Application of new bacterial regrowth potential method for water distribution system – a clear evidence of phosphorus limitation

Conventionally, biologically available organic carbon is assumed to be the sole limiting nutrient for bacterial regrowth in drinking water distribution system. A new method, termed bacterial regrowth potential (BRP) method, was developed in The University of Tokyo to take into account of other possibly limiting nutrients. The BRP method is a bioassay, produced results within 5 days, had the capability of identifying the limiting nutrient in the sample using the indigenous inoculum, and supported the attached bacterial growth due to incubation of test tubes in a rotating plate. The BRP method was applied for a survey of Tokyo metropolitan drinking water distribution system. The system was distributing around 6×106 m3/d through 14,616 km long pipes (pipes were interconnected between ten treatment plants) for around 11 million people. Before distribution, the water had undergone different treatment trains but all had chlorination as the last step. Mostly the trains included conventional rapid sand filtration system. Exceptions were that one had advanced treatment such as ozonation and biological granular activated carbon in between coagulation/sedimentation and rapid sand filtration, two had slow sand filtration, and two others had ground water supply. The survey covered the whole area with a total of 28 samples collected from different points at two different times over a three month period (April–June ‘95). The results of the survey had clearly illustrated that inorganic nutrients were limiting in more than half of the samples analyzed. In the rest of the samples biologically available organic carbon was limiting. With the use of phosphorus instead of inorganic nutrients, the limiting inorganic nutrient was confirmed to be phosphorus. Hence, proper analysis of the water for its actual limiting nutrient and thus the proper control measure is proposed.