The term "purification" has specific connotations which implies that there will be no remaining contamination. In the great outdoors as well domestically, we never really purify water, we treat it in order to reduce levels of contamination down to a level where our bodies can cope. It is certainly misleading to suggest that we are making water pure by adding chemicals such as chlorine and iodine.

UPDATE

This Google Trends analysis shows that Water Treatment is a significantly more searched for term.

http://www.google.co.uk/trends/explore#q=water%20treatment%2C%20%20water%20purification&cmpt=q