Biological Colonization of Granular Activated Carbon Filters in Drinking‐Water Treatment

Abstract

A study of biological filtration on granular activated carbon (GAC) has been performed at the Neuilly‐sur‐Marne (France) treatment plant using experimental pilot filters. Bacterial colonization of GAC was monitored in three carbon filters having varying depths and filtration velocities, but similar empty bed contact time (EBCT). About three months were required to colonize the GAC in the filters and a steady state bacterial biomass of around 2 μgC/cm3 GAC was reached. During the colonization period, removal of nonbiodegradable organic carbon (NBDOC) by filtration decreased from 50% to less than 10% showing a gradual saturation of GAC adsorption capacity. However, biological processes take turn with adsorption to ensure a significant removal of biodegradable dissolved organic carbon (BDOC) during the study period (more than 200 days). Filters operating under different conditions did not show any difference in the BDOC removal. This indicates that biological removal of organic matter in GAC filters having a given EBCT is independent of filtration rate (in the range 6 m/h–18 m/h). The maximum bacterial density in the filter effluent water was 1.5×105bacteria/mL during the colonization phase; this seemed to be linked to a lag in establishing a protozoan population, which, at equilibrium, efficiently removed bacterial biomass produced within the filters. Once established, the efficiency of this complex microbial assemblage was remarkable; the removal of about 0.25 mgC/L of BDOC resulted in the release of less than 0.5×105bacteriapermillileter, around 0.0015 mgC/L as bacterial biomass in the effluent.