French to English Tech/Engineering Chemistry; Chem Sci/Eng Wastewater treatment RPs

I think I probably posted this question in the wrong 'specific field' as it really should have been chemistry instead of engineering. So, whilst I have had some interesting and helpful answers, and at the risk of objections, I am re-posting it in the Chemistry; Chem Sci/Eng field in the hopes that a Chemistry expert will indeed pop by! This is the only question I had on an otherwise finished translation.

My text is about recommended practices for the elimination of iron and manganese from drinking water.

I know that 'domaine d'existence' is usually a mathematical term and that GDT translates it as simply 'domain' (offering with 'domain of definition' and 'support' as synonyms). Given this definition, the text below now seems slightly ambiguous to me, as it could be using mathematical terminology - and it does mention 'co-ordinates' - or it could be using 'domaine d'existence' to mean simply 'environment' or 'milieu' or 'the domains in which the forms exist'.

Les traitements du fer et du manganèse sont basés sur une conversion de la forme soluble (dissoute) en une forme oxydée insoluble (précipité) qu’il faut ensuite séparer de l’eau. La maîtrise des procédés d’élimination passe par la connaissance des domaines d’existence de leurs différentes formes. Ces domaines d’existence sont définis dans un système de coordonnées comportant le pH et le potentiel-rédox (mV/H2).