Regrowth in the drinking water distribution system is a primary concern for water utilities. The disinfection process, although normally efficient for primary inactivation, is not always enough to discourage microbial regrowth if sufficient substrate is available. Previously, the available substrate in drinking water has been assessed by culturing the sample with suspended bacterial seeds at specific counts and relating increased bacterial counts to a concentration of a specific substrate. Besides being labor intensive, this approach, termed assimilable organic carbon (AOC), does not account for the indigenous bacterial population or the biofilm aspects of degradation. Joret et al. and Gimbel and Maelzer overcame these problems by using indigenous populations attached to a sand media and measuring decreases in dissolved organic carbon (DOC); Joret et al. in a batch system and Gimbel and Maelzer in a recirculating batch reactor. Both systems were site specific i.e., the acclimated biofilm and samples were from the same treatment plant. The objectives of the study were to develop, assess and demonstrate a non-site specific system for the determination of the biodegradable DOC concentration in drinking water, and show applications to samples from several treatment plants.