EARLIER this year householders in one area of the UK were warned not to use tap water after it was found to contain abnormally high levels of chlorine.

Severn Trent Water warned 3,700 Derbyshire households not to use any tap water after the utility discovered abnormally high levels of chlorine in one of the local

In the UK we are lucky to have water supplies on tap. Bradford has two water treatment works which feed the whole of the city: Chellow Heights in Haworth Road, Bradford and Graincliffe in Otley Road, Bingley.

Water arriving at the works comes from Scar House and Angram reservoirs in Upper Nidderdale, Upper Barden reservoir near Skipton and the River Wharfe at Lobwood. Water extracted from the river is pumped to Chelker Reservoir between Ilkley and Skipton. Chellow Heights has its own reservoirs, beside the treatment works.

Across the Yorkshire region reservoir levels are around 97 per cent full. It takes just 36 hours for reservoir water to leave the treatment works.

“It is a vital resource and we are hugely privileged,” says Dave Marshall, Yorkshire Water’s product and process area manager. He explains the process whereby water reaches our taps.

When “raw” water arrives at the treatment works it flows through a huge metal screen which catches large leaves, twigs and other large items before it enters a mixing tank where a coagulant - a chemical which produces positive charges - is introduced. This acts like a magnet, attracting loose particles within in the water and causing them to stick together, forming larger masses which are more easily removed. “It changes from a liquid to a thicker sludge-like material,” says Dave.

This substance, known as ‘floc’, then goes through sedimentation, where the larger particles are allowed to settle, so the water on top is cleaner.

“We add chlorine which oxidises the water, allowing it to be filtered,” says Dave. “We calculate the amount of chlorine we add. It acts as a disinfectant, killing all bugs in the water.”

He adds: “We use about a tenth of the amount you would use in a swimming pool. You can’t taste it but in some instances you may get a slight smell.

“There are a lot of impurities in the water, and chlorine helps to remove them,” explains Dave.

Filters filled with sand remove any remaining smaller particles from the water. Manganese, which occurs naturally in the water promoting the growth of bacteria, is also removed.

Dangers which you might find from ‘raw’ water entering reservoirs can include bacteria such as E.coli, and the microscopic parasite Cryptosporidium that can cause respiratory and gastrointestinal illness, and Salmonella

“This can happen due to factors such as contamination by animals and chemicals contained in pesticides,” explains Dave. Some pesticides cause several types of bacteria to thrive and spread.

Water making its way to Chellow Heights can take up to 16 hours to arrive. “If it is coming from Pateley Bridge it is travelling more than 30 miles down the Nidd Aqueduct, “ says Dave.

An amazing feat of engineering using overground channels, tunnels and pipes, the aqueduct uses only gravity to move water, supplying 21 million gallons (95,000 cubic metres) of water each day to the treatment works.

It goes on to two large clean water tanks at Chellow Heights, each containing around 150million litres. “This allows the chlorine to do its job and remove all the bacteria.”

From leaving Scar House to gushing out of the tap in a Bradford household can take up to three days. “It is generally about 36 hours before it leaves the treatment works, then it travels on to supply many places.

Regular samples are taken to guarantee water quality. “We make in the region of 450,000 tests every year,” says Dave. “Water treatment in the UK is controlled by some of the tightest regulations in the world.”

Different reservoirs deliver different water of different qualities due to the ground conditions in the catchment. “The colour may be darker, mainly due to peat,” says Dave. “It is worse during periods of heavy rain or heavy rain following a dry period.”

Across Yorkshire, around 45 per cent of water is supplied from upland reservoirs. River water is harder, containing more dissolved salts, calcium or magnesium, and tastes slightly more bitter.

Water can be moved around the region via YW’s grid - a network built in response to the 1995 drought. “It enables us to move water across the region if one area is suffering from shortages,” says Dave. The grid also connects underground service reservoirs.