By Ralf König
Summary: More and more, people recognize the viability UV in water treatment. UV disinfection can be applied efficiently to ultrapure uses, but only if there’s a general understanding of key components. Several are discussed below along with details on how each play a part in the overall efficacy of UV.
Ultrapure water is needed for many production processes in the microelectronics and pharmaceutical industries. The microelectronics industry creates ever smaller, more powerful components to satisfy ever-increasing performance and market demands. Conducting track separation is now so small that dust particles, bacteria or carbon residues between the tracks can cause processor chips to malfunction or fail completely. And the health requirements of pharmaceutical products mandate very specific quality parameters regarding physical, chemical and biological purity.
Suspended solids can be reliably removed from the inflow water by using ultra-fine filters. Filtration of bacteriologically contaminated inflow water, however, presents greater difficulties. Bacteria can grow through even 0.2 micron (mm) filters within a very short time and, thus, interfere with the process cycle.1 For this reason, microorganisms must be destroyed or at least inactivated. An emerging alternative method for this is ultraviolet (UV) disinfection. If chemicals are used (e.g., chlorine compounds), there’s a risk they’ll interfere with other processes such as ion exchange resins and reverse osmosis (RO) membranes. There’s also a concern over disinfection by-products (DBPs) such as trihalomethanes (THMs) formed by the reaction of chemical disinfectants with organics in the water. Organic residues in the process water, expressed as TOC (total organic carbon) may already be present in the inflow water in dissolved form in very small amounts or they may form from the biomass of microorganisms during the disinfection step. Organic carbon derived from biomass must also be reliably degraded, as it may accumulate on printed circuit boards, resulting in formation of burn points causing product malfunction.
One benefit of UV is that it has no DBPs or chemical residual. Invisible UV rays of the type used for disinfection in water treatment and other processes are present in sunlight and are thus part of our natural environment.2 Like X-rays, microwaves and infrared rays, they belong to the electromagnetic spectrum, situated between visible light and high-energy X-ray radiation (see Figure 1). UV radiation is differentiated into UV-A radiation (315-400 nm), which is responsible for the familiar skin pigmentation, UV-B radiation (280-315 nm), which is responsible for the synthesis of vitamin D in our skin, and UV-C radiation (200-280 nm), which has a microbicidal effect—optimal at 254 nm. The nucleotides of DNA absorb an especially large amount of energy in the UV-C range (see UV & DNA). For this reason, very effective disinfection can be achieved by the selective use of UV-C radiation. The vacuum range of UV (180-185 nm)—the range in which ozone is also produced as a by-product—may be considered as part of the UV-C range. The UV destroys the ozone but that nanosecond in its destruction allows for brief formation of hydroxyl (OH) radicals, which are the most powerful oxidants behind fluorine and offer enhanced disinfectant and oxidative properties to the process.
The UV dosage needed for disinfection is the product of the intensity of the radiation—sometimes referred to as irradiance—and the exposure time is expressed as: UV dosage (J/m²) = UV intensity (W/m²) × exposure time(s). Some scientific studies3,4 have shown that a UV dosage of 400 joules per square meter (J/m2)—sometimes shown as millijoules per square centimeter (mJ/cm2)—is needed to kill the great majority of known microorganisms and viruses. More recent studies have proven lower dosage levels to be effective against Cryptosporidium and Giardia. In the end, the effectiveness of an installed UV disinfection system is determined by the biodosimetric method, which involves determining the efficiency with which the UV radiation kills test organisms in defined suspensions.
Ultrapure water is produced in a sequence of individual steps, of which UV disinfection and oxidation with ozone are two of the most commonly used mechanisms. Both are sometimes carried out in tandem since UV also destroys ozone residues that can also be detrimental to downstream materials. The most important steps in producing ultrapure water are described here, as is use of UV and ozone disinfection in the process.
Process water preparation
Raw water needed for the preparation process—usually municipal water or water from user-owned wells—undergoes pretreatment to reduce particulate matter that might interfere with UV transmittal and disinfection of microbial content (see Figure 2). Suspended solids are filtered out and water hardness is reduced in softening plants. A UV disinfection step can be included in pretreatment to lower microorganism concentration in the inflow water. The water is then desalinated by passing it through an RO or deionizer unit.
Next, inflow water is subjected to UV disinfection followed by further process steps. Microorganism content of the inflow water after UV treatment must not exceed 1 colony-forming unit per liter (cfu/L) for microelectronics applications or 100 cfu/L for pharmaceutical ones. High-performance UV reactors, capable of delivering radiation dosages between 850 J/m2—85 mJ/cm2—(when used to destroy residual ozone; see below) and more than 2,000 J/m2 (when used to destroy TOC), are needed to achieve such extremely low microorganism counts. This may seem high to some, but is recommended in tandem use of UV for disinfection and reduction of organic components. Water is circulated through ring lines, to which the take-off points are connected.
As well as disinfecting the water, the UV radiation must also reduce its TOC content relative to the initial concentration. This is done by oxidation with electrolytically generated ozone. The water is then enriched with ozone in a bypass (see Ozone Technology). A basic ozone concentration of 0.05 mg/L is maintained. In this case, a target residual TOC level is less than 500 parts per billion (ppb) for pharmaceutical applications and 0.5 ppb for microelectronic applications. The ozone used to produce ultrapure water should only be generated electrolytically, otherwise accidental introduction of impurities into the production process, e.g. from air or oxygen bottles, cannot be excluded.
Organic carbon content also can be reduced with UV systems fitted with special oxidizing UV lamps. These lamps generate UV radiation at 185 nanometer (nm). At this wavelength—which belongs to the vacuum UV range (180-185 nm) where ozone is also produced—hydroxyl (OH) radicals are formed in water. These radicals take part in a chain reaction in which the organic carbon is oxidized. Incompletely oxidized charged carbon fragments are then efficiently removed by mixed-bed ion exchangers. Ozone concentration also must be reduced until it’s below detection limits. For this reason, more UV systems are positioned upstream of the ultrapure water take-off points as residual ozone scavengers.
Technology & disinfection
Water to be treated flows past special high-performance UV lamps. High-energy UV radiation is generated by briefly vaporizing mercury inside the lamp and igniting the liberated gasses. The ionized mercury emits very intense UV radiation at 254 nm. Modern UV lamps contain a solid indium amalgam instead of pure liquid mercury. The highly volatile mercury in this amalgam is liberated by applying an electrical voltage. The vaporization is reversible, i.e. after the lamp has been switched off, the vaporized mercury returns to its original state.
UV lamps used today are different in terms of amalgams used and gas pressures generated when they’re in operation. A distinction is drawn between low-pressure lamps, medium-pressure lamps and high-pressure lamps. Only the first two, however, are used for UV water disinfection. The low-pressure lamps have a gas pressure of less than 1 millibar after mercury vaporization and thus operate with a vacuum. The gas volume in the lamp is small in comparison with medium-pressure lamps. For this reason, the gas mixture—or ionized mercury in the gas mixture—emits only one sharp line at 254 nm. The other wavelength bands and lines are very weak under these conditions. The mercury line at 254 nm, or rather its energy, is needed to inactivate the DNA when the low-pressure technique is used. Due to low power input, only a slight amount of heat is generated by low-pressure lamps, which operate at temperatures between 40°C and 110°C.
Medium-pressure lamps differ from low-pressure ones in that the composition of the amalgam differs. In addition, more mercury gas is generated. Under operating conditions, the pressure of the vaporized gas in the lamp reaches almost 1 bar. A complete band spectrum is emitted, of which only the lines around 254 nm are needed. The remaining energy is dissipated in the form of heat. The operating temperature is therefore between 600°C and 1,000°C, i.e. much of the energy is lost and the whole system becomes very hot. As a result, medium-pressure lamps consume large amounts of energy and have a shorter service life than low-pressure lamps. On the other hand, medium-pressure UV systems need fewer lamps, i.e. they’re smaller and more compact. In terms of energy consumption, it’s more effective to use a low-pressure lamp than a medium-pressure one. The UV-C radiation yield can be more than 40 percent compared with the 10-to-15 percent achieved with medium-pressure lamps.
Electronically regulated ballasts are useful for the operation of the UV lamps. When a lamp is on, the ballast regulates the electronic soft start and subsequent high-frequency modulated lamp output. This control system extends useful life of the lamps and enables very low energy consumption levels. An actively controlled power-switching circuit (with UV sensor) enables lamps to be operated at optimal performance at all times. As a result, the emitted UV dosage and, thus, the disinfection performance are always optimal, irrespective of supply voltage fluctuations, lamp aging and variations in outside temperature.
The integrated UV sensor is a key component of a UV system. To ensure optimal disinfection, performance of the lamps—the UV intensity in the water—must be continuously measured by means of a UV sensor. This sensor immediately transmits an alarm signal if the intensity falls below a given value and disinfection is incomplete. The UV sensor enables disinfection performance to be monitored continuously online. Online monitoring of other disinfection methods (e.g., chlorination) is either not possible or is very costly. The UV sensor is characterized by temperature-stable operation and resistance to aging, while specially adjusted filters give it the ideal sensitivity for the application for which it’s used. The sensor reacts to UV radiation just as sensitively as bacterial DNA, i.e., it’s as UV-sensitive as the microorganisms themselves.
A complete UV system
The UV system discussed above is tailored to requirements of the microelectronics and pharmaceutical industries and can disinfect potable and process water as well as reliably destroy residual ozone. All system components (quartz reactor, UV lamp and electronics) are housed in a compact high-grade stainless steel enclosure. The actual UV reactor consists of a quartz glass tube with UV lamps arranged in parallel to the flow. The tube geometry is such that there is no “dead space,” i.e. the UV lamps aren’t integrated in the medium. Special reflectors focus the UV radiation in the middle of the tube so that effective disinfection is achieved. This reactor is designed for ultrapure water applications. In this configuration, it satisfies all Food and Drug Administration requirements. The high-performance, low-pressure UV lamps used are extremely stable with regard to temperature fluctuations. Such fluctuations in the water have only a slight influence on radiation efficiency.
Use of UV technology with ultrapure water treatment involves no chemical residue or unhealthy by-products, yet acts as an effective disinfection—and sometimes oxidative—agent. The specific effect of the UV rays on the DNA of the microorganisms selectively kills these organisms. Electrolytically generated ozone reliably oxidizes organic carbon residues in the water and prevents microbial recontamination in the short term. There are thus strong arguments in favor of the combined use of UV and ozone technology as an essential system component of ultrapure water preparation.
- FIGAWA, “UV-Bestrahlung in der Trinkwasser- und Abwasserbehandlung zu Desinfektionszwecken”, Technische Mitteilungen der FIGAWA (Bundesvereinigung der Firmen im Gas- Wasserfach e.V.) Nr. 20/98, Arbeitskreis UV-Wasserbehandlung.
- J. Kiefer, “Ultraviolette Strahlen,” De Gruyter, Berlin, 1977.
- FIGAWA, “UV-Desinfektionsanlagen für die Trinkwasserversorgung—Anforderungen und Prüfung,” DVGW Arbeitsblatt W294, 1997.
- O. Hoyer, “Testing performance and monitoring of UV systems for drinking water disinfection,” International Water Association, Journal of Water Supply, Vol. 16, 1998.
- Lubert Stryer, “Biochemie,” Spektrum Akademischer Verlag GmbH, Heidelberg, Germany, 1991.
About the author
Ralf König is head of public relations at WEDECO AG Water Technology in Düsseldorf, Germany. He can be reached at +49 211 951-9618, +49 211 951-630 (fax) or email: firstname.lastname@example.org.
UV & DNA
It’s important to discuss exactly how UV inactivates microbial contaminants. Primarily, this is through its biological effect on an organism’s DNA.
DNA is a very long macromolecule with a complex structure.5 It consists of numerous polynucleotides, sugar groups and phosphate groups. Genetic information is coded in the sequence of the nucleotide bases. Four bases (adenine, thymine, cytosine and guanine) are available for coding this information. The coded information is transmitted enzymatically by the process of DNA replication. This consists of two steps. The first is known as transcription and involves the production of RNA. The actual protein synthesis is started from this RNA template and is referred to as translation. The three-dimensional structure and the functionality of the synthetized proteins depends on the DNA strand being read error-free. An error in the fundamental sequence of the nucleotide bases, e.g. due to the action of UV radiation, causes an error in the amino acid sequence and results in the formation of a defective protein. The resulting disruption of the cell’s metabolic processes causes it to die (see Figure 3).
UV radiation also interferes with replication. When a cell divides, the DNA polymerase reads the DNA strand step by step. If the three-dimensional structure is flawed—e.g. by the formation of radiation-induced thymine dimer, an adduct (pyrimidine-pyrimidone-(6-4) adduct) or a hydrate—replication stops. Cell division, and thus propagation, ceases and the organism is no longer able to reproduce. This replicative necrocytosis reliably prevents the spread of pathogens.
In general, disinfection is deemed complete when 99.99 percent of the organisms initially present have been eliminated—or a 4 log microbial reduction. A UV disinfection system must be designed to achieve this.
Ozone is a colorless, extremely toxic and unstable gas. An ozone molecule consists of three oxygen atoms. Ozone breaks down spontaneously into O2 + 1/2 O2, liberating energy (+284 kJ), and has a half-life of 3 days (at 20°C). This instability gives ozone its special properties. As an oxidizing agent, ozone is second only to fluorine. Carbon is rapidly oxidized to carbon dioxide at normal temperatures. Many organic dyes are bleached and oxidized by ozone. The toxicity of ozone is partly attributable to the oxidative decomposition of unsaturated fatty acids in the organism. This harmful effect of ozone is utilized for disinfection purposes against lower organisms such as viruses, bacteria and fungi.
Ozone rapidly decomposes back into oxygen, cannot be effectively stored and must therefore be generated in situ. Most modern ozone generators produce ozone from oxygen or air by the method of silent electrical discharge known as a corona discharge. This involves applying a high voltage between two concentrically arranged electrodes, which are separated by a dielectric and two discharge chambers, through which gas flows. Some of the oxygen molecules in the input gas break down in the electric field and immediately attach themselves to free oxygen molecules, forming ozone.
An alternative to this industrial-scale method of ozone production is electrolytic ozone generation, which is frequently used in the ultrapure water production sector. The ozone is generated in the circulating water, thus excluding any accidental introduction of impurities, i.e., from the air or the oxygen system used such as an oxygen bottle.