Growth of bacteria in drinking water distribution and storage systems can lead to the deterioration of water quality, violation of water standards, and increased operating costs. Growth or Regrowth results from viable bacteria surviving the disinfection process and utilizing nutrient in the water and biofilm to sustain growth. Factors other than nutrients that influence regrowth include temperature, residence time in mains and storage units, and the efficacy of disinfection. Tests to determine the potential for bacterial regrowth focus on the concentration of nutrients. Not all organic compounds are equally susceptible to microbial decomposition; the fraction that provides energy and carbon for bacterial growth has been called labile dissolved organic carbon, biodegradable organic carbon (BDOC), or Assimilable Organic Carbon (AOC). Easily measured chemical surrogates for AOC are not available now. As alternative to chemical methods, bioassays have been proposed. Assimilable Organic Carbon (AOC) is that portion of the biodegradable organic carbon that can be converted to cell mass and expressed as a carbon concentration by means of a conversion factor. In this study, two organisms, namely Psuedomonas fluorescens strain P17 and Spirillum species NOX were selected for the AOC determination. The growth of the bacteria was determined by periodic colony counts with spread plate technique on LLA (Lab-Lemco nutrient agar) cultivation medium until the growth reached maximum (maximum colony count, Nmax). Results showed that AOC follows a trend based on the climatic and seasonal changes (local climate) with peaks in summer and low during winter season and vice versa in term of AOC removing capability. In addition to confirm AOC removal rate in biofiltration bed was evaluated with a test column containing the same filling materials, Granular Activated Carbon (GAC). Long term test showed that GAC would last for forty weeks without any special treatment. Other result showed that biofiltration bed has a better removal efficiency rate 72% (average based on four year), than the test column 49% since it experience frequent back-washing, thus maintaining a healthy removal rate. In the test column change in total organic carbon was quite abnormal. AOC yearly distribution was also studied and differentiated into four stages. AOC removal of each stage was 48%, 70%, 83% and 77%. Total organic carbon concentration was much higher in the effluent 384 than influent 334 £gg C/L; later methionine was found in water sample (effluent) which strongly suggests that the indigenous microbes had been reducing organic material such as cystein to methionine thus increasing the organic carbon content of the effluent. The microbial growths inside the GAC test column is entirely based on the long term feed of water at the treatment plant. Several other parameters such as Scanning Electron Microscope (SEM), Excitation Emissions Fluorescence Matrix (EEFM), Molecular Weight and Amino acids detection were selected and coupled with the AOC to shed light on the working mechanisms of both GAC as filtration material and characteristics of indigenous microbes towards the removal of organic contaminants and changes they can bring about to the quality of clear water.