Distillation is a long established process for water purification in which water is heated until it evaporates and the vapor condensed and collected. The equipment is relatively inexpensive but it is very energy intensive - typically it uses 1kW of electricity per liter of water produced. Depending on the design of the still, distilled water can have a resistivity of around 1 MΩ-cm and will be sterile when freshly produced if purpose built equipment is used, but will not remain so without very careful storage. Moreover, volatile impurities such as carbon dioxide, silica, ammonia and a variety of organic compounds will ´carry over´ into the distillate.
What are the drawbacks of distillation?
Distillation only produces purified water slowly. It is not an on-demand process. Due to this, a quantity of water must be distilled and stored for later use. This storage of the distillate can be problematic if the container in which the water is stored is not made of an inert material. Ions or plasticizers will leach out of the container and re-contaminate the water. In addition, bacteria grow very well in water that has been standing for some time.
To maintain sterility, sterile storage bottles are used and the collected water autoclaved, but once the bottle is opened it is exposed to bacteria and contamination begins. In hard water areas stills require frequent acid cleaning, due to scale build-up, unless the feedwater is pre-treated by softening or reverse osmosis.