How we clean water
We supply 1.8 billion litres of clean drinking water to over eight million people in our region every single day. That’s because our customers always need water. However as rainfall is completely unpredictable we often rely on storage reservoirs. Here’s how we clean our water from the reservoir and beyond.
We use mesh screens (similar to a sieve) to remove sticks, water weeds and other large objects. Often, we force air into the water by running it down a tower, spraying it into the air or bubbling it. The air helps to get rid of some smells and gas from the water, and allows some dissolved metal salts to separate and to be filtered out.
At this stage there are still some very tiny pieces of matter in the water, along with colour and bacteria. We add a precisely controlled amount of a coagulant like ferric or aluminium sulphate. This reacts with the material in the water, making it all stick together to form what's known as 'floc'. To make this flocculation happen, water and the coagulant have to be mixed together very quickly and thoroughly in a special device called a 'flash mixer'.
The floc forms itself into sludge and is separated in a specially designed tank called a clarifier. This sludge layer is called the ‘sludge blanket’. To control the blanket, sludge is periodically drained off, concentrated and removed for safe disposal.
The water is filtered slowly through very fine sand or sand-like material. This removes any remaining particles.
For some river waters, we use ozone gas injection as a treatment stage. This can work alongside GAC treatment, where the water is filtered through carbon granules to take out any undesirable traces of natural organic materials. The carbon in these filters is removed and regenerated by heat as necessary so it can be used again.
The water is filtered, thoroughly cleaned and disinfected to make sure no harmful bacteria remain. A small, controlled amount of chlorine is the most effective method, and provides essential protection. This method has been used for over 100 years across the globe. In our region, 99.9% of tap water tested passes the requirements of the UK, EU and other world bodies.
Final adjustments are made to the pH levels of the water during the treatment process to make sure it's not too acidic or alkaline. This helps protect pipework, fittings and the water itself. All water is checked for quality, bacterial levels, smell and taste before it’s fed from the treatment works into a huge mains pipe network nearly 46,000 kilometres long. This forms the grid around our region, and it’s this grid that you access directly from you home every time you turn the tap on.