Innovation in air flotation leads to solid alternative to a legacy process

(Originally published in the June 2022 issue of Water Environment & Technology magazine. All rights reserved.)

By George Tchobanoglous, Harold Leverenz, and Dan Zeller

The total solids content of waste activated sludge (WAS) varies considerably, depending on the operation of biological reactors and clarifiers. Thickening is a procedure used to increase the solids content of WAS by removing a portion of the liquid fraction. To illustrate, if waste activated sludge, which is typically pumped from secondary settling tanks with a content of 0.4% to 0.8% solids, can be thickened to a content of 4% solids, then a fivefold decrease in sludge volume and a corresponding increase in digester retention time is achieved.

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The focus of this paper is on the performance of a relatively new process known as Suspended Air® flotation (SAF®), employing aphron-based technology for the thickening of WAS. Aphrons are used because they are resistant to coalescence, and their chemistry can be tuned to adsorb constituents based on surface charge. In addition to achieving high WAS solids separation efficiency, based on onsite testing, the principal advantages of the SAF® process include relatively small process footprint, low power requirement, low chemical usage, and the ability to handle high concentrations of suspended solids (up to 16,000 mg/L in this study).

Based on the results from a Kansas facility, the SAF® process is an attractive alternative to the legacy dissolved air flotation (DAF) process. Because of the effectiveness of the SAF® process, as described in this paper and in other studies, it is anticipated that it will find broader applications in the field of wastewater management.

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