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 e stimated using Yeast Extract Agar plate counts. Plate counts in the di stribution system showed patterns of spatial and seasonal variation. T he spatial pattern was that plate counts increased through the distrib ution system until approximately 30 to 40 hours retention time and rem ained constant further through the distribution system. The seasonal p attern was that plate counts were low in the winter and had large peak s in the summer and autumn. Assimilable organic carbon (AOC) concentra tions were measured in the second and third years of the study using a n adenosine triphosphate (ATP) assay. There appeared to be no relation ship between AOC concentrations and the spatial and seasonal variation in plate counts. The lack of correlation may have been caused by a la ck of sensitivity in the AOC technique. Another explanation is that th e increase in plate counts through the distribution system was due to an increase in the culturability of bacteria on plate count media, rat her than an increase in bacterial numbers. Bacteria may not have grown through the distribution system and therefore not utilised the AOC.