Oset Water Treatment Plant

Oset Water Treatment Plant

Water Supplies in Oslo Oslo Water and Sewerage Works (OWSW) supplies the population and industry within the city boundaries with drinking water. OWSW also supplies the municipality of Ski and has emergency supply agreements with Nedre Romerike Waterworks and the municipality of Oppegård. In total OWSW supplies water to 535,000 persons in Oslo and 20,000 persons in Ski. The raw water is supplied from the large watersheds in the forests surrounding Oslo. The water is treated at four plants, Oset, Langlia, Alunsjøen and Skullerud. On average, OWSW produces 110 million cubic metres of drinking water every year. Skjærsjøelva Maridalsvannet Skarselva OSET The Water Supplied to Oset The Maridalen watershed is Oslo s most important water source, with a catchment area of 252 square kilometres. The tributaries to of the lake Maridalsvannet are the rivers Skjærsjøelva and Skarselva. Maridalsvannet is the intake reservoir for Oset Water Treatment Plant and has an annual average inflow of 184 million cubic metres of water. Every year OWSW extracts around 95 million cubic metres for drinking water purposes. The surplus water runs to the river Akerselva. Akerselva

Oset supplies the city with 3000 litres of water per second. Oset Water Treatment Plant Oset is Oslo s largest plant for drinking water production and has the capacity to supply the whole city. Under normal conditions, the plant covers 85% of the city s drinking water needs. This means 95 million cubic metres per year, 250,000 cubic metres a day or 3,000 litres per second.

10 1 2 3 1 Main raw water intake at 35 m depth; also a reserve intake at 15 m. 2 Coarse filter and fish barrier with 4 mm mesh openings. 3 Raw water pumping station with 12 centrifugal pumps. Capacity 11,800 litres per second and 5 m lifting height. 4 Aeration plant with 10 cubic metres per second capacity. The water s oxygen content is increased from 85 to 95%.

The Treatment Process 12 10 10 10 11 To eastern parts of the city via high level underground basin. 4 5 7 8 9 6 To central and western parts by gravitation. 5 Pre-chlorination Two retention basins with a total capacity 6 of 18,000 cubic metres. 7 30 microstrainers in perlon fabric with a mesh size of five thousandths of a millimeter. Capacity is 300 litres per second per strainer. Rinsing water consumption is approximately 2.5% of water flow. The microstrainers remove some 5 tons of particles each year. 8 4 post-chlorination and balancing basins, each with a capacity of 12,000 cubic metres, 10 Ventilation plant with air filter. Temperature in the underground corresponding to about five hours chambers is approximately 10 C all year consumption. round. 9 High-pressure drinking water pumping 11 Control room for monitoring the station with eight pumps, each with a treatment process. capacity of 315 litres per second and 125 m 12 Administration building with offices, lifting height. changing rooms and canteen.

Drinking Water from Oset The untreated water from the lake Maridalsvannet is clear and without noticeable odour, taste or colour. It therefore undergoes only a simple treatment with aeration, microstraining and disinfection (chlorination). To guarantee a healthy water, small quantities of chlorine gas are added: 0.3-0.5 milligram per litre. The acidity (ph) of the drinking water is slightly below the requirements of the Drinking Water Regulations. As the governmental regulations also require two hygienic barriers as a safety measure, Oset needs to be rebuilt. The Water Distribution in Oslo The water leaves Oset and joins a dense network of pipes which supply 85% of Oslo s consumers. The other 15% are divided between the south-east of the city, and two smaller areas in the north and in the west. Two main pipes supply the inner city and western areas. The eastern parts of the city are supplied via a high level underground reservoir. To bring the water all the way to the consumer, Oslo has an extensive distribution network. Some 1,550 kilometres of public water mains. 29 pumping stations transport water from lower to higher areas. 153 reduction valves and 20 distribution reservoirs provide even water pressure throughout the day.

The Production Plants and Their Supply Areas 1 Water- Intake lake/ Percentage treatment Catchment area of annual plant production 4 2 1 2 LANGLIA Langlivann/ 2% Nordmarka (Max 7% when reserve capacity is used) ALUNSJØEN Alunsjøen/ 3% Lillomarka 3 SKULLERUD Elvåga/ 10% Østmarka 4 OSET Maridalsvannet/ 85% Nordmarka 3

A quick look back... Construction work in the thirties: the construction of the city s first tunnel with personnel access, for public installations including water and sewage pipes. The Maridalen watershed have always been the main source of Oslo s water supply. The oldest public water supply known in Oslo was established after a conflagration had devastated the city in 1624. It was a wooden pipe from the river Akerselva to Akershus Fortress. Gradually the pipe network was extended - for instance, to the Royal Castle in 1845, which was the last wooden pipe that was installed. Over time, the water intake was moved higher up the Akerselva due to increased industrialisation, causing for instance sawdust in the water. In 1866, Oset in Maridalen became the main intake and the lake Maridalsvannet was conserved for drinking water supplies. The first treament plant was taken into use in 1924. This can still be put into operation at short notice as a reserve. The new Oset Water Treatment Plant was opened in 1971. Oslo kommune Water and Sewerage Works Phone +47 02 180