Granular activated carbon (GAC) used in the water industry as a filter material for the adsorption of organic compounds supports extensive colonization by microorganisms. The GAC filter-adsorber provides the environmental conditions that encourage microbial growth. Having a biological treatment process in drinking water production carries an implied health threat from opportunistic pathogens that may survive the disinfection barrier. There is some concern that microbially populated carbon particles are released in the product water of GAC filter-adsorbers. A pilot plant consisting of three GAC filter beds operated in parallel was assembled to explore the biological activity associated with GAC filter-adsorbers and evaluate the release of carbon fines in the product water. A polycarbonate filter apparatus was used to capture particulate matter in the product water for evaluation. Plate count data indicated lower application rates released more microbes to the product water than higher application rates. Backwashing the adsorbers with chlorinated water did not reduce colonization in the long term (beyond 30 days). Colonization reached a steady-state level of 10^5 to 10^7 CFU/g (dry weight) on the carbon and 10^4 CFU/mL in the product water. Coliform organisms were not found to propagate in the filter-adsorber. The average concentration of fines was estimated as 1.0 ug/L and scanning electron microscopy revealed that the particulate supported extracellular growth that could enhance survivability of microbes, but made the constitution of the matter indeterminable.