The aim of this project was to investigate the influence of the configuration and operation of wastewater treatment plants (WWTPs) on the denitrifying population in active sludge and to investigate how NosZ-genes can be used as molecular markers in the identification of these denitrifiers. The influence of the operation of WWTPs was investigated by studying the effect of adding different types of substrate to the active sludge, and monitoring the denitrification rate and the population of denitrifying bacteria in the active sludge. The substrates used were acetate, casaminoacid, and a combination of these substrates. The active sludge tested was from three wastewater treatment plants located in the north of Jutland: Aalborg WWTP-east (AAØ), Aalborg WWTP-west (AAV) and Aabybro WWTP (Aabybro). The results of these experiments indicated that with the addition of acetate, denitrification was faster. It was also noted that the combination of substrates resulted in the denitrification rate in the three WWTPs being similar or higher than with acetate, indicating that the denitrifiers have different substrate preferences. In the case of casaminoacid a low denitrification rate for the three WWTPs was found. The configuration of the WWTPs seemed to have a great influence on the denitrification-rate. In AAØ and AAV the configuration of the plants includes a “sidestream-hydolysis” which gives a longer adaption-time and also by means of hydrolysis and fermentation provide the denitrifiers with easily accessible organic matter. To identify the NosZ-carrying denitrifiers, experiments were carried out using PCR and computerized analysis of the NosZ-carrying sequences with alignments of known sequences in GenBank. The alignments resulted in a sequence-library that was converted into a phylogenetic tree, which showed great similarity to phylogenetic trees based on 16S rRNA and NosZ from previous investigations. Also, it was observed that β-protobacteria is dominant in the activated sludge of the three WWTPs. Moreover the results confirm that there was a difference between bacteria grown in the laboratory and bacteria that live in the active sludge. The investigation also indicated that the choice of primers and length of sequences have a significant influence on the reliability of the results, and that identification using the NosZ-gene is possible, but should be supplemented with other analyses, like quantitative FISH analysis.