**8. Dissolved air flotation (DAF)**

The removal of pollutants and colloidal particles by injecting compressed air into the effluent is called "Liquid Air Flotation" or "Degradation" of DAF wastewater. As a result of this action, finely released bubbles cling to colloidal particles suspended in the sewer, causing the particles to float on the surface where they are collected by the skimmer and discharged into a sludge storage tank (**Figure 7**). Due to the high efficiency of the DAF degradation system, this equipment is widely used in various industries such as oil and gas refineries, food factories, and chemicals [7]. DAF is an alternative to sedimentation [9].

In the design process of the DAF wastewater degradation machine, several steps are considered for the removal of oil and colloidal particles suspended in water, including [10]:


To increase the efficiency of the DAF wastewater degradation system, it is better to use coagulants according to the fluid type and particulate matter in it. To do this,

**Figure 7.** *Dissolved air flotation system configuration [7].*

### **Figure 8.**

*Coagulation and Flocculation Process and Mixing Essentials [11].*

### **Figure 9.** *Dissolved air flotation thickener [9].*

add to the inlet effluent of coagulants and polymers to make the load of the particles. For this purpose, spiral tubes are used, which on the one hand, coagulants are injected in different parts, and on the other hand, an air mixture with pressurized liquid is injected into these tubes for better mixing.

In the DAF grease trap system, some of the cleaned and cleared water is inserted into the flocculants pipes and part of it is transferred directly to the floating unit.

Excess treated water is also removed from the DAF system. Clearness of the outlet water from the DAF unit is one of the criteria for the proper operation of this system.

In some cases, coagulation and flocculation tanks are used instead of flocculants tubes. In this case, the coagulants are mixed with effluent in the coagulation tank for about 5 minutes and then injected into the DAF. This process is superior to tubular flocculants when the input effluent is not neutralized and there is enough time to neutralize and on the other hand it is easily controlled by the user.

The objective of the Coagulation Process is to create micro-floc from particulate matter, agglomerate the particles, so that they can settle and move to the sedimentation process. At the Flocculation Process stage, another chemical can be added, a polymer, which helps keep the macro flocs together. Essentially it helps make the bridge between the micro flocs stronger when forming the macro-flocs. A Flocculator is three or more basins with separate mixers. At each stage there are various sizes of flocs, as the size of the flocs increase, the power on the mixers is decreased, to prevent shear, and not damage the flocs (**Figures 8** and **9**).
