This study conclusively demonstrated the presence of significant levels of assimilable organic carbon (AOC) in the drinking water distribution system of Bursa Province, utilizing ATP luminescence as a superior alternative to traditional bacterial counting methods. AOC is a critical factor, as it directly serves as a carbon and energy source for heterotrophic bacteria, raising concerns about microbial regrowth in water systems with detectable organic matter. Researchers established a robust calibration curve from the luminescence values of reference bacteria subjected to varying acetate carbon concentrations. This curve effectively transformed maximum luminescence values into precise equivalents of acetate carbon. The results were compelling: AOC concentrations in Zone C1 averaged 133 µgC/L using the traditional cultural methods, while ATP luminescence revealed elevated levels of 188 µgC/L. Despite the correlation coefficient of 0.823 between the two methods, the luminescence approach consistently returned higher AOC values. Crucially, both methodologies confirmed that the AOC levels were more than sufficient to support microbial regrowth within the distribution system. These findings establish ATP luminescence as a highly effective and time-efficient method for AOC assessment, allowing for better management of water quality and proactive measures against bacterial proliferation in drinking water systems.