A wastewater treatment plant controls the level of pollution reaching the environment. Yet, despite being the most common aerobic route for treatment of wastewater, the activated sludge process is not utilized to its full potential. This is mainly due to the lack of knowledge base correlating the microbial community in the activated sludge to its degradative performance. In this study, the activated biomass at the treatment site was monitored for five consecutive months. Even though operational parameters were kept constant, the microbial community was observed to change after 3 months. This shift was seen to correlate with 25 % loss of degradative efficiency. Target oxygenases were monitored at two time points, and results indicated that the dominating pathway operating in the common effluent treatment plant (CETP) is the degradation of chlorinated aromatics. This study demonstrates the change in degradative efficiency in a CETP with the change in microbial community and analyzes the parameters influencing the microbial community of activated sludge.