Carbon absorption is the most widely sold method for home water treatment because of its ability to improve water by removing disagreeable tastes and odors, including chlorine. Because of these
attributes, carbon filters are very well-suited for homes that use
municipally treated water. Only a few carbon filter systems have been certified for the removal of lead, asbestos, volatile organic chemicals (VOCs), cysts, and
coliform.

There are two types of carbon filter systems, each with advantages and disadvantages: Granular activated carbon and solid block carbon.

Granular Activated Carbon

• Activated carbon effectively removes many chemicals and gases, and in
  some
  cases it can be effective against microorganisms. However, generally it will
  not affect total dissolved solids, hardness, or heavy metals.
  
• Any granular activated carbon filter has three inherent problems.

  1. It can provide a base for the growth of bacteria. When the carbon
  is fresh, virtually all organic impurities (not organic chemicals) and even
  some bacteria are removed. Accumulated impurities, though, can become food
  for bacteria, enabling them to multiply within the filter.
  2. Chemical recontamination of granular activated carbon filters can
  occur in a similar way. If the filter is used beyond the point at which it
  becomes saturated with the impurities it has absorbed, the trapped
  impurities
  can release from the surface and re-contaminate the water, with even higher
  concentrations of impurities than in the untreated water. This saturation
  point is impossible to predict.
  3. Granular carbon filters are susceptible to channeling. Because the
  carbon grains are held (relatively) loosely in a bed, open paths can result
  from the buildup of impurities in the filter and rapid water movement under
  pressure through the unit. In this situation, contact time between the
  carbon
  and the water is reduced, and filtration is less effective.

Solid Block Carbon
These are created by compressing very fine pulverized activated carbon with
a
binding medium and fusing them into a solid block. The intricate maze
developed within the block ensures complete contact with organic impurities
and, therefore, effective removal. Solid block carbon filters avoid the
problems just discussed with granular carbon filters.

Block filters can be fabricated to have such a fine porous structure that
they filter out coliform and associated disease bacteria, pathogenic cysts
such as giardia, and lighter-weight VOCs. Block filters eliminate the
problem
of channeling. Also, they are so dense that they do not allow the growth of
bacteria within the filter.