Sludge Blanket Considerations In Final Clarifiers, by William H. Boyle, P.E.

In the activated sludge process, the final clarifier follows the aeration unit and its primary purposes are to separate, by gravity, the solids from the mixed liquor flow from the aeration system, to collect the settled solids at the bottom, and to return some of these solids to the same aeration basins (return activated sludge) as well as waste some to the solids handling system (waste activated sludge).

It is important to know and evaluate how activated sludge physically acts at the bottom of the final clarifier because sludge deposition and removal are critical to the mass loading (lb/day/ft²) design factor as well as to the concentrations of return and waste sludge. Examination of the logic and data from field installations provides an insight into the sludge blanket level and its solids concentration. Such an examination may show reasons for discrepancies in certain solids flux models and other clarifier design assumptions.

Logic:

    Water seeks its own level.

    Water is a fluid (liquid).

    Activated sludge acts as a fluid (liquid).

    Therefore: Activated sludge seeks its own level.

According to technical literature mixed liquor is denser than the free water in a final clarifier. This fact has been substantiated by studies showing that the heavier mixed liquor, when entering the final clarifier basin, drops to the floor or sludge blanket. It then travels along the floor until it hits an obstruction; normally the peripheral wall for the center feed clarifier. This wall effect is a major process concern when designing final clarifiers. There are few reported instances where the solids in a final clarifier do not reach the wall. It is evident that activated sludge does not settle as a discreet particle (such as grit or primary solids). Rather, the activated sludge settles in a hindered zone of influence and is not governed solely by Stokes law of settling. Activated sludge has no choice but to seek its own liquid mass level in a clarifier if no mechanical or hydraulic limitations are imparted on it.

Sludge Concentration Variance in the Basin 

The sludge, most likely, will not be a homogeneous solids mixture in the sludge blanket, nor will the sludge blanket have a constant concentration of solids per unit area of the tank floor. At two Wisconsin wastewater plants, for example, data and measurements of solids in final clarifiers show random solids contour lines of where the sludge settles. 

Since sludge in each plant is different, the relative concentration will vary, but the ‘random’ pattern of the contours probably will not. Obviously, these contour lines will not be a perfect circle about the center line of the basin. A theoretical removal point will pass through as many as eight different solids contours for half the basin. It would be most difficult to design a sludge collector for rapid adjustment to each different concentration point. 

Even if there were well defined radial concentration points, it would be impossible to say where the heavy versus the light sludge would be at any given time. These contour lines and concentration points would be very ‘flow’ dependent and could vary throughout the day and hour.