Water is an absolutely essential part of nearly all cleaning processes. It is the basis for automated dishwashing. Water transports the cleaning agent and rinse aid, it enables the items being washed in the dishwasher to be cleaned mechanically, and — in combination with the cleaning agent’s properties — it holds particles of dirt suspended in a cleaning agent solution. This prevents the dissolved particles from being redeposited on the surface of the items being washed. Water also transfers heat to the items being washed, ensuring perfect rinsing and drying results.
Optimal washing results cannot be achieved without the proper water quality. That makes it especially important to adjust the water quality to meet commercial dishwashing requirements.
Water can contain both solid materials and dissolved substances. Examples of solid materials include sand, rust and tiny particles of dirt from the plumbing system, which can damage the dishwasher or even cause parts like the magnetic valves to fail. The installation of a suitable filter system can help protect against that. Dissolved substances include gases, minerals, salts and organic components. Dissolved gases are primarily those contained in the air: nitrogen, oxygen and carbon dioxide. They have no effect on the results of washing.
Minerals and salts do have a considerable effect on the quality of the water. If the water contains high levels of minerals and salts, this can cause spots to form on the items washed as well as corrosion and deposits.
Different water qualities are generally categorised according to their overall hardness. The overall hardness of water is represented by its carbonate hardness (also called “temporary hardness”) and its non-carbonate hardness (also referred to as “permanent hardness”).
The carbonate hardness consists of calcium and magnesium ions that are generated by hydrogen carbonates. When water is boiled, they can cause lime to precipitate and create build-up of what is called “boiler scale”. The non-carbonate hardness consists of calcium and magnesium ions that are not generated by hydrogen carbonates. Examples include magnesium chloride. Even when boiled, these minerals remain in solution and do not precipitate.
Soft: less than 1.5 millimoles of calcium carbonate per litre, or less than 8.4° of German hardness (dH)
Medium: between 1.5 and 2.5 millimoles of calcium carbonate per litre, or between 8.4° and 14° of German hardness (dH)
Hard: more than 2.5 millimoles of calcium carbonate per litre, or higher than 14° of German hardness (dH)
Cleaning agents contain ingredients that prevent precipitation of minerals that form hard water deposits (e.g. GASTRO STAR FR 45). For overall hardnesses of 0.54 mmol/l (equal to 3° dH), special water treatment should be performed for reasons of cost-effectiveness. However, even water with an overall hardness of less than 0.54 mmol/l (3° dH) does not absolutely guarantee good washing results. Only if the total salt content of the water is low can spot-free cleaning results be achieved.
There are various methods of treating water depending on the water quality. They are divided into softening, partial deionisation and complete deionisation .
Softening involves using an ion exchanger to replace the hard water minerals (calcium and magnesium ions) contained in the water with sodium ions. This removes all the minerals that can precipitate as lime scale from the water. To allow the ion exchanger to perform optimally, it occasionally needs to be refilled with water softener salt (sodium chloride). It should be expressly pointed out that simple water softening still involves the possibility of visible mineral residue being left on the washed items. This will generally dissolve in water.
Since softening cannot lower the total amount of salt contained in the water itself, there is no way to avoid performing partial or complete deionisation of water with extreme salt content in order to achieve optimum washing results. Examples of this method include a two-state ion exchange system or a so-called “mixed-bed exchanger”. This removes all the cations and anions from the water. Another suitable method of deionisation is reverse osmosis, during which water is treated using a membrane.
You can find additional information about water quality and appropriate settings for spot-free washing results available for download free of charge from the website of the Arbeitskreis Gewerbliches Geschirrspülen (Working Group for Commercial Dishwashing): Praxishandbuch (Practical Handbook), Chapter 05, “Wasserqualität” (Water Quality): www.akggs.de.
There you can also find more information on commercial dishwashing with helpful tips and more detailed articles on the following subjects:
– Organising and planning dishwashing systems
– Commercial dishwashers
– Dosing technology
– Water quality
– Process chemicals
– Items made of metal
– Items made of glass
– Items made of plastic
– Environment and sustainability