Excerpt from Dr. Falkinham paper:
(9) Beware of Granular Activated Carbon/Charcoal (GAC) Water Filters. GAC filters are
widely marketed and sold directly to consumers to reduce the bad taste of drinking
water. They work by binding chlorine. other disinfectants, metals, and organics that
impart a bad taste to water. However, they promote the growth of NTM without
preventing their passage, as shown in an EPA study (Rodgers et al., 1999). The pores
of GAC filters are not small enough to prevent bacterial passage; the tortuous path of
movement merely delays passage for a while. NTM are quite happy in GAC filters;
they attach and grow on the carbon-bound organics and metals as they are resistant to
the disinfectant. The manufacturer’s recommendation for replacement of the filters is
based on the capacity to remove disinfectants, metals, and organics; not on preventing
passage of bacteria. In our hands, the recommended time to replace a GAC filter is
longer than the time when high NTM numbers pass the filter. If you want to remove
disinfectants, metals and organics from drinking water, install a GAC filter upstream
of a 0.2 micrometer filter.