Abstract: A water purification system includes one or more UV light sources that produce germicidal UV light and provide the UV light to a given amount of fluid contained in a chamber as a batch or as flowing through the chamber. An inner surface of the chamber, that may be reflective, is coated with a thin plastic film, such as polypropylene, that is highly transmissive to UV germicidal light. The thin plastic film separates the inner surface from the fluid, and the UV light passes through the thin plastic film to reach the fluid that is being purified in the chamber. The thin plastic film may be melted at relatively low temperatures to provide heat sealing of the surface to produce the chamber. Alternatively, the thin plastic film may be readily shaped as a bag or pipe that contains or directs fluid flow.