Separation of oil from water by dissolved air flotation

Synthetic industrial effluent was prepared by stabilising low concentrations of oil (Catenex 11, Shell, UK) in aqueous dispersion with a non-ionic surfactant (Span 20) and dissolved air flotation (DAF) was used to clarify this wastewater. Operating parameters including saturator pressure, recycle ratio, and air to oil ratio, were investigated to find conditions that would enhance the efficiency of the DAF separation. The zeta potential of the emulsion droplets and flocculated oil droplets was determined and aluminium sulphate and four different cationic polyelectrolytes were used to destabilise the system. Measurements indicated that a saturator efficiency of about 90% was achieved and it was found that increasing the working pressure of saturator had less effect in the separation of oil. droplets than increasing recycle ratio. Optimum conditions for separation are obtained with an air to oil ratio of 0.0075 corresponding to a recycle ratio of 10%. The zeta potential measurements indicate that oil droplets are negatively charged between pH 5 and 10. In addition the zeta potential measurements of flocculated oil droplets were found to depend strongly on the concentration of the polyelectrolyte. Although an inversion of the oil droplets charge was observed even at very low polyelectrolytes concentration. The polyelectrolytes were ineffective in enhancing the separation. This effect can be related to the structure of the polyelectrolytes and to the adsorption mechanism. In contrast, when aluminium sulphate was used it was found that it is important to decrease the magnitude of zeta potential in order to decrease electrostatic repulsion so that the emulsion was destabilised prior to flotation. Under these conditions DAF yields near complete oil separation even at moderate working pressure and recycle ratio when the oil droplets are destabilised. (C) 2002 Elsevier Science B.V. All rights reserved.