What are the environmental health and safety issues that need to be considered when developing an aqueous cleaning process?
Aqueous cleaners, by definition, use water for cleaning and rinsing. This has both advantages and disadvantages. Water is an inherently environmentally sound and substantially safe chemical to work with. It is a recyclable natural resource. Yet, as populations grow, clean surface water will become increasingly scarce. Water can also be a transport medium for various polluting or hazardous chemicals that may derive from the use of aqueous cleaning in specific instances.
One way to look at the environmental health and safety of a cleaning process is to consider:
- How hazardous is the cleaning process?
- How hazardous is the effluent resulting from the cleaning process?
- How sustainable-in terms of energy and resources-is the process?
Of course, some aqueous cleaners do contain hazardous ingredients that may be used to clean hazardous soils, which may produce hazardous and polluting waste. But by the same token, industrial cleaning may involve the use of an aqueous cleaner with no hazardous ingredients used to clean the same hazardous soil, resulting in a clean and relatively safe process.
Improvement in safety can be achieved by eliminating the source of the hazardous soil in the process. Going a step further, waste can be reduced by integrating soil recycling, cleaning solution recycling and/or rinse water recycling into the cleaning process. It is possible to design a so-called "zero-discharge" system with no fluid effluent, limited volatile effluent and reduced solid waste by recycling cleaning and rinsing solutions using filters. In order to move toward a clean, safe and sustainable process, however, one would need to eliminate the hazardous soil and replace it with a non hazardous biodegradable soil. Then, after the water used in the cleaning and rinsing process has been recycled sufficiently, for energy efficiency, the now nonhazardous soil in the effluent would not pose no environment threat. Any water released could safely be incorporated into the natural water cycle (surface water evaporates to form clouds, which later precipitate as rain, and return as surface water).
It is, generally much more difficult to clean safely and sustainably using nonaqueous cleaning methods. Many nonaqueous cleaners are themselves health hazards, water pollutants, or air pollutants. Certainly not all nonaqueous cleaners are hazards and/or pollutants, but most lack a basic natural means, such as the water-cycle, of purifying and/or recycling key ingredients. Of course, this is an oversimplification. Given enough time, almost anything can complete a natural cycle of synthesis and decay. However, here, we are considering processes achievable in the course of a human lifetime.
One might argue that the carbon cycle, the nitrogen cycle, the oxygen cycle and other elemental cycles are involved in the decomposition and purification of ingredients used in nonaqueous cleaners. In fact, some of these cycles are also involved in the purification of ingredients found in aqueous cleaners but to a lesser degree. All of the cycles, noted above, involve multiple chemical transformations. They are slow processes in which chemicals may remain in one state for many years before degrading to a purer form. For example, during the nitrogen cycle, nitrogen remains in the air typically for years. Likewise, carbon takes the form of geological carbonates for extensive periods of time during the course of the carbon cycle, in some cases for millennia. Oxygen also remains tied up in the form of geological carbonates. These elements simply do not cycle rapidly, to a pure state, the way water does.