UV in high-purity water
The term: ultrapure water
In the field of analytics, biotechnology and other laboratory applications it is necessary for several reasons to use water of very high purity requirements. "Normal" water of fresh-water quality contains a series of impurities leading to faulty results in these applications - be it in analytics or in works in the microbiological field. Standards have been prepared by several bodies (e.g., ASTM, DIN) in which the requirements of ultrapure water have been quantified. Acc. to ASTM, water is deemed ultrapure water if it meets the following criteria (as a rule, the standards also list other qualities of lower requirements):
parameter: limit value
- conductivity: 0,056 µS/cm (at 25 °C)
- resistance: 18,0 MW x cm (at 25 °C)
- TOC: 100 ppb (with UV lower values possible)
- endotoxins: < 0,03 EU/ml
Conductivity and resistance are affected by ions occurring in water. The TOC (Total Organic Carbon) is formed by organic impurities. Moreover, the water may contain impurities caused by bacterial infestation.
Processes of water treatment
The water can be raised to the quality described by applying several stages of purification. Ion exchange and reverse osmosis as well as electrodeionisation are adopted as stages of pre-purification. This renders very good separation of particles, colloids and organic compounds (reverse osmosis) as well as a maximum separation of ionic compounds (ion exchanger). Both procedures combined reduce conductivity to a range of 0.5 µS/cm (ion exchange). Reverse osmosis lowers the TOC value further. Based on these qualities achieved in pre-treatment, ultrapure water can be generated in the systems of the various manufacturers.
Steps of UV treatment in ultrapure water systems
Apart from the disinfecting effect, TOC degradation gains primary importance when applying UV technology in the field of ultrapure water, in order to further scale down the TOC concentration achieved by pre-treatment to the values required for ultrapure water. The TOC plays a crucial role when assessing the organic quality of ultrapure water. Since it cannot be measured directly, conductivity is relied upon as a parameter.
UV oxidation makes use of the effect that UV irradiation, especially at a wave length of 185nm, generates hydroxyl radicals from water. These radicals oxidise the organic constituents down to CO2 and H2O. Thus the UV reactor is used for degradation and the determination of TOC at the same time. Special UV lamps are applied in the reactors, which, apart from the wave length of 254nm relevant to disinfection, also emit UV irradiation of 185nm required for TOC degradation.
An important criterion for the design of UV systems is, apart from the qualitative property of the UV lamp, to emit a wave length of 185nm, i.e. irradiation H [J/m²], which is at 1,200 J/m² and higher (e.g. up to 4,000 J/m² in microelectronics) for TOC degradation in ultrapure water systems.