Most water filters use reverse osmosis (RO) to purify drinking water. Experts agree that there should be at least 4 cleaning steps starting with a sediment pre-filter that gets rid of the initial large particles, moving towards a carbon block pre-filter that removes chlorine to make the water taste better. No wonder chlorine. It needs to be removed so that it does not damage the RO membrane. This is followed by the RO membrane where the pressure forces water through the membrane and leaves the contamination behind. The fourth is a carbon block post filter that removes residual odors and flavors so that the water is good for drinking. Depending on the amount of water used in the home and the quality of the local water, these filters should be replaced every 6-9 months. If the flow of water decreases, it is a sign that the water filter is clean. There are models that have 9 stages of filtration.