Typical source waters for various purposes include mountain streams, lower reaches of rivers, municipal wastewater, treated municipal wastewater, hazardous waste sites, etc. Each has a typical ‘‘proﬁle’’ of water quality, and some kind of treatment is always required prior to use in order to meet the criteria or standards of that use. Then, after a use of water, and prior to discharge to rivers, lakes, and seas, the product water must meet the standards and=or criteria established for such discharge. The treatment train selected depends upon the particular combination of source water available and product water required. The source water quality available and product water quality required, along with cultural, economic, and operation factors, is the treatment ‘‘context.’’ Thousands of such combinations are possible, making each treatment context unique.