TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: Treatment killing the microorganism comprises the addition of a biocide or a treatment with electromagnetic radiation (preferably biocide, especially hydrogen peroxide). The concentration of the substance is measured on several occasions during and after the treatment killing the microorganisms and the concentration as a function of time is used as an indication of the type of microorganisms.Preferred Substance: The measured substance is an enzyme (preferably catalase). EXAMPLE - A culture of microorganisms (bacteria) was cultivated in a tank filled with white water from a deinking plant at a paper plant using recycled fiber. The concentration of the microorganisms was lowered to zero by an addition of peroxide. The tank was left to stand for 12 hours, which resulted in an almost zero concentration of the catalase. Water from this tank was pumped to a test apparatus, which used catalase as the substance secreted by the microorganisms and hydrogen peroxide both as a biocide and for determination of the catalase concentration. At the initial level, the water contained low concentration of bacteria, free catalase and hydrogen peroxide (about 15 parts per million). To this added water (50 liters) containing bacteria. The bacteria were killed or inactivated by hydrogen peroxide initially present. Commercial catalase was added to remove the remaining hydrogen peroxide. This allowed a study of the degradation of the added catalase and removal of hydrogen peroxide triggered the growth of bacteria. The concentration of bacteria before the growth started was measured to be less than 10 cfu (colony forming units)/ml. At this period of time, the concentration of free catalase was high and that of the bacteria was low. The triggered bacterial growth phase was also promoted by the high concentration of the oxygen resulted. The depletion of the surplus oxygen resulted in the death of some microorganisms. The circulation of water released above the surface in the cultivation tank maintained a constant supply of oxygen and allowed the progress of the growth phase. At the final phase the added oxygen was consumed at a greater pace, which resulted in a decrease in oxygen concentration. At this stage the concentration of free catalase was low. While the bacterial concentration measured was found to be 106 cfu/ml. The relationship between the first measured concentration and the final measured concentration was used as the measurement of the concentration of the living microorganisms present in the sample.