The formation of undesirable chlorinated by-products is related to increasing concentration of ammonia in raw water, present study was investigated in water treated with chlorine where humic acid was used as trihalomethane (THM) precursors. The objective of this study was to clarify the possibility of the biodegradation of THMs adsorbed on granular-activated carbon and determine the adsorption capacity in the bioregenerated granular activated carbon (GAC) by different input ozone concentration. When raw water was reacted with chloride, the yield of THMs was increased as a function of the input amount of chloride. As the input amount of chloride in raw water increased by two or five-fold to remove NH3, the chloroform of THMs was also significantly increased five or ten-fold and the dibromodichloride was increased five-fold. It was found that chloroform was significantly removed in biological activated carbon (BAC) treatment in comparison with ozone treatment, and the removal efficiency of THMs in BAC was reduced by 65% in the BAC column following ozonation due to the biological degradation on the surface of the activated carbons depending upon input ozone concentration.