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 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, it is common to design TOC reduction systems using a "flow-rate adjustment" of 10% to 25% the flow-rate of a UV disinfection system. For example, a TOC system designed for a flow-rate of 25 gpm could require the same number of UV lamps as a disinfection system designed to handle 100 gpm to 250 gpm.

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.

Advanced Oxidation Processes and Ultraviolet Light

UV can also be used for TOC reduction as part of an Advanced Oxidation Process (AOP). In general, Advanced Oxidation refers to processes that utilize hydroxyl radicals as a primary oxidant. For ultraviolet systems, AOP refers to the process of using UV photolysis of ozone (O3) and/or hydrogen peroxide (H2O2) to create hydroxyls.

UV advanced oxidation processes benefit from synergies that occur when O3 and/or H2O2 in water are exposed to the appropriate wavelengths of ultraviolet light. These synergies allow ultraviolet AO systems to generate increased quantities of oxidizing hydroxyls, with less UV energy, and lower concentrations of O3 and/or H2O2. Therefore, ultraviolet AOP systems are more efficient, and capable of oxidizing more TOC's in a given flow-rate of water than would be possible for any of the three processes working alone.