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Basically there are two kinds of raw water that are used for water supply in the UK, one being 'ground water' from wells, boreholes or springs and 'surface water' which is drawn from impounding reservoirs and rivers. Portsmouth Water is fortunate to benefit from the vast expanse of the South Downs. Formed of chalk, the porous nature of the Downs allows the rainwater to seep down through the strata and flow through fissures towards the sea. During the summer months much of the rain will quickly evaporate or be absorbed by vegetation but the winter rainfall soaks into the ground and so replenishes the groundwater reserves. All year round the water flows through the cracks and fissures on its way towards the sea. To take advantage of the water flowing through the chalk, Portsmouth Water have drilled boreholes and wells at 18 locations throughout the catchment area. Up to 3.5m in diameter, and some as deep as 137m, they intercept the water flowing underground and powerful pumps bring the raw water to the surface.
Situated at Havant and Bedhampton, near the shore of Langstone Harbour is a unique set of 25 springs in two groups, just 6m above sea level. Thought to be the largest spring water supply in the country, possibly in the world, they help keep the cost of our water one of the lowest. 23 of the springs are used by the Company. The yield of the springs varies according to the time of year. In winter when rainfall replenishes the chalk quicker, the yield can rise as high as 170 million litres of water per day, but during the autumn of a 'drought year' this can fall to around 53 million litres a day. Some 30% of the water supplied by the Company comes from the Havant and Bedhampton springs. For Geological details of the Havant and Bedhampton Springs, click here. [file size 200k]
An abstraction on the River Itchen provides the Company with up to 45 million litres a day. For environmental reasons the river intake is sited just above the tidal limit at West End near Southampton. Not far from the treatment works is a large raw water reservoir holding up to 135 million litres. This safeguards the water supply in the case of fluctuation in river quality as well as accidental pollution of the river. How our water is treated Boreholes and wells - ground water Water taken from underground wells or boreholes is usually of high quality as it has been naturally filtered during its journey through the soil and rocks. At most sites a carefully controlled dose of chlorine gas is all that is required to guard against bacteria. At some sites we have to give the raw water a heavier dose of chlorine to kill off any bacteria; afterwards we reduce the chlorine level before the water goes into supply. Springs - ground water Water taken from the springs at Havant often contains some small particles and therefore goes through a filtration process at our Farlington Treatment Works to remove them. River water - surface water The river water by comparison is dirtier. It undergoes extensive treatment which is carefully monitored. Chemicals are added which bind the small pieces of debris in the water together. These form a blanket of sediment in the settlement tanks and as the water passes through the blanket so it removes further particles. Any remaining particles are removed by a filtration process. Finally chlorine is added to ensure a safe and wholesome water supply. This is done at our River Itchen Treatment Works. For further more detailed information on our Treatment Plants, click on the links below:
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