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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.

In a typical five-stage washer system, such as the one shown in Figure 1, stage one uses 140°F 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 130°F 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.

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

Reverse

osmosis

moves

finisher

forward

Unable to reach its
quality goals with city tap
water, a Milwaukee-area
coater installs a
reverse-osmosis system
that provides cleaner
pretreatment for a
stronger finish while
using less materials
and labor.

Heralded as a great place on a great lake, Milwaukee rests on the shores of Lake Michigan. This proximity to water played a part in crowning this metropolis the beer capital of the world. The birthplace of Miller, Pabst Blue Ribbon, Schlitz, and a namesake brew, Old Milwaukee, great beers came from this great lake city. But what proved to be a boon for city brewers became a bane for a local finisher.

Ad-Tech, Watertown, Wis. has four coating lines--two liquid and two powder--to serve the Milwaukee metro area. Customers include metal fabricators, plastic molders, machine shops, metal stampers, and manufacturers involved in the lawn and garden and medical industries.

In addition to offering coating choices, the lines handle different parts, substrates, and production volumes. As a result, the lines use different pretreatment systems. One of the powder lines and one of the liquid lines have a five-stage power washer, consisting of a multimetal cleaner, rinse, iron phosphate, rinse, and a reverse osmosis (RO) final rinse with nonchrome seal. The other powder coating line has a seven-stage pretreatment system, consisting of an alkaline cleaner, rinse, laser scale remover, rinse, iron phosphate, rinse, and an RO seal. The other liquid line uses a five- stage immersion system.

Ad- Tech's five-stage washer has never needed to be descaled in the 3 years of operation since the company started using reverse osmosis (RO) water. Previously, tap water caused scale to build up on nozzles and risers (inset), distorting the spray pattern and leaving the parts dirty. Poor cleaning resulted in adhesion problems.

Something in the water

Previously, the finisher used tap water for its washers. Finishing fundamentals proved difficult to achieve with the city water pretreatment. In terms of performance, the company struggled to get good salt-spray results. Tests needed to be run constantly in a struggle to achieve 200 hours resistance. Salts, minerals, and other materials in the water hampered production because they absorbed a lot of the pretreatment chemicals, making them less effective and leaving solids on the parts from the final rinse. Therefore, the wash systems required more chemicals to get acceptable parts cleaning.

Washer maintenance proved to be even more of a burden than achieving acceptable finishes. Twice a year, the company had to descale the washer to remove the mineral-and-salts buildup on the tank and canopy walls, headers, risers, and nozzles. Workers dumped the baths (stages vary from 600 gallons to 2,000 gallons) and added a descaler to remove the buildup. This involved heating the tank. After the scale was removed, the descaler needed to be neutralized and sent to waste treatment. Next, workers refilled the tanks to rinse them. Then the effluent needed to be flushed out and neutralized. "There's a lot of time, plus a lot of cost [involved]," said Steve Loritz, operations manager. "Maintenance is downtime, and you need to schedule it on weekends. Then, you're paying people overtime to come in and do that type of work."

On a weekly basis, workers needed to check for plugged nozzles. To do this, workers entered the washer to inspect every nozzle-a time-consuming task because some washer stages have 150 nozzles. In addition, the risers and nozzles are narrow, 1 1/4 inches and 1/16 inch in diameter, and plugged easily, resulting in poor spray impingement. Instead of spraying a nice soft v-shaped fan pattern, partially blocked nozzles spewed a stream of solution that left parts at best, poorly cleaned, at worst, knocked off the racks and lying on the washer floor. "This hurt too," Loritz said. "We were losing parts and coming up short on orders."

When the company used tap water, washers needed to be descaled twice a year. Workers checked for plugged nozzles and risers weekly. With the reverse-osmosis system, the company hasn't had to descale its washers in 3 years.

The RO system uses membranes to filter water; removing salts and minerals out of it. These membranes reject 98 percent of the incoming total dissolved solids. The system processes 10 gallons per minute and has two 3,OOO-gallon storage tanks.

 
Looking for a cleaner clean

'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.

Ad- Tech finishes parts for
metal fabricators, plastic
molders, machine shops,
stampers, and manufacturers
involved in the lawn and
garden and medical industries
in the Milwaukee metro area.

Getting it all and more

The addition of the RO system has addressed Ad- Tech's concerns about coating performance and operating efficiencies. Pretreatment with RO water provides excellent corrosion resistance. Coatings getting 200 hours salt-spray resistance with tap water pretreatment now achieve 600 hours with RO pretreatment; some high-end coatings went from 400 hours to 1,400 hours. Most of the company's coating requirements are easily achieved with the RO water. Before, all pretreatment parameters had to be perfect. It was challenging, sometimes impossible to meet certain requirements depending on the soils that were on the steel.

In terms of production, the company’s pretreatment chemicals are more effective with RO water than with tap water. The solids in the tap water, such as salts and minerals, absorbed a lot of the chemical. The RO water makes the chemicals more aggressive by removing these solids. As a result, the finisher's chemical consumption dropped by 20 percent. The laser scale remover is more effective in the RO water than in the city water and allows chemicals to remove the carbonation buildup around welds. Chemical bath life has doubled and dissolved solids are about 30 times less with RO water than with tap water.

In addition to reducing chemical use, the RO system enables the finisher to handle water efficiently. Workers cut back overflowing the rinse stages by as much as 50 percent. Saving water saves money. "If you're on city water like we are, you get billed for water that you use and as it gets treated and goes to sewer," Loritz said. "So the more water you use, the more sewer costs also."

The RO water also demands less of the company's in-house waste treatment system, requiring less chemistry than tap water to treat it and get it within the city's limits to discharge to sewer. As a result, the finisher uses fewer chemicals for pH adjusting, flocking and coagulating, and everything else needed in waste treatment.

In addition, the system has substantially reduced washer maintenance. Washer operation efficiency has increased and labor demands have dropped. "We've been running this [seven-stage] washer for 3 years, and we've never had to descale anything yet," Loritz said. "We're not scaling up the heat exchangers or burners, and the heat transfer is much better."

Just as a host of local breweries once fought for the discerning palette of the beer drinking populous, Ad-Tech finds itself surrounded by powder coating shops looking to tap into their customer base. Loritz says that the RO system gives his company the edge on the competition by boosting customer satisfaction and by eliminating some overhead.

Effective chemicals provide quality pretreatment that gives better adhesion and coating performance. As a result, the shop has cut down on reworks. Less rework translates into quicker turnaround times to customers. "After being in the business as long as we were (since 1978), we knew the RO system would get us to the next step," Loritz said. "It makes us unique over other finishers. Not that the tap water is just lousy pre- treatment, it's not. But the RO water just brings everything to another level." PC


Water system design: TBS Industrial
Water Treatment, Slinger, WI.
414/333-1807. www.tbsales.net


Reverse osmosis machine: Osmonics,
Minnetonka, MN. www.osmonics .com


Pretreatment chemicals: JohnsonDi-
versey Industrial/DuBois, Sharonville,
OH. 513/326-8800. www.johnson
diversey.net
 

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

Replacing deionizers with reverse
osmosis technology to purify water
for multistage washers

Thomas Borcherding TB Sales



Many powder coaters use reverse osmosis and deionization technology to achieve spot-free rinsing of products before applying powder coatings. This article focuses on the replacement of deionizers by continuous membrane processes such as reverse osmosis. It explains the history of reverse osmosis technology and details how the process can effectively remove total dissolved solids from water. The article compares a basic five-stage pretreatment system with various reverse osmosis systems. The article goes over the advantages of reverse osmosis technology and emphasizes how important it is for companies to get involved early in the planning of water treatment equipment.

Many pretreatment systems used in the powder coating industry rely on reverse osmosis (RO) and deionization (DI) technologies for water treatment. However, RO is replacing DI as the technology of choice for many coaters. As an example, one powder coating company recently replaced it portable exchange DI vessels, used for spot-free final rinse water to the washer, with an RO system capable of producing 10,000 gallons per day of purified water. The portable exchange DI vessels needed replacing every several days, costing the company $3,000 per month. With the RO system now in place, the company can save $36,000 per year and can enjoy consistent process-water quality and quantity, which, in turn, guarantees the smooth function and operation of the company's multistage washer. The only operating cost of an RO unit is for the electricity needed to run the motor. In this case, the powder coating company pays about $500 annually in electricity for the RO unit.
Although the technological advances in powder coating chemistry and related equipment are significant, the pretreatment process (where parts are prepped 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. In this article, we'll explore the evolution of spot-free rinse water systems and their impact on washer system design and operation.

Removing total dissolved solids
The traditional DI system uses caution and anion ion-exchange resins to remove the dissolved ions from the water supply. The resins are inert, synthetic plastics, developed in the late 1950's, which have an affinity for positively and negatively charged ions. When water passes through these resins, it removes 99% of the dissolved salts. As electrolytes in the form of dissolved salts in water are removed, the water is less capable of conducting electricity. This causes the water's resistance, measured in ohms, to increase. Therefore, resistivity and conductivity are used to measure the purity, or quality, of the water. Table 1 shows specific conductivity, specific resistance, and approximate electrolytes in water.
Measuring water quality

Specific conductivity
(in microohms)

Specific resistance
(in ohms)

Approximate electrolyte content
of sodium chlorine
(in parts per million)

0.1

10,000,000

0.04

0.2

5,000,000

0.08

1.0

1,000,000

0.40

2.0

500,000

0.80

4.0

250,000

1.60

6.0

166,000

2.50

8.0

125,000

3.20

10.0

100,000

4.00

20.0

50,000

8.00

30.0

33,333

14.00

40.0

25,000

19.00

50.0

20,000

24.00

60.0

16,666

28.00

70.0

14,286

33.00

80.0

12,500

38.00

90.0

11,111

43.00

100.0

10,000

50.00

200.0

5000

100.00

High dissolved-solids levels in rinse water can cause flaws in a powder finish by leaving behind a residue or spot on the substrate surface as the water droplets evaporate. Spot-free final rinse water for typical powder coating lines must have resistivity greater than 50,000 ohms. This corresponds with a total dissolved solids (TDS) level of less than 8 parts per million (ppm). A DI unit will remove 99 percent of the dissolved solids from feedwater, yielding more than 100,000 ohms resistance.

RO is a relatively new technological development; the first commercial TO systems date to the 1970's. In the years since then, the technology had matured. Today's systems represent viable methods for reducing the concentration of materials dissolved in water. The technology has found applications in widely diversified fields including

  • 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.

Using a five-stage washer
In a typical five-stage washer (shown in Figure 1), stage one uses 140 degree F hot water and alkaline cleaner to remove cutting oils and degrease the parts. Stage two is an ambient-temperature rinse using city water. Stage three is a surface preparation using 130 degree F chemical solution containing iron phosphate. Stage four is another ambient-temperature city-water rinse. Stage five uses fresh DI water for a spot-free final rinse. A 5- to 10-gallon-per-minute (gpm) flow rate of fresh DI water passes through the last set of spray nozzles and will continuously overflow from the washer tank to the drain. This once-through DI design provides adequate rinse-water quality in the range of 200,000 to 500,000 ohms of resistivity.

One problem with this design is that the load capacity on the DI equipment is equal to the TDS in the raw city water being treated. Frequent regeneration and maintenance are required for the once-through DI system to work properly. An optional sixth stage uses a recirculated DI water design to send fresh DI water to the final-rinse spray nozzles. The water then cascades back to stage five, which is now a recirculated DI water rinse. The used DI water can now be brought back to the DI unit at flow rates that are a fraction of the pump flow rate in stage five. In this manner, the water will continuously polish and maintain reduced TDS quality by removing any carryover dissolved solids that come from stage four city-water rinse dripping off parts as they travel to stage five. Depending on the size and shape of the parts being washed, this carryover water can create a significant load on the capacity of the DI system. A typical recirculated DI water treatment system would incorporate a duplex unit (one unit in service, the other in standby) for a continuous 24-hour-a-day operation.

Today's multistage spray washers require final-rinse-halo flow rates of DI water from 2 to 20 gpm. Usually, you consider the size, shape, and contact time within the rinse for each part when designing the washer. Because 2 to 20 gpm is a relatively low flow rate, smaller commercial-size equipment can be used for the production of spot-free rinse water. The combination of RO and DI equipment will most often provide the most environmental advantages, along with the lowest costs (in terms of capital and operation) by eliminating the frequent regenerations (acid and caustic chemicals) typically used by a traditional once-through DI system.

Designing an RO system
The following four system designs have evolved from each other. They offer cost savings and other operating advantages over the once-through DI unit.

Large-capacity single DI. This is a large-capacity single DI unit with a carbon filter and optional waste-neutralization tank. The system, shown in Figure 2, can run 8 to 16 hours per day with single DI and can regenerate its capacity during off-peak hours. This system has the lowest capital cost among systems but is less efficient than other systems. This was the standard design 30 years ago.

Recirculated duplex DI. This is a duplex DI system for continuous, 24-hour-a-day operation. The used DI water recirculates back through the carbon filter while the online DI unit remains on standby. Built-in resistivity monitors automatically initiate the regeneration sequence. In recirculation mode, the DI water will approximate 1.5 million ohms. This design, shown in figure 3, saves on chemical costs and recycles water; however, you need to include the capital cost of the second DI unit when considering this design. Make-up water in the recirculation loop is the only load on the capacity of the DI unit.

DI with RO make-up. This system, shown in Figure 4, includes single or duplex DI units with a small receiving tank added to the recirculation loop. In this loop, the water level is maintained in the receiving tank (not the washer) using RO as the make-up water source. The RO device removes 97 percent or more of the make-up TDS, thereby reducing the regeneration frequency of the DI that much more. This is the most efficient system design for high-volume DI water usage and high rinse-water flow rates.

RO with portable exchange deionizer vessels. In this system, shown in Figure 5, the RO device is the primary roughing unit. It removes 98 percent of raw-city-water TDS, no matter what the incoming TDS is, into relatively large volume storage. This stored RO water is the continuously recirculated through portable exchange deionizer (PEDI) vessels. This system is highly efficient and uses no chemical to regenerate the DI tanks on site. Water quality is the same in this system as it is with the system that uses DI with RO make-up because daily make-up volumes are smaller. Here, the storage tank volume dictates the available fresh DI water for fast refill of washers if they need replenishing.

The RO system with the PEDI vessels has become the most common water treatment system used for washers. Many advances in the water treatment industry have led to the success of the RO machine. Several water treatment equipment manufacturers have focused on engineering and research. These companies don't accept the industry premise that RO membranes are as disposable as razor blades. These companies' attitudes are bolstered by dozens of patents, several of which deal with machine design and the process of membrane cleaning. This technology administers cleaning agents to the feed and permeates sides of the membranes. The agents remove scale and biogrowth- (microorganisms such as algae and bacteria that develop on a film which can restrict the flow of water through the RO membrane) from the membrane.

Because the thin-film-composite membranes are pH-resistant and mechanically durable, many manufacturers use a combined pH-and osmotic-driven process to affect cleaning. This makes cleaning quick, safe, simple, and reliable. It also creates a preventative process so that the entire membrane surface is recovered with each cleaning. For an experienced operator, total cleaning time takes 1 hour. The cleaning process restores the membrane to its original flux rate and diverts the system's permeate water to drain, which flushes out the debris before the water comes back on line. The clean-in-place system provides the following advantages to the user;

  • 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