Dissolved air flotation (DAF) in the sewage treatment is based on this physical principle that substances with a lower density in a solution come to the surface. Therefore, with the help of this method, it is possible to separate all lightweight materials from sewage. In the package DAF API, CPI, fat and oil will be floated due to their lightness with proper time on the sewage surface and simply collected and refined.
dissolved air flotation (DAF) is widely used in sewage treatment processes, especially dairy, starch and slaughterhouses. Degreasers are used in various ways, such as gravity flotation, API, CPI, grease traps or advanced technology, which is the same as compressed air flotation (DAF), depending on the type of suspended fat or solvent used in sewage treatment.
In wastewater treatment of machine-building industries and restaurants in which fat is in suspended for, a simple degreaser package will be used. In the treatment of dairy industry waste, due to its high concentration of oil and grease, both simple and advanced degreasers, including CPI or API, plus DAF is used to separate both suspended and solvent type fats in the sewage.
Choosing the type of oil removal, dissolved air floatation The choice of oil refining and oil separation in sewage treatment depends on several factors.
If the volume of the sewage is low and the diameter of the oil particle is high and also the oil concentration is relatively high, gravitational oil removal including API and CPI, will be used more in sewage or wastewater treatment.
If the volume of the sewage and the concentration of oil is high and the diameter of the oil particle is small, the Dissolved Air Floatation (DAF) method is used in sewage treatment.
If oil and fat particles are digestible in a sewage treatment in a biological process, and the concentration of oil and fat in the wastewater is low, biological methods are used. Because the smaller the particles diameter, the greater their degradability. Of course, in the process of biological treatment of wastewater, to achieve the proper efficiency, the concentration of microorganisms should be as high as 10 times the concentration of oil and fat in the sewage.
Description the simple oil removal process of CPI and API in sewage treatment In simple oil removal method, the sewage is stored for a relatively long time in a pond, in which case the particles whose specific gravity is less than the specific gravity of the fluid are transferred to the surface of the pond and then collected by the foam droplet. This method is commonly used to float large suspended oil particles, which it includes API (American Petroleum Institute) and Corrugated Plate Interceptor (CPI) method.
In the design and treatment of wastewater by the CPI method, the media pages are set diagonally and in multi-centimeters. The application of these plates is the separation of suspended oil particles from the sewage bed. Oily particles will be floated on the surface of the sewage or effluent due to their low density relative to the liquid, and in some cases, the integration of the chemical coagulation system with the simple oil removal method is also effective. Advantages of CPI system is the occupied space and low maintenance cost. Also, since there are no moving parts, its productivity will be high.
Describing the process of sewage treatment using dissolved air floatation (DAF) The basis of the dissolved air floatation DAF's advanced oil removal system is the treatment of sewage oil and grease separation through the entry of pressurized air bubbles into the liquid phase. The air bubbles stick to solid waste particles in the wastewater and the floating force between the fraction of fat and gas bubbles is so high that it raises the particle of fat to the surface of the sewage, so that particles with a density more than the liquid can be was forced to float to the level of the DAF and filter out sewage or wastewater. oil removal method using dissolved air floatation is mainly possible in two ways.
• When sewage TSS is very high, degreasing method using compressed air (DAF) and chemical coagulation in sewage treatment will be used together
• When the TSS is low in sewage, the degreasing method using compressed air(DAF) is used.
The most important components of dissolved air floatation method The wastewater in the first stage of the dissolved air floatation package enters a fast mixing tank the so-called coagulation. By adding chemicals (coagulants, such as chlorophyll or alum), the sewage coagulates and then the sewage enters the slowly mixing tank, which is injected with the aid of polymer coagulant flocculation therefore flocculation occurs in the DAF process in the sewage treatment process. During these two stages, suspended particles and agents regarding TSS is formed in the form of heavy debris and flock that can be deposited and precipitated in the dissolved air floatation unit.
The sewage will then be introduced into the degreaser compressed air tank. In this reservoir part or all of the sewage is always in recycle. The flow through the pump under pressure into a compartment is saturated. In this compartment, by means of compressed air supplied from a compressor, the sewage is saturated from the air and then enters dissolved air floatation unit.
As mentioned above, in the DAF unit, the air is saturated with atmospheric air and causes the oil and fat particles to rise to the surface of the sewage. There is a skimmer (or surface skimmer) on the surface of the waste water that separates fats and oil on the surface and causes sewage or wastewater to be treated in a DAF unit.
Applications of DAF, API, CPI in the treatment of sewage The oil removal method is used in the treatment of sewage in many industries. In the treatment of sewage or wastewater that works on the basis of the biological (aerobic and anaerobic) method, degreasing is known as a pre treatment because the present oil and grease cannot be removed with biological methods therefore they interfere with the biological process. Its necessarily to use dissolved air floatation as a pre-treatment method before the entrance of sewage as well as wastewater into these systems and units.
Some of the most important uses of oil removal in the treatment of sewage include the following industries:
• Oil removal for treating sewage and dairy plants with products such as milk, butter, yogurt and dough and cream to separate fat and insoluble or suspended oil
• Dissolved air floatation for treatment of sewage and dairy wastewater to separate fat and soluble oil
• Separation of oil and grease in the machine tool industry by DAF or CPI method
• Separation of oil and fat in sewage of restaurants
• dissolved air floatation by injecting chemicals in wastewater treatment of starch and glucose industries due to its high fats and suspended particles.
• dissolved air floatation by injecting chemicals in the treatment of petrochemical wastewater