Derby Water Hardness & Quality Report (2026)

Derby's water supply is managed by Severn Trent Water, drawing primarily from the River Derwent and its associated storage reservoir, Carsington Water near Ashbourne — Severn Trent's largest reservoir, completed in 1992 with a capacity of over 35 billion litres. The Derwent rises in the Bleaklow moorland of the Peak District and flows south through Derwentdale, abstracting water that blends moorland softness with some limestone influence from the middle reaches. Water is treated at Ambergate and other Severn Trent facilities in Derbyshire before distribution to Derby and the surrounding urban area. The city's water supply infrastructure has been closely tied to the Derwent valley since the 19th century.

Derby's hardness of 140 mg/L (9.8°Clark) is a product of the mixed Derwent catchment geology. The upper Derwent drainage from the Peak District gritstone moorlands contributes soft water, but the middle and lower Derwent flows through Carboniferous Limestone terrain in the Derbyshire Dales — one of England's most dramatic karst landscapes — where calcium carbonate dissolution significantly raises the mineral content. Carsington Water's feeder catchment similarly traverses Triassic and Jurassic limestone outcrops. The blended result falls in the moderately hard classification of the Drinking Water Inspectorate (DWI).

Limescale is a regular feature of Derby homes. At 140 mg/L, kettles accumulate a noticeable white limescale deposit over four to six weeks, and monthly descaling with a commercial descaler is advisable. Combi-boiler efficiency is mildly to moderately affected over time, and annual boiler servicing should include a limescale check for the heat exchanger. Showerheads and taps develop steady limescale deposits. Washing-up liquid lathers adequately but noticeably less well than in the softer-water North West. Using Calgon monthly in the washing machine and a descaler cartridge inline with the boiler is sufficient protection for most Derby households.