Water purification based on granulated active carbon filters (GAC) is more and more popular recently. It's caused by simplicity, low overall costs and high efficiency of this method in disposal of the removing organic water contaminations. To achieve full productivity, granulated active carbon filters must be settled by microorganisms living in water. Living cells of bacteria use many organic water contaminations as a source of carbon. Mostly, from the GAC filters are isolated Pseudomonas, Acinetobacter, Flavobacterium or Bacillus genera. To establish working carbon filters true activity, defining amount of living bacteria cells at the granulated carbon volume unit is required. Today's researchers propose for that purpose both classical microbiological methods based on the bacteria culture and sophisticated colorimetric analysis of the enzymatic reactions conducted by microorganisms. Some of them are method using FDA reduction by esterases to the fluoresceine efficient to establish amount of living bacterial cells, method with TTC which is reduced to red formasan by bacteria, quantitative indication of ATP in samples or usage of glucose labeled by carbon 14 C isotope.