An alternative to secondary settling tanks offering itself is flotation with additionally improved separation efficiency. Very small gas bubbles of only few thousands of a millimetre attach to the activated sludge flocks and very fine turbidities. These gas-solids agglomerates become much lighter in density than water and therefore float quite fast to the water surface in the flotation tank. Such very small gas bubbles are generated by the principle of dissolved air flotation. They are produced by injecting air under pressure into a recycle water flow where they become completely dissolved before they are released again through pressure relief in a flotation cell. Due to the small size of the bubbles, even turbidities can be transported to the surface of the flotation tank that could not be separated during their residence time in a secondary settling tank.
Improved process and energy balance of the overall plant
The solids rising with the gas bubbles accumulate at the surface of the flotation tank to up to 5% solids content and can be returned to the system as return sludge. Alternatively, the thickened sludge can be removed as surplus sludge and passed on to downstream treatment stages.
A sludge flotation plant can be fed with up to 5m³ activated sludge per m² surface and hour. In practice, significantly better effluent values are achieved than with secondary settling tanks. Moreover, flotation plants need much less space. The slightly higher energy costs for operating a dissolved air flotation plant relativise due to the improved overall process.
Flotation systems are optimally suitable especially as retrofits where space is limited or for the refurbishment of existing plants without any major construction work. Flotation plants are also increasingly applied as tertiary treatment stage for so-called ‘polishing / P-reduction’.