Optimizing the characteristics of granular activated carbon(GAC)can improve the performance of biologically activated carbon(BAC)filters,and iodine value has always been the principal index for GAC selection.However,in this study,among three types of GAC treating the same humic acid-contaminated water,one had an iodine value 35％lower than the other two,but the dissolved organic carbon removal efficiency of its BAC was less than 5％away from the others.Iodine value was found to influence the removal of different organic fractions instead of the total removal efficiency.Based on the removal and biological characteristics,two possible mechanisms of organic matter removal during steady-state were suggested.For GAC with poor micropore volume and iodine value,high molecular weight substances(3500-9000 Da)were removed mainly through degradation by microorganisms,and the biodegraded organics(soluble microbial by-products,＜3500 Da)were released because of the low adsorption capacity of activated carbon.For GAC with higher micropore volume and iodine value,organics with low molecular weight(＜3500 Da)were more easily removed,first being adsorbed by micropores and then biodegraded by the biofilm.The biomass was determined by the pore volume with pore diameters greater than 100 μm,but did not correspond to the removal efficiency.Nevertheless,the microbial community structure was coordinate with both the pore structure and the organic removal characteristics.The findings provide a theoretical basis for selecting GAC for the BAC process based on its pore structure.