In this study the role of higher organisms and the effect of their predation on bacterial size distribution as well as on floc size variation were investigated in the membrane-separation activated sludge system. The pilot plant study was conducted by setting a membrane-separation bioreactor (MBR) in a wastewater treatment plant in Tokyo, Japan. A large number of predator organisms were observed in the MBR dominated by ciliated protozoa. The dispersed bacterial size distribution was obtained by using the combination of florescent in situ hybridization (FISH) and image analysis technique. With the increase of predator species especially sessile ciliates and free-swimming ciliates, reduction of smaller bacterial population (1 μm or so) was significant. It was found that, bacteria and predators are negatively correlated i.e. with increasing number of predator organisms the number of dispersed bacteria decreased. This effect of predation was confirmed while observing the floc size distribution with an automatic particle distribution measuring instrument simultaneously. It was observed that when the higher predator organisms (protozoa and metazoa) were high in number, the percentage of floc groups less than 10 μm (optimum prey size for most of the ciliate species) were less and vice versa. From this study it can be concluded that, mainly the smaller flocs which mostly consist of small (<1 μm) dispersed bacterial species, were grazed heavily by the predator population in the membrane-separation activated sludge.

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