Today's green cleaning process uses less energy, produces fewer air and groundwater contaminants, and is safer for employees. During the last 25 years, the green revolution has resulted in a massive shift from solvent to aqueous cleaning, reducing emissions of ozone-depleting chemicals as well as usage of suspected carcinogens. But at what cost? Increased energy consumption, increased water usage, and increased waste treatment and effluent discharge to the environment come to mind. Also, a larger process footprint and, in some cases, inadequate cleaning results have all contributed to taking the bloom off the rose of aqueous cleaning. The next phase of the green revolution has, ironically, led us back to solvent cleaning. With the introduction of new solvents and high-efficiency equipment, solvent cleaning has become the new gold standard in going green.
Popular new solvents such as n-Propyl bromide (nPB), hydroflourocarbons (HFCs) and hydroflouroether (HFE) offer the end user low-energy-consuming, environmental- and user-friendly cleaning options when compared to aqueous cleaning. They are extremely effective cleaners, particularly for precision cleaning of electronic components and other demanding operations.
Products that are nPB-based have similar characteristics and physical properties as other halogenated solvents such as 1,1,1 trichloroethane and trichloroethylene, but without the adverse effect on the environment. For example, most producers of nPB-based products have developed products with no flash point and extremely low ozone depletion potential (~0.009). In fact, most nPB-based products are not reportable under SARA 313, not regulated as a hazardous waste under RCRA, and not regulated as a hazardous air pollutant under NESHAP. In May of 2007, nPB-based products were SNAP approved by the U.S. EPA (docket ID No. EPA-HQ-OAR-2002-0064 can be found at regulations.gov) as suitable replacements for Class I and Class II ozone depleting substances. Most manufacturers of nPB-based products recommend a PEL of 25-100 ppm. These PELs are easily achievable using today's high efficiency vapor degreasers.
Like nPB-based products, HFC- and HFE-based products typically are safer and use lower amounts of energy than aqueous cleaning. Neither of these alternatives has a flash point or ozone depletion potential; however, both have global warming potential. nPB-based products are usually considered to be better cleaners due to a higher Kauri-Butanol (KB) value. HFCs and HFEs are SNAP approved and are not considered hazardous air pollutants.
When the goal is green cleaning, a number of factors should be considered. How much energy does the cleaning process consume? How much air and ground water pollution does the process create? To what hazards does the cleaning process expose employees? Is the cleaner used once or several times before disposal? How much space, water, time and equipment does the cleaning process require? By addressing these areas, a company can bring its cleaning processes closer to optimal environmental efficiency.
Energy Consumption
The discussion in Washington these days is all about energy. "Cap and trade," "reduced dependence on foreign oil" and other common phrases are dominating the dialogue. Today's green cleaner must be low in energy consumption.
Aqueous degreasing systems usually require at least one heated cleaning stage and a heat-driven drying process. Today's solvent degreasing systems only require heat to the solvent boil sump, which is generally much smaller in size than the aqueous cleaning tanks. Parts come out of a vapor degreaser clean and dry, so there is no rinsing or drying required. Solvent systems generally require a fraction of the running energy of an aqueous system.
Air and Ground Water Contaminants
Modern aqueous cleaners used for metal cleaning are detergent formulations containing surfactants, builders, sequestrants, anti-corrosion agents, solvents and other specialty additives. Waste generated from an aqueous degreasing process is rarely suitable for immediate discharge into the environment and generally requires some treatment (such as pH adjustment, oil separation, or metal precipitation) first. Although little data is available on this subject, aqueous cleaners are generally considered to present a low risk to the environment. Depending on the product formulation, air emissions and ground water contaminants are not a big concern.
Today's solvent systems have also been engineered to be free of hazardous air pollutants (HAPs). They also have low/no ozone depletion potential. Many users of TCE are faced with the challenge to replace it by 2010. Fortunately, most systems can be easily converted to use nPB, HFE or HFCs. The system simply needs to be drained and cleaned. A reputable supplier can retrofit the system for the new solvent, charge it up, and it's ready to go.
Employee Safety
Employee safety with aqueous systems usually means that the user must wear protective eyewear and gloves to protect against splashes. Limited data is available regarding workplace exposure to aqueous cleaners, though some components such as ethanolamines, glycol ethers, d-limonene and some builders do have defined occupational exposure limits. While some aqueous cleaners can be hazardous, the most common danger is burns: caustic, acid, or high-temperature. In some cases, the mist from an aqueous cleaner can cause rash or drying of the skin, and inhaling the mist can cause respiratory problems. Still, aqueous cleaners are usually considered to present a relatively minor health threat to employees.
Today's solvent systems no longer contain suspected or probable carcinogens, as is the case with perc and methylene chloride. Modern solvent systems combine a safer solvent (like nPB, HFE or HFC) with efficient equipment and implementation of best practices. Solvent system should be set up so that workers are not exposed to excessive amounts of solvent. Exposure can be measured in parts per million in the air surrounding the equipment where the employee works. For most solvents, maintaining an eight-hour time-weighted average of 25-100 ppm is well within the safety zone and easily achievable. Once a safer solvent is selected, best practices include ensuring that the heater, chillers and thermostat are working properly and that the parts are lowered into and raised out of the vapor degreaser at a rate of 11 feet per minute or less. Many companies use monitoring badges to detect how many ppm their employees are exposed to. Data from these badges confirm and document that employee exposure remains at or under the recommended limits.
Recyclability
Aqueous systems have an expected useful life based on measures of alkalinity, acidity, soil loading, or a combination thereof. Users often will employ filters to extend the life of an aqueous cleaning bath. Then, the bath must be disposed of. This is true for every stage of an aqueous cleaning system—the cleaning stages, the rinsing stages and (in some cases) the rust preventive stage. These baths must either be hauled off to a waste treatment facility or dumped (with applicable permit) at the local publicly owned treatment works (POTW) facility. The majority of aqueous waste goes to the local POTW for treatment. The primary job of a POTW is to treat human sewage, first for health reasons and second for environmental purposes. Other jobs, such as treating storm runoff and industrial wastewater, have been added to the mix. Once the waste arrives at the local POTW, it is treated for metals, oils, phosphates, bacteria and other contaminants. Many stages of filtration and clarification are performed, and then the resulting effluent either goes into a river, the local drinking water, or another water supply. The solid waste generated from this process generally goes to a landfill.
Today's solvent systems are different. The solvent in the degreaser is recycled over and over until it is considered too dirty for further use. At this point, the boil sump is drained and hauled away by a licensed waste hauler and recycling facility. The waste is distilled to get any remaining solvent out, which is reclaimed and sent back to industry by recyclers for reuse. The resulting oil-containing liquid still bottoms are fuel-blended with other flammable waste streams, such as mineral spirits, lacquer thinners, oils and greases. This fuel blend is sent to cement kilns, where it is burned to get fuel value. This fuel blend replaces other energy sources such as gas, oil or coal.
Any ash and metal contaminants that are generated from this process fall into the cement and are locked up in the final concrete product. This concrete matrix seals them out of the environment and prevents them from being leached into the environment.
Consumption of Resources
Footprint—A typical aqueous cleaning system has at least one cleaning stage, several rinses, (in some cases) a rust preventive stage, and a drying stage. The entire process can be accomplished in a conveyorized system or in a batch cleaning process using a series of tanks. The footprint on these systems is many times larger than a typical vapor degreaser, with which the cleaning and drying occurs in one place.
Waste Treatment—Aqueous cleaning systems require waste treatment when they are spent. This process can be started on-site (with a proper permit and equipment) with the resulting waste sent to the local POTW, or it can be outsourced to an off-site facility. Either way, the final waste product will then be discharged back into the water supply in some form.
Solvent waste is taken to a solvent recycler, where the solvent is distilled out and sent back to industry for re-use. The still bottoms are fuel-blended and sent out to be used as fuel in cement kilns.
Filtration—Aqueous systems are constantly filtered, requiring the ongoing purchase and disposal of filters. Depending on what is actually being filtered out, the filters may need to be disposed of as a hazardous waste. Solvent degreasing systems do not have this requirement. They require no mechanical filtration as the distillation process continuously separates clean solvent from the soils.
Moving Forward
Industry-leading companies are always looking for ways to be more responsible and more productive. As the green revolution marches on, and health and environmental stewardship become more of a driving force in business, solvent cleaning has seen resurgence in popularity. Safer solvents, combined with highly efficient equipment, have resulted in cleaning processes that consume very little energy, are safe for the environment and employees, cost effective, and deliver top cleaning performance. The result is that today's solvent cleaning systems are often the answer for those companies who seek to reach the gold standard in green cleaning. PC
Scott Mondi works for Poly Systems, USA. Mr. Mondi's background includes more than 21 years of experience in aqueous and solvent cleaning. He can be reached at mondi@solvon.com. For information from Poly Systems USA Inc., phone 800-696-9520 or go to solvon.com.
