Is tap water not pure ?

Tap water in the UK is 'wholesome' but not pure. That is the definition that water supply companies must work to according to the water supply regulations. What comes out of the tap varies widely in total dissolved solids (TDS) and hardness across the country, and reflects largely where your water comes from


  • Water in the midlands, south east, north east and south of the Great Britain and on the island of Ireland is hard to very hard with total dissolved solids of about 400 mg/l
  • Water in the north west of England, mid and north Wales and the far south west of England is generally low TDS and soft
  • Water in the north of Scotland and on the islands of Scotland is soft and low TDS but is peaty with a relatively high organic content

It is your statutory right to request an analysis of the water coming out of your tap. Just contact your local water company. 


Definitions of 'pure' water

What you call pure water very much depends upon your industry, below you can see that there are a number of different definitions

Semi-conductor fabrication - ultrapure water

Semi-conductor fabrication demands the highest purity of water available, so-called ultra-pure water. To produce water of this quality requires considerable investment not just to make it, but to maintain it at that purity. It is so pure that to expose it to air would render it 'out of spec'. The conductivity of this water is approximately 0.06 uS/cm2 (or its resistivity is '18 mega ohm'). This water is 10-100 times purer than 'pure water' used by window cleaners

Water for injection

Water for injection must be 'pyrogen free' and prepared by distillation technique. 

Deionised water

This is the type of water used by window cleaners and by laboratories. It is commonly used also for boiler feed water in industry and for battery top up and also various specialist uses. Definitions vary widely but generally we work to a specification that deionised water has less than 1 part-per-milion total dissolved solids, or a conductivity of less than 2 uS/cm2 (microsiemens per centrimetre squared)

De-alkalised water

This is water that has been stripped of its bicarbonate alkalinity

Soft water

This is water with the calcium and magnesium content removed


Industrial reverse osmosis system

Deionised, demineralised, distilled or pure?

Effectively these are all the same thing, as all of the above signify a water that has substantially reduced levels of total dissolved solids. Of all of the above definitions distilled is the only one that defines the process of manufacture


Reverse osmosis

Ion exchange process


Please let us have your questions and we will list them here for others to see