There are some water problems which are not corrected by activated carbon filtration. If the water contains large amounts of magnesium and calcium (hard water), softening is still necessary because an activated carbon unit will not remove hardness. It will not remove dissolved metals such as iron, lead, manganese, and copper or chlorides, nitrates, and fluorides. Small activated carbon units can remove only small portions of hydrogen sulfide.
These filters are not effective against bacteria. In fact, they may promote bacterial growth especially when not used for a few days or when not changed at proper intervals. Some manufacturers claim that filters containing silver discourage the growth of bacteria within the filter. However, research shows that silver-impregnated carbon units do not significantly reduce bacteria problems and may increase the silver content in drinking water up to 0.028 mg/l.
Even with these limitations, activated carbon filters can significantly improve water quality. Carbon filtration can remove more than 90% of cadmium, chromium, manganese, mercury, silver, and tin. It removes many objectionable tastes and odors. It is effective on turbidity, but more economical sand or fiber filters should be used if this is the only problem. It is most effective for removal of chlorine and potentially dangerous and carcinogenic organic compounds, which may be present in a water system as a result of chlorination or industrial pollution.
The efficiency of any activated carbon filter is dependent on the "useful flow rate" of the filter and estimated filter lifetime, which are governed largely by the size of the filter and the amount of carbon it contains. There are two basic types of carbon filters: sink-mounted, which are attached to the faucet outlet, and in-line models connected to the cold water supply line to the house or just beneath the sink depending on the degree of the problem. Quite often the effective lifetime of a carbon filter can be short, which requires the filters to be replaced frequently. To determine the lifetime of a unit requires knowledge of mean and peak flow rate, residence volume of unit, carbon surface area to volume ratio, and the concentration of the various contaminants in the water. To prevent replacing the filter too seldom (contaminants not removed) or too frequently (costly) professional help by trained water quality experts or a continuous testing program for the water is needed, which is usually cost prohibitive. Some filters use powdered activated carbon embedded in a felt-like pad and others use granular activated carbon. It has been found that powdered carbon has a tendency to "unload" certain chemicals after it becomes saturated and, therefore, units containing granular activated carbon are recommended.