Wastewater Treatment: Flotation and Sedimentation Techniques
Wastewater Treatment: Flotation and Sedimentation
Flotation is a process for separating solids of low density or liquid particles from a liquid phase. The separation is accomplished by introducing a gas (usually air) in a liquid phase in the form of bubbles. The liquid phase is subjected to a pressurization process to achieve an operating pressure ranging from 2 atm in the presence of sufficient air to obtain air and water saturation. Then, the air-saturated liquid undergoes a process of depressurization to atmospheric pressure by passage through a pressure-reducing valve. In this situation, due to depressurization, small air bubbles emerge from the solution. Suspended solids or liquid particles (e.g., oil) float because these small bubbles associate with them, forcing them to rise to the surface. Concentrated suspended solids can be separated from the surface by mechanical systems. The clarified liquid can be separated near the bottom, and part of it can be recycled or not.
Types of Flotation
- Cavitation Air Flotation (CAF) vs. Aeration: In aeration at atmospheric pressure, air bubbles are introduced directly into the liquid phase by diffusers or submerged turbines. In all these systems, it is possible to improve the degree of elimination and performance through the introduction of chemical additives.
- Dissolved Air Flotation (DAF): This is based on the principle of air solubility in water under pressure. It consists essentially of subjecting the flocculated raw water to pressure for a time in a container, simultaneously introducing compressed air and shaking the whole by various means to achieve air dilution in water. Subsequently, water is depressurized under appropriate conditions, shedding large numbers of micro air bubbles. They cling to the flocs in sufficient quantity so that their buoyancy exceeds the reduced weight of the flocs, elevating them to the surface, where they are constantly or periodically removed by various mechanical means. In order to save energy on one side and, on the other, in order to minimize the possible destruction of flocs in the turbulent process of creating micro-bubbles, the total flow of treatment is not normally pressurized, but a partial stream of clarified water is recirculated, enough to create the micro-bubbles needed for the process.
- Vacuum Flotation: Air-saturated wastewater is introduced directly into the aeration tank, or air is allowed to flow into the suction of a pump. By applying a partial vacuum, the dissolved air leaves the solution as tiny bubbles. Bubbles and solid particles that are attached to the surface then ascend to form a layer of foam that is removed by a mechanism of surface scratching. The sand and other heavy solids that settle to the bottom are then transported to a central bowl of sludge for removal by pumping. In the event that the facility is provided for the removal of the sand and if the mud has to be digested, it is necessary to separate the sand from the mud in a sand classifier before pumping to the digesters.
Sedimentation in Wastewater Treatment
Sedimentation is the separation, by the action of gravity, of suspended particles whose specific gravity is greater than that of water. It is one of the most widely used unit operations in wastewater treatment. The terms “sedimentation” and “settling” are used interchangeably.
Types of Sedimentation
- Sedimentation of Discrete Particles: This refers to the sedimentation of suspended particles in a low concentration of solids. The particles settle as individual entities; there is no substantial interaction with neighboring particles. Its application is in eliminating the sands from the wastewater.
- Flocculent Sedimentation: This refers to a very dilute suspension of particles that are added or flocculate during the sedimentation process. By joining, the particles increase in mass and settle faster. Its implementation is in removing a fraction of the suspended solids in raw wastewater in primary sedimentation tanks and at the top of the secondary clarifiers. It also eliminates chemical flocs in sedimentation tanks.
Lamellar and Tubular Plate Decanters
Analysis of Lamellar Decanters and Tubular Plates: These have been developed as an alternative to shallow sedimentation tanks. Used in both existing and specially designed tanks, these decanters are shallow tanks containing devices made of packets of parallel plates or small plastic tubes, and different geometries are used to improve the characteristics of sedimentation tanks. Their use has posed numerous problems, mainly caused by odors due to biological growth, the accumulation of oils and fats, and filling.