Flotation in Water and Used Water Purification

This chapter provides an overview on diverse flotation technologies with the main discussion point being dissolved air flotation (DAF). Flotation is a clarification process which can be described as the opposite to sedimentation. Instead of precipitating the particles, flotation relies on the attachment of bubbles onto suspended matter, hence driving them to the surface of the water against gravity. The contaminated water is firstly coagulated and flocculated, and flows by gravity into the flotation tank where the flocs collide with bubbles and the newly formed aggregates rise to the water surface. The top sludge layer is removed while the clarified water is withdrawn from the bottom. Many parameters influence the clarification performance of DAF, the most important being the coagulation pretreatment and bubble size. The addition of chemicals and pH adjustment during coagulation modifies the particle properties and thus insures an optimal bubble attachment. In comparison, bubble size heavily influences the flotation performance. Large bubbles generate turbulent flows which can disturb the top sludge layer. In contrast, bubbles that are too small have a reduced rise velocity and thus decrease the overall performance. However, for DAF applications small bubble sizes are preferred. Dissolved air-flotation is mainly employed for the removal of algae and turbidity and possesses many applications such as sludge thickening, drinking water production, primary or secondary clarification of diverse industrial effluents, and seawater pretreatment. Furthermore, with the addition of surfactants the removal of dissolved ions can also be achieved. The flexibility of DAF should not be underestimated. Flotation comes in many variants and can be tailored to the desired purpose. This is further exemplified with two case studies.