The Lime Kilns & Water Softening
Twyford Waterworks is unique in retaining its water softening plant and equipment. The water from Twyford is exceptionally pure but also exceedingly hard. Hard water causes lime scale and between 1903 and 1968 the water was 'softened' to remove calcium hydrogen carbonate from the water. It is these hydrogen carbonate ions which form the calcium carbonate scale when the water is heated. Chalk from the quarry was burnt in the kilns to create quicklime which, in turn, was slaked with water and added to the raw well water. The slaked lime reacted with the hydrogen carbonate ions, precipitating calcium carbonate out of the water in the softening tank. Finally the water was filtered before being pumped to the reservoirs.
The Lime Kilns
Twyford boasts a set of five intermittent open flare lime kilns. This rather antiquated design of kiln for burning chalk to produce lime has its origins in Roman technology which had been superseded over two hundred years before the first kilns were built in 1903. However, for making pure lime without any waste material entering the drinking water, this technique proved superior. Wagon loads of chalk and coal were brought up the incline railway to be loaded into the kilns through doorways on the charging floor, which were then bricked up. Once the burn had taken place the lime was taken to the mixing room where it was added to water to create a milk of lime suitable for dosing the water from the wells. The mixing drum and the incline railway are powered by a pair of water powered engines.
The Softening Tank
In this deep covered tank the well water mixed with the milk of lime and it was here that the 'softening' took place. The water would pass through the tank and over a baffle wall at the far end before being filtered. With softening taking place the bottom of the tank would fill up with chalk sludge which had to be removed. Periodically the tank would be allowed to drain down and the wet slurry was then pumped under the road to create the artificial bank now covered by trees.
The Filter House
Once hosting a total of thirteen filter tanks the Filter House had its contents scrapped before the site had been preserved. However, amazingly for such a rare process, the Trust was able to rescue a replacement set of seven from another waterworks and these have now been installed. An area of this building is being restored to represent the Haines system for water filtration. Each filter tank contains twenty filter elements; thin cloth covered perforated metal boxes. The softened water passed through the cloth, into the box and then out through a slot in the bottom. On leaving the tank the water entered the clear water tank from which it was pumped to the reservoirs.
The Filter House also contains displays of small pumping engines and pumps including the 1896 Crossley Gas Engine which is run on Open Days.
Last Updated: Tuesday 26th April 2016