Enhanced biological phosphorus removal (EBPR) is a biological wastewater treatment process facilitated by polyphosphate-accumulating organisms (PAO). The absence of isolates that have the PAO phenotype has limited the scope of studies into the physiology of these industrially significant and metabolically unique organisms. This thesis outlines findings into the physiology and ecology of EBPR in mixed microbial cultures, which contribute to the fundamental understanding of the process.
The first experimental approach used in these studies was to investigate the microbial abundance of identified PAOs and GAOs in full-scale and lab-scale EBPR processes, and correlate these data with chemical monitoring methods both at a “macroscale” and “microscale”. The “macroscale” studies consisted of process optimisation experiments that found propionate to be a more effective and stable carbon source than acetate. The “microscale” study investigated the activity of Competibacter, growing in dense aggregates. This study discovered that the structure of the granules affected the distribution of activity by limiting the supply of oxygen and that the activity of the Competibacter in turn affected the structure of the aggregate.
The second experimental approach was to target key facets of the microbial physiology of PAOs and GAOs at a molecular level. Environmental gene expression studies were used to investigate the stimulus for the expression of a putative Accumulibacter polyphosphate kinase gene (ppk). This study found that the expression of this gene was repressed by high external phosphate concentrations, which suggests that the pho regulon is functioning in Accumulibacter. In another study, previously published models were integrated and elaborated to develop a model for the membrane transport processes in PAOs and GAOs, which give them the unique ability to sequester VFA without an electron acceptor. These studies confirmed that the proton-motive force (PMF) drives the uptake of VFA by both PAOs and GAOs and postulated fundamental differences in the molecular mechanisms that PAOs and GAOs use to create a PMF in the absence of respiratory electron transport. The studies also explain the molecular basis for findings in other studies that PAOs have a competitive advantage over GAOs at increased pH.
The third experimental approach was to attempt to isolate organisms significant to EBPR. Some measure of success was achieved: colonies of Competibacter were obtained in pure culture but the growth could not be sustained further than the growth of micro-colonies just visible to the eye.
EBPR microbiology, like many other subjects of inquiry in environmental microbiology, has benefited greatly from developments in molecular methods to identify and describe microbial communities. However, the investigation of microbial physiology in the environment remains a challenge; this thesis has taken up that challenge. Discoveries regarding the benefits of propionate as a carbon source and the basis for the competitive advantage that PAOs derive from an increased pH have potential application for practitioners of EBPR plants. Furthermore the findings make a contribution to the fundamental understanding of the physiology of EBPR organisms that may in the future lead to entirely novel approaches to EBPR optimisation.