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K.J. Ives Professor of Public Health Engineering University College London The aggregation of small particles in liquids, to form flocs which are large enough to settle, or to be filtered, is a common operation in industrial processes, and water and wastewater treatment. This aggregation, given the general title flliocculation in this book, may be brought about by the addition of chemicals to reduce the stability of the original suspension, by neutralising electrical forces of repulsion, by the addition of chemicals (polymers) to link particles by bridging action, by the addition of chemicals which form particles to increase collision proba bilities, and by the input of energy leading to hydrodynamically induced collisions. The particles undergoing flocculation may range from colloidal in the nanometer size range, through micro scopic (micron) size, up to visible particles in the millimeter size range; that is a total size range of six orders of magnitude. Consequently the colloid chemist and the hydrodynamicist are both concerned with the interactions that take place, and to them the engineer must turn, to obtain the fundamental information ne cessary for the process design and its associated hardware.