Minerals in the water will build up on the quartz sleeve and on the vessel. This coating must be cleaned off periodically because it reduces the amount of UV light reaching the water, therefore reducing disinfection performance. Depending on the water quality, it is advised to clean the unit at least every time the lamp is changed.The provision of clean water is an important issue to solve and there is no one solution, thankfully. Different areas have different problems and resources and no one solution can be applied to all. In developed countries, water is often pumped from a nearby lake or from groundwater and extensively treated in several stages in large plants to ensure safety. Water is commonly filtrated, sometimes in several stages with chemicals added to completely eliminate the smallest particles of pollutants and to speed up the process. Chemicals are then filtered out again. Filtration in various forms is so far the only method to clean water, apart from UV-purification, whether it be a traditional slow sand filter or a reverse osmosis system or a modern, large scale filter in a water-purification plant.