Cation-dependent flocculation in a Flavobacterium species predominant in activated sludge

Abstract

The predominant bacterium of activated sludge classified as belonging to the genus Flavobacterium showed good flocculent growth in the presence of both calcium and magnesium ions, although capsular material or gelatinous matrix was not detectable in the flocs. The bacterium accumulated a large amount of poly-beta-hydroxybutyrate when grown on glucose, but not on peptone, in spite of good flocculent growth on both substrates. When the flocs formed during growth phase were suspended in deionized water and shaken for a few minutes, they disintegrated perfectly, and a uniformly dispersed cell suspension was obtained. Furthermore, when the dispersed cell suspension thus obtained was added with each of various mineral salts and shaken for a few minutes, the cells flocculated again, the resultant supernatant solution being almost clear. Even if the dispersed cells were killed by heat or treated by trypsin, they did not lose the ability to form flocs when added with mineral salts. Since the electric charge of the cell surface was negative, a possible mechanism of floc formation was suggested as follows: negatively charged surfaces of adjacent cells are bridged by ionic bonds intermediated by cations. The bacterium utilized a relatively wide variety of organic compounds and showed high metabolic activity comparable to that of naturally activated sludge. These properties, along with floc-forming ability, were considered to be factors for making the bacterium predominant in activated sludge.