Air Flotation Systems

Air flotation is one of the oldest methods for the removal of solids, oil & grease and fibrous materials from wastewater. Suspended solids and oil & grease removals as high as 99%+ can be attained with these processes. Air flotation is simply the production of microscopic air bubbles, which enhance the natural tendency of some materials to float by carrying wastewater contaminants to the surface of the tank for removal by mechanical skimming. Many commercially available units are packaged rectangular steel tank flotation systems; shipped completely assembled and ready for simple piping and wiring on site. Models typically range from 10 to over 1000 square feet of effective flotation area for raw wastewater flows to over 1000 gallons per minute. Complete systems often include chemical treatment processes. A dissolved air flotation (DAF) system can produce clean water in wash operations where reduction of oil and grease down to 2 mg/1 is achievable in certain applications. In addition to municipal and heavy industry applications, DAF has found a home with commercial vehicle washing, industrial laundries, food processing. Vehicle wash applications need not be confined to automobiles and trucks, but can extend to buses, tank cars and many other types of vehicles. There is a broad and varied market for DAF. Excessive oil and grease (especially emulsified oils), plus high levels of suspended solids and metals are good candidates for our DAF systems. Another example is an industrial laundry, where there is the need for good waste treatment systems. A DAF system can be used in a variety of food processing applications, including Vegetable Oils, Animal and Seafood Fats, Red Meat Butchering, Poultry processing, and Kitchen and Equipment Washing. Oil Drilling on and offshore is under pressure from the USEPA. The removal of the oils from the dirty water which surfaces when drilling wells cannot be dumped. DAF technology has proven very effective in this industry. Some waste stream problems found in shipyards and aboard ships are oily bilge water and solids containing copper and other heavy metals used in marine paints. DAF is a proven method in these applications as well. In metal finishing operations DAF will remove cadmium, chromium, lead, zinc and other toxic heavy metals from a waste stream. Finally, when combined with other treatment processes DAF can be applied upstream of large water treatment systems to handle contaminated water before it mingles with the rest of the waste stream.

Air Flotation systems for separating oil and solids from wastewater include:

ODissolved Air Flotation

ODispersed Air Flotation

O Induced Air Flotation

<=> Diffused Air Flotation

Designs include rectangular, circular, cross flow and inclined parallel plate systems.

When the primary target is oil removal, we should distinguish between the forms of oil. There are two forms of oil that we find in wastewater. Free oil is oil that will separate naturally and float to the surface. Emulsified oil is oil that is held in suspension by a chemical substance (Detergents - Surfactants) or electrical energy. When making an evaluation, free oil will normally separate by gravity and float to the surface in approximately 30 minutes. Emulsified oil is held in a molecular structure called a micelle and will not separate on its own. Hence, there is the need for a more sophisticated method of treating suspensions containing emulsified oils. A good way to see how DAF technology works is to fill a glass full of water from the tap and observing the tiny, almost microscopic bubbles in the water. Dissolved-air flotation uses the same principle in order to introduce tiny bubbles into water. These bubbles form because the water inside the pipes, which is at high pressure, had somehow dissolved enough air that the water becomes supersaturated with air when the pressure drops before the water falls into your glass. As a result, the excess air precipitates out in the form of tiny bubbles. These bubbles are much smaller than we produce by other means of dispersing air in water.

The flotation process was developed in the mining and coal processing industries as a way of separating suspended solids from a medium such as water. As noted above, the flotation process has found uses in other fields, such as wastewater treatment. The process introduces fine air bubbles into the mixture, so that the air bubbles attach to the particles, and lift them to the surface. Dissolved-air flotation uses a particular way of introducing the air bubbles into the flotation tank. A dissolved-air flotation machine dissolves air into the water to be treated by passing the water through a pressurizing pump, introducing air, and holding the air-water mixture at high pressure long enough for the water to become saturated with air at the high pressure. Typical pressures are 20-75 psig. After saturating the water with air at high pressure, the water passes through a pressure-relief nozzle, after which air precipitates as tiny bubbles. This process for creating air bubbles has two advantages over other processes. Dissolved-air flotation typically produces bubbles in the 40-70 micron range, whereas in normal foam fractionation, a bubble of 500 microns is considered small. The smaller bubbles have much more surface area for their volume than do the larger bubbles. A particular volume of air has 10 times the surface area when distributed as 50 micron bubbles as it does when distributed as 500 micron bubbles. Looking at this fact another way, you need 10 times the air flow with 500 micron bubbles as you need with 50 micron bubbles in order to achieve the same air-water interfacial area.

The second, and probably more important, advantage of producing bubbles by

precipitation is that the process provides a more positive attachment between air bubbles and the particles or globules that you want to remove. Particles and globules in the water act as nucleation sites for the precipitation process; the precipitating air seeks out these sites to begin bubble formation. This is better than relying on chance encounters between waste particles and large bubbles introduced by some other means.

A typical DAF process is not simply a physical separation technique. One must consider the entire treatment process, which is based on chemical coagulation, clarification and rapid sand filtration. This process train is widely accepted and is very applicable to the treatment of colored and turbid surface waters for municipal and industrial applications. Normally the clarification stage employs DAF. The suspended solid matter in the chemically treated water is separated by introducing a recycle stream containing small bubbles which floats this material to the surface of the tank. This is achieved by recycling a portion of the clarified flow back to the DAF unit. The recycle flow is pumped to higher pressure and is then mixed with compressed air. The flow passes through a tank where the air dissolves to saturation at the higher pressure. When the pressure is released at the clarifier, the dissolved air precipitates as a cloud of micro bubbles which attach to the particulate matter causing it to rise to the surface.

The DAF process is particularly well suited for the removal of floe formed in the treatment of low alkalinity, low turbidity, colored water. This type of floe tends to be very fragile and voluminous, making traditional gravity sedimentation inefficient. Flotation processes do not require large, heavy floe in order to achieve efficient solids removal. This results in lower chemical dosages and reduced time required for flocculation. The compressive forces applied to the sludge by the buoyant bubble/floc agglomerates result in greatly reduced volumes of waste-water from the clarification process. This enhances the efficiency of chemical use and reduces the volume of residuals to be treated. An equally important benefit of the technology is the efficiency of the clarification process. Since the performance of the filtration process is directly affected by the amount of solids in the clarified water, the high degree of solids removal achieved in the DAF results in an overall increase in system performance. In order to meet stringent standards for turbidity removal in potable water applications, this high performance is essential.

In recapping, DAF is the process of removing suspended solids, oils and other contaminants via the use of bubble flotation. Air is dissolved into the water, then mixed with the wastestream and released from solution while in intimate contact with the contaminants. Air bubbles form, saturated with air, mix with the wastewater influent and are injected into the DAF separation chamber. The dissolved air then comes out of solution, producing literally millions of microscopic bubbles. These bubbles attach themselves to the particulate matter and float then to the surface where they are mechanically skimmed and removed from the tank. Most systems are versatile enough to remove not only finely divided suspended solids, but fats, oils and grease (FOG). Typical wastes handled include various suspended solids, food/animal production/processing wastes, industrial wastes, hydrocarbon oils/emulsions, and many others. Clarification rates as high as 97 % or more are achievable. The basic flow scheme for a DAF system is illustrated in Figure 25.

Porsche 911 Auto Heat Exploded View
Figure 25. Conventional DAF process scheme.

The conventional DAF saturation design relies on a recycle pump combined with a saturation vessel and air compressor to dissolve air into the water. This type of system is effective, however it has the drawbacks of being labor intensive, is expensive, and can destabilize its point of equilibrium, creating "burps" due to incorrect, loss or creeping of EQ set-point in the saturation vessel. Such designs are slow to recover and can upset the flotation process. Air transfer efficiency is roughly 9 % with 80 % entrainment. This operational methodology can result in an increase in chemical use, labor costs, downtime, effluent loadings, production schedules due to the EQ loss. To overcome these shortcomings, some equipment suppliers have devised operational and control schemes that are best categorized as pollution prevention techniques.

A variation for one vendor is shown in Figure 26. The design and control of the system takes into consideration the following parameters: flow rate, water temperature, waste characteristics, chemical pretreatment options, solids loading, hydraulic loading, the air to solids ratio. Units are designed on the basis of peak flow rate expected.

Chemical pretreatment is often used to improve the performance of contaminant removal. The use of chemical flocculants is based on system efficiency, the specific DAF application and cost. Commonly used chemicals include trivalent metallic salts of iron, such as FeCl2 or FeS04 or aluminum, such as A1S04. Organic and inorganic polymers (cationic or anionic) are generally used to enhance the DAF process.

Rocycie Bypass

Rocycie Bypass

Suction Volvo -

Pump & Motor

Figure 26. Variation of DAF. Vendor scheme of Pan America. Source: Downloaded from site www.panamev.com.

Suction Volvo -

Pump & Motor

Figure 26. Variation of DAF. Vendor scheme of Pan America. Source: Downloaded from site www.panamev.com.

The most commonly used inorganic polymers are the polyacrylamides. Chemical flocculant concentrations employed normally range from 100 to 500 mg/Liter. The wastewater pH may require adjustment between 4.5 and 5.5 for the ferric compounds or between 5.5 and 6.5 for the aluminum compounds using an acid such as H2S04 or a base such as NaOH. In many applications, the DAF effluent requires additional pH adjustment, normally with NaOH to assure that the effluent pH is within the limits specified by the POTW.. The pH range of the effluent from a DAF is typically between 6 and 9.

One mg/Liter of flocculant in 1 million gallons of water treated per day is 8.34 Lbs of material.

The mechanism by which flocculants work and enhance DAF is as follows. Attachment of most bubbles to solid particles can be effected throughsurface energies while others are trapped by solids or by hydrous oxide floes as the floe spreads out of the water column. Colloidal solids are normally too small to allow the formation of sufficient air-particle bonding. They must first be coagulated by a chemical such as the aluminum or iron compounds mentioned above. . The solids are essentially absorbed by the hydrous metal oxide floe generated by these compounds. Often, a coagulant aid is needed in combination with the flocculant to agglomerate the hydrous oxide floe, to increase particle size and improve the rate of flotation. Mechanical/chemical emulsions can also be broken through the application of pH and polymer reactions.

The material that we recover from the surface of the DAF is referred to as the "float". The float often contains 2 to 10 % solids. These solids will need to be dewatered before ultimately finding a home for them. The subject of dewatering is covered in Chapter 12.

pH adjust Typical Process Flow

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Flotation Systems pH adjust Typical Process Flow

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Flotation Systems

TO SEWER OR RÏUSL

Figure 28. Continuous flotation system flow sheet. (Courtesy of Beckart Environmental, Inc., Kenosha, WI, email: [email protected])

TO SEWER OR RÏUSL

Figure 28. Continuous flotation system flow sheet. (Courtesy of Beckart Environmental, Inc., Kenosha, WI, email: [email protected])

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