All processes for particle removal in potable water treatment require proper design and control of solution and interfacial chemistry and of interparticle collisions. This is also true for most processes used to remove particle-reactive pollutants such as natural organic matter (NOM) and arsenic. The treatment processes involved span a range from conventional plants with coagulation, flocculation, sedimentation and packed bed filtration through direct Filtration, flotation, solids contact systems and microsand facilities and on to various membrane technologies. The "collisions" provided by each of these technologies differ in important ways. It follows that the "chemistry" most appropriate to each of them can also differ significantly and substantially. Similarly, the aggregation and settling of particles in rivers, lakes, estuaries and the ocean and also the deposition of colloidal particles in ground waters depend on surface and solution chemistry and on collisions of suspended particles with each other or with other media such as aquifer materials. This paper presents a discussion of the similarities and the differences in the chemistry and the collisions appropriate to some of these aquatic systems. It includes a focus on the colloid chemical role of natural organic matter in potable water treatment and in natural waters.
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