Dormancy is a topic that is researched extensively for various microorganisms, especially in the field of medicine. However, not many studies have been conducted for the dormancy of microorganisms in the wastewater treatment sector. Treatment processes like the NEREDA® technology use Phosphate Accumulating Organisms (PAO) to remove phosphate from the wastewater. If dormancy affects these bacteria, it can cause the wastewater treatment plants to function at a lower capacity. Reviving dormant PAOs can increase treatment capacity and treat the wastewater during high influent loading without effluent deterioration. Studying the optimal conditions for granule storage without inducing dormancy can be advantageous during granule transport for inoculation of reactors during their start-up phase. This study presents the first independent assessment of the possible dormancy of PAOs in Aerobic Granular Sludge (AGS).

In this research thesis, dormancy is a survival strategy opted by PAOs to persist substrate limitation. There are two different pathways to dormancy that has been explored here. Responsive switching and starvation are studied as they express fast and slow transition respectively into dormancy. Lag faced during resuscitation of dormant cells is considered proportional to the length of dormancy in responsive switching.

The experimental phase of the research can be interpreted in three parts: first through method development, second through method testing and third through the application of the developed method. The method was developed to understand dormancy in PAOs by observing differences in their maintenance and activity rates. This method was then tested on granules where dormancy was induced by lowering the temperature to 4ºC and anaerobically storing without feed. This method was then applied to newly sampled granules from a full-scale reactor to study dormancy in the PAOs. The developed method increased the activity of the former granules by 60% but did not raise the activity of the latter. Maintenance rates increased by 167% and 68%, respectively, in the granules. Although dormancy could not be sufficiently proved by the method, starvation of PAOs in Utrecht NEREDA® reactors was possible.