Abstract: An apparatus for exposing a liquid to bactericidal radiation is disclosed. The apparatus includes a source of ultra-violet radiation and a non-opaque housing enclosing the source of ultra-violet radiation to prevent exposure of the source to the liquid and permit exposure of the liquid to the radiation. Channels conforming to the shape of the housing are provided. Projections are formed in the channels to induce turbulent plug flow such that when the apparatus is inserted into a flow of liquid to be treated, the channels act to confine and direct liquid about the housing and the projections act to establish a continuous, cyclical flow in the channels between the housing and the channel walls to ensure exposure of the liquid to a lethal dose of the ultra-violet radiation of the source. Methods utilizing the apparatus for exposing a liquid to bactericidal radiation are also disclosed.