Activated Sludge Thickening by
Dissolved Air Flotation
G. A. ETTELT, Chemical Engineer
The Metropolitan Sanitary District of Greater Chicago
Chicago, Illinois
INTRODUCTION
Sludge thickening became a major and urgent research problem at the
Chicago Southwest plant when the Zimmermann Wet Oxidation (Zimpro) Process
was first considered for the ultimate disposal of activated sludge. In order to
maintain this combustion process on an energy basis, the sludge must contain a
minimum COD of 30 gms per liter, whereas a COD of 60 gms per liter would enable the process to be self-sustaining. This corresponds to the approximate activated sludge solids concentration of three and five per cent, respectively. Increases in the sludge handling capability of the combustion system would also be
provided by the attainment of more thickened sludge. In addition, the performance of the digestor units, presently under construction, would benefit from the
greater feed solids.
Thickening of activated sludge by settling is not adequate since concentrations of only 1.2 to 2.5 per cent solids are obtained. If we utilize 50 per cent
by weight of preliminary sludge, five per cent solids are possible. However, the
activated sludge handling capacity is then decreased notably. Preliminary sludge
disposal is not of primary concern in our present operation. Moreover, the supply
of preliminary solids would be limited accordingly. Obviously, another means of
solids thickening is needed and attention was directed to the flotation technique.
Flotation process has long been employed in industry, especially in mining
and refineries for two-phase separation. Generally, the process is applied to
systems where there is a low concentration of insoluble or immiscible particles
suspended in a bulk liquid. Either the suspended particles are quite small (nearly
colloidal) or they have a density comparable to that of the bulk liquid. Air
bubbles are introduced into the system to combine with the particles resulting in
an aggregate with a density sufficiently less than the bulk density to effect flotation.
MECHANICAL FLOTATION
The mechanical flotation process, also described as dispersed air flotation,
utilizes air bubbles formed either by agitation, e.g., cavitational action of impellers, or by passage of compressed air through screens or diffusers. Bubbles
usually are relatively large compared to the solids (1), producing heavy foaming
at the surface. From experiments conducted with the Denver Laboratory mechanical flotation cell (2), maximum solids concentration of only 2.8 per cent was
obtained. Recovery of the total solids in the feed as floated solids ranged from
40 to 80 per cent. However, these results were achieved with the use of collector and frother agents and at high air input. The gain in solids thickening by
this process over settling was not significant.
DISSOLVED AIR FLOTATION
Interest in dissolved air flotation process followed from literature reports on
laboratory experiments with sewage sludges showing greater solids thickening by
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