WASTE ACTIVATED SLUDGE THICKENING
BY COUNTERCURRENT FLOTATION
Jenoe T. Albert, Technical Director
Liquid/Solids Separation Equipment
Ingersoll-Rand Co.
Nashua, New Hampshire 03061
INTRODUCTION
An extensive development program has been conducted by Ingersoll-Rand on dissolved-
air flotation thickening. Laboratory research showed that the well known countercurrent
flow concept when applied to flotation made a major improvement in the performance of
a flotation thickener. The countercurrent flow pattern achieves two to three times higher
solids loading per effective flotation area than conventional plug-flow type flotation
thickeners.
The development history of this new type flotation unit provides information on basic
development and operating data generated with various-size development units.   This paper
includes a comparison study showing performance data of both the plug and countercurrent flotation thickeners operating side-by-side on brewery, textUe and municipal waste
activated sludges.   Performance data on commerical size countercurrent flotation thickeners are also presented.
Several methods are avaUable to thicken waste activated sludges.   Gravity thickening,
dissolved-air flotation and centrifugation are among the most frequently used methods.
Dissolved-air flotation was introduced in the 1930's in Scandinavia for industrial use and
later for wastewater treatment; however, in the U.S. only in the last decade has flotation
gained wide acceptance.   This dewatering method has proved to be one of the most efficient methods to thicken waste activated sludge up to 4 to 6% when the suspended solids
in the feed is less than 1.0%.
The commerical flotation thickeners used today are noted for their large surface areas.
The majority of these thickeners have a 100 to 300 ft2 flotation area.   However, a 5000 ft2
steel fabricated unit is not considered unusual to be installed in a wastewater treatment
plant.
To increase the solids loading capacity of the conventional flotation thickeners per effective flotation area, Ingersoll-Rand Co. conducted basic research on the subject. Among
the various flotation systems tested in the laboratory, the countercurrent flotation system
proved to be the most efficient system by tripling the solids loading capacity per effective
flotation area of the plug-flow type flotation systems. Basic development information on
the countercurrent flotation system on laboratory, pUot and commerical scales wUl be discussed in this paper.
CONVENTIONAL FLOTATION THICKENING
In the flotation process liquid is super-saturated with air at 30 to 70 psig and is released
continuaUy into a flotation tank at near atmospheric pressure.   The air comes out of solution in the form of micro size bubbles.   These bubbles are contacted by and become attached to suspended solids particles.   The specific gravity of these agglomerates is less than
water, which causes their rise to the surface, forming a float blanket which is removed by
a top scraper.
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