Flotation using air

Method diagram

  

Method and installation description

The aim of flotation is to remove oils, fats, suspended matter and undissolved particles from wastewater. Flotation can be used as a pre-treatment in the purification process, for biological purification, for example, but is often also used as the only step in the purification process.

The diagram shows the most commonly used set-up. This is referred to as dissolved air flotation (DAF). Air is dissolved in water under pressure (5-7 bar). When water saturated with air is released into the floatation unit, small bells (20-100 µm) are formed which interact with floccule-shaped particles. This causes the total density to become less than that of the water, whereby the particles float to the surface. The saturation of water with air is normally carried out in a pressure vessel or by injecting air into the suction-side of the pump. Other less commonly used possibilities include spraying water in an unpacked or packed saturation column (saturator). A DAF can be set up with or without effluent recycling.

Coagulants and/or flocculants are generally added prior to flotation in order to enhance the flotation effect. This converts smaller particles into larger particles with better floating qualities. A package is sometimes placed in the flotation unit, which allows particles to unite more easily and better highlights the flow pattern – this package separates the particles from the water flow. A DAF unit features a scraper at the top, which scrapes away the floating layer. This thickened and scraped floating layer is disposed in a tray or container that is located next to the flotation unit. The sludge can be further thickened and/or diluted.

Specific advantages and disadvantages

A major advantage, compared to a sludge separator, of removing particles using a DAF unit is that a DAF unit generally realises a higher solids content. Another advantage of the DAF unit is that it separates a wide variety of substances. Again, substances that dissolve effectively are not removed by this system. The additional cost of chemical support aids (coagulants and flocculants) is generally relatively high, particularly if high yields are required.

Application

Sectors in which DAF installations are implemented, include:

  • Food sector: Flotation is often used for fat removal in biological treatments in meat processing, slaughterhouses and dairy products;
  • Pre-purification of water that is released in barrel cleaning or tank cleaning.
  • Removal of mineral oil and other difficult to dissolve hydrocarbons from wastewater and polluted rain water, including in oil refineries. Floatation can be accompanied by stripping volatile substances, like H2S, NH3 and VOCs;
  • Separation of sludge in biological purification, as an alternative for sludge sedimentation.

Boundary conditions

DAF units work at environment temperature and pressure. The capacity varies from 100 litres/hour to in excess of 1.000 m³/hour. The dosage of coagulants and/or flocculants is determined by the amount of pollution.

Effectiveness

Flotation can be implemented for removing the following parameters:

  • Suspended matter (50-90 %);
  • Animal and plant oils and fats (80-99%);
  • COD (50-90 %).

Support aids

Coagulants (like iron chloride, polyaluminium chloride, short molecular cationic polymers, and polytannines) and flocculants (like organic poly-electrolytes, in the groups anionic, cationic and non-ionic polymers), which can be dosed prior to the DAF or in the DAF.

Environmental issues

Sludge is released as by-product. Components may be emitted into the air (strip gases: H2S, NH3). This could cause odour problems.

Costs

Case study 1 (2007)

volume 70 m³/hour, DAF installation with pipe flocculator, poly electrolyte-creation unit and dosage pump and sludge pump: € 105.000 investment cost

Case study 2 (2008)

volume 30 m³/dag, DAF installation with dosage pumps, measurement and regulation equipment, sludge tank and  controllers:  € 73.600 investment cost

Comments

Flotation is suitable for separating lighter substances, particles or floccules when the aim is to primarily remove heavy particles or floccules via sedimentation.

Complexity

A certain amount of experience is required to correctly configure a flotation set-up on the basis of visual observations. It is best to establish chemical doses via prior lab tests.

Level of automation

Can be fully automated.

References

  • Baeyens J., Hosten L. and Van Vaerenbergh E., Wastewater purification, Environment Foundation - Kluwer Editorial, 1995
  • EIPPCB, Reference Document on BAT in Common Waste Water and Waste Gas Treatment / Management Systems in the Chemical Sector, draft February 2009 (revision upon release)
  • VITO-SCT, revision of technical notes WASS, 2009

 Version February 2010