Introduction

The current regulatory climate is causing both large and small companies to reconsider their existing cleaning methods. The use of chlorinated solvents in metal and electronics parts cleaning is no longer the primary method of choice. Over the last five to 10 years, the consequences of past usage of these cleaning systems have led to traumatic and expensive experiences of many companies, municipalities and government agencies. With heightened environmental awareness, more improvements are still required. The Environmental Protection Agency has declared that source reductions leading to pollution prevention will be the primary area of focus in environmental compliance over the next decade and beyond.

In the realm of precision cleaning, aqueous cleaners are emerging as safe and effective alternatives to solvent degreasers. Switching to water-based cleaners, however, creates a new waste stream which tends to be high in oil and grease and potentially RCRA hazardous. An article appearing in this journal described how one company, Hamilton Standard, switched from a vapor degreaser to an aqueous cleaning technology. A more impressive example comes from R. B. White, which accomplished the same feat and has been using the same cleaning solution for four-and-a-half years without dumping its cleaning tank. This process of reclaiming and reusing spent solutions is the basis of closed-loop cleaning technologies. This article will discuss closed-looping of aqueous cleaning processes as a means to make them more economical and environmentally friendly. In support of this, we will present case studies exemplifying the process and documenting the results.

Aqueous Cleaning Combined With Closed-Looping

In general terms, aqueous cleaning combines a water-based cleaning solution with mechanical cleaning action. In particular, alkaline cleaners are viewed as the most viable substitutes for chlorinated solvents because they are capable of removing nearly any type of contaminant. The selection of the most appropriate aqueous cleaning method is dependent upon the nature of the contaminants and the desired level of cleanliness. Factors affecting the cleaning process include cleaning temperature and time, type of mechanical action, fixturing of the parts and the cleaner concentration and additives. The three methods for providing mechanical energy are immersion/agitation, spray and ultrasonic, operated either in a batch or continuous mode.

In order to appreciate the value of close-looping an aqueous cleaning system, careful consideration of several factors would be helpful. Cleaning solutions get contaminated with dirt and oil, limiting their cleaning capability. Periodic dumping and replacement of the cleaning solution is required. Closed-looping, as understood here, is incorporating into the cleaning system the technologies by which contaminants are removed from the cleaning solution and/or rinse waters, and this solution is subsequently "looped" back into the process so it can be used again in cleaning/rinsing. This allows for extended recycling and reuse of a given volume of both cleaning solution and rinse water in the aqueous cleaning system.

In some instances, a dirty bath can be cleaned, its chemicals recovered for reuse, and, in the process, its aqueous waste will be cut as much as 99 percent. The parts are initially immersed in the cleaner bath. The contaminated cleaner solution is continuously pumped through a particulate filter to a process tank where tramp oils are skimmed. The solution is then passed through a membrane filtration module, which removes emulsified oils and other contaminants. The permeate containing the cleaner components is recycled to the original cleaner bath, while the contaminant-laden retentate is sent back to the process tank.

The process tank eventually accumulates oil and other contaminants such that it must be emptied and disposed. The continued use of the process tank is a function of its productivity. This is a result of the flux through the membrane: too large a drop in flux means lower productivity. In the field, this is usually measured by the number of times the cleaner bath is "turned over" within a given period of time. When the flux has dropped below a designated level (this will be an arbitrary value, depending upon the site), the process tank will usually be shipped off for disposal (either as hazardous or non-hazardous waste, depending upon the contaminant content).

The closed loop system for the rinse section is more extensive. Because the rinsed part must be left as clean as possible, the rinse water must likewise be free of contaminants. While few places spend extensive time and money on this, the closed loop rinse water configuration is well-established and results in the production of highly purified rinse water. Depending upon the particular requirements, this configuration can be modified to reduce capital costs.

Potential Methods of "Closing the Loop"

Since the recycle design of aqueous cleaning systems relies on effective ways to separate the contaminant from the aqueous detergent, the differences in methods depend upon how separation is accomplished. Both physical techniques and chemical techniques are important in the design of closed-loop systems. The first consideration is to optimize the actual cleaning system. After this, the contaminants (oils, greases, etc.) can be physically separated from the aqueous phase. The contaminants separated will usually contain residues of the cleaners and surfactants from the cleaning solution. Some methods for accomplishing the separation are vacuum distillation, sedimentation of insoluble contaminants, reverse osmosis and membrane filtration, although it is important to note these methods have widely varying price tags associated with the processes, which may make one method more likely than another. The Illinois Hazardous Waste Research and Information Center (HWRIC) has helped dozens of companies implement membrane filtration. This alternative offers the advantage of being able to function as a "kidney" for the aqueous cleaning system by removing contaminants from the cleaning solution without taking the solution out of the process.

Membrane Filtration Technology

Periodic replacement of the bath creates a waste disposal problem. Current disposal options for spent aqueous cleaning solutions include hauling off-site, incineration on- or off-site, direct discharge (to a publicly owned treatment works) or pretreatment prior to discharge. Depending on the physical characteristics of the bath solution, the life of the bath can be further extended by skimming contaminants off the top, settling heavier fractions to the bottom, or filtering out suspended species.

In removing contaminants from the cleaner bath, the useful life of the cleaner is extended and the quantity of waste disposed is minimized. Although the aqueous degreasers do not carry all the risks and liabilities associated with the disposal of waste organic solvent cleaners, there are important concerns which must be weighed before an aqueous cleaning system with closed looping is adopted. Cleaning efficacy is paramount (i.e., the parts must be as clean as necessary), and discussions on this topic are abundant. Secondly, available methods for removing contaminants from the cleaner baths must be examined. Finally, economics must be examined.

Membrane filtration is a technology using filters capable of separating contaminants from clean detergent. The crux of the technology lies in the ability of these filters to separate molecules on the basis of differences in their sizes, shapes and charges. Membrane filtration is actually numerous techniques, including nanofiltration, multiple membrane filtration, reverse osmosis, microfiltration and ultrafiltration. Basically, the names refer to different-sized pores in the membrane filters.

While simple in concept, successful implementation of membrane filtration hinges upon selection of the right filter material and on choosing operating conditions to minimize the plugging of the filters. The case studies included in the next section illustrate the benefits resulting from successful implementation of a membrane filtration system.

Economic Considerations

Total capital costs and operating costs have been estimated for a variety of closed-loop systems. The initial costs to make the switch may seem large. However, close-looping an aqueous cleaning system can result in money saved, improved quality, increased productivity and dramatically reduced waste chemical generation.

From discussions with industry, D'Ruiz estimated costs for four typical aqueous cleaning systems. These systems include varying degrees of recycling. While the volume is reduced over non-closed-looped systems, disposal costs are included for systems where baths must eventually be dumped and the waste water must be treated as a hazardous waste. Depending upon the capacity and complexity of the aqueous cleaning system, capital costs alone may exceed $200,000. Operating costs are comparable to those of a similar sized vapor degreaser.

The table notes some of the major costs associated with a switch to closed-loop aqueous cleaning. By evaluating all the costs (both obvious and hidden), a company may decide that the promise of long-term profits outweigh the short-term costs of switching to a closed-loop system. Incorporation of a closed-loop system brought savings to those companies profiled here and the payback from investment for those companies was within two years.

The return on investment (ROI) will vary among technologies and will depend upon the contaminant-content to be dumped. The economic factors which must be considered prior to switching to a new technology are varied and involve numerous cost/benefit scenarios, which will not be discussed here. This article is designed to point out some factors that have not often been considered.

Case Study #1

R. B. White, a metal fabricator in Bloomington, IL, had been using a phosphatizing/degreasing bath at its facility for more than four-and-a-half years. Extended use resulted in the buildup of dirt and oil in the bath, compromising product quality. The bath had to be dumped every three or four months, with the replacement process requiring a full day of lost production time, and disposal costs approaching $15,000 a year. HWRIC engineers and chemists discovered that the dump water contained less than one percent oil and grease. The remaining 99 percent was composed of valuable chemicals and water. Center staff reasoned that if a method could be found to separate the valuable chemicals from contaminants, both chemical and disposal costs would be drastically reduced.

After the highly encouraging study results, a full scale system was installed at a cost of $12,000. R. B. White has since operated the system for more than four-and-a-half years without dumping the bath, and has dramatically cut its chemical consumption. The company projects more than $200,000 in savings over the next 10 years.

Case Study #2

Radio Flyer manufactures children's wagons at its facility in Chicago, IL. The company contacted HWRIC engineers regarding a waste problem associated with degreasing the wagons prior to painting operations. The degreasing operation resulted in discharging approximately 6,000 gallons of spent solution to the sanitary sewer every two weeks. HWRIC staff conducted a site assessment of the facility and determined that incorporating an ultrafiltration system into the degreasing process might significantly extend the life of the solution, reducing waste discharges.

The filtration unit was effective at maintaining contaminant levels at less than 0.02 percent over the monitored period. Based on these results, Radio Flyer chose to install a permanent ultrafiltration system as an in-process recycling technique. Radio Flyer anticipates more than $50,000 in savings each year from the reduction in chemical use and waste disposal costs. Additionally, the waste volumes from dumping the degreasing tanks will be lowered by approximately 75 percent.

Case Study #3

Eco Finish manufactures various metal parts using stamping forming processes at its facility in Montgomery, IL. Metal shaped parts are cleaned, phosphatized and painted prior to shipment to the customer. The cleaning/phosphatizing operations at the plant generated a waste water that had to be disposed. HWRIC designed and installed a closed-loop system using ultrafiltration membranes to continuously recycle the cleaning solution.

Implementation of ultrafiltration has resulted in significant cost savings and waste water reduction. It enabled Eco Finish to continue operating at its facility, since it is not connected to municipal sewer facilities and had no practical and economical means of disposing its spent degreasing/phosphatizing solution.

Case Study #4

Harris Corporation's Broadcast division is located in Quincy, IL, and manufactures radio and television communications systems and equipment. The company successfully reduced raw materials cost and minimized its hazardous waste generation through a phased replacement of a vapor degreasing system with an aqueous cleaning system combined with a closed-loop ultrafiltration system.

In January, 1996, the vapor de-greasing tank was removed and the aqueous cleaning system was in-stalled. Allowing for total costs, the company projects a payback period of 1.7 years (on an initial capital investment of $142,700) and a projected 10 year savings of more than $500,000.

Case Study #5

Superior Plating, Inc. in Minneapolis compared the paybacks for the replacement of a 1,1,2-trichloroethylene vapor degreaser with an immersion aqueous cleaning system. The line originally cleaned 15,500 sq. ft. of plated surface per week. Without cleaning solution recovery, the payback period for the aqueous cleaning equipment based on operating savings ($13,288/yr.) was 1.13 years. When a cleaner recovery system incorporating a ceramic filter was installed, the annual operating savings increased to $26,719. The payback period for the total system (immersion tank and recycler) became 1.35 years.

Pollution Elimination

The current practice of closed-loop aqueous cleaning does not eliminate the problem of a waste stream. It instead allows for reducing the quantity of waste disposed by continuously reusing the cleaning chemicals and reducing the volume of the non-hazardous component of the waste stream. The reality of the future in cleaning is that all contamination will be removed from a cleaned part. This contamination, itself, in the end, will be transformed into the simple chemicals of water, carbon dioxide and/or the basic elements.

Conclusion

Initial motivation for considering - or for adopting - a closed-loop system would include regulatory benefits, fees or elimination of hazardous waste. Finding a system which offers the best ROI may not necessarily result in implementing a closed-loop system. Size, contaminants and cleaning demands might lead to options aside from closed-looping. However, this article's focus was on closed-loop aqueous cleaning.

Closed-loop aqueous cleaning is a proven technology, found to be effective at reducing waste volumes by both concentrating the sludge accumulated in the cleaning process and extending the life of the cleaner bath by an average of seven to 10 times. By incorporating closed-loop aqueous cleaning to reduce chemical use and waste generation, companies will position themselves at the forefront of both cleaning technology and current government regulations.

The next goal to pursue involves how to completely recover the water used in the cleaning and to produce zero waste (total closed-loop). While this is not yet a reality, it can be viewed as the ultimate vision of the cleaning process.

References

1. Fava, J.S.; Page, A. "Application of Product Life-Cycle Assessment to Product Stewardship and Pollution Prevention Programs," Wat. Sci. Tech, 1992, 26, 275-287.

2. Jenkins, R.W. "Closing the Loop on Aqueous Cleaning at Hamilton Standard," Precision Cleaning, Nov. 1995, 18-22.

3. Technology Update, Nov. 1994, The Hazardous Waste Research and Information Center.

4. The Massachusetts Toxics Use Reduction Institute "Closed Loop Aqueous Cleaning," 1995, Tech. Report 29.

5. Lindsey, T.C.; Ocker, A.G.; Miller, G.D. "Recovery of an Aqueous Iron Phosphating/Degreasing Bath by Ultrafiltration," J. Air & Waste Manage. Assoc. 1994, 44, 697-701.

6. Bailey, P.A. "The Treatment of Waste Emulsified Oils by Ultrafiltration: Proceedings of the Filtration Society," Filtration and Separation, Jan/Feb 1977, 47.

7. Quitmeyer, J. "Aqueous Cleaners Challenge Chlorinated Solvents," Pollution Engineering 1991, 23, 88.

8. Nelson, W.M. "The Key to Successful Aqueous Cleaning is...Water," Precision Cleaning, April 1996, 30-34.

9. D'Ruiz, D.D. "Aqueous Cleaning as an Alternative to CFC and Chlorinated Solvent-Based Cleaning," Park Ridge, NJ: Noyes Publications, 1991, pp. 62-71.

10. Karrs, S.L. and McMonagle, M. "An Examination of Paybacks for an Aqueous Cleaner Recovery Unit," Metal Finishing, September 1993, 91, 45-50.

11. HWRIC, "Harris Corporation: The Road to Continuous Improvement, 1993-1996."

12. Karrs, S.L. and McMonagle, M. "An Examination of Paybacks for an Aqueous Cleaner Recovery Unit," Metal Finishing, September 1993, 91, 45-50.

13. Seelig, S.S. "Cleaning and Drying in the 21st Century," Metal Finishing, March 1996, 20-23.

About the Author

William M. Nelson received his Ph.D. in organic chemistry from The Johns Hopkins University. After spending numerous years in academia, Dr. Nelson is currently an alternative process chemist for the Hazardous Waste Research and Information Center, where his research focuses on the design of chemical processes which lead to pollution prevention. He is a member of the American Chemical Society, American Society for Photobiology, American Association for the Advancement of Science and the International Ozone Association. He may be reached at (217) 333-8940, by fax at (217) 333-8944 or via e-mail at billn@hwric.hazard.uiuc.edu