Water treatment

We treat and supply more than 330 million litres of drinking water to 1.3 million customers every day.

Where does your water come from?

Around 80% of our water comes from deep underground in Dorset and Wiltshire. The remainder comes from our 12 surface water reservoirs, which are fed by streams and rivers mainly in the Somerset area.

The treatment process your water goes through depends on whether it comes from groundwater sources or reservoirs.  

Water from groundwater sources

Groundwater is the water found underground in the cracks and spaces in soil, sand and rock which is naturally replenished by surface water from precipitation, streams and rivers.

In our region, an underground layer of water-bearing rocks – such as chalk, limestone and sandstone – hold a huge amount of groundwater. These rocks are also known as aquifers.

Boreholes, which have been submerged up to 120 metres into the rocks, enable us to pump this water to the surface.

The water then arrives at our water treatment centres. Because water sourced from aquifers is usually free from impurities, it requires less treatment.

We inject the water with a solution of chlorine gas to kill off any bacteria and in some cases aerate it to replace carbon dioxide with oxygen before supplying it to our customers. 

Water flowing over rocks in nature

Water from reservoirs

Reservoirs store water from rain that falls during the wetter parts of the year in preparation for when there is less rain. They have been built for over 5,000 years.

Reservoir water needs a higher level of treatment than groundwater, so we use several different processes to purify it to the high quality required by law.

Water arrives at our treatment works and flows through extremely fine filters to remove algae, insects, leaves, branches and any other objects.

Next, the water is fed into a chamber where chemicals are added to make any particles stick together.

Air is pushed up through the water forcing the particles to the surface where they are then removed.

After getting dosed with chlorine and lime to remove certain contaminants, the water flows into fine sand filter beds which trap any remaining particles.

The remaining water goes through a filtration process which removes any pesticides and impurities that are present.

The water receives a final dose of chlorine to kill any remaining germs and bacteria before it flows into our storage reservoirs and is supplied to our customers.