Sludge removal, flotation

Flotation The side stream of the aerated water is mixed with the main stream of unclarified water and fed into a flotation tank. The fine air bubbles adhere to the floes and rise to the surface where they form a stable layer of sludge. For good flotation results flow turbulences in the tank have to be kept to a minimum. Sludge removal, clarified water outlet The formed stable sludge is removed from the water surface and discharged. The clarified water exits from the bottom of the tank. 

Water Treatment. Flotation in water treatment is used both for the removal of dissolved ions such as Cu ", Cr ", or (PO or surfactants and suspended soHds as in the case of sludge treatment. The final product in this case is purified water rather than a mineral concentrate. Furthermore, water is treated either for drinking purposes (potable water preparation) or safe disposal to the environment. 

Dissolved Air Flotation. Dissolved air flotation (DAF) is used to separate suspended soflds and oil and grease from aqueous streams and to concentrate or thicken sludges. Air bubbles carry or float these materials to the surface where they can be removed. The air bubbles are formed by pressurizing either the influent wastewater or a portion of the effluent in the presence of air. When the pressurized stream enters the flotation tank which is at atmospheric pressure, the dissolved air comes out of solution as tiny, microscopic bubbles. Dissolved air flotation is used in many wastewater treatment systems, but in the United States it is perhaps best known with respect to hazardous waste because it is associated with the Hsted waste, K048, DAF flotation soflds from petroleum refining wastewaters. Of course, the process itself is not what is hazardous, but the materials it helps to remove from refining wastewaters. 

In dissolved air flotation, air bubbles float the sludge, which is then removed by a scraper. It is generally appHcable to large volumes of waste-activated sludge. 

Flotation. Flotation (qv) is used to remove suspended soHds from wastes and for the separation and concentration of sludges (52,53). The waste flow is pressurized in the presence of sufficient air to approach saturation. When the pressurized air—Hquid mixture is released to atmospheric pressure in the flotation unit, minute air bubbles are formed. As they rise in the Hquor the sludge floes and suspended soHds are floated to the surface where the air—soHd mixture can be skimmed off. 

In contrast to vacuum flotation, dissolved-air flotation units can be operated on a continuous basis by the application of pressure. This consists of pressurizing and aerating the process stream and introducing it into the flotation vessel that is maintained at the atmospheric pressure. The reduction of pressure results in the formation of fine air bubbles and the collection of fine particulates to be floated and removed as sludge. 

As with thickening, air flotation is enhanced by the addition of polymers. Flotation has been successfully used with wholly inorganic metal hydroxide sludges. Polymers and surfactants are used as additives. Engineering details on air flotation equipment has been developed by and is available from various equipment manufacturing companies. Liquid removed during thickening and flotation is usually returned to the head end of the plant. 

Different technologies have been developed in recent years to treat the wastewaters contaminated with heavy metals. Chemical precipitation, coagulation-flocculation, flotation, ion exchange, and membrane filtration can be employed to remove heavy metals from contaminated wastewater.6 However, they have inherent limitations in application mainly due to the lack of economical feasibility for the treatment of large volumes of water with a low metal concentration. Furthermore, the major disadvantage of conventional technologies is the production of sludge.9... 

Full-scale trials with the secondary flotation clarifier were conducted in summer 1980, with excellent pilot results in solids removal, sludge consistency, and water clarity. Table 27.1 documents partial operational data. The following are the conclusions drawn from the investigation ... 

Sepaflot A process for removing solids and oil from wastewater by a combination of Activated Sludge treatment and dissolved air flotation. Offered by Lurgi. 

Dissolved air flotation (DAF) is a process commonly used in refineries to enhance oil and suspended solids from gravity-separator effluent. In some refineries it is used as a secondary clarifier for activated sludge systems and as a sludge thickener. The process involves pressurizing the influent or recycled wastewater at 3-5 atm (40-70 psig) then releasing the pressure, which creates minute bubbles that float the suspended and oily particulates to the surface. The float solids are removed by a mechanical surface collector. 

This is a process for concentration of sludge by removing water. Either gravity thickening or dissolved air flotation thickening can be used. The thickening efficiency depends on the nature of sludge to be processed [15]. 

In the absence of 02, Fe is oxidized to Fe2+ and Fe(OH)2 is formed. However, Fe(OH)3 precipitates more easily than Fe(OH)2 and, thus, the injection of 02 facilitates pollutant removal. The injection of gas also helps in the flotation process. Vik et al. [162] have described a process in which H2 is produced at the cathode and Al is oxidized to Al(III) ions at the anode. The OH generation from H2 evolution produces precipitation of the metal hydroxides and also contributes to the coagulation-flocculation process. In addition, the hydrogen gas bubbles result in the flotation of the sludge formed. The principles of such a process are depicted in Fig. 24. 

There has been very little publication/information on the treatment of palm oil refinery effluent. Osenga (41) introduced a treatment process consisting of a cross flow interceptor (CFl) for oil separation, physical and chemical treatment, and air flotation units to remove the floes followed by a batchwise activated sludge process for the liquid effluent treatment. This process also requires close supervision in order to achieve the desired treatment efficiency. 

As will be described in the next subsection, the use of chemical flotation in this application may be less desirable considering other processes. DAF has been applied effectively as a trimmer process following other treatment facilities for FOG removal requiring a 100-mg/L FOG limit, which can sometimes be accomplished using food grade-approved polymers. In that event, the recovered materials can sometimes be sold as a low-grade feed fat rather than disposed of as a waste sludge. 

A variety of industrial equipment has been used to carry out the flocculation process and the separation of the flocculated materials producing a well-defined sludge suspension layer, which can be removed. Some plant operates with sludge feedback to enable more efficient adsorption. Sludge flocks can also be separated by flotation. 

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