Ultraviolet Light (UV)
Total oxidizable carbon (TOC) 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. Given high purity water specifications that typically require TOC levels below 10 ppb (parts per billion), significant reduction is often necessary. 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 flow rate. 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.
Elimination of bacteria in high purity water systems, while highly desirable, is all but impossible given technologies available today. At best, it can be reduced and its proliferation minimized. Disinfectants such as ozone or chlorine, while highly effective, cannot be tolerated in high purity water other than as part of periodic cleaning and disinfection regimens.
Intimate exposure of bacteria to a wavelengths of 200-300 nanometers (nm) in the UV spectrum will reduce bacterial proliferation by altering cellular physiology in a manner as to minimize or prevent cellular reproduction. The mechanism is described below.
All living organisms contain DNA (Deoxyribonucleic Acid), a nucleic acid that contains the genetic instructions used in the development and functioning of living organisms. The main role of DNA is the long-term storage of information and DNA is often compared to a set of blueprints, since it contains the instructions needed to construct other components of cells such as proteins and RNA. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information.
Ribonucleic Acid (RNA), present in cell nuclei, plays several important roles in the processes that translate genetic information from deoxyribonucleic acid into protein products. An important aspect of this role is in cellular reproduction.
When organisms are subjected to UV light of sufficient dosage in the range of 200-300 nm, DNA and RNA molecules absorb the UV. The absorption of UV radiation at 254nm causes the formation of dimers along the DNA strands. Dimers are A_A (adenine – adenine), GG (guanine-guanine), T-T (thymine-thymine) compounds as opposed to the normal formation of A-T-G-C- etc. The net effect is disruption of the RNA nucleic material so as to prevent cellular replication. So, while bacteria are not typically killed outright, the failure to replicate prevents proliferation.
Dosage is a function of wavelength, intensity and time of exposure. If the dosage is not optimized, then complete inhibition of replication will not occur. An insufficient UV dosage may cause limited damage to the DNA, which can, under favorable conditions, repair itself using repair enzymes.
Ultraviolet disinfection systems typically utilize thin cylindrical bulbs capable of generating narrow band wavelength UV light in the 254 nm range, placed in close proximity to high-purity glass tubes through which the water flows. The unit must be sized to provide adequate dosage at the specified flow rate. The water to be treated must be sufficiently free of turbidity (sediment), iron, manganese and any colorant that could affect transmission of the 254 nm UV light. Manufacturers’ specifications should be consulted.
Many design and service considerations come to bear, including pretreatment of the water upstream of the UV, sizing the UV system, over-temperature during times of no flow, UV intensity monitoring, automatic “scrubbing” of the quartz tube(s), bulb replacement frequency, downstream filtration for removal of inhibited bacteria, UV “traps” on inlet and outlet to prevent UV degradation of connecting piping, and many others.