An extensive three-year urban hydrology programme was carried out in Finland. Data were collected and analysed on the quantity and quality of precipitation and runoff water at seven urban test catchments. During rainfall events, on the average, the proportion of surfaces generating direct runoff was found to account for 50-80 per cent of the proportion of paved surfaces in the residential catchments, and for 80-90 per cent in the city centres. Under Finnish conditions, the pollutant loads discharged to recipients with untreated runoff waters are relatively minor in comparison to those of purified waste waters, regarding organic matter and nutrients. Further measures in the Finnish sewage works should thus be focused on other factors, such as increasing the effectiveness of the existing waste water treatment plants, and decreasing the amount of leakage inflows to sewer networks.

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