Reprinted from Powder Coating Magazine= Volume 14 / Number 3 = April 2003

REVERSE-OSMOSIS SYSTEMS

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

• Coolant
• Heat transfer
• Rinsing purposes
• Cleaning purposes
• 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.

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.

In summary

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

Endnotes

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 [info@mail.wqa.org]

Editor's note

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.

Reprinted from Powder Coating Magazine= Volume 13 / Number 5 = August 2002  

CASE HISTORY

'To meet more of its customers' needs and to be more cost-competitive, the company looked for an alternative to pretreat parts. The company considered deionized (DI) water but determined that it was too costly to process the water and retrofit its mild steel tanks to handle DI. The finisher's chemical supplier suggested an alternative. The supplier had been collaborating with a process design firm on a pretreatment package to lower chemical costs and raise finish quality. This piqued Ad- Tech's interest enough to have the water systems engineer come in and propose a system. Initially, the firm installed a pilot sys- tem that ran for a month. Seeing this pilot unit in action convinced Ad-Tech that this system addressed and remedied their pretreatment issues. Next, the design firm assessed their operation and sized a system accordingly.

Making more with less

The chosen system uses reverse osmosis (RO) water. Tap water goes through a carbon filter that removes chlorine and then goes through a softener. Next, the water goes through the RO system. RO membranes filter the water, removing salts and minerals. The membranes reject 98 percent of the total dissolved solids (TDS) in the water. As a result, the city water went from 330 TDS down to about 7 TDS, leaving less dissolved solids on the part and eventually under the coating finish. The RO system could also be used to continuously remove organic contaminant, suspended solids and colloidal particulates, and inorganic dissolved salts from the used rinse water returned from the washer. Recycling rinse water saves thou- sands of gallons of process water daily. With the previous method, the company overflowed more used water to the drain. Now, the company uses RO water in all of the final rinses, in the immersion sys- tem, and in the seven-stage spray washer.

The RO system makes 10 gallons of RO water per minute. The processed water is held in two 3,000-gallon process tanks. When a washer needs water, a valve opens on one of the process tanks-both are under pres- sure-and a pump feeds the washer. Another pump circulates the water in the holding tanks to eliminate bacterial growth. If the finishing lines were running 100 percent, water demand would be 8 to 9 gallons per minute. The system keeps up with that. In addition, 6,000 gallons of RO water are stored in the holding tanks, ready for use as fast refill water when and if the washer stages are dumped and need to be replenished.

Besides its processing capability, the RO system needs little maintenance. Workers change the particulate filter monthly and perform a daily visual check to make sure pressure, product flow, drain flow, and other parameters are where they're supposed to be. In the 3 years since the system has been operating, the RO membranes haven't needed any service.

Reprinted from Powder Coating Magazine= Volume 11 / Number 3 = April 2000

• Treatment of waste
• Concentration of fruit juices
• Treatment of drinking water
• Production of high-purity process water for use in numerous industrial applications

RO technology uses a high-pressure pump to force water through a semipermeable membrane. 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 unit can remove 97 to 98 percent of the TDS found in incoming feedwater. To calculate the approximate product water electrolyte content from an RO unit, simply multiply the raw water TDS by 0.02. In most instances, RO product already has 100,000 ohms resistivity and is considered spot-free (DI) water.

Developing RO technology
Early attempts to introduce RO for industrial use met with reliability and performance problems, associated mainly with

• High pressure requirements to achieve reasonable fluxes
• Limits of membrane service life
• Lack of operating experience
• Lack of guidelines

Following this rather problematic 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 employ RO instead of the traditional ion-exchange systems. Furthermore, by using specific newly developed membranes, RO technology has worked successfully in other applications, including large-scale desalination. Thus, when technical performance and cost issues related to RO were satisfactorily resolved, the widespread introduction of RO took place in a remarkably short time.

As in all developing technologies, many have interpreted the role RO treatment should play and what its interaction with existing technologies should be. In particular, one school of though considers RO a competitor of ion-exchange technology; many references to comparisons of the two treatment methods can be found in literature of the past 2 decades. Regardless, the market penetration of RO technology in the front end of water systems is an example of the current trend towards replacement of ion-exchange operations by continuous membrane processes. In the past 2 years, my company (TB sales) has installed more than 25 RO systems and only two DI systems.

• Restores the membrane to its original flux rate
• Sustains membrane performance
• Increases membrane service life
• Saves money
• Decreases downtime
  

RO membranes often come with manufacturers' warranties and can last as long as 6 to 8 years if maintained properly. Membranes need to be changed when water flow drops 15 to 20 percent from the original flow rate or when cleaning the membranes no longer restores the original flow rate.


Understanding the whole-washer RO concept
We’ve seen RO technology produce DI water quality (less than 10 ppm TDS). We also know that the only operating cost for an RO machine is the electricity to run the motor. So, you may ask, why not treat all the water for the entire washer? Reduced TDS water used to be expensive to produce with a traditional DI system. However, the RO machine, operating at a cost of hundreds of dollars annually, can now remove 98 percent of the incoming raw waters dissolved solids, compared with 99 percent removal by a DI system that costs thousands of dollars annually to operate. Purified water isn’t just for final rinse any longer. It’s now cost effective to use purified water for the whole washer, including daily make-up water and fast refill water. The advantages include

• Elimination of scale on heat exchangers
• Elimination of sludge buildup from insoluble salts
• Reduction of chemical use for phosphates and cleaner

Consider the following example of a typical five-stage washer with purified water on the final rinse only. City water with 400 ppm is used for continuous overflow rinses in stages two and four. The overflow rates must be maintained at approximately 3 gpm to keep the rinse water diluted. However, we know that the TDS in stages two and four will never be less than 400 ppm in this manner. By using RO water with only 10 ppm TDS, we can substantially reduce the overflow rate until the operating TDS of stages two and four are somewhere in the rage of 100 ppm, this can be monitored by an operator with a handheld TDS meter. In most instances, overflow rates can be reduced by as much as 50 to 60 percent. This means you produce less wastewater and you use less wastewater treatment chemicals.

Next, let's look at the heated stages, one and three, that loose water in evaporation. If stage three has 2,000 gallons of iron phosphate and requires about 500 gallons a day of make-up water, then the TDS for the entire 2,000-gallon bath increases by approximately 100 ppm per day. This causes a corresponding change in conductivity. You can see that after 1 to 2 months of operation, the TDS content will have increased to over 2,000 ppm. Because only the water evaporates, the dissolved solids remain in stage three. If conductivity is the parameter we’re trying to monitor in stage three, then it becomes literally impossible to control the TDS. Eventually, everything in stage three must be dumped to drain and recharged with new chemical.

Because the RO machine will remove approximately 98 percent of the raw-city-water TDS, the resulting RO product water in this example will have 8 ppm. Naturally, by removing 98 percent of the TDS, we can eliminate the buildup or concentration of dissolved solids; the effect of fighting against conductivity is now reduced.


Planning ahead
Getting involved early in the construction planning is the best way to provide the needed water treatment equipment. Unfortunately, many project planners and designers don’t think about water quality for a finishing line until late in the design process. Nonetheless, awareness of water quality for pretreatment is increasing as more information becomes available about contaminants and their various water supplies. Water quality is becoming a primary design criteria for multistage washers, just as energy efficiency is the primary driver in designing heating systems. This has created the need to apply whole-washer treatment approaches to new