GAC's ( Granular Activated Carbon )

1) Whole house filters (both fiber and AC) usually have a fairly large pore size (typically larger than 10 - 15 microns) and will NOT effectively trap harmful biological contaminants.  Fiber filters will not remove any organic contaminants, and the water typically moves through a whole house granular activated carbon (GAC) filter too fast to have all of the organic chemicals removed.  

2) The popular GAC pitcher or faucet mount filters would be completely useless in removing most kinds of seriously harmful contaminants.  The pore size is too large to trap harmful particulates, like the biological contaminants discussed above, and there is too little activated carbon to remove all organic contaminants that might be in the water.  In addition, channeling (described below) would reduce the effectiveness of the filters even further.

3) Larger GAC filters, used in counter-top and under-counter filtration systems, also have extremely large pores that will not remove small, harmful particulates.  Although activated carbon is good at removing a wide spectrum of organic chemicals, all GAC filters can suffer from a phenomenon called channeling where the water pressure forces channels to open up in the loose carbon granules.  Some of the water, following the route of least resistance, will flow through the channel and not come into contact with the carbon filtration medium.  Consequently, some of the water flowing through a GAC filter may not have been filtered at all, and there is no way of knowing if the water still contains some harmful contaminants.

The effectiveness of carbon filters to reduce contaminants is affected by the factors affecting adsorption listed above and three additional characteristics of the filter, contact time between the water and the carbon material, the amount of carbon in the filter, and pore size .

The length of contact time between the water and the carbon material, governed by the rate of water flow and the amount/volume of activated carbon, has a significant effect        on adsorption of contaminants. More contact time results in greater adsorption. 

The amount of carbon present in a cartridge or filter affects the amount and type of contaminant removed. Less carbon is required to remove taste- and odor-producing        chemicals than to remove trihalomethanes. 

Because of the filter characteristics discussed above, the most effective Point of Use activated carbon filters are large SBAC filtration systems, and the least effective are the small, pour-through pitcher filters.

Activated carbon filter cartridges will, over time, become less effective at reducing contaminants as the pores clog with particles (slowing water flow) and the adsorptive surfaces in the pores become filled with contaminants (typically not affecting flow rate). There is often no noticeable indication that a carbon filter is no longer removing contaminants, so it is important to replace the cartridge according to the manufacturer's instructions. The overall water quality (turbidity or presence of other contaminants)also affects the capacity of activated carbon to adsorb a specific contaminant.

It is important to note, particularly when using counter-top and faucet-mount carbon filtration systems, that hot water should NEVER be run through a carbon filter. I have seen warnings about possible damage to the filter from hot water. Perhaps more importantly, hot water will tend to release trapped contaminants into the water flow potentially making the water coming out of the filter more contaminated than the water going in.

The most effective carbon is the solid block carbon.

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