Reverse osmosis water purification is different to water filtration because it leaves you with pure water. This reverse osmosis system removes impurities including bacteria, viruses, hormones, chemicals and other impurities. The onboard computer system monitors any impurities in parts per million (ppm) via its onboard computer. If the level of impurities increases then it is time to change filters. The recommended filter change intervals are only a guide, the actual interval will depend upon the contamination levels of the inbound supply. The purified water is stored in a tank 1st Stage Filter (WLP17): This filter removes sediment particles and should be changed at least every three months 2nd Stage Filter (WLP18): This filter contains Granulated Activated Carbon (GAC) and removes Volatile Organic Chemicals (VOC), it should be changed every six months 3rd Stage Filter (WLP19): This filter contains the Reverse Osmosis (RO) membrane and removes any remaining bacteria, viruses, chemicals and hormones. This filter should be changed every 12-18 months