Hazardous wastes separation processes

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. 

Particle size distribution relating to gas cleaning is well understood in the industry. This section deals with general rules of thumb. Certain important issues not included in this section are flue gas desulfurization, flue gas denitrification, hazardous waste gas cleaning, waste incineration gas cleaning, and removal of CO2 from flue gas. All these topics have special requirements, which must be considered separately in the design process. 

Likewise, fly ash from power plant combustors often contains small amounts of metals or their oxides, which require costly disposal in the ever-shrinking number of approved hazardous waste landfills. Thus, there are economic incentives to recover the metal values as well as to reduce the costs of ultimate disposal. Here, too, the metal content is low, and research is needed to develop economical separation processes. In principle, advances in this area could be translated into recovery of metal values from mine tailings. 

A large fraction of the hazardous waste generated in industry is in the form of dilute aqueous solutions. The special challenges of separation in highly dilute solutions may be met by the development of new, possibly liquid-filled, membranes by processes involving selective concentration of toxic chemicals on the surfaces of particles or by the use of reversed micelles. 

The two SCFs most often studied—CO2 and water—are the two least expensive of all solvents. CO2 is nontoxic and nonflammable and has a near-ambient critical temperature of 31. UC. CO2 is an environmentally friendly substitute for organic solvents including chlorocarbons and chlorofluorocarbons. Supercritical water (Tc = 374°C) is of interest as a substitute for organic solvents to minimize waste in extraction and reaction processes. Additionally, it is used for hydrothermal oxidation of hazardous organic wastes (also called supercritical water oxidation) and hydrothermal synthesis. (See also Sec. 15 for additional discussion of supercritical fluid separation processes.)... 

Because all depolymerization processes will generate waste that may be classified as hazardous waste or at least chemical waste, it will always be economically preferable to separate as much non-PET material from the PET material as is practical. Traditional bottle washing procedures can produce used bottle flake that is clean enough to be used to make more bottles or somewhat less clean and less expensive material. Technologies have been proposed to dissolve the polyester scrap in appropriate solvents to separate PET from other materials such as cotton fiber or magnetic tape components [19]. 

In addition, oily sludge from a wastewater treatment facility that results from treating sour wastewaters may be a hazardous waste (unless recycled in the refining process). These include API separator sludge, primary treatment sludge, sludge from various gravitational separation units, and float from dissolved air flotation units. 

According to a USEPA survey, many of the more than 150 separate processes used in petroleum refineries generate large quantities of hazardous wastes. Typical wastes generated from refinery processes include bottom sediments and water from crude storage tanks, spent amines, spent acids and caustics, spent clays, spent glycol, catalyst fines, spent Streford solution and sulfur. 

Following biological degradation, the extract is exposed to photochemical degradation, which removes uranium from solution as polyuranate. The metals and uranium are captured in separate treatment steps, allowing for the separation of wastes into radioactive and nonradioactive waste streams. This treatment process does not create additional hazardous wastes and allows for the reuse of the contaminated soil. The technology has been the subject of bench-scale tests and is not currently commercially available. 

Membrane Separation Processes for Treatment of Hazardous Waste... 

Chemical/physical treatment processes are those in which a chemical reaction is used to alter or destroy a hazardous waste component. Chemical treatment techniques can be applied to both organic and inorganic wastes, and may be formulated to address specific target compounds in a mixed waste. Typical chemical treatment processes include oxidation-reduction reactions such as ozonation, alkaline chlorination, electrolytic oxidation and chemical dechlorination. Physical treatment processes separate waste component by either applying physical force or changing the physical form of the waste. Various physical processes include adsorption, distillation, or filtration. Physical treatment is applicable to a wide variety of waste streams but further treatment is usually required. 

The cast grids are made into battery anode and cathode plates by the application of a lead oxide paste of 70 percent lead oxide (PbO) and 30 percent metallic lead. Lead ingots are tumbled in a ball mill wRh airproducing lead oxide and fine lead dust (referred to as leady oxide ). Leady oxide particulates are entrained in the mill exhaust air, which is treated sequentially by a cyclone separator and fabric filter. The used fabric fiRer bags are shipped to a RCRA-permRted commercially operated hazardous waste landfill located in Colorado. The leady oxide production process does not produce wastewater. 

Specifically, the criteria for a successful procedure are five-fold. First, the metals must be removed to or below levels acceptable from an environmental standpoint. Second, the target elements must be separated and recovered in sufficient purity (> 95%) and yield (> 95%) to make it economically feasible to return them to commercial use rather than storing them as hazardous waste. Third, the process should use operating chemicals that are readily available, inexpensive, and pose minimal environmental hazard. Fourth, in its operation the process should result in a significant volume reduction and add no undesirable ions to the system. Finally, the process should be inexpensive, environmentally friendly, and easy to operate. The MRT separation system achieved high metal recovery and product purities as indicated in Table 3. Effluent water quality met the EPA Gold Book specifications... 

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