introduction to Dissolved Air Flotation

Almost all treatment processes involve the use of some separation method for the removal of suspended matter.  

In sewage treatment, the aim of the primary treatment is to remove the influent suspended solids. In the secondary treatment stage, micro-organisms transform soluble and colloidal organic matter into biological flocs, which are separated from the water.

In chemical treatment the addition of coagulants transform colloidal matter into chemical flocs which also must be separated from the wastewater.


Biological and chemical flocs normally sink and can thus be separated by sedimentation.  However, as the density of the flocs generally is close to that of the water, they sink slowly; large settling units are consequently required. 


Most problems in sewage treatment plants can be traced to separation problems, caused by too light flocs or fermentation of the biological sludge specially in hot climates.

In all chemical-physical treatment process’s the use of coagulants and polymers in the appropriate concentration, result in a solid to liquid separation.

By using a polymer with ionic charges opposite to that of the suspended solids, the solids surfaces are neutralised with a subsequent electrochemical bridging and trapping of micro air bubbles, that results in an extremely strong absorption between the solids and the micro bubbles. The apparent specific gravity of the resultant floc is between 0.4 to 0.6.Thus the solids separate from the liquid and instantaneously float.


Rising velocity together with flow rate determine the required separation area. In plate separators, a large separation area in a small volume is obtained, where the effective area is then reduced to the horizontally projected area of the plates. Laminar flow is maintained between the plates resulting in predictable performance.

A combination of a screening and a dissolved air flotation can result in COD removals up to 85%, free fat removal of over 90% and suspended solids removal over 95%. Exact reductions are dependent on degree of pollution and wastewater origin. A combination of screening, flocculation and dissolved air flotation can result in COD removals of more than 90% and fat removals up to 98%.

Sludge production, from a dissolved air flotation system, is generally < 1÷1,5% of the treated effluent. This value is dependent on the treatment parameters and chemical dosage.  The consistency of sludge, obtained from a DAF process, generally reaches a SS content between 4 and 8 %, depending on the chemicals added to the process as well as the sludge blanket layer regulation.


‘Off the Shelf’ (or entirely prefabricated) and compact plants allow the purification process to start quickly, and at the same time requiring a small amount of space.  The DAF process, with respect to traditional technologies that make use of sedimentation, has the following significant advantages:

  • High velocity separation and short retention time, resulting in very compact space-saving equipment;

  • Allows immediate start up of the entire process line, with automatic start and stop according to the effluent flow rate of the targeted waste stream;

  • Provides for higher quality and homogeneity of the treated effluent, which allows, under many circumstances, the recycling of the treated water;

  • Easily thickens bulking sludge and performs well even under adverse weather conditions;

  • Produces improvements in the thickening of the sludge, therefore increasing economy in storage and/or waste removal;

  • Produces greater efficiency in anaerobic digestion, due to the high concentration of solid in the sludge, which can be freely adjusted between 3 to 6 percent, thus avoiding the need for further thickening.

  • Standard range of fully prefabricated machines cover flow rate ranges 
    from 700 to 400.000 litres/hour.

    DAF tanks are completely built in stainless steel AISI 304 or 316 for specific requirement.   Pipes and every part in contact with the liquid is also made is stainless steel or in polpropylene.   

    Each model can be equipped with : 

    • On-line flocculation reactor

    • Automatic or semiautomatic chemical facility for powder products

    • Dosing pumps for chemicals with manual or automatic and proportional control connected to pH or Folw meters

    • General process automation cabinet with PLC and OP control

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