Page 77 - Soil and water contamination, 2nd edition

64 Soil and Water Contamination

Box 3.I Wastewater treatment plants

Wastewater purification in treatment plants occurs in multiple stages. The purpose of
the primary mechanical treatment is to reduce the amount of organic solids in the water
through settling and filtering. Primary treatment can reduce the solid particles by 30 to
60 percent and oxygen- demanding waste by 20 to 40 percent (Marsh and Grossa, 2002).
Secondary biological treatment involves a further settling and filtering of the wastewater,
plus aeration to accelerate the oxidation of organic matter by bacteria. Primary and
secondary treatment together can remove up to 90 percent of the oxygen-demanding
waste. However, most secondary treatment systems remove only 50 percent of nitrogen,
30 percent of phosphorus and even lower percentages of heavy metals and organic
compounds. To remove these remaining pollutants, a tertiary treatment is applied, which
includes the following actions (Marsh and Grossa, 2002):
• flocculation and settling to remove phosphorus and suspended solids;
• chemical adsorption of organic compounds;
• advanced filtering such as reverse osmosis to remove dissolved organic and inorganic
substances;
• application of disinfectants such as chlorine, ozone, or UV light to kill pathogenic
bacteria and some viruses.

Though tertiary treatment is effective and can usually remove up to 95–98 percent of the
pollutants, it is very expensive.

In modern western countries, a high proportion of households and industrial premises are
connected to sewage systems and wastewater treatment plants. In Northern and Western
Europe the connection to wastewater treatment plants was 80 percent on average in
1987 (EEA, 1998, Farmer, 1999) (see Table 3.Ia) and this number is still increasing. The
proportion of households connected to tertiary treatment was considerably lower, but EU
legislation was passed in 1991 to promote the ongoing upgrade from secondary to tertiary
treatment works.

Table 3.Ia Urban wastewater treatment in EU member states (in 1987) (Farmer, 1999).
Country Total Connection Rate*: Mechanical treatment
Sewerage and treatment in treatment works only
Denmark 98 % 8 %
France 50 % No data
Germany 90 % 2 %
Greece No data 1 % (1985)
Italy 60 % No data
Luxembourg 91 % 14 % (1985)
Netherlands 89 % 7 %
Portugal 11 % 4 %
Spain 29 % (1985) 13 % (1985)
United Kingdom 84 % 6 %

* Note that the households that are not connected to wastewater treatment plants do not necessarily discharge the untreated
wastewater into the surface water. Most of the wastewater is treated locally in septic tanks.

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