2009_Book_FoodChemistry

23 Drinking Water, Mineral and Table Water
23.1 Drinking Water
Drinking water should be clear, cool, colorless
and odorless, free from pathogens (low in microorganisms),
perfect with regard to taste, cause
no materials corrosion, and contain soluble substances
only in narrow limits and minerals normally
in concentrations of less than 1 g/l. In individual
countries, criteria have been defined by
law for the quality of drinking water, especially
limiting values for microorganisms and contamination.
As an example, limiting values stipulated
in the German decree on drinking water are
presented in Table 23.1.
Drinking water is recovered from springs,
groundwater, and surface water. In sparsely populated
areas, springs and brooks provide water
that can be used without further pretreatment.
Frequently, however, the water available does not
fulfil the requirements and must be laboriously
purified.
In dry areas, drinking water is obtained by desalting
brackish or sea water. The usual processes applied
are reverse osmosis with the use of semipermeable
membranes for slightly saline brackish
water and multistage evaporation, mainly as flash
evaporation, for sea water.
23.1.1 Treatment
To remove suspended particles, the water is first
filtered through gravel and sand layers of different
grain size. Humic acids, which may color water
yellow to brown, are flocculated with aluminium
sulfate. After clarification, the quality of the water
is improved still further, if required, by the application
of the following processes.
Water should not contain more than 0.2mg/l of
iron, which is present as the bicarbonate, and
0.05 mg/l of manganese (Table 23.1). The iron
can be eliminated as iron (III) hydroxide by aeration.
In this process, manganese also precipitates
as MnO 2 if the pH is higher than 8.5. Biological
processes have also been developed for deferrization
and demanganization. Free carbonic
acid must be removed because it attacks pipes.
The deacidification process applied depends on
the hardness of the water and on the concentration
of free carbonic acid. The usual process involves
aeration and filtration through carbonate
rock (e. g., marble or magnesite).
The disinfection of water is mostly achieved
by chlorination or ozonation. At a pH of 6–8,
the chlorine gas passed into the water forms
practically only HClO and ClO − which, together
with the dissolved Cl 2 , are expressed as free
chlorine. In the case of superchlorination for
the killing of very resistant microorganisms,
the excess chlorine (>0.1mg/l of free chlorine)
must be withdrawn with the help of SO 2 ,
Na 2 SO 3 ,Na 2 S 2 O 3 and filtration through calcium
sulfite or coal. Disinfection with ozone has
the advantage that due to its decomposition
into oxygen, no chemicals remain in the water.
Interfering odor- and taste-active substances
are eliminated by filtration through activated
carbon.
Overly high conventrations of nitrate (limiting
value in Table 23.1) can be reduced by bacterial
denitrification, ion exchange, or reverse osmosis.
The fluoridation of drinking water is discussed
in 7.3.2.10.
23.1.2 Hardness
The total water hardness refers to the total concentration
of alkaline earths calcium and magnesium
in mmol/l. The concentrations of strontium
and barium, which are usually very low, are not
considered. The following is valid for conversion
to German degress of hardness ( ◦ d): 1 mmol/l
hardness = 5.61 ◦ d. Factors for conversion to the
degree of hardness of other countries are given in
Table 23.2.
H.-D. Belitz · W. Grosch · P. Schieberle, Food Chemistry 986
© Springer 2009