Water has been called both the matrix of life and the universal solvent. In biotechnology, the two functions are fulfilled to a unique degree : water not only forms the basis of nutrient media for cell culture, but acts as a vehicle for the active ingredients of biopharmaceutical products. It also plays a vital part in downstream separation and purification techniques, as well as in rinsing and cleaning-in-place procedures. Moreover, in quality control and research and development laboratories, water has yet another role to play as a key analytical chemical However, the life-support and solvent properties that make water such an invaluable process fluid are a double-edged sword. Not only does raw water contain a host of microbial and chemical contaminants, but purified water can become recontaminated by micro-organism and impurities leached from the distribution pipework or introduced from elsewhere into the water purification system.