Dissolved air flotation unit (DAF)

Dissolve air flotation "AKEP"

Introduction

Flotation is a primary procedure and it is applied on waste effluents in order to remove solid particles from liquid as well as during the procedure of sludge thickening. The principle of the separation lies in “Stocks’ Law” and more specifically in

1. The difference of the relative density of the solids compared to the liquids

2.The formation of solid aggregates during coagulation

3. The supply of air bubbles that promote the ascent of the formed aggregates to the surface of the tank.

The supply of air bubbles help the formed solid aggregates emersion and then the latter are removed by a scraper into a tank for further treatment.

Shape of a flotation tank

The shape of the flotation tank depends upon flow. For small flows up to 40m3/h the preferred shape is square whereas for greater flows the shape is circular.

Flotation equipment

A complete flotation unit consists of

1. COAGULATION UNIT. Coagulation is performed in two tanks equipped with agitators. In the first tank, which is equipped with a flexuous agitator (40rpm), the solids begin to form aggregates with the addition of chemicals. In the second tank the aggregates, with the help of slower agitation and the addition of chemicals, are allowed to form larger entities. The formed aggregates are visible to the naked eye. The size of the tank, retention time, chemical additives and agitation speed varies and depends upon the type, the flow and the chemical identity of the waste effluent.

2. MAIN FLOTATION TANK. The coagulated wastes are transferred through pipes into the main flotation tank. In an apparatus located by the flotation tank a small portion of air is diffused in a given amount of water. The water then expands in the flotation tank under pressure. The coagulated formations have to obey “Stokes’ Law”, therefore the emersion of the former is achieved. The process is very quick and it takes around 15min for the aggregates to arrive on the surface of the tank.  The shape, size and amount of supplied air bubbles depends upon the chemical identity of the waste. Then a scraper removes the aggregates and transfers them into another tank for further treatment.

3. DEVICE FOR MIXING WATER WITH AIR. The necessary amount of air is produced in a regular compressor. Then through pipes it is transferred to the water line. The mixture of water-air is homogenized in a pressurized chamber and finally is diffused in the flotation tank.

4. CHEMICAL ADDITIVES PREPARATION UNIT. Chemical additives are necessary in order to form suspended particles of the effluent. Two preparation units with agitators are installed next to the two coagulation tanks, where the dilution of polyelectrolyte takes place. The addition of chemicals into the tanks is performed with the use of dose pumps which are fully automated.

5. TANK FOR THE FORMED SLUDGE. The formed sludge produced by the emersion of the aggregates is removed from the surface of the flotation tank with a scraper and it is transferred through pipes into a tank for further treatment. The exact shape and size of the tank varies.