FLOC CHARACTERISTICS IN WATER AND WASTEWATER TREATMENT

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Abstract
The behavior of an aggregate of colloidal particles, or floc, in a water treatment process is dependent upon its physical characteristics, including size, strength, density, and permeability. It is no exaggeration to say that the success of a water treatment operation can be gauged by the properties of the aggregates produced. Size and density are particularly important in sedimentation processes where the rate of solids subsidence constrains plant operation. Strength and permeability are indicators of the ease with which sludge may be dewatered for ultimate disposal. An experimental study of how floc characteristics are interrelated has been completed. In the case of kaolin-polyacrylamide aggregates, physical properties are largely determined by the conformation of the macromolecular bridges. Measurements of the viscosity of the dilute polymer solution under varied pH and ionic strength conditions have shown that macroion extension is greatest at a pH of about 6.5 for minimal counter-ion concentrations; the addition of a small amount of NaCl, however, moves the point of maximum extension to a pH of about 10. Under conditions more representative of coagulation processes, floc permeability Increased sharply as the flocculator pH was elevated beyond about 9.7. Minimum permeability for this type of floc was observed at a pH of about 9. The influence of the various chemical parameters is profound, and it now seems certain that “best” plant operation will represent a compromise between treatment objectives and process efficiency.