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Understanding water quality
Understanding water quality

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4 Treatment of water supplies

Water quality standards are necessary to ensure that supplies are suitable for use. The quality required depends on the intended use: drinking water, for example, needs quality standards different from those for industrial or irrigation water. The World Health Organisation (WHO) has set guidelines for drinking water (1993), and so has the EU (1998) in Directive 98/83/EC (Table 4a). The standards cover:

  • bacteria;
  • chemicals such as metals, nitrate and pesticides;
  • the way water looks and how it tastes.

However, in many developing countries, which have a piped water supply only in some areas, there are no drinking water standards; water is often taken from the nearest river, and may be of very poor quality, leading to disease. A better source in these areas is pumped groundwater, as groundwater is often less polluted than surface water, but again there is no control on quality.

Table 4a EU Drinking Water Directive 98/83/EC: selected parameters and concentrations.
Guide value/mg l-1Maximum acceptable/µgl-1
pesticides, individual0.1
pesticides, total0.5
Other parameters
pH range6.5-9.5
The full standards include many other substances not listed here. 1 mg 1-1= 1000 µg l -1; 1 mg 1-1= 1 ppm;

Activity 5

This activity looks at the chemical composition of bottled drinking water. Bottled waters are often described on the label as mineral waters, a scientifically inappropriate term, as the scientific meaning of mineral water is high-TDS groundwater, which would not be suitable for drinking. Bottled water is perhaps surprisingly not subject to the general EU Drinking Water Directive, but has its own directive.

  • a.Examine the label of any bottled water you have at home (or visit a shop to examine the label of such a bottle) and write down the dissolved substances it contains, with their concentrations (EU regulations require bottled waters to give this on their labels).
  • b.Compare your list with the EU drinking water standards in Table 4. Does your bottled water exceed the EU guide values or maximum acceptable concentrations for any dissolved substance in drinking water?


Table 4b Concentrations of dissolved substances in Buxton and Perthshire natural mineral water
Concentration of dissolved substances/mg l-1
nitrateless than 0.11
p H7.4
  1. I had two different types of bottled water at home: 'Buxton Carbonated Natural Mineral Water' and 'Mountain Spring Natural Mineral Water, Perthshire'.
  2. Neither of the bottles breaches the EU guidelines for the substances listed in Table 4a.

The quality of the water required for industrial processes depends on the process so there is no general set of standards for industrial water equivalent to the standards for drinking water. High-pressure boilers require water of the highest quality, with more rigorous standards than for drinking water, whereas low-quality water, such as seawater, is usually adequate for cooling.

There are also no general standards for irrigation water; but there are three main quality criteria, as given below:

  1. The maximum TDS of water that can be used for irrigation is usually 3000 mg l-1, but this is not a precise limit, as different crops have different salt tolerances. Few fruit trees, for example, will tolerate much salt (2500 mg l-1 is usually the limit), vegetables and most cereals have a moderate salt tolerance (3500 mg l-1), and grasses, cotton and date palms have a high tolerance (6000 mg l-1).

  2. Water with a higher sodium concentration than the combined calcium and magnesium concentrations is also generally unsuitable as it may damage the soil structure. The sodium ions can replace calcium and magnesium ions in the soil, which reduces the permeability, and the soil will be sticky when wet and very hard when dry.

  3. Toxic substances, if present in more than very small quantities in irrigation water, will prevent plant growth. For example, a concentration of more than about 1mgl-1 of the element boron will restrict growth.