Water can contain a large amount of different contaminants that require different treatments to remove them. Particulates can be removed by different filtration systems that remove particulates based on their size. These can range from particles of 1mm size and larger (particle filtration), down to an effective pore size of 0.001 to 0.01 micron (nanofiltration), or reverse osmosis which removes large molecules selectively. Particulate filters can remove microbes like bacteria (0.2 -30 micron in size) or viruses (0.003 to 0.05 micron in size) and even endotoxin and RNAses based on the pore size of the filter in the system.
Ions like dissolved salts can be removed by distillation, or more commonly nowadays, by deionization, a process that is also often called ion exchange. Water used in the laboratory usually is of ASTM type I or II grade, with one being the more pure.
Organic compounds can be removed by treatment of the water with activate 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 decrease the Total Organic Carbon content (TOC) by oxidizing organic carbon compounds into less harmful compounds.
UV radiation treatment devices usually employ a low pressure mercury lamp as a source of UV-radiation. The lamps create UV light of a range of wave lengths. Lamps with a regular quartz sleeve will let UV light of a wavelength of 254nm pass. This light is absorbed optimally by DNA in microorganisms and kills them this way.
UV lamps with an ultrapure quartz sleeve will let UV light of two wave lengths pass:
- 254nm for sterilization purposes and
- 185nm for additionally higher energetically UV light
This light can oxidize organic compounds and thus reduce the TOC to values of under 5ppb. Oxidizing of organic compounds, however, 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 that it produces reduces resistivity of the water and an UV-oxidizer cannot remove any ions, so that most water purification systems employ a deionizing step after the UV-oxidization-treatment.
Whether you need UV treatment and if yes, which type depends on the application you will be using your water for. Analytical applications like HPLC, gas chromatography and mass spectrometry can be influenced by the presence of organic compounds which increase the background and decrease the sensitivity of the method. Single organic compounds can produce false peaks in the chromatogram. For these methods water is needed that has a TOC as low as possible. Most analytical laboratories therefore 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-oxidizer step.
Organic compounds can be removed based their size to a large amount by filtration. The biggest reduction in TOC, however is only possible using a UV oxidizer additionally to filtration. Some systems allow to monitor not only resistivity, but also TOC continuously. This can be very important since many deionizer/UV-radiation systems have a higher deionzing than oxidizing capacity which can lead to an organic compound breakthrough that will not be detected by the resistivity monitor.
Biological applications like the preparation of media and buffers for tissue culture require sterile and pyrogen-free water. The highest quality water without traces of endotoxin that can interfere with cell growth and results of experiments can be obtained with a water purification system that includes deionizing, filtration and UV-sterilization/oxidation steps.
Food preparation business like breweries and producers of fruit and vegetable juices and ciders require sterilization of the endproduct. Sterilization by ultrafiltration is often not an option for these applications as the end product contains particulate matter that is important for the appearance and the taste of the product (e.g. yeast in unfiltered beer, fruit pulp in unfiltered apple cider and orange juice). This can be efficiently achieved with a UV sterilizer that employs a 254nm wavelength.