Assimilable organic carbon concentrations and bacterial numbers in a water distribution system

A three year study was conducted to investigate bacterial growth in a drinking water distribution system in the UK. Bacterial numbers were estimated using Yeast Extract Agar plate counts. Plate counts in the distribution system showed patterns of spatial and seasonal variation. The spatial pattern was that plate counts increased through the distribution system until approximately 30 to 40 hours retention time and remained constant further through the distribution system. The seasonal pattern was that plate counts were low in the winter and had large peaks in the summer and autumn. Assimilable organic carbon (AOC) concentrations were measured in the second and third years of the study using an adenosine triphosphate (ATP) assay. There appeared to be no relationship between AOC concentrations and the spatial and seasonal variation in plate counts. The lack of correlation may have been caused by a lack of sensitivity in the AOC technique. Another explanation is that the increase in plate counts through the distribution system was due to an increase in the culturability of bacteria on plate count media, rather than an increase in bacterial numbers. Bacteria may not have grown through the distribution system and therefore not utilised the AOC