Ultraviolet TOC Destruction
Total Oxidizable Carbon Reduction
TOC (Total Oxidizable Carbon) is undesirable in high purity water systems because it serves as a food source to support bacterial proliferation and may interfere with downstream processes. TOC's are present in most municipal water systems, especially those that are surface-sourced, at levels of 1-20 ppm (parts per million) or more. High purity water specifications typically require TOC levels below 10 ppb (parts per billion). This means significant TOC reduction is often necessary. The use of activated carbon or organic-scavenging ion exchange resins may help reduce TOC levels as part of pretreatment; however, these typically do not provide reduction to ppb levels and are generally not appropriate as part of the high purity water recirculation and polishing system.
For final treatment and polishing, the chosen system must be capable of achieving and maintaining ppb TOC levels and must not impart impurities to the water. Meeting these criteria, ultraviolet TOC destruct systems are often used for the effective reduction of this group of compounds (these are also referred to as "TOC burners"). The destruct mechanism is explained below.
Ultraviolet TOC destruct systems typically utilize thin cylindrical bulbs capable of generating UV light in the 185 nm range, placed in close physical proximity to high-purity glass tubes through which the water flows. UV light possesses considerable energy and is absorbed by compounds and organisms at most wavelengths. However, certain wavelengths are more effective due to energy and absorption characteristics so as to induce maximum effect. For example, bacteria are most susceptible at 254 nanometers (nm). At a wavelength of 185 nm, the increased energy and adsorption sensitivity of oxidizable organic compounds leads to formation of hydroxyl free radicals in varying degrees of photochemical excitement. These hydroxyl (OH-) free radicals break various chemical bonds of organics, which in turn produce chain reactions, oxidizing most organics into carbon dioxide and water, the basic building blocks of all organic compounds.
Because of technology limitations, TOC destruct bulbs, while designed to produce light of 185 nm, emit most of their energy in the 254 nm range. For this reason, TOC units are typically sized 6-8 times larger than UV disinfection systems for the same flowrate. Sizing is also dependent on the TOC load in the water and the design specification for the treated water. Properly applied and sized, ultraviolet systems are capable of achieving <0.5 ppb TOC.
TOC destruct systems are typically placed in the recirculation loop of high purity water treatment systems downstream of "primary" or "secondary" mixed bed deionization polishers. Since TOC reduction results in production of carbon dioxide, the conductivity increases (resistivity decreases) due to formation of carbonic acid, which is ionized. Therefore, standard practice is to place a final "polishing" mixed bed deionizer containing very highly rinsed, low TOC resin, downstream of the TOC destruct unit. To reduce bacteria in the system, UV disinfection units operating at 254 nm are often placed after the final polishing deionizer.
Many design and service considerations come to bear, including TOC concentration in the raw water, sizing the TOC destruct system, in-line TOC monitoring, bulb replacement frequency, quartz tube replacement, UV "traps" on inlet and outlet to prevent UV degradation of connecting piping, and many others. Applications Engineers at WaterProfessionals® can help design and specify these systems and answer questions.