FRP pressure vessels are composite tanks. A blow moulded seamless liner of non – permeable thermoplastic is over wounded with miles of epoxy reinforced fibre glass laminate for top strength and performance. The tanks are used for water softening, filtration and de-ionization. The processes where the tanks are used for reaching from active carbon filtration – iron removal – multimedia filtration – softening – demineralization -portable exchange – special ion exchange processes. The used materials and the production process allow an almost unlimited corrosion resistance for the applications in the water treatment as well as for a many diluted chemicals.