This is not a simple answer. The method of choice is dependent on whether the WFI is used as final rinse water, or is actually used in the production process. The specified water quality has to be achieved, not only at the output of the water treatment plant, but also at the points of usage, so due attention must be paid to the storage and distribution of the WFI produced by the water treatment system. The extremely important aspects of manufacture in accordance with GMP mean that the manufacturer should pay great attention to the details.
A detailed analysis of the feed water needs to be performed, and then suitable pre-treatment and storage and distribution systems will need to be assessed. The minimum quality of source or feed water for the generation of WFI is Drinking Water, as defined by the US EPA, EU, Japan or WHO. This feed water may be pre-treated to render it suitable for subsequent distillation (or a suitable validated process according to the monograph). The finished water must meet all of the chemical requirements for Purified Water as well as an additional bacterial endotoxin specification.