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Flotation for WWTPs (DAF)

For:industrial waste water, thickening of excess activated sludge


Dissolved air flotation (D. A. F. or DAF) is a process used to separate the oil and grease that flows out with the waste water produced by industry (mainly food industry).



  • Lower investment and operating costs for the subsequent purification stage (activation)


Principle of DAF

Dissolved Air Flotation is a separation process used to separate the solid phase from the liquid.

The principle of separation by flotation the suspension is air microbubble aggregation with flakes resulting suspension.

The resulting assembly has an overall specific gravity less than water, it is thus discharged to the surface.

Flotation foam grows on the surface, then is mechanically removed.

Ascending speed of aggregates formed during flotation is one or two rank higher than the original sedimentation speed of these flakes, this allows to work at high surface load of flotation as compared to sedimentation.

Technical specifications

The first step is pre-treatment mounted rotary screen, placed ahad of flotation unit EP DAF.

The rotary screen is used for continuous separation of suspended solids from the wastewater (e.g. parts of the bones, skin, bristles, glass, paper etc.). The main work area is a rotary drum screen. Water is delivered by a pump or gravitationally flows on a rotary drum screen. Mechanical contamination that exceeds the dimension of the slot remains on the outer surface of rotary drum screen, then is stripped off and the spatula falls into the container. Water free from mechanical impurities is drained from the bottom of rotary drum screen.

In the case of physical flotation process flotation is not intensified any chemical agent and to remove impurities are only using microbubble air.

Physical flotation

Flotation unit is brought into the operating state - flotation tank is filled with pre-treated waste water from rotary screen, then circulation pump turns on, upon displacement of the pump is supplied with air. The pressure in the air valve is 5-6bar.

The stream of the circulated exhaust air-saturated water from the air valve is fed into the flotation space. The air contained in the waste water is circulated in the inlet section releases, due to pressure changes, in the form of very fine bubbles (size 30-50 µm).

Gradually throughout the volume of the inlet section of the flotation chamber and generates fine air dispersion. There is a physical process of flotation, when tiny air bubbles adhere to the particles of pollution and cause the buoyancy necessary to plot these particles to the surface of the flotation tank.

Due to the large amount of air bubbles, this process is very intensive, so that the surface layer gradually develops floated sludge.

Water, which is devoid of contaminant particles by flotation, flows through the lamella blocks. Blocks facilitate the process of sedimentation of particle pollution, which can not be separated by flotation. The sediment settles in the bottom of the flotation tank.

Treated water by flotation and sedimentation lamella built-flows, invades over the edge to discharge trough outlet manifold flows of flotation unit.

Flotation foam is wiped from the surface of the flotation tank by scraping device into drain trough and that flows into the container.

Sediment is discharged through the bath to trap sediment.

Chemical and physical flotation

In this case, the flotation process is intensified in addition a suitable dosage of the flocculant or coagulant.

For this purpose, the flotation unit to add the chemical units (dissolving and storage tanks for preparing chemical solution). Part of the storage tank is a dosing pump. Before flotation unit is associated with a pipe used for mixing wastewater with a coagulant (flocculant), which helps to remove organic contamination (SOD,SS).

Furthermore, the flotation unit is completed by a tubular mixer which, is linked to the supply pipe flotation unit, used for mixing of influent wastewater with a chemical solution. Treatment of these chemical additives is achieved precipitation emulsified pollution, sludge flocs formed are then easily separated by flotation.

Coagulants may be used alternatively:

ferric sulphate Fe2(SO4)3
ferric chloride FeCl3
aluminum sulphate Al2(SO4)3
Polyaluminium chlorid PAX

Cationic polymer flocculants could be used eg.:

Praestol A7.112
Praestol 524 BC
Zetag 92

PH adjustment can be dosed slaked lime, soda or calciumhydroxide.

The optimal dose and type of chemical agents are determined only on the basis of coagulation and flocculation test.

Description of the flotation unit EP-DAF

Flotation tank

is divided into  inlet part, flotation and sedimentation space, trough for discharging flotation foam and outlet.


is used for wiping the flotation foam from the surface of the flotation tank to the discharge chute. Mops are attached to chains that are stretched between the sprockets. It is driven by electric. transmission

Tubular mixer

Use d for mixing with influent wastewater chemical agents. Places into the supply line to the flotation unit.

Recirculation circuit

volume portion flotation tanks circulates, while there is a saturation of the circulating air volume of wastewater . It consists of a circulating pump suction and discharge piping. Part of the discharge pipe is air vlave, from the offgas system of hoses.  These hoses distribute circulated water saturated air into the flotation chamber flotation tanks.

Chemical units

used for the preparation and dosing of flocculant or coagulant. These chemical additives are used to intensify the process of flotation. Cause precipitation fouling and emulsified flakes which are readily separated by flotation.

Expected reduction of pollution

Flotation is significantly reduced especially extractable impurities (lipids), significant reductions are also indicators of organic pollution (BOD, COD). Pollution reduction efficiencies are given in the following table:

Parameter Physical flotation Chemical and physical flotation
Extractable impurities 60 - 75 % 80 - 90 %
SS 60 - 75 % 80 - 90 %
BOD5 25 - 40 % 55 - 65 %
COD 25 - 40 % 55 - 65 %

Technical specification

    EP-DAF-5 EP-DAF-10 EP-DAF-20 EP-DAF-30 EP-DAF-40 EP-DAF-50
Maximum flow m3/h 5 10 20 30 40 50
Total volume m3/h 2 2.5 4.5 6 11 11.5
Useful volume m3/h 1.7 2 3.8 5.3 8 10
Area level m2 1.8 2.7 5.4 4.7 4.7 8.5
Weight without filling kg 500 800 1000 1400 1600 1800
Weight with filling kg 2500 3300 5500 7400 11100 13300
Installed power kW 4.37 4.37 5.87 7.87 11.37 11.37
Lenght L m 2.7 2.7 3.9 3.4 4.4 4.4
Width B m 1.5 2 2.3 2.5 2.5 2.9
Height H m 2 2 2.2 2.6 2.6 2.6
Inlet A   DN100 DN100 DN100 DN200 DN200 DN200
Outflow purified water B   DN100 DN100 DN200 DN200 DN200 DN200
Outflow of flotated sludge C   DN100 DN100 DN200 DN200 DN200 DN200
Sediment outflow D   DN80 DN80 DN150 DN150 DN150 DN150
    EP-DAF-60 EP-DAF-70 EP-DAF-80 EP-DAF-90 EP-DAF-100
Maximum flow m3/h 60 70 80 90 100
Total volume m3/h 12 15 17 19 24
Useful volume m3/h 10.8 12.5 14.3 16.3 22
Area level m2 9 10,3 11,7 10 13
Weight without filling kg 2000 2300 2600 2800 3000
Weight with filling kg 14000 17300 19600 21800 27000
Installed power kW 15.37 15.37 15.37 15.37 15.37
Lenght L m 4.6 5.2 5.7 6.3 6.2
Width B m 2.9 2.9 2.9 2.9 2.9
Height  H m 2.8 2.8 2.8 2.8 3.5
Inlet A   DN200 DN200 DN200 DN200 DN200
Outflow purified water B   DN200 DN200 DN250 DN250 DN250
Outflow of flotated sludge C   DN200 DN200 DN200 DN200 DN200
Sediment outflow D   DN150 DN150 DN150 DN150 DN150

Design of Flotation Unit

The flotation unit we will design according to the specific conditions of operation!

An important design parameters is the degree of pollution of wastewater inflow, desired output quality purified water and flow needed equipment.