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Minimising the use of chemical products in the field of ultrapure water treatment

Increasing occupational safety through softening and membrane degassing processes

A wide variety of water treatment processes are available for the technical utilisation of municipal water for industrial purposes.

Depending on the application, operating conditions and the technical qualifications of the operator’s personnel, a detailed consideration of the selected process is required.

The following are examples of water softening processes and processes for reducing the carbon dioxide (CO2) content in water.

Treatment is particularly necessary for hard and very hard water in order to protect the downstream system components, such as the membranes of the reverse osmosis system, from damage caused by deposits.

The best-known methods for this include softening using ion exchangers and hardness stabilisation by adding polyphosphonic acids.

By using a water softening system with a technically accurate design and in combination with integrated online monitoring of the water hardness, softening can be reliably realised, regardless of any fluctuations in water hardness and consumption volumes. At the same time, the resin bed of a softening system has a filter effect that is not provided by chemical hardness stabilisation. The operator of the system only has to ensure that sufficient regenerating salt is added to the brine tank at regular intervals. All other processes run fully automatically thanks to state-of-the-art control technology.

The main advantages of softening over hardness stabilisation include the following aspects:

  • In the case of hardness stabilisation, the absence of the chemical product used in the ultrapure water must be verified. This is not necessary in the case of softening.
  • The use of hardness stabilisers for high water hardness must be carefully checked.
  • Adjustment to fluctuating water hardness is not necessary when realising the regeneration of the softening system via online monitoring of the water hardness.
  • The absolute value of the hardness slip through the membranes of the reverse osmosis is significantly higher when hardness stabilisers are used, but is not detectable in water softening systems.
  • The contamination of the first stage as well as the concentrate treatment of reverse osmosis with high concentrations of hardness stabilisers is eliminated with softening.
  • Significantly reduced handling of chemicals, as regular container changes are not necessary with softening.

To achieve low conductivity values, the membrane process of reverse osmosis has established itself as the first process stage. Due to the shift in the so-called lime-carbonic acid equilibrium during “desalination” using reverse osmosis, CO2 is expelled, the pH value falls into the acidic range and the conductivity of the permeate increases as CO2 is not retained by the membrane. In order to counteract possible corrosion in downstream system components and an excessive load on further treatment stages, such as the EDI, the balance must be restored. Membrane degassing or the dosing of caustic soda (NaOH) is usually used for this purpose.

The advantages of membrane degassing over the dosing of caustic soda include the following points:

  • Low space requirement and significantly reduced labour requirement because no storage containers for chemicals or container changes are necessary.
  • No need to raise the pH value to > 8.5 as with NaOH dosing.
  • Significantly reduced handling of chemicals/gases, as only a single connection is required.
  • No scaling of iron and manganese oxides on the membranes, as the installation takes place behind the reverse osmosis membrane.
  • By using membrane degassing, the Co2 limit value required for the operation of an EDI (usually < 5 mg/l) is reliably undercut. The process is therefore suitable for all processes that do not require an absolute absence of CO2.

Both softening and membrane degassing are processes that do not involve the use of chemicals. This also means that no ingredients are added to the water that are not natural components.

Complicated interventions, such as changing chemical containers or venting the dosing pump for the chemical products, are no longer necessary. Due to the elimination of these activities and the elimination of potential sources of danger, such as leaks in dosing hoses, these processes can be regarded as much more suitable from an occupational health and safety perspective and should therefore be prioritised.

It should be noted that, from an occupational safety perspective, it is always necessary to check whether a tried and tested, less hazardous process is technologically available.