size greater than 40 μm (Zouboulis and Avranas 2000). There are three basic
configurations for the dissolved air flotation process, namely (Shammas and Wang
2016):
(a) Full flow pressurization system, where the entire wastewater feed stream is
pressurized before introducing to the flotation tank. This system is commonly
used for the feed streams with a high suspended solid concentration of >800 mg/
L and those require large volumes of air bubbles.
(b) Partial flow pressurization system, where only about 30–50% of the wastewater
feed stream is pressurized and directly introduced to the flotation tank. This
system is suitable to be employed in the applications with low suspended solid
concentrations that have low air requirement.
(c) Recycle flow pressurization system, where 15–50% of the treated wastewater is
pressurized, recycled and mixed with the raw wastewater feed. This system is
more frequently used compared to the other two configurations in the oil
treatment applications where coagulation and flocculation are necessary ahead
of the flotation process (Al-Shamrani et al. 2002; Zouboulis and Avranas 2000).
The efficiency of the dissolved air flotation is closely related to the capturing of
particles by the rising bubbles. Hence, it is known that the production of air bubbles
with small sizes is particularly important for the dissolved air flotation process.
Microbubbles, which are generally defined as the bubbles with the diameter of less
than 50 μm, has attracted considerable interest as they have lower rising velocity
which provides a longer residence time in the flotation tank, and large interfacial area
(Zheng et al. 2015; Agarwal et al. 2011). These characteristics allow greater chances
for the bubble-particle collisions and consequently leads to high particle capturing
efficiency. The bubble size can be affected by the saturator pressure. For instance,
Shannon and Buisson (1980) reported an average bubble size of 66 μm at the
pressure of 210 kPa, whereas it was 42 μm at 350 kPa.
Air/solid ratio is another important parameter to be considered in the design and
operation of the flotation system. In the oily wastewater treatment system, the
air/solid ratio is defined as the mass of air precipitated per unit mass of wastewater
solids (oil particles in this case). The air/solid ratio governs the rising rate of the
bubble-particle agglomerates in the dissolved air flotation system (Bratby and
Marais 1975; Wang et al. 2010). The efficiency of the oil removal of the air flotation
system would be afflicted if the air employed is less than the optimum amount. On
the other hand, power would be wasted if too much is employed. Hence, the
optimization of the air/solid ratio is utterly important in designing the dissolved air
flotation system.
The recycle ratio has significant influence over the air/solid ratio (Bratby and
Marais 1975; Wang et al. 2010). The mixing of the pressurized recycle flow with the
wastewater feed changes the concentration of the supplied air bubbles and affects the
bubble-particle collisions (Al-Shamrani et al. 2002). This consequently affects the
removal of the bubble-particle agglomerates. In addition, some other design variables such as rising velocity of the particle, hydraulic design of the flotation
chamber, type, and concentration of the dissolved materials, suspended matter and
11 Oily Wastewater Treatment
359
configurations for the dissolved air flotation process, namely (Shammas and Wang
2016):
(a) Full flow pressurization system, where the entire wastewater feed stream is
pressurized before introducing to the flotation tank. This system is commonly
used for the feed streams with a high suspended solid concentration of >800 mg/
L and those require large volumes of air bubbles.
(b) Partial flow pressurization system, where only about 30–50% of the wastewater
feed stream is pressurized and directly introduced to the flotation tank. This
system is suitable to be employed in the applications with low suspended solid
concentrations that have low air requirement.
(c) Recycle flow pressurization system, where 15–50% of the treated wastewater is
pressurized, recycled and mixed with the raw wastewater feed. This system is
more frequently used compared to the other two configurations in the oil
treatment applications where coagulation and flocculation are necessary ahead
of the flotation process (Al-Shamrani et al. 2002; Zouboulis and Avranas 2000).
The efficiency of the dissolved air flotation is closely related to the capturing of
particles by the rising bubbles. Hence, it is known that the production of air bubbles
with small sizes is particularly important for the dissolved air flotation process.
Microbubbles, which are generally defined as the bubbles with the diameter of less
than 50 μm, has attracted considerable interest as they have lower rising velocity
which provides a longer residence time in the flotation tank, and large interfacial area
(Zheng et al. 2015; Agarwal et al. 2011). These characteristics allow greater chances
for the bubble-particle collisions and consequently leads to high particle capturing
efficiency. The bubble size can be affected by the saturator pressure. For instance,
Shannon and Buisson (1980) reported an average bubble size of 66 μm at the
pressure of 210 kPa, whereas it was 42 μm at 350 kPa.
Air/solid ratio is another important parameter to be considered in the design and
operation of the flotation system. In the oily wastewater treatment system, the
air/solid ratio is defined as the mass of air precipitated per unit mass of wastewater
solids (oil particles in this case). The air/solid ratio governs the rising rate of the
bubble-particle agglomerates in the dissolved air flotation system (Bratby and
Marais 1975; Wang et al. 2010). The efficiency of the oil removal of the air flotation
system would be afflicted if the air employed is less than the optimum amount. On
the other hand, power would be wasted if too much is employed. Hence, the
optimization of the air/solid ratio is utterly important in designing the dissolved air
flotation system.
The recycle ratio has significant influence over the air/solid ratio (Bratby and
Marais 1975; Wang et al. 2010). The mixing of the pressurized recycle flow with the
wastewater feed changes the concentration of the supplied air bubbles and affects the
bubble-particle collisions (Al-Shamrani et al. 2002). This consequently affects the
removal of the bubble-particle agglomerates. In addition, some other design variables such as rising velocity of the particle, hydraulic design of the flotation
chamber, type, and concentration of the dissolved materials, suspended matter and
11 Oily Wastewater Treatment
359