The next purification step is reverse osmosis which removes 90 to 99% of all the contaminants found in water
. This is the heart of any well designed water purification system because it effectively removes a broad range of contaminants. However, the tight porosity of the reverse osmosis membrane limits its flow rate. Therefore, a storage container is used to collect water from the system and distribute it to other points-of-use such as polishing systems. These systems purify pretreated water like reverse osmosis by removing trace levels of any residual contaminants. When this happens, the water is elevated where the quality of pretreated water becomes "ultrapure" and may be fit for human consumption.Pretreatment removes contaminants that may affect purification especially reverse osmosis.