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7.1.2.      Trend in urban wastewater treatment

During the last century increased population growth and increased wastewater production, coupled with a greater percentage of the population being connected to sewerage systems, initially resulted in most European countries in increases in the discharge of pollutants into surface water. Over the past 20 to 35 years, however, the biological treatment (secondary treatment) of waste water has increased, and organic discharges have consequently decreased throughout Europe. During the last 20 years tertiary (advanced/more stringent) treatment with nutrient removal (phosphorus and nitrogen) has been introduced at many waste water treatment plants resulting in markedly lower nutrient discharge to receiving waters.