AIR FLOTATION UNITS

Description

Flotation techniques are used to separate solids and oils from liquid effluents. The mixture may be treated with coagulation and flocculation agents to encourage the agglomeration of the solids or oil. When air or gas is injected into the mixture, bubbles attach themselves to the agglomerated particles and float them to the surface to form a blanket. This blanket is removed from the surface before the particles have a chance to become resuspended in the clarified liquid.

There are two different types of air flotation unit. Induced air flotation uses an aeration device such as a venturi nozzle to introduce air bubbles into the tank. In dissolved air flotation (DAF), a proportion of the clarified liquid is diverted to a saturating device. Air or gas is saturated in the liquid at 400-800 kPa. When the pressurised liquid is released into the DAF tank, clouds of very fine bubbles of 30-60 µm diameter are formed. This white water is capable of floating much finer particles than the coarser bubbles produced by induced air flotation. Clarified water is removed from the centre of the unit.

The blanket of floating material is removed by brushes, scrapers or baffles. During flotation, heavier material falls to the bottom of the unit where it forms a sludge. This has to be removed periodically either automatically or manually.

The air flotation tank can be rectangular or circular and may be relatively shallow. Where space is limited, shallow tanks can be stacked. Deeper tanks can incorporate parallel plate technology to enhance oil separation.

Application

Air flotation can be a rapid continuous method of removing solids, including insoluble COD, or oils from a liquid stream. Units are used in the paper industry and in the, milk, meat, chemical and drinking water industries. DAF is particularly suitable for streams that are to be recycled and where the solids are difficult to remove by filtration or centrifugation.

Air flotation units can be designed for flows of 4-3000 m3/hour and will reduce the suspended solids concentration of wastewater from 3000 mg/l to 100 mg/l. When combined with screening, air flotation can separate 90 % of the BOD and 98 % of the fat from a waste stream.

Sizing

Typically a 50 kW pump and compression system is required, and the power consumption is about 3 kWh for each cubic metre of wastewater to be treated. Induced air flotation has a lower capital cost than DAF, but cannot achieve the more difficult separations, such as emulsified oil.