Synergy of Flotation and Filtration in Wastewater Treatment and Recycling, 1994 Recycling Symposium Proceedings

D.B. Guss

A flotation clarifier is very effective for removal of light contaminants and various flocculated solids from filtrates and wastewaters. Air is dissolved in water under 5 atmospheres of pressure. The air is then released by decompression to form very small bubbles which combine with flocculated solids, andeffect a very rapid separation of the solids by a gentle flotation action. The separation is faster than that which can be achieved by settling, and does not require dense, heavy flocculated solids. Separated solids are thicker, and easier to dispose of than settled sludge. Compared to screening, flotation clarification can remove much finer solids because the gentle separation does not tear apart flocculated solids. In contrast to a mechanical screen, the mechanism does not plug, because the “mechanism” consists only of air bubbles. For these reasons, and because the most troublesome contaminants naturally float, most process designers of paper recycling systems choose to use flotation for removal of suspended solids in effluents. Mechanical screening and filtration are also vital in a secondary fiber process because it is necessary to separate solids by size, and the mechanical barrier of the screen or filter gives a guaranteed particle retention or pass-through. Figure 1 illustrates the particle size retention of flotation and screening as used for effluent treatment in secondary fiber processing systems.

When flotation is combined with screening or filtration, the resulting combined process is synergetic. This means goals can be accomplished which are not achievable in either process by

itself. The advantages and particle separation ability of each process can be combined to produce the most economical and effective separation. Three of these synergetic combinations are described in the sections that follow:

- Fine screening followed by flotation

- Flotation followed by screening

- Flotation followed by sand filtration