Defining the Problem

Unlike other field assessments, the pollution prevention assessment focuses on determining the reasons for releases and discharges to all environmental media. These reasons

FIG. 3.3.1 Pollution prevention hierarchy.

Most Preferred Approach

Least Preferred Approach

FIG. 3.3.1 Pollution prevention hierarchy.

can be identified based on the premise that the generation of emissions and waste follow recurring patterns independent of the manufacturing process (Chadha and Parmele, 1993).

Emissions and waste are generated due to process chemistry, engineering design, operating practices, or maintenance procedures. Classifying the causes into these four generic categories provides a simple but structured framework for developing pollution prevention solutions.

Data gathering, area inspections, and tools for identifying the source of waste are discussed in Section 3.2. In addition to the main chemical processing unit, the assessment team should also investigate the storage and handling of raw materials, solvent recovery, wastewater treatment, and other auxiliary units within the plant.

For many continuous processes, the source of an emission or waste may be an upstream unit operation, and a detailed investigation of the overall process scheme is necessary.

For example, impurities may be purged from a distillation column because of the quality of the raw materials used or undesirable products generated in upstream reaction steps.

Similarly, identifying and understanding the fundamental reasons for waste generation from a batch process requires evaluating all batch processing steps and product campaigns. This evaluation is especially important since batch operations typically generate emissions of varying characteristics on an intermittent basis.

Start up and shutdown and equipment cleaning and washing often play a key part in generating emissions waste, especially for batch processes. The related operations must be carefully observed and evaluated during problem analysis activities.

Emission sources and operations associated with batch processes are not always obvious and must be identified with the use of generic emission-generation mechanisms. In general, emissions are generated when a noncondens-able such as nitrogen or air contacts a volatile organic compound (VOC) or when uncondensed material leaves a process.

Thus, for batch processes involving VOCs, processing steps such as charging the raw material powders, pressure transfer of the vessel's contents with nitrogen, solvent cleaning of the vessel's contents with nitrogen, and solvent cleaning of the vessels between batches should be closely

FIG. 3.3.2 Waste minimization techniques.
FIG. 3.3.3 Methodology for multimedia pollution prevention assessments. (Reprinted, with permission, from N. Chadha, 1994, Develop multimedia pollution prevention strategies, Chem. Eng. Progress [November].)

observed. The operator may leave charging manholes open for a long period or use vessel cleaning procedures different from written procedures (if any), which can increase the generation of emissions and waste. The field inspection may also reveal in-plant modifications such as piping bypasses that are not reflected in the site drawings and should be assessed otherwise.

The unit flow diagram (UFD) shown in Figure 3.3.4 is a convenient way to represent the material conversion relationships between raw materials, solvents, products, byproducts, and all environmental discharges. The UFD is a tool that systematically performs a unit-by-unit assessment of an entire production process from the perspective of discharges to sewers and vents. This visual summary focuses on major releases and discharges and prioritizes a facility's subsequent pollution prevention activities.

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