There are two primary varieties of carbon filters. Carbon block filters use a densely packed piece of activated charcoal to attract heavy metals out of the water. Granular-activated carbon filters use sand-sized carbon granules that have more overall surface area than a carbon block. Carbon filters are highly effective at removing metals due to the high surface area of the charcoal portions of the filter, and the carbon molecules themselves, which have a large molecular structure with which to contain free-floating metals.
Reverse osmosis filters use a multistage process to eliminate everything from heavy metals to bacteria. The filter uses pressure to force water into a micro-porous filter, or "membrane module." The water molecules are able to pass through the membrane while the metal ions and other contaminants are removed. These types of filters are often more expensive than a simple carbon filter, but result in purer water through the removal of non-toxic contaminants such as salt.
Ceramic filters are extremely effective at removing all contaminating particles. They most commonly use nano-silver particles, which excel at removing heavy metals, microorganisms and other impurities. These filters also use a micro-porus filter in their design, and can stop particles as small as 1/100,000th of an inch. Like reverse osmosis filters, ceramic filters are more expensive than disposable carbon tap-based filters. Distributors argue the added cost is offset by the heightened purity of the water.
Distillers are typically high-quality filters designed to last decades. Some are connected directly to the water supply when others are considered counter-top. They use electricity to filter out heavy metals and other inorganic compounds. However, they are expensive and produce water quite slowly, making them an unpopular choice for residential use. Distillers also generate a significant amount of heat as a by-product of the filtration process, all of which is wasted energy that contributes to electricity costs.