The website of the US Pharmacopoeia (USP) comprises a document with frequently asked questions and answers on the subject of pharmaceutical water. The not very well known page contains some very interesting points.
In total, there are 21 frequently asked questions on the topics of water for pharmaceutical or analytical use. Many questions deal with the off-line testing of parameters such as microbiology or TOC, and how samples need to be handled. In many places further explanations are given on the chapters <643> (TOC) and <645> (conductivity), which are missing in the actual monographs. It is also explained why the US Pharmacopoeia no longer defines requirements for the test criteria nitrate or pH. These were deleted several years ago and are covered by compliance with the conductivity test point. In another interesting point, the weak points or critical points in a WFI system are explained. For example, for sub-loops of hot systems that are used cold, the hot phase during sanitization should be sufficiently long. Especially the water enclosed in heat exchangers requires sufficient time to become sufficiently hot. The same applies to water in dead legs. The problem of an increased endotoxin content after or despite the distillation is also addressed. This can happen with poorly maintained components of the upstream purification steps, as well as with strongly fluctuating raw water quality.
On the USP website you can find the Q&A Document on the subject pharmaceutical water.