas with a high hardness level (greater than 3.5 mmol/l). In particular a water supply with a hard water has negative effects at the consumers:

  • causes an increase in the cleaning work required at surfaces,
  • results in a notable increase of energy consumption at heat exchangers,
  • notably increases the amount of detergent used,
  • reduces the service life of piping and fittings.

Partially increased heavy metal emissions from the drinking water installations are furthermore ecologically relevant.

For this reason consumers often use local on-site water softening devices that usually function according to the cation exchanger principle. However, these systems often cause high follow-up costs in the range of € 0.8 to 1.0/m³ of decalcified water (depreciation across 10 years, salt consumption and maintenance). In addition, the sewage is polluted with regeneration salt.

One way out is centralised softening in a water treatment plant for which various procedures are available:

  • Slow decarbonisation (SDC)
  • Rapid decarbonisation (RDC)
  • CARIX process
  • Nano-filtration process (NF) and reverse osmosis (RO)

Relevant factors for the choice of the process are, in addition to the composition and the quantities of the contents, in particular also the pre-treatment, the treatment capacity, the recycling of the waste materials, the conditions for the discharge of the unavoidable waste water as well as any post-treatment that may be required.

Rapid decarbonisation in a fluidised bed

Rapid decarbonisation is considered the most inexpensive and environmentally friendly process for centralised softening of drinking water. In rapid decarbonisation the lime is removed in a so-called fluidised bed reactor.
Advantages of softening in a fluidised bed

  • Softening from hardness range 4 to 2
  • Avoiding or reduction of scale in warm-water systems
  • Low energy requirement
  • More pleasant and skin-tolerant water
  • Lower consumption of regeneration salts
  • Lower washing agent and detergent consumption
  • Low waste-water pollution
  • Magnesium contents stay constant
  • Pellets can be used in agriculture
  • Additional costs for end consumer: Only € 0.10-0.15/m3

The dosing of alkalis (depending on the raw water quality - sodium hydroxide or lime milk) disturbs the lime-carbonic acid balance so that the lime precipitates and grows into pellets at crystal nuclei. The pellets swim in the reactor that has an upward current, grow and are removed from the reactor from time to time. The systems operate fully automatically.

RDC with sodium hydroxide dosing

In the so-called "Amsterdam process" softening is carried in the fluidised bed reactor by dosing sodium hydroxide (NaOH). An extremely intensive mixing-in is achieved by means of special nozzles that causes the lime to precipitate spontaneously and grow into pellets. The sodium hydroxide increases the sodium contents of the water slightly. Garnet sand is usually used as the promotion material.
Advantages of this process:

  • The exit turbidity is extremely low
  • As a rule no downstream filter stage is required
Centralised softening with sodium hydroxide dosing
Centralised softening with sodium hydroxide dosing

RDC with lime milk dosing

In the newly developed process softening in the fluidised bed reactor is based on the dosing of lime milk (CaOH). Dosing is carried out here via dosing lances directly into the reactor inlet. Here too the lime grows into pellets. In lime milk dosing a filter system has to be located downstream for clarification.
Advantages of this process:

  • Sodium contents of the water remains constant
  • Less expensive than the Amsterdam process
Centralised softening with lime milk dosing
Centralised softening with lime milk dosing
Information centre water treatment
ContactManfred Brugger

Tel. +49 751 6009-47
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Further information

In the field of centralised softening of drinking water we cooperate with the Ingenieurbüro Eppler from Dornstetten, Germany. More information via the above link