Predator grazing effect on bacterial size distribution and floc size variation in membrane-separation activated sludge

In this study the role of higher organisms and the effect of their predatio n on bacterial size distribution as well as on flee size variation were inv estigated in the membrane-separation activated sludge system. The pilot pla nt study was conducted by setting a membrane-separation bioreactor (MBR) in a wastewater treatment plant in Tokyo, Japan. A large number of predator o rganisms were observed in the MBR dominated by ciliated protozoa. The dispe rsed bacterial size distribution was obtained by using the combination of f lorescent in situ hybridization (FISH) and image analysis technique. With t he increase of predator species especially sessile ciliates and free-swimmi ng ciliates, reduction of smaller bacterial population (1 mum or so) was si gnificant. It was found that, bacteria and predators are negatively correla ted i.e. with increasing number of predator organisms the number of dispers ed bacteria decreased. This effect of predation was confirmed while observi ng the floc size distribution with an automatic particle distribution measu ring instrument simultaneously. It was observed that when the higher predat or organisms (protozoa and metazoa) were high in number, the percentage of flee groups less than 10 mum (optimum prey size for most of the ciliate spe cies) were less and vice versa. From this study it can be concluded that, m ainly the smaller flocs which mostly consist of small (<1 <mu>m) dispersed bacterial species, were grazed heavily by the predator population in the me mbrane-separation activated sludge.