How do the water purity standards differ for the various uses?

The standards differ because different usages require different standards. For example, drinking water requires chemical treatment and biological treatment, while water for steam generation (Industrial) requires separation of ion and dissolved gasses to prevent either corrosion or fouling. If the water is used for processes, the treatment will focus on removing impurities that have great impact on the balance of the process. Another reason why the standards are different is because of the treatment cost that differs based on each standard.