Physical processes

Features of the DAF Grease Trap

• Ability to remove more than 99% of emulsified and free oils and suspended solids

• Ability to remove more than 75% of chemical pollution (COD) from wastewater

• Clarification of effluent due to removal of colloidal particles

• Easy transportation and installation due to integrated package design of the DAF unit

• Ability to produce relatively solid, low-moisture sludge

• Capability to remove detergents from wastewater

Advantages of DAF

• Advanced manufacturing technology

• Reasonable cost

• Minimum required space

• Odor removal

Applications of DAF

• Refineries and petrochemical industries

• Food and dairy industries

• Processes whose wastewater contains high amounts of oil

• Wood and paper industry wastewater

• Textile industries

• Leather tanning industries

When grease and oil are present in wastewater in emulsified form, or when their specific gravity is not sufficient to allow easy flotation, dissolved air is used for flotation. Fine air bubbles attach to oil and grease droplets and particles, assisting their flotation. This process requires dissolving air in the wastewater and generating very fine bubbles.

In this method, separation is achieved by introducing fine gas bubbles (usually air) into the liquid phase. Air bubbles attach to solid particles, and the buoyant force of the particle–bubble assembly becomes large enough to cause the particle to rise to the surface. In this way, particles with a density greater than the liquid can be forced to float to the surface. The flotation of particles with a density lower than the liquid (such as oil dissolved in water) can also be facilitated by this process.

The use of gas or air bubbles for separating mineral particles and for treating oil-containing wastewater is widely practiced. In general, the flotation process consists of four main stages:

• Generation of bubbles in oily wastewater

• Collision between gas bubbles and oil droplets suspended in water

• Attachment of oil particles to gas bubbles

• Rise of the air–oil assembly to the water surface, where oil (and accompanying suspended solids) is collected

In a DAF system, the wastewater is pressurized to saturate it with air. Pressurization of the wastewater is carried out in three ways:

• Pressurizing the entire flow:
  In this system, all incoming raw wastewater is pressurized and saturated with air. Compared to the other two methods, this approach dissolves the highest amount of air, resulting in the greatest probability of proper attachment between particles and air bubbles. However, due to the need for larger saturation systems, there is a higher risk of floc breakage caused by pump operation and during pressure reduction.

• Pressurizing a portion of the flow:
  In this system, part of the incoming raw wastewater is diverted to the pressurization system. The main advantages include reduced pumping cost, greater system capacity to handle flocculated flow, and reduced floc breakage. A common drawback of this system and the first method is floc shearing or oil emulsification when the incoming flow undergoes pressure reduction. At equal pressures, the amount of dissolved air in this system is lower than in full-flow pressurization due to the lower wastewater flow rate.

• Pressurizing the return flow:
  In this system, 20–50% of the treated wastewater is returned to the pressurization system. Therefore, floc breakage or re-emulsification of oil in the incoming flow is avoided. If the hydraulic load (based on influent flow rate) is not to be changed, a larger flotation basin must be used due to the addition of the return flow to the total flow.