Flotation is a physical process of removing not only oil and grease, but also fine and light suspended particulates from wastewater. Flotation has particular appeal to food wastewater treatment because this source of wastewater contains a substantial amount of oil/grease floating on the surface. The particulates in wastewater that do not settle well and take too much time for settling are also good candidates for flotation treatment. Flotation is achieved by introducing gas (usually air) in the wastewater stream through either pressure-dissolved air in the feed or direct air diffusers or vacuum. The air bubbles attach themselves to the particulates causing the particulates and oil/grease to aggregate and rise to the surface where the particulates are removed by mechanical skimmers. For oil and grease removal, the emulsified oil and grease in wastewater present a problem for utilization of flotation technology. It is critical to break up the emulsion before applying flotation to the wastewater. Adjustment of pH value, heat, and additions of chemicals are used to destabilize the emulsion. Dissolved air flotation (DAF) is commonly used in food wastewater treatment. The wastewater feedstock is first pressured with air in a closed tank, and after the waste-water is discharged into a tube in the center of the DAF tank, the air bubbles carry the solids in the wastewater to the surface where they are skimmed by skimmers and moved to a discharge trough where they are collected. Treated water exits from the lower sections of the DAF through the riser tubes. Heavy solids that do not float settle to the bottom where a bottom skimmer moves them to a discharge point for removal. Flotation is also used to concentrate sludge from biological processes. Flocculants (see the section "Coagulation and Flocculation" below) of inorganic (FeCl2, FeSO4, or AlSO4) and organic (carrageenan, chitosan, and lignosulfonic acid, or their derivatives) as well as polymeric (polyacrylamides) nature sometimes are added to aid flotation operations. As a food wastewater treatment strategy, flotation (DAF in particular) has its own advantages and disadvantages. The following are the advantages:
• Reduces oil, grease, and suspended particulates
• Reduces suspended solids and settling solids if equipped with a bottom sweep
• Does not require excessive maintenance or management
58 Food and Agricultural Wastewater Utilization and Treatment Disadvantages include the following:
• Does not remove the BODs associated with soluble materials
• Disposes and/or treats floats
• Capital and operating costs could be high
In a DAF system, wastewater is first mixed with flocculants and pressurized to a pressure of several atmospheres, followed by the release of pressure to the atmosphere level by a valve. As the minute bubbles in the order of 50X100 microns resulting from depressurization rise to the surface, the particles, oil, or grease adhere to the air bubbles to be carried upward. A mechanical skimmer on the top then removes the float. A bottom sweep is sometimes employed to stir up settleable particulates to aid flotation operation (see Fig. 3.5). For larger flotation systems, a portion of the effluent from the DAF system is recycled (at least 10%) and mixed with the fresh feed of the DAF system.
The oil, grease, and suspended particulates removed by the DAF system are concentrated in the float and must be used or disposed of properly to avoid pollution. DAF float can be disposed of by
• Applying it to land (A permit is required.) See also Chapter 7 regarding land applications.
• Depositing it in a landfill. (This practice is banned in some states.) Regulations usually require that the material be easily handled by the machinery in the landfill.
• Using as ingredients for animal feeds. But, the flocculants used must be approved by the FDA, and the float often contains too much liquid for most direct feeding applications or for uses as ingredients for animal feed manufacturing (high energy cost for drying).
• Mixing with sludge for further treatment.
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