Water can contain a large number of different contaminants that require different treatments to remove them. Particles can be removed by different filtration systems that remove particles based on their size. These can range from particles of 1 mm and larger (particle filtration) to an effective pore size of 0.001 to 0.01 microns (nanofiltration) or reverse osmosis which selectively removes large molecules. Particulate filters can remove microbes such as bacteria (0.2 to 30 microns in size) or viruses (0.003 to 0.05 microns in size) and even endotoxins and RNA depending on the pore size of the filter in the system.
Ions, such as dissolved salts, can be removed by distillation or, more commonly today, by deionization, a process also often referred to as ion exchange. The water used in the laboratory is usually ASTM grade type I or II, one of them being the purest.
Organic compounds can be removed by treating water with activated carbon. UV purification systems can, depending on the type of system, sterilize water by killing microorganisms such as bacteria and viruses (UV sterilizers) or even lower total organic carbon (TOC) content by oxidizing organic carbon compounds into less harmful compounds. . UV radiation treatment devices typically use a low-pressure mercury lamp as the source of UV radiation. The lamps create ultraviolet light of a range of wavelengths. Lamps with a normal quartz sleeve will pass ultraviolet light with a wavelength of 254nm. This light is optimally absorbed by the DNA in the microorganisms and kills them in this way. UV lamps with an ultrapure quartz sleeve will pass UV light of two wavelengths: 254nm for sterilization purposes and higher energy UV light with a wavelength of 185nm. This light can oxidize organic compounds and thus reduce the TOC to values below 5ppb. However, the oxidation of organic compounds can only reduce a certain amount of TOC and therefore should be used as a final polishing step after ultrafiltration to achieve the lowest TOC counts. The CO2 it produces lowers the resistivity of the water and a UV oxidant cannot remove any ions, which is why most water purification systems employ a deionization step after the UV oxidation treatment.
Whether it needs UV treatment, and if so, what type, depends on the application you will be using the water for. Analytical applications such as HPLC, gas chromatography, and mass spectrometry can be influenced by the presence of organic compounds that increase background and decrease method sensitivity. Individual organic compounds can produce false peaks in the chromatogram. For these methods you need water that has as low a TOC as possible. Therefore, most analytical laboratories use ASTM Type I water (water with a resistivity of more than 18 MΩ/cm2) and an ultrafiltration device to remove bacteria in addition to a UV oxidant step. Organic compounds can be removed based on their size in a large amount by filtration. However, the greatest TOC reduction is only possible using a UV oxidant in addition to filtration. Some systems allow monitoring not only resistivity, but also TOC continuously. This can be very important as many deionization/UV radiation systems have a higher deionization capacity than oxidation, which can lead to ingress of organic compounds that will not be detected by the resistivity monitor.