Reduction of Total Oxidizable Carbons (TOC’s) in water is achieved via three types of reactions initiated by UV that work to destroy and/or remove oxidizable carbons.
The primary UV/chemical reaction is an oxidation process that begins when high-energy 185 nm UV dissociates water molecules, thereby creating hydroxyls (free OH- radicals). The hydroxyls created by UV are highly reactive and readily combine with other molecules, such as the hydrocarbon molecules that make-up TOC’s. When hydroxyls combine with the TOC hydrocarbons they form water and carbon dioxide molecules; TOC’s are destroyed and the oxidation is complete.
The second type of UV reaction that works to remove TOC’s is one whereby the ultraviolet photons dissociate organic molecules directly. This result is TOC removal/ TOC reduction by means of destruction. A third UV reaction occurs when deionization is added downstream of a UV reactor. Ultraviolet energy will ionize TOC’s, which allows for subsequent removal by a deionization system.
One of the challenges of using ultraviolet energy for TOC reduction is that 185 nm UV does not transmit as well through water as does 254 nm UV. Therefore, a very high UV dose is required for TOC reduction as compared to standard disinfection. As a thumb rule, a minimum UV dose of 1500 J/m2 is required for TOC reduction.
A side benefit of using UV for TOC reduction is that the TOC lamps will generate significant levels of 254 nm output and consequently provide high levels of microbial reduction (disinfection), in addition to the TOC reduction.