One of the most frequently implemented processes in the water treatment technology is filtration through a biosorption bed. Techniques based on biochemical processes involving bacteria result in obtaining high quality water. There are a number of different materials used as filler material for biological filters. Carbon deposits are the most popular, due to their high effectiveness. The problem with the use of this process is the leaching of microorganisms from the biofilm and the biological stability of water thus obtained. There is a need to develop quick methods to assess the microbiological quality of this water. Modern techniques for determining the amount of microorganisms, such as flow cytometry and luminometry may be just the right tools. The water collected for testing came from the Water Treatment Station located in the region Podkarpackie. The microbiological tests carried out in the analyzed water samples taken after the filtration process on granular activated carbon. Both traditional culture method and modern techniques used to determine the number of microorganisms (flow cytometry, luminometric ATP assay) demonstrated an increase in the number of microorganisms in the examined waters (in water after the filtration process and in water introduced into the water supply network) after the incubation process for 3 and 7 days at 15 and 22 C.