• Source reduction (process modification)
• Separation and volume reduction
• Exchange/sale as raw material
• Energy recovery
• Secure ultimate disposal (landfill)
Source: Reprinted with permisstion from G.D. Combs, 1989, Emerging treatment technologies for hazardous waste, Section XV, Environmental Systems Company (Little Rock, AR).
to the environment. Technological options for waste handling depend upon waste type, amount, and operating cost. Figure 11.13.1 aligns categories of industrial wastes with the treatment and disposal processes usually applied.
Numerous chemical, physical, and biological treatments are applicable to hazardous wastes. Many such treatment processes are used in by-product recovery and volume reduction processes. All wastes should first be surveyed and characterized to determine which treatment or destruction process should be used.
Hazardous wastes may be organic or inorganic. Water will dissolve many of these substances, while others have limited solubility. Sodium, potassium, and ammonium salts are water soluble, as are mineral acids. Most halo-genated inorganics, except fluoride, are soluble; while many carbonates, hydroxides, and phosphates are only slightly soluble. Alcohols are highly soluble, but aromat-ics and long-chained petroleum-based organics are of low solubility. Solubility is critical in chemical treatment processes.
The following treatment alternatives are detailed in Figure 11.13.1.
Low-concentration effluents and other wastewaters usually require modest capital and operating costs to treat before discharging into municipal sewers. Strong acids and alkalis can be neutralized to prevent characterization as hazardous wastes under the RCRA cor-rosivity criteria. Frequently, industrial water may be acid or basic, requiring neutralization before any other treatment. It may be feasible to mix an acidic waste stream with a basic stream to change the pH to a more neutral level of 6 to 8.
As heavy metals are virtually impossible to destroy, they must be managed by immobilization techniques. After heavy metals have undergone fixation processes and are nonleachable, they can be placed in landfills.
Reactive wastes and toxic inorganics, such as hexavalent chromium and aqueous cyanide-bearing wastes, must be handled carefully prior to the chemical treatments and separation processes that will make them environmentally acceptable. Hexavalent chromium is highly toxic. When it is reduced to trivalent chromium it can be precipitated as chromium hydroxide, which is much less toxic and more acceptable for subsequent recovery or disposal. A common method for treating aqueous cyanide waste is alkaline chlorination.
Should inorganic waste streams contain sufficient amounts of metals or other potentially valuable resources, recovery via physical and chemical processes is highly desirable. Recovery potential must be studied on a case-by-case basis, considering the estimated value of the quantities available, the market acceptance of the recovered materials, and the public perception of recycling and reusing such waste products.
Organic wastes such as solvents, resins, paints, sludges, and chemicals offer considerable recovery potential. Separation techniques such as distillation or extraction can recover valuable hydrocarbon streams for energy or chemical process industry use. However, organic re covery processes still produce a concentrated but significant volume of hazardous waste that eventually must be destroyed or landfilled. The destruction of hazardous wastes, such as chlorinated hydrocarbons and pesticides, that cannot be eliminated or recovered involves incineration or biological treatment. Incineration is the third alternative in the EPA's preventive hierarchy, after source reduction and recycling. It is preferred because it eliminates potential problems in landfill disposal or other interim waste management processes. Biological treatment also offers the potential for complete destruction of biodegradable hazardous wastes. The development of specialized microbes for efficient destruction systems eliminates the need for landfill disposal. Ultimate disposal of products from hazardous waste management facilities will affect the air, water, and land. There is simply no way to avoid placing the waste byproducts of our society and technology into our air, water, and land.
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