Waste treatment

DEFINITION: Processing that alters the physical, chemical, or biological composition of waste in order to concentrate or neutralize it

Given the potential environmental hazards posed by improper disposal of wastes and the ever-increasing problem of lack of sufficient landfill space, the need for cost-efficient and effective methods of waste treatment is widely recognized.

Rapid advances in technology and industrialization have resulted in the discharge of increased quantities of wastes into the environment. Adverse environmental effects develop if the concentrations of wastes exceed the natural capacity of air, water, and land systems to assimilate them. In the United States, where pollution control is exercised by the federal government, virtually all of the environmental control legislation has been written and passed since the end of World War II, when dramatic increases in urban density and industrialization occurred.

The principal wastes associated with industrial and municipal facilities can be categorized as organic and inorganic, solid (suspended and dissolved), acid and base, and hazardous. Organic wastes are oxygen-demanding and lower the amount of dissolved oxygen in the receiving waters. Suspended solids settle and cause benthic deposits. Acid and base wastes destroy natural buffers, and acid rain has the potential to have devastating impacts on aquatic life and forests. Hazardous wastes—which include explosive, flammable, volatile, radioactive, toxic, and pathological wastes—can cause serious damage to people or property. The storage, collection, transportation, treatment, and disposal of such wastes require special caution.

Waste treatment processes can generally be divided into three categories: physical, chemical, and biological. Physical treatment is used to concentrate wastes, reduce volume, and separate different components for further treatment or disposal. Chemical treatment is used to precipitate, detoxify, or destroy the hazardous properties of wastes. Biological treatment utilizes microorganisms to stabilize organic wastes.

The degree of treatment required for any waste stream depends on the characteristics of the waste, the maximum discharge limit, and the final disposal requirements imposed by regulatory agencies. Waste treatment facilities utilize a number of processes to achieve the desired degree of treatment. More than twenty types of physical treatment processes—also called unit operations—are commonly used for handling wastes.

Removal of wastes is achieved by physical forces. Common examples of physical treatment processes are sedimentation, centrifugation, flotation, evaporation, drying, distillation, stripping, carbon adsorption, ion exchange, membrane processing, freeze crystallization, and solidification. Sedimentation, centrifugation, and flotation processes are typically used to remove suspended solids. Evaporation, drying, and distillation are utilized to concentrate wastes. Stripping removes ammonia and volatile organic compounds. Carbon and resin adsorption is used for removal of organic solute from aqueous waste streams. Their applications include the separation or removal of phenols, fats, colors, pesticides, carcinogens, and chlorinated hydrocarbons. Membrane processes such as reverse osmosis, ultrafiltration, and electrodialysis utilize synthetic membranes to concentrate industrial and hazardous wastes; such processes are also used in the desalination of brackish water. In freeze crystallization, the waste stream is cooled so that pure water (in the form of ice crystals or solid ice) can be separated from the contaminants, which concentrate in liquid. Solidification is the transformation of hazardous waste into a nonhazardous solid product through fixation or encapsulation.

Chemical processes are used for the treatment of industrial wastes, usually in conjunction with other methods to achieve desired end results. Common chemical treatment processes used for waste treatment include neutralization, oxidation, reduction, precipitation, hydrolysis, catalysis, chlorinolysis, electrolysis, photolysis, and incineration. Neutralization is the adjustment of the pH level through the introduction of either acids or bases. This process has a wide application in the treatment of wastes from many industries. Oxidation is used mainly for detoxification of hazardous wastes. Chlorine, ozone, hydrogen peroxide, and potassium permanganate are excellent oxidizing agents often used in the presence of ultraviolet light. Reduction is achieved through the use of a reducing agent such as sulfur dioxide. As an example, hexavalent chromium, a very toxic substance, is reduced to trivalent chromium, which is much less toxic, and then precipitated. Hydrolysis, catalysis, chlorinolysis, electrolysis, and photolysis are destructive processes used to break chemical bonds. Incineration is the thermal destruction of hazardous organic wastes.

Biological waste treatment processes involve biochemical reactions that take place in or around microorganisms. Generally, organic compounds are decomposed in suspended or attached growth reactors. The most common biological waste treatment processes use biological reactors to stabilize organic matter, followed by the separation of solids and liquid. The waste solids, referred to as sludge, are combined with other solids and treated separately. Anaerobic sludge digestion is used for stabilization of sludge and high-strength wastes. With proper controls, biological waste treatment processes are reliable and environmentally sound; no chemicals are added to the wastes, and operational costs are relatively low.

Bibliography

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