Ultraviolet (UV) systems are used for the effective reduction of organics, commonly referred to as TOC (total oxidizable carbon). Reduction of TOC is accomplished by incorporating a 185nm UV system appropriately designed and sized as well as strategically located in conjunction with other equipment. Carbon dioxide is a typical by-product of TOC reduction process, resulting in a drop in the resistivity of water. While most organic molecules are oxidized into carbon dioxide and water molecules, other more resistant species become weakly ionized or charged, after absorbing the UV. This is why polishing deionization (DI) beds are typically placed downstream of the TOC reduction units, so that they not only remove the charged/ionized organics, but also restore the resistivity to the water.
This is the most common application of UV light in water treatment. A microelectronics water system could have several locations where UV equipment would be installed. Some typical locations of installation would be post-carbon filter and pre-RO (reverse osmosis). When installed downstream of the carbon bed and/or directly upstream of the RO unit, a UV system can significantly reduce the microbial counts by destroying at least 99.9% of the bacteria present in the influent stream. Disinfection is also recommended for the process distribution loop and pre-storage tank.
Ozone is commonly used in the pre-treatment area of a water system, as well as for sanitizing process and re-circulating systems. Prior to the point-of-use, the residual ozone needs to be destroyed to ensure the process water is not compromised. Because it is a non-chemical, fast acting mechanism, UV technology is the preferred method for ozone destruction. After considering the appropriate variables, a properly sized UV unit can be guaranteed to destroy the ozone to non-detectable limits, insuring the integrity of the process and the product. A dosage of 90 mJ/cm2 is recommended for destruction of ozone residuals of 1.0ppm or less.
While the addition of chlorine and chloramines to city water may control bacteria levels, they have undesirable effects on the degradation of membrane filtration or RO. Popular methods of removal, such as carbon beds or chemical injection, have proven to be problematic. Sodium metabisulfite involves replacing one chemical with another and creates food for micro-organisms, while carbon beds can be inefficient, vulnerable to channeling and provide breeding grounds for micro-organisms. UV solves these problems while destroying chlorine, using a small footprint and reducing maintenance costs.