Coming clean and cutting costs
WWT's review of water treatment during the last half-century took Peter Minting to Chew Valley reservoir, which for 50 years has supplied Bristol Water's cleanest surface water
Bristol Water began work on the Chew Valley water supply scheme after being given the go-ahead by the government in June 1950. The project was initiated to improve and expand water supplies in the south Bristol area. Bristol Water saw an opportunity to create a clean, plentiful supply of water by flooding the Chew Valley and the water company was able to persuade ministers of the necessity of the scheme in view of Bristol’s growing population.
The contract to begin construction was awarded to AE Farr, and work started in November 1950. The aim was to begin partial filling in 1953 with completion in 1955. Some people who lived in the area were forced to move, and so a re-housing scheme was set up in conjunction with Clutton District Council. During the five years it took to complete the reservoir, archaeologists worked on a number of sites in the area now flooded. Interesting finds included a Roman Villa, Bronze Age graves and medieval mills.
Budgeting for such a major project in the years following World War Two turned out to be quite a challenge, as supplies of building materials, steel and other manufactured goods were still being rationed by the government. But this did not stop Bristol Water planning for increases in demand and work continued apace until 1956 when the Chew Valley scheme was officially declared open by the newly-crowned Queen Elizabeth II. Despite limitations on capital investment water companies were still legally required under the Civil Defence (Water Supplies) Regulations to cope with any wartime conditions, including the possibility of nuclear attack during the height of the Cold War.
As part of the Chew Valley scheme a new WTW was built at Stowey to treat water from the reservoir. Stowey WTW is still working in much the same way as it did in the 1950s and is typical of 1950s process design. To this day water from Chew Valley Lake receives fairly limited treatment at the works, as the raw water in the reservoir is so clean. Most other sources used by Bristol Water require a great deal more treatment. Water entering Purton WTW, for example, undergoes several more treatment stages as the water is taken from the River Severn and is much more likely to be polluted. Purton is Bristol Water’s largest WTW, with a huge output of 165Ml/d in comparison with Stowey’s maximum 27Ml/d. Chew Valley Lake is surrounded by agricultural land but has not been identified by the Drinking Water Inspectorate (DWI) as a source at high-risk of Cryptosporidium contamination. According to Stowey’s plant manager Daniel Warren: “There are plans afoot to introduce ozonation at Stowey but these are still some way off.” Ozonation already takes place at Purton which makes the supply more expensive. Mr Warren pointed out: “It is much cheaper to use water from Stowey as it requires less treatment, so Bristol Water always tries to use as much water from here as is reasonably possible.” Chew Valley Lake is full at the moment but when supplies diminish in drier summers Bristol Water can switch to more expensive sources. This year it will not be necessary as 2000 has been exceptionally wet. 1950 was also a very wet year, in fact the wettest since 1927.
Water is pumped to Stowey WTW from Chew Stoke pumping station which is north of Chew Valley Lake. Once at the works it passes by gravity through a pair of microstrainers. The water flows through rotating drums made of a fine steel mesh (30µm), which traps the particles. If the mesh starts to block up the drums automatically rotate faster to maintain the necessary flow. Jets of water directed at the top of the drums clean them at each revolution.
Microstraining is followed by a zigzag-shaped primary settlement channel which is baffled to promote settling. After this stage the process has undergone some change. Originally the water passed into a giant cistern, but this has now been abandoned as the mechanics became unreliable. Now flow to the main filtration stage of the works is controlled by an automatically actuated Rotork valve.
Settling is followed by a series of eight rapid gravity filters (RGFs) of a design widely used at WTWs throughout the 1950s and 1960s. Similar filters can be seen at many WTWs of this age, and many water companies have not replaced them although many have been refurbished. Backwashing at Stowey is automatically controlled but is relatively infrequent due to the high quality of the incoming water. Cleaning is carried out by passing compressed air upwards from high-pressure nozzles in the base of the tanks to loosen the sand and gravel. Clean water is then pumped through the bed and drained away to remove waste material trapped in the filter.
After the RGFs water enters four slow sand filters, two of which were added after the initial construction. The 2.4m-deep ponds each cover over 2,000m2 and like the RGFs contain a top layer of sand and a gravel base. Freshwater organisms which live on top of the fine sand form a slime layer known as the Schmutzdecke. A host of microorganisms live in the sand bed which help to break down any unwanted substances and reduce nutrient levels. Cleaning of the sand beds takes place at intervals of around 90 days, the sand being taken away and cleaned at Barrow WTW by contractor RE Hill.
Once through the metre-thick sand bed the water enters a series of channels which lead onto the chlorine contact tank. Chlorination for one hour kills any remaining bacteria and viruses. Excess chlorine is removed by the addition of sulphur dioxide to achieve a residual level of 0.2-0.4mg/l. Orthophosphate is also added to prevent lead dissolving into the water from lead pipes.
Low lift pumps then take the treated water to the clear water tank. Water in the clear water tank, which also includes some treated water from Sherborne Spring, is then pumped to the service reservoirs. Eleven semi-automatic electric pumps deliver the water to service reservoirs at Knowle, Coley, Clutton and Stanton Wick. As Stowey is no longer manned it is essential that the plant can operate effectively if equipment fails. Mr Warren, who visits the works regularly along with eight other sites said: “The pumps have been known to overheat, but if one fails a standby pump will automatically take over.”
Bristol Water can make use of numerous water sources in Avon and Somerset. As the area is largely underlain by limestone aquifers, boreholes and natural springs are commonly used in addition to surface water supplies. There are three large reservoirs in Bristol Water’s territory, Chew Valley Lake, Blagdon Lake and Cheddar Reservoir. Cheddar Reservoir is filled from borehole sources but Blagdon, like Chew Valley Lake, was created by the damming of a small river. Blagdon, which is around 10km to the west of Chew Valley, is much older and was first filled in 1891. It took a team of workers, horses and carts eight years to complete. The 4.8km2 Chew Valley Lake, which has a catchment area of 50km2, holds around 20,000Ml while the 1.8km2 Blagdon Lake holds 10,000Ml.
Although the reservoirs provide the cleanest supply of surface water, they are insufficient to meet modern demand. By far the biggest single water source for Bristol Water is the River Severn. The Severn feeds the Sharpness canal, from which Bristol Water is licensed to take up to 235Ml/d, more than half the company’s available supply. Up to 165Ml/d of this is treated at Purton WTW, which is Bristol Water’s most advanced plant. The process includes ferric sulphate coagulation, RGFs, two stages of ozonation and chlorination and granular activated carbon (GAC) filtration to remove complex hydrocarbons.
In common with other water companies Bristol Water has cut the number of personnel responsible for asset maintenance. Downsizing has been driven by the water company’s need to save money and by technological improvements which allow an increasing amount of plant management to be carried out from a distance.
Stowey’s control room is not one of the most advanced but the monitoring systems which have been introduced since the 1980s have proven reliable. The plant is now visited daily by Mr Warren for around two hours, to ensure all the systems are operating correctly. Mr Warren said: “In the 1980s there was a plant superintendent at Stowey, along with three full-time maintenance personnel. Now the plant is not even managed 24hrs a day. It’s a far cry from the 1950s when even the smallest of pumping stations such as Axbridge were manned round the clock.” The decline in staff numbers began almost as soon as the post-war construction phase was over. Shift workers were made redundant at Blagdon Lake’s WTW in the 1950s when automatic shutdown equipment was installed to cope with burst mains and the reduction has continued ever since.
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