Smaller metazoa appearing in biological treatment process are divided into two types. One is filter feeder such as Rotatoria and another is detritus feeder such as Oilgochaeta. To make clear the role of smaller metazoa in water purification, Philodina erythrophthalma and Aelosoma hemprichi were used as Rotatoria and Oligochaeta individually in this experiment. Smaller metazoa were cultivated under static and dark conditions in LE (Lettuce and egg yolk extract) medium containing several bacteria. The run of experiment consisted of only bacteria (B), bacteria+A. hemprichi (BA), bacteria+P. erythrophthalma (BP), and bacteria+A. hemprichi+P. erythrophthalma (BPA) . Purification efficiency was estimated by TOC, viable count and suspended solid in cultivation medium. The results obtained from this experiment were as follows.
(1) In the B system, the turbidity is high and the efficiency of organic matter removal is low because of existence of the large amount of dispersed bacteria.
(2) In the BA system, the amount of biomass decreases because the detritus feeder such as A. hemprichi feeds on the detritus, but the turbidity is still high and the efficiency of organic mater removal is not so high because of the presence of dispersed bacteria.
(3) In the BP system, the biomass and turbidity decrease and the efficiency of organic matter removal increases, since the filter feeder such as P. erythrophthalma feeds on the dispersed bacteria.
(4) And in the BPA system, the amount of biomass decreases most remarkably along with the turbidity reduction and improved efficiency of organic matter removal, because the coexistence of A. hemprichi and P. erythrophthalma whose feeding mechanisms and food habits differ from each other, promotes their predatory activities on the dispersed and flocculated bacteria.
It can be concluded that the detritus and filter feeder smaller metazoa in bio-film are contributing greatly to water purification and reducion of biomass.