What is one characteristic of PAOs that makes them denser than non-PAOs?

PAOs, or polyphosphate-accumulating organisms, have the distinguishing feature of storing phosphorus in their cells, which contributes significantly to their density. This storage occurs in the form of polyphosphate granules, allowing PAOs to accumulate higher levels of phosphorus compared to non-PAOs. The presence of these granules increases their weight and density, which is advantageous during the treatment process, as it enables them to settle more effectively during sedimentation.

The other characteristics listed do not specifically relate to the enhanced density of PAOs. Although a higher metabolic rate and increased oxygen tolerance might play roles in their overall activity and environment, they do not directly influence density. Similarly, longer growth cycles pertain to their reproduction and life span but do not account for the physical density of the organism itself.