A3
Water treatment in the household
The
usual
method
for
softening
in
the
household
is
the
partial
removal
of
hardness
formers
by
ion
exchange.
A
partial
flow
of
the
water
is
passed
through
ion
exchange
columns
and
the
hardness
formers
calcium
and
magnesium
are
completely
removed.
This
partial
flow
is
then
mixed
again
with
the
main
flow,
which
contains
the
original
hardness, so that the water has a lower hardness after mixing.
You can carry out a simple mixing calculation here:
e.g.:
Main stream
50 %
22 °dH
3.92 mmol/L
Secondary flow
50 %
0 °dH
0 mmol/L
Total flow
100 %
11 °dH
1.96 mmol/L
Softening in the household
Softening in the household is referred to as post-treatment of the drinking water, which may have already been treated centrally before distribution. If the water has already been softened at the waterworks, this is referred to as centralized softening.
How an ion exchanger works
The ion exchanger (in this case a cation exchanger) consists of an ion exchange resin that contains exchangeable ions. In the usual case, these are sodium ions Na + . The hardness formers Ca 2+ and Mg 2+ are completely exchanged for these sodium ions, so that the water then no longer contains any hardness formers, but the sodium concentration in the water is increased by an equivalent amount.
Figure: Diagram of partial flow softening
The
ion
exchanger
has
a
certain
capacity
up
to
which
it
can
absorb
the
hardness
formers. When this capacity is exhausted, it must be regenerated.
Common
salt
NaCl
(sodium
chloride)
is
used
for
regeneration
(ion
exchanger
in
the
Na
form).
The
Ca2+
and
Mg2+
ions
in
the
ion
exchanger
are
exchanged
for
Na+
ions
again
and
the
Ca2+
and
Mg2+
ions
as
well
as
the
chloride
ions
Cl-
are
discharged
into
the waste water. The ion exchanger is then ready for operation again.
In
household
appliances,
this
regeneration
runs
automatically.
The
operator
only
notices a water drain. However, salt must be topped up regularly.
Cation
exchangers
can
also
be
regenerated
with
hydrochloric
acid.
However,
this
type
of regeneration is not suitable for private households.
Figure: Ion exchanger in operation
Figure: Ion exchanger during regeneration
Softening in the dishwasher
Softened water is also used for the last wash cycle to ensure that no limescale stains remain on the dishes in the dishwasher's final rinse. There is an integrated ion exchanger cartridge in the dishwasher for this purpose. The process is similar to the softening described above. The dishwasher must also be regularly topped up with salt for regeneration. Some dishwasher tabs (3 in 1) often make the addition of salt superfluous.Water hardness in the dishwasher
Hard water in the washing machine leads to hard laundry when using soaps. However, limescale precipitation on the heating elements when the water is heated is particularly problematic here too. However, the water is not softened in this case, but rather so-called hardness stabilization is carried out. The hardening agents remain in the water, but are converted into a form in which they cannot precipitate. Substances are added to the detergent (complexing agents) which bind the hardness formers to themselves in a complex bond and thus prevent them from precipitating. Such complexing agents are, for example, polyphosphates, sodium aluminum silicates and nitrilotriacetate (NTA). These substances are harmful to the environment to varying degrees.Softening
If limescale has built up in household appliances for heating water, it must be removed regularly to maintain their functionality. This is particularly important for heating coils, as heat transfer is hindered and energy consumption is increased. In principle, decalcification takes place by reversing the precipitation reaction. If the pH value in lime-saturated water is increased (decrease in H+ concentration), lime precipitates. If the pH value is lowered (increase in H+ concentration), limescale dissolves again. Acids are therefore used to dissolve limescale. The easiest way to do this in the household is with acetic acid (vinegar). Household vinegar (5%) is normally sufficient. Descaling with vinegar essence (25%), which can also be diluted accordingly, is less expensive. For sensitive appliances, such as fully automatic coffee machines, you should use the agents recommended for this purpose. These often contain citric acid or lactic acid. Household cleaners for removing limescale stains also have an acidic pH value. The effect of the acid during descaling can be recognized by the carbon dioxide released, which foams up violently in some cases and forms gas bubbles.
Figure:
Chemical reaction sequence
during decalcification
Figure: Dissolution of lime by acid with CO
2
formation
In
order
to
compensate
for
the
disadvantages
of
hardness
in
drinking
water,
water
treatment is often carried out in households with hard drinking water.
This
is
referred
to
as
post-treatment
of
the
drinking
water
compared
to
centralized
treatment at the waterworks.
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