Total Organic Carbon (TOC) is found in all water supplies as naturally occurring organic matter. Over many years of experience, medium or low pressure UV treatment has become recognised as an effective process step in the breakdown and reduction of TOC in water intended for use in pharmaceutical, health care, power generation and micro-electronics applications. In the latter case, ultra-pure water is used to wash silicon wafers on which even small deposits of carbon compounds on the wafers can severely affect their performance.
UV treatment is often just one part of the process of TOC reduction; it may be used in line with additional processes such as activated carbon, ultrafiltration, reverse osmosis and de-ionisation. Most TOC reduction by UV occurs at wavelengths of below 200 nm, by the production of hydroxyl radicals (•OH) from water molecules. Germicidal wavelengths (200 nm – 280 nm) play little role in this process. There is some direct absorption by some organic molecules, but this pathway of TOC reduction is not the major one. The hydroxyl radical is a strong oxidizing agent (only fluorine is stronger). It reacts by:
• hydrogen abstraction (saturated aliphatics)
• addition to double bonds (unsaturated & aromatics)
• electron transfer (primarily with inorganics)
• radical-radical reactions (e.g., •OH + HO2 •)
The end products of the process are water and carbon dioxide.
The short wavelengths required for the production of hydroxyl radicals are absorbed very strongly by water (this absorption is the reason why the radicals are produced) and the effective path lengths within a UV chamber are short. UV systems designed for TOC reduction therefore require special geometries and high purity quartz materials that are partially transparent to short UV wavelengths. The UV doses that are required are considerably higher than those used for disinfection applications.