Investigations of the microbial diversity and dynamics in activated sludge using molecular methods

Wastewater treatment is necessary to reduce the health risks and environmental impacts associated with discharge of untreated wastewater. The most common way to treat wastewater in wastewater treatment plants is through the activated sludge process. Although the main principle of the process has been the same since its usage began 100 years ago, there has been a continuous development and modern wastewater treatment plants can be designed to remove not only organic material but also nitrogen and phosphorus by exploiting the properties of different microorganisms. However, as the demands on the wastewater treatment plants are increasing, either by lowered accepted effluent concentrations of nutrients or by increased volumes of wastewater, there is a need for further development of the processes. For this development to be possible, an increased understanding of the factors governing the composition and dynamics of the microbial communities in the wastewater treatment plants, is regarded as fundamental. The research presented in this thesis focused on the investigation of the diversity and dynamics of the microbial community in the activated sludge of a large wastewater treatment plant. Novel tools and methods for the analysis of data from a DNA-fingerprinting method, terminal restriction fragment polymorphism analysis, were developed and used for longitudinal studies of Bacteria and Archaea in the activated sludge. The archaeal community was determined to be less diverse, present in lower numbers and more static than the bacterial community. Methanogens, likely entering the sludge with the recycled water from an anaerobic bioreactor, dominated the archaeal community. The most abundant bacterial classes were the Alphaproteobacteria and Betaproteobacteria, which are both commonly found in varying proportions in wastewater treatment plants. However, which of these two phyla that was the most abundant, was found to be highly dependent on the method used to describe the diversity. Seasonal variations in the bacterial community composition were observed and could be explained by the seasonal variations in temperature. A major operational change, bypassing of the primary settlers due to maintenance work, also coincided with changes in community composition. Thus, both operational parameters, such as treatment plant design, and environmental parameters which cannot be controlled, such as temperature, appear to be shaping the bacterial community in the activated sludge. Changes in both the archaeal and bacterial community composition coincided with observed changes in activated sludge floc properties. However, further studies are required to determine if these observations were due to causal relationships.