Tannin s are natural organic materials (humic and fulvic acids) formed primarily by the decomposition of plants. In surface waters (lakes, rivers) and shallow wells, tannin s dissolve in the water causing a light yellow to dark tea colour in the water. (Tea and wine have tannic acids and thus the dark colour). Tannin s also stain plumbing fixtures such as bath tubs and toilet sinks, just like iron does.
Tannin s are not considered a health risk, although most people would prefer not to drink water that has a yellow or tea colour to it. Tannin s also severely limit the performance of UV disinfection systems and must be removed if UV treatment is required.
Rust is formed when dissolved iron (ferrous) oxidizes to a particle form (ferric). Homes on urban water supplies get rust from corrosion of old distribution piping which is common in many Canadian cities. Homes on private wells can have naturally occurring rust or through the well system. Rust is not considered as a health contaminant but is recommended to be removed for aesthetic reasons (taste, odour, colour, sediment and protection of plumbing devices such as water heater). Since rust is a particle, it can be removed by sediment filters. Homes on private wells that have high ferrous iron (dissolved) should consider a whole house iron reduction system.