Recently, it has been suggested that oxygen transport limitations within activated sludge floes may induce the onset of bulking phenomena (9). Furthermore, several groups have recently employed the concept of an effectiveness factor, as developed from chemical engineering science, to correct for diffusional limitations within microbial particles or slimes (l,3,4-6, 10). To the writer’s knowledge, all the major literature in the environmental engineering area which applies to this topic has been cast in terms of the use of a Thiele modulus (11) to estimate the value of the effectiveness factor.