The problem of sewage sludge composting is the subject of many scientific investigations. If the process of composting runs properly, a consequent dominance of determined types and groups of microorganisms in connection with the successive changes in temperature occurs. In this paper, changes in the number of mesophilic and thermophilic bacteria were examined in the process of composting sewage sludge together with all sorts of additions. Influence of the differences in the composition of compost on bacteria development dynamics was also examined. Colonies of bacteria isolated on selective culture mediums were the base for determining the total number of bacteria in the compost. Relation between the activity of dehydrogenases and the number of mesophilic and thermophilic bacteria was also examined. It was estimated that the composition of compost and pH did not interact with the number of mesophilic and thermophilic bacteria during the composting process. Existence of relation between the decrease in the number of microorganisms and the increase in T (temperature) was observed. On the basis of the obtained results, it was also found that the activity of dehydrogenases positively correlated with the number of mesophilic bacteria.

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