The objectives of this project were to: evaluate the benefits of ultraviolet light (UV) and ozone and determine how to integrate the two disinfection systems efficiently; compare medium pressure UV light (MP-UV) with the combination of ozone and MP-UV to determine their effectiveness in meeting CT requirements for inactivation of Giardia, Cryptosporidium, and viruses, especially in cold water; and compare the effects of MP-UV and the combination of ozone and MP-UV on the formation of disinfection by-products, DBP precursors, and the biostability/bacterial regrowth potential. Protozoan parasites are a major public health concern as evidenced by waterborne outbreaks of Cryptosporidium in the early 1990s. USEPA regulators have proposed new regulations which require site-specific means corresponding to the source water for the removal or inactivation of Cryptosporidium. For water systems with vulnerable sources, UV disinfection offers a cost effective approach. Utilities that have already installed ozone systems and those considering a multiple barrier approach to disinfection are interested in the effectiveness of UV alone and UV combined with ozone.