Flotation, unlike sedimentation, is a solid-liquid or liquid-liquid separation technique used to separate aggregates with a lower density, from the liquid containing them. These aggregates are eventually recovered as scum (floated muck) off the units' surface.
Categorization of flotation
When the density differential between the aggregates and the water is naturally adequate for separation to occur, it is referred to as natural flotation. When outside help (gas, air, nitrogen, or natural gas, and eventually reagents) is used to facilitate the separation of particles that are naturally floatable, it is referred to as assisted flotation (but having an inadequate separation rate).
When does floating occur?
Floating is said to be induced when the density of a particle, which was previously larger than that of the liquid, is intentionally decreased to assure its flotation. This method takes use of some solid (or liquid) particles' ability to combine with gas bubbles (typically air or nitrogen) to generate "particle-gas" pairs that are less dense than the liquid in which they are scattered. As a result, the applied phenomena is triphasic (gas-liquid-solid) and will be influenced by the physical-chemical features of the three phases, particularly the affinity between their interfaces (more or less hydrophobic).
Average bubbles are 2 to 4 mm in diameter, small bubbles are a few m to 1 mm in diameter, and microbubbles are 40 to 70 m in diameter.
In the realm of water or wastewater treatment, the word "flotation" refers to flotation generated by microbubbles comparable to those observed in "white water" given by a tap on a water main rich in gas dissolved under high pressure. Dissolved gas flotation is the name of this procedure (DAF).
How does bubble velocity affect the design of the WWT system?
The Stokes equation may be used to determine a gas microbubbles rising velocity through water in laminar operating conditions:
‘g’ stands for gravitational acceleration;
‘d’ is the diameter of the bubble,
‘g’ is the density of gas,
‘L’ is the liquid density, mL,
‘μ’ is the viscosity in a dynamic state.
Particle-bubble composite and rising velocity
The Stokes equation is still valid when:
‘g’ is substituted by ‘ρa’ = particle-bubble "composite" density, which is directly proportional to the gas volume/composite volume ratio. Adjustment is made for the form or sphericity of the "gas bubble-particle."
The size of the bubbles and Volume
Microbubbles are essential for floc separation; in fact, a 1.2 mm bubble contains 8,000 times more gas than a 60 m microbubble; as a result:
If we desire a proper distribution of bubbles throughout the whole portion of the unit, utilizing bubbles that are a few millimetres in diameter would result in a significantly greater gas flow rate than employing microbubbles, and this big amount of gas would cause very disruptive turbulent currents.
Microbubbles and increase in concentration
The likelihood of a collision between solid particles and bubbles increases as bubble concentration rises. Furthermore, because of the low microbubble rising velocity in comparison to the fluid mass, microbubbles will attach effectively to the flocs delicate particles.
Microbubbles and volume
Larger bubbles (fine or even medium bubbles) can be used to distinguish voluminous particles that are lighter than water, hydrophobic, and float naturally from those that we merely want to improve their flotation and keep the cake on the top. This is true when it comes to fat separation.
To ensure flotation, a minimum amount of gas is necessary!
The following equation may be used to compute the minimal volume of gas ‘Vg’, with a density of ‘g’, necessary to assure the flotation of a particle with a volume ‘Vs’ and a density ‘s’ in a liquid with a density ‘L’:
The significance of floc quality
Because microbubbles are 10 to 100 times bigger than colloids, they collide infrequently, and coagulation-flocculation is a necessary pre-condition for their flotation (and to their settling).
Furthermore, because the microbubble/floc, mixing zone is usually turbulent, the floc must be robust enough to withstand shear pressures.
Technical assistance and guidance
Netsol Water collaborates with a wide range of international organizations to tackle complicated problems by strategically integrating modern treatment technology with practical solutions. We can also provide a good framework of WWT plant, among other services.