Masters Theses

Date of Award


Degree Type


Degree Name

Master of Science


Civil Engineering

Major Professor

Gregory D. Reed

Committee Members

Chris D. Cox, George Hyfantis


The focus of this investigation was the determination of air flotation parameters to perform solid liquid separation treatment in an activated sludge treating grease waste by promoting filamentous bacteria. The treatment was intended to achieve a pretreatment level, previous to the application of the typical activated sludge process.

Generally solid liquid separation in activated sludge is performed by gravitational settling, but it is been demonstrated that the presence of an overgrowth population of filamentous microorganism in activated sludges hinder the settling properties of it. For this reason, gravitational settling may result in an inefficient treatment. Therefore, an alternative treatment has to be applied.

The treated waste in this project has a density lighter than water; additionally filamentous microorganisms tend to trap fine bubbles therefore causes floating. These specific conditions make air flotation a suitable alternative technology. The ranges of minimum operation for Dissolve Air Flotation (DAF) controlling parameters (air pressure, recycle rate, rise rate, air to solid ratio, and solids recovery) were determined. For this purpose a descriptive model for the treatment was developed. The research determined that the traditional gravitational settling was not adequate for separating the biomass from the liquid phase. In contrast dissolve air flotation showed that it was possible to achieve the pretreatment level (300 mg/L in the final effluent).

In the analysis the correlations between the parameters under study (air pressure and recycle rate) and the response variables (rise rate, air to solid ratio, suspended solids in the subnatant, and solids recovery) were determined. With the relationships defined, the minimum ranges for the parameters of operation of a DAF unit were determined. The results showed that in order to achieve the pretreatment level air pressure should range between 45-70 psi and the recycle rate between 30-35%. All the values for the rest of the response variables were determined to be within the ranges of the typical application of DAF, therefore the same equipment used for the typical application can be used to apply this treatment under these specific conditions.

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