SOI/DT 2006-01 dmm
11/71
599 37 47-13
3.6 Water
Water is the most important element in the washing process, and is fundamental to the final result. Ideally,
the water used for the wash should have the following composition:
Clear and transparent, a low level of hardness, absence of manganese, low iron and mineral salt
content.
The water may contain various substances (mineral and others) in varying quantities:
Fe (iron), Mg (manganese), Si (silicon), Na (sodium), Ca (calcium), K (potassium).
Some of these substances, if they exceed a certain level or are present in combination, may lead to the
formation of ferrous oxide, which causes the well-known phenomenon of "rust spots".
In addition, if present in excessive quantities in the water, some substances may react with the chemicals
contained in the detergent, altering its characteristics and preventing it from performing with full efficiency.
During the heating phase, calcium and manganese - if present in excessive quantities - react at
temperatures in excess of 60°C, producing calcareous substances that, suspended in the washing solution,
may adhere to the fabrics.
These calcareous substances may precipitate, forming a scale build-up on the internal components of the
appliance (tub, heating element, filter body etc.).
3.6.1 Formation of lime scale and ferrous oxide
As rain falls through the atmosphere, it dissolves the gases that it contains: when it comes into contact with
carbon dioxide, the rain absorbs the gas and transforms it into droplets of dilute carbonic acid (H
2
O+CO
2
=
H
2
CO
3
).
When the carbonic acid falls onto calcareous rocks, it reacts with the limestone to form a solution of Calcium
bicarbonate (Ca(HCO
3
)
2
.
The problems of rust spots deposited on the washing are due mainly to the presence of ferrous oxide in the
water. It is difficult to determine the quantity of ferrous oxide originally present in the water and the quantity
that forms by reaction. If a filter with a very fine mesh is installed on the tap supplying the appliance, ferrous
residue will be deposited after only a few days. This is one of the major obstacles to achieving a satisfactory
washing result.
3.6.2 Hardness
of
the
water
According to current conventions, the hardness of the water refers to the concentration of calcium and
magnesium ions. In general, a distinction is made between total hardness, permanent hardness, temporary
hardness, alkaline (carbonate) and non-alkaline hardness.
The total hardness indicates the concentration of calcium and magnesium, while the temporary hardness
refers only to magnesium and calcium bicarbonate, which precipitate when the water boils.
Permanent hardness is caused by all the salts which, after boiling, do not precipitate in the same way as
carbonates, but instead remain suspended in the solution (thus including sulphates, chlorides and calcium
and magnesium nitrates).
The alkaline (or carbonate) hardness refers to soluble bicarbonates, hydroxides and carbonates. The excess
hardness with respect to the alkaline hardness is referred to as the non-alkaline (non-carbonate) hardness.
The hardness of the water is caused by soluble calcium (Ca) and magnesium (Mg) salts, expressed as
calcium carbonate, measured in "°F", and calcium oxide, which is measured in "°D".
From a toxicological viewpoint, hard water does not appear to be harmful to human beings. On the contrary,
it has been observed that the presence of calcium and magnesium helps to prevent certain illnesses such as
hypertension and cardiac arrest.
However, at an industrial level, hard water may cause scale to form on mechanical parts due to the
presence of carbonates, sulphates and alkaline-terrous metal silicates. This scale build-up can significantly
reduce the efficiency of a machine. Especially at risk are components such as heat exchangers, boilers,
domestic appliances (washing machines, dishwashers etc.), because the reaction is endothermic. In other
words, the formation of carbonates is facilitated by an increase in temperature. In the dyeing industry,
calcium and magnesium may cause certain colouring agents to precipitate, thus causing uneven distribution
of the colour on the fabrics.
