Why are Stiebel Eltron drinking water filters recommended to be used only with municipal tap water supply?

Stiebel Eltron drinking water filters (except pre-filter) are developed to be used for metropolitan tap water supply that is compliant with Ministry of Health of Thailand standards. While drinking water filters are designed to remove harmful contaminants from piping systems to provide clean and safe drinking water, serious contamination of pretreated water might affect the effectiveness and the lifespan of the water filters.