These filters can be acceptable for uses that call for semi-effective filtration, such as shower filters. Activated carbon that is compressed into a solid block does an effective job of removing lead, chlorine, giardia, asbestos fibers, trihalomethanes, pesticides and organic chemicals. They're also dense enough to prevent channeling and growth of bacteria. A reverse osmosis system also wastes a lot of water. Three to 12 gallons go down the drain for every gallon purified.
Different systems are available.
Some vent the boiling water in such a way that VOCs are eliminated. Distillers also waste a lot of water, unless you divert it, cool it and use it in the garden. They produce only one to three gallons in several hours and draw a lot of electricity.