Industrial Waste Treatment
Industrial waste treatment
Many different types of solid, liquid, and gaseous wastes are discharged by industries. Most industrial waste is recycled, treated and discharged, or placed in a landfill . There is no one means of managing industrial wastes because the nature of the wastes varies widely from one industry to another. One company might generate a waste that can be treated readily and discharged to the environment (direct discharge )ortoa sewer in which case final treatment might be accomplished at a publicly owned treatment works (POTW). Treatment at the company before discharge to a sewer is referred to as pretreatment. Another company might generate a waste which is regarded as hazardous and therefore requires special management procedures related to storage, transportation and final disposal.
The pertinent legislation governing to what extent wastewaters need to be treated before discharge is the 1972 Clean Water Act (CWA). Major amendments to the CWA were passed in 1977 and 1987. The Environmental Protection Agency (EPA) was also charged with the responsibility of regulating the priority pollutants under the CWA. The CWA specifies that toxic and nonconventional pollutants are to be treated with the Best Available Technology (BAT). Gaseous pollutants are regulated under the Clean Air Act (CAA), promulgated in 1970 and amended in 1977 and 1990. An important part of the CAA consists of measures to attain and maintain National Ambient Air Quality Standards (NAAQS). Hazardous air pollutant (HAP) emissions are to be controlled through Maximum Achievable Control Technology (MACT) which can include process changes, material substitutions and/or air pollution control equipment. The "cradle to grave" management of hazardous wastes is to be performed in accordance with the Resource Conservation and Recovery Act (RCRA) of 1976 and the Hazardous and Solid Waste Amendments (HSWA) of 1984.
In 1990, the United States, through the Pollution Prevention Act , adopted a program designed to reduce the volume and toxicity of waste discharges. Pollution prevention (P2) strategies might involve changing process equipment or chemistry, developing new processes, eliminating products, minimizing wastes, recycling water or chemicals , trading wastes with another company, etc. In 1991, the EPA instituted the 33/50 program which was to result in an overall 33% reduction of 17 high priority pollutants by 1992 and a 50% reduction of the pollutants by 1995. Both goals were surpassed. Not only has this program been successful, but it sets an important precedence because the participating companies volunteered. Additionally, P2 efforts have led industries to rigorously think through product life cycles. A Life Cycle Analysis (LCA) starts with consideration for acquiring raw materials, moves through the stages related to processing, assembly, service and reuse , and ends with retirement/disposal. The LCA therefore reveals to industry the costs and problems versus the benefits for every stage in the life of a product.
In designing a waste management program for an industry, one must think first in terms of P2 opportunities, identify and characterize the various solid, liquid and gaseous waste streams, consider relevant legislation, and then design an appropriate waste management system. Treatment systems that rely on physical (e.g., settling, floatation, screening, sorption , membrane technologies, air stripping) and chemical (e.g., coagulation, precipitation, chemical oxidation and reduction, pH adjustment) operations are referred to as physicochemical, whereas systems in which microbes are cultured to metabolize waste constituents are known as biological processes (e.g., activated sludge , trickling filters , biotowers, aerated lagoons, anaerobic digestion , aerobic digestion, composting ). Oftentimes, both physicochemical and biological systems are used to treat solid and liquid waste streams. Biological systems might be used to treat certain gas streams, but most waste gas streams are treated physicochemically (e.g., cyclones, electrostatic precipitators, scrubbers , bag filters, thermal methods). Solids and the sludges or residuals that result from treating the liquid and gaseous waste streams are also treated by means of physical, chemical, and biological methods.
In many cases, the systems used to treat wastes from domestic sources are also used to treat industrial wastes. For example, municipal wastewaters often consist of both domestic and industrial waste. The local POTW therefore may be treating both types of wastes. To avoid potential problems caused by the input of industrial wastes, municipalities commonly have pretreatment programs which require that industrial wastes discharged to the sewer meet certain standards. The standards generally include limits for various toxic agents such as metals, organic matter measured in terms of biochemical oxygen demand (bod) or chemical oxygen demand , nutrients such as nitrogen and phosphorus , pH and other contaminants that are recognized as having the potential to impact on the performance of the POTW. At the other end of the spectrum, there are wastes that need to be segregated and managed separately in special systems. For example, an industry might generate a hazardous waste that needs to be placed in barrels and transported to an EPA approved treatment, storage or disposal facility (TSDF).
Thus, it is not possible to simply use one train of treatment operations for all industrial waste streams, but an effective, generic strategy has been developed in recent years for considering the waste management options available to an industry. The basis for the strategy is to look for P2 opportunities and to consider the life cycle of a product. An awareness of waste stream characteristics and the potential benefits of stream segregation is then melded with the knowledge of regulatory compliance issues and treatment system capabilities/performance to minimize environmental risks and costs.
[Gregory D. Boardman ]
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