Reprinted from Powder Coating Magazine= Volume 14 / Number 3 = April 2003
Using reverse-osmosis systems to recycle used rinse water
Thomas Borcherding TB Sales
Although powder coatings are dry, companies that apply them require large amounts of water to expedite the finishing process. Despite the fact that water was cheap and plentiful at one time, finishers now know it's not an infinite resource that can be squandered. By using reverse osmosis (RO) and deionization (DI) technologies, finishers have found ways to reduce their water consumption. This article focuses on RO technology to recycle used rinse water. It also discusses the history of water use in the powder coating industry. It then explains how RO and DI technologies came to play a role in water treatment and how RO can be used to recycle as much as 90 percent of the rinse water used in surface preparation.
The powder coating industry demands a high volume of purified water to support production. Finishers use purified water to clean and rinse parts. Simultaneously, finishers face the need to reduce the total volume of wastewater from their plants. As companies begin to find out that many of their waters can be reused internally by containing them and then repurifying them, finishers are also finding that the process involved is so simple, it's often cost-effective for them to reclaim the water. This is particularly true when membrane processes are used to treat water because the water recovery for reuse is extremely high, typically around 80 to 95 percent.
Water Use Reduction is becoming essential to the powder coating industry. Finishers have reduced their water consumption through reuse and reclaim processes that have resulted in a lowered total amount of water used. Water use reduction, if implemented correctly, translates to a reduction in cost. However, this cost saving is only one reason to reduce water use. A growing customer demand for industrial environmental responsibility has prompted several companies to develop environmental management systems (EMS) and to become International Standards Organization (ISO)1 14001 certified.
Certification in the ISO 14001 standard is partly based on the requirements of a company's EMS to identify the environmental aspects derived from their operations, set objectives and targets to minimize significant aspects, and to commit to continual improvement. By reducing water consumption, companies have reaped many benefits. For instance, companies have been able to reduce costs, increase the efficiency of their operations, and reduce the consumption of natural resources in today's environmentally conscious community. Finishers can make significant reductions in wastewater discharge by eliminating an unneeded prerinse step or by counterflowing rinse baths and then reclaiming the used rinse water.
Finishing businesses take water availability for granted
With regard to water and related issues, the powder coating industry is just emerging from its infancy. Powder coaters began by using water as a utility. They used water for
*Chemical solution preparation
During the industry's infancy, water was relatively abundant and inexpensive. Because water was plentiful and cheap, finishers took it for granted. As the industry developed during the past 2 decades, relatively large quantities of water were used on a once-through basis. Even high-purity final rinse water wasn't conserved, recycled, or reused in the same application. Industrial times weren't only good, they were great. The use of water in the powder coating industry continued to grow; simultaneously, the reuse and conservation of water continued to be neglected.
Then, a change took place. Water became the target of the public eye. In addition, two other interesting results put pressure on water-related issues. First, because the industry grew rapidly and because corrosion science had a tendency to be widely misunderstood, older plants and equipment, such as washers, began to leak. Bear in mind that some of these plants were built rapidly and often with less than the most sophisticated engineering. Multiple materials came in contact with each other, corrosion resulted, and leaks occurred. Leaks, sludge, scale buildup, and plugged nozzles all caused downtime. Downtime threatened to ruin the economical aspect of powder coating.
Second, a related and even more misunderstood issue occurred with water. The amount and type of contaminants found in raw water supplies throughout the country varied considerably2. Mineral hardness, bicarbonate alkalinity, and total dissolved solids (TDS) content began to affect washer chemistry. That, in turn, affected powder adhesion.
Finishers had a tendency to overlook water problems because they considered water a utility rather than a product. An industry that started out by taking water for granted had now grown to where water had become a major problem. Now, the management of water promises to dominate the next 2 decades of changes in the powder coating industry. The use of water in washers is complicated by the wide variety of materials finishers have used through the years in their water-transporting systems and by the chemicals finishers have been adding to the water to control unwanted waterformed scale or debris, biological fouling, and many types of corrosion. By adding chemicals, cleaners, phosphates, and sealers, along with the natural chemical concentration that occurs when water is evaporated, the powder coating industry is finding it necessary to staff its plants with personnel who can manage water or to outsource the job to companies capable of fulfilling this function as service vendors.
Whatever the case, powder coating operations generate water and wastewater that must be contained, managed, and disposed of appropriately. The costs associated with these processes are no longer trivial; they're significant production expenses. Finishers can no longer make the assumption that water is an inexpensive commodity.
The first step in water and wastewater management is awareness. As companies become aware of ways to reuse their water internally by containing and repurifying it, they're also discovering that the process is so simple, it's often cost-effective for them to reclaim the water. This is particularly true when membrane processes are used to treat water because the recovery rate of water for reuse is extremely high, typically 80 to 95 percent. Conservation practices may be new to the industry, but they're becoming a way of life. Companies constructing new plants are instructing their engineers to keep water conservation, reuse, and recycling in mind when drawing up designs. This trend is likely to continue.
Reverse osmosis replaces deionization systems
Many pretreatment systems used in the powder coating industry rely on reverse osmosis (RO) and deionization (D!) technologies for water treatment. [See "Replacing deionizers with reverse osmosis technology to purify water for multistage washers" Powder Coating, vol. 11, no. 3 (April 2000), p. 23.] However, RO is replacing DI as the technology of choice for many coaters. Although the technological advances in powder coating chemistry and related equipment are significant, the pretreatment process, where parts are prepared for coating, can't be overlooked. This includes, as a minimum, spot-free final rinsing of the product before powder application. In addition, with many newer washers, pretreatment also includes purification of the water used in the entire washer.
RO is a relatively new technological development. The first RO systems date back to the 1970's. In the years since then, the technology has matured. Today's systems represent viable methods for reducing the concentration of materials dissolved in water. The technology has become applicable in widely diversified fields, including drinking water, fruit juices, waste treatment, and the production of highly pure process water for use in numerous industrial applications. RO technology uses a high-pressure pump to force water through a semipermeable membrane made of plastics. The water molecules are small enough to pass through the membrane, leaving behind the larger metal ions and mineral salts. In this manner, an RO machine can remove 97 to 98 percent of the TDS found in incoming feed water.
Early attempts to introduce RO for industrial use met with reliability and performance problems mainly associated with the high pressure required to achieve reasonable fluxes, the limits of membrane service life, the lack of operating experience, and the lack of guidelines. Following this rather questionable introduction, viable RO technology, based on a new generation of membranes and a better understanding of operating requirements, was eventually introduced in the 1980's. The commercial introduction of RO then rapidly evolved so that today, most new and many retrofit water systems use RO instead of the traditional ion-exchange systems. Furthermore, by using specific and newly developed membranes, RO technology has worked successfully in other applications, including wastewater treatment.
In a typical five-stage washer system, such as the one shown in Figure 1, stage one uses 140F water and alkaline cleaner to remove cutting oils and to degrease the parts. Stage two is an ambient-temperature city water rinse that continuously overflows to drain. Stage three is a surface preparation cycle using 130F chemical solution containing iron phosphate. Stage four is another ambient-temperature city water rinse, also overflowing continuously to drain. Stage five uses a traditional deionizer for fresh DI water to achieve a spot-free final rinse. Many improvements have been made to this system design in recent years, including replacing the final rinse water with an RO system (see Figure 2) that not only provides spot-free rinse water quality but also does this more efficiently and less expensively than the deionizer. Moreover, the continuous overflow to drain from the rinse stages can be reduced and sometimes eliminated when using RO water for the rinse baths.
Recycling rinse water requires separation
How do you recycle the used rinse water without overflowing it? To recycle the rinse water, you need to separate the used rinse water into a bulk storage tank so that you can repurify it. This stored water can now be continuously circulated through a series of filters, each specifically designed to remove certain impurities such as iron, mineral hardness, chlorine, organics, turbidity, suspended colloidal particulates, and dissolved salts. Figure 3 shows the whole washer RO system design.
The system is similar to the RO system discussed previously, but with the addition of two more storage tanks, you can now separate the used rinse water returned from the washer and collect the concentrate waste stream from the RO machine for reuse.
In this manner, water going out to the washer is purified RO water (Tank 1) and all of the used rinse water (Tank 2) returned from the washer must pass through the RO equipment. The concentrated salts from the RO reject stream are also collected (Tank 3) and passed through the RO machine one more time. By adjusting the recovery rate of the RO machine, you can recycle and reuse as much as 90 percent of the rinse water being used in your five-stage washer. Typically, the final 10 percent is sent to an evaporator or other means of disposal when you need to meet zero-discharge requirements. This system works well when the TDS of the used rinse water can't exceed a concentration of 200 milligrams per liter (mg/L). The final concentrate to the evaporator will then stay below 800 to 1,000 mg/L, which in many instances is still cleaner than the raw water (city water) that you started with.
Figure 4 shows an RO reclaim system at Tuthill Transport Technologies, Brookston, Ind. The system includes four 1,500-gallon storage tanks, an activated carbon filter, duplex water softener, and a 10,000-gallon-per-day RO machine. The used rinse water is continuously circulated through the equipment. Purified water is then sent back to the washer. This allows the company to recycle 90 percent of its used rinse water with the remaining 10 percent going to the evaporator. The system meets zero-discharge regulations.
This article has discussed the advantages of RO technology as well as the advantages of incorporating RO into a new system design. The concept of having a whole washer RO with reclaim offers several advantages. For example, the whole washer RO system
*Recycles as much as 90 percent of used rinse water
*Reduces dumping and the need to recharge chemical stages with more chemicals
*Eliminates scale buildup on heat exchangers
*Reduces sludge buildup
*Provides a spot-free final rinse
*Increases finish quality by increasing powder adhesion
Consider these benefits and find ways to incorporate RO technology into your finishing process. This way, you'll be making economical as well as ecologically wise decisions. PC
1. International Standards Organization, located at 1, rue de Varembe, Case postale 56, CH-1211 Geneva 20, Switzerland; 011-4122-749-0111, fax 011-4122-733-3430; Web site is at [www.iso.ch]. ISO 14001 is the international standard for environmental management systems.
2. The Water Quality Association (WQA) provides detailed information about the quality of water supplied by municipal water systems in the US. For more information, contact the organization at: WQA, International Headquarters and Laboratory, 4151 Naperville Rd., Lisle, IL 60532; 630/505-0160; Web site is at [www.wqa.org]; E-mail address is [email@example.com]
For further reading, see articles listed under the Surface preparation headings in the "Index to Articles and Authors 1990-2002," Reference and Buyer's Issue, Powder Coating, vol. 13, no. 9 (December 2002) and check the Powder Coating Web site at [www.pcoating.com].
Thomas Borcherding is president of TB Sales, PO Box 99, Slinger; WI 53086; 414/333-1807. He is a distributor for Osmonics RO machines and specializes in designing and installing custom built industrial RO systems. He studied chemical engineering at the University of Wisconsin-Milwaukee, is the author of several articles for trade publications, and speaks at seminars and other educational programs.
Borcherding TB Sales