A slender type diatom is observed. The settling behavior and surface electrostatic situation of the diatom are discussed through the consideration of physiological activity and observation by SEM. By considering the settling behavior, one side of slender type diatoms such as Nitzschia and Synedra is heavier than the other side. The settling velocity is related to physiological condition of algae, for example the settling velocity in the endogeny phase is faster than it is in the logarithmic growth phase. The surface charging of diatoms is increased (more negative) by the existence of mucous algal borne substances at the surface of diatoms. It influences the neutralization. Aluminium hydroxide adsorbs at some specified points on the surface of diatoms, then the area covered aluminium hydroxide is spread. It is related to floc formation ability of the diatoms by bridging action.

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