RO: NEW THINKING ON COUNTERING BIOLOGICAL RISKS

With the growth of Reverse Osmosis (RO) into a common method of Point-Of-Use (POU) treatment, it can be easy to forget how implausible that proposition would have seemed not too long ago. From its humble beginnings as a method of rendering seawater potable, RO has come a long way.

With the ascendency of RO, the public no longer bemoans the lack of "flavor," recognizing instead that water should have no taste, odor or texture.

On the technical end, rejection rates are higher than ever. The variety of membrane types and materials available is broader than ever. Advances have made RO feasible at ever decreasing line pressure levels and ever increasing temperatures. With modern system designs, fluctuations in source water variables, such as pH, presence of volatile organics and chemical contaminants can all be handled readily.

POST-TREATMENT CONCERNS: THE NEW FRONTIER

The most recent, and perhaps most significant RO enhancement has less to do with advances in membrane technology, but with the industry's improved understanding of RO's limitations. Through the efforts of industry publications, such as WC&P, manufacturers and dealers are beginning to understand that RO is not, by itself, properly regarded as a method of drinking water disinfection; that the purity of RO treated water can itself present an added risk of post-treatment re-contamination; and that membrane bio-fouling and grow-through can result in sullied product water.

The thankful consequence of our growing understanding is the implementation of newer, more intelligent system designs to guard against post-treatment re-contamination and bio-fouling, and to provide collateral sources of disinfection when biological hazards are suspected.

ADDRESSING BIO-FOULING and GROW-THROUGH

Membrane manufacturers, initially slow to acknowledge the risks of bio-fouling and pathogen grow-through, have since been in the forefront of combating the phenomenon. Higher Quality Control and Quality Assurance standards, coupled with newer materials, better packaging and seemingly more effective non-toxic preservatives (i.e. the chemicals, most often liquid, in which membranes are packed), appear to have reduced the number of complaints that most of us now hear.

Ironically, another major advance has been the improvement in what had been a fading technology, cellulose tri-acetate (CTA) and its various close relatives, marketed under a variety of product designations. While still not the equal of thin film membranes in various ways, it is significant that CTA membranes, now capable of sustaining higher temperatures and higher pH, and with better rejection rates, can now be substituted for thin film membranes in more applications than ever.

Thin film membranes are chlorine intolerant, with a maximum tolerance of < 0.1ppm. Passing chlorine through Thin Film Composites rapidly destroys the membrane. CTA membranes, by contrast, tolerate common residual chlorine levels (< 2.0 ppm) rather well. As a result, when using adequately chlorinated source-water, the residual chlorine acts to continually preserve and disinfect the membrane, cutting down on CTA bio-fouling. That is, without residual chlorine, CTA may have a worse biofilm or grow-through risk than its thin film cousins, but with it, the levels can be comparable.

PURE WATER CAN MEAN PURE RISK

Reducing bio-fouling is only one step in assuring biologically safe drinking water. It is virtually inevitable that RO-treated water will be stored after treatment. With rare exception, the process is simply too slow to lend itself to contemporaneous production and usage. It is during storage that RO water is at its greatest risk of biological contamination.

It is not in spite of the product water's purity that risks remain, but because of it. Gone is the residual chlorine from the source water, a powerful inhibitor of pathogen growth. Such pathogens may enter the system in any number of ways, from the source water, contaminated components, through the air (in atmospheric systems), or through drainage or spigot back contamination.

For the reasons discussed above, in thin film systems, all source water chlorine will necessarily have been removed. While now changing, most CTA systems as well still remove the source water chlorine before storage, whether or not they also post-filter. Those who ascribe to the largely unscientific "naturalist" theories of water treatment might also argue that the elimination of minerals diminish the water's natural preservative properties.

PRESERVATION, POST-TREATMENT and DISINFECTION

Two options are available to ensure the maintenance of the initial purity of RO treated water: Preserving with residual chemical disinfectants, or post-treating at or immediately prior to the point of dispensing.

Using Source Water Residuals

By far, the easiest approach to preservation is to take advantage of any residual free chlorine found in the source water. Most municipal water supplies world-wide still use chlorination as the principal method of disinfection. Residual chlorine levels of between 0.5 ppm and 1.5 ppm are most common. This residual, if passed through a CTA membrane and not removed, acts as an effective preservative.

Chemical Disinfectants

The alternative approach to preservation is to add back a chemical disinfectant immediately downstream of the membrane. This approach enjoys the distinct advantage of addressing RO's shortcomings as a method of disinfection. By coupling RO with disinfection, not only is the product water preserved effectively, but the system is equally effective against source water that is micro-biologically contaminated. Chlorine, ozone and iodine are particularly effective for this purpose.

In either case, preservation adds two logistical dilemmas for system designers: ensuring chlorine tolerant storage vessels and removing the residual disinfectant prior to consumption. The former requires a careful consideration of materials selected, especially when using bladder tanks. The latter entails the need for post-filtration, using (depending upon the chemical disinfectant chosen) carbon, KDF™, iodine removal media or, in the case of ozone, a catalytic filter.

However, the back pressure exerted by the post-filter will reduce net membrane pressure. That is to say that the water forced backward when hitting the filter requires higher line pressure to force the water through the membrane. That is an undesirable consequence from the perspective of RO production, in particular where line pressure is low, or water temperature or TDS (and consequently osmotic pressure) is especially high.

Thus, the use of either an in-line pump or, whenever possible, gravity fed filters, may therefore be needed. The gravity fed systems obviously enjoy a cost advantage, as well as eliminating the need for an electrical supply.

UV as an Alternative

A second approach is using Ultraviolet irradiation (UV) downstream of the membrane. This approach has only recently become practical at the POU as a result of advances in UV technology that enabled manufacturers to construct small UV modules.

RO treated water is fed through a chamber containing a UV lamp. This may be an ideal application for UV treatment. UV has come under increasing criticism due to inconsistent kill rates and the risks of shadowing (i.e. pathogens "hiding" behind the shadow of sediment or clumps of bacteria, thus evading the direct UV light beam). However, in this application, the membrane will have already removed sediment, total dissolved solids and, at a minimum, the larger clumps of bacteria. Thus, there is less risk of microorganisms evading the UV light by "hiding" behind larger materials.

As noted above regarding the use of pumps, the obvious problems with this approach are the need for a reliable supply of electricity (which can't be taken for granted in some underdeveloped nations) and the added cost. It is also especially important to perform the treatment at the very last moment prior to dispensing, since UV has no residual effect after the lamp. This may conflict, however, with the desire to carbon post-filter after the UV, as some consumers complain that plastic UV housings can emit a foul taste.

CONCLUSION

One of the hallmarks of a maturing industry is learning more about the weaknesses of established products and improving rather than obsoleting them with new technology. With the global acceptance of RO, the water treatment industry has begun to recognize and acknowledge the microbiological risks. The recognized need for secondary disinfection following RO treatment, whether as a method of preserving biologically safe water or disinfecting contaminated water, is growing, and the industry has acted responsibly in beginning to address them.

The following reflects the published specifications of comparably rated thin film and CTA membranes of one manufacturer only, and is offered exclusively for comparative purposes.

Feature	Thin Film	CTA
FEATURES
pH Range	4.0 - 11.0	3.0 - 9.0
Maximum Pressure	17.2 bar	17.2 bar
Maximum Chlorine Tolerance	<0.1 ppm	2.0 ppm
Maximum Feed Flow Rate	2 gpm	2 gpm
Minimum Brine Flow rate	5-9 x permeate	5-9 x permeate
Maximum Feed Turbidity	1.0 NTU	1.0 NTU
REJECTION RATES
Sodium Chlorine	99.2%	96.2%
Sodium Nitrate	98.3%	88.2%
Magnesium Sulfate	99.5%	98.2%
Sodium Bicarbonate	98.2%	98.2%
Magnesium Chloride	99.2%	94.1%

Return to Articles Menu

However, few people recognize that the history of the POU cooler is now over 75 years old!

1922: THE DUNBAR PATENT

Water treatment specialists, seeking to enhance their own business, have long recognized the merit of combining modern methods of POU treatment with the convenience and universality of the bottled water cooler.

In 1922, W. E. Dunbar applied for what later issued as U.S. Patent 1,547,105. The Dunbar patent, in essence, converted a standard 5 gallon water bottle into a pour-through filtration system; a device not unlike a giant Brita in the form of a bottle. A series of patents issued over the following 5 years covering variations on this theme.

PLUMBED SYSTEMS ARRIVE

However, such devices required frequent manual filling, limiting their utility and popularity. With the advent of plumbed systems and advanced in-line treatment methods, automated systems became practical alternatives. However, since no appliance existed that allowed such a marriage, dealers began rudimentary retro-fits, the forerunners to the modern "bottle-less" or "flat-top" cooler.

By the mid-1980s, enough of these jury-rigged designs existed in the market place to catch the attention of creative Elkay engineers. They designed more standardized and reliable retro-fit kits and, eventually, complete POU coolers. Other major cooler manufacturers were quick to follow suit. Ebco, soon to be the leading seller of such systems, went so far as to acquire Hydrotech, a well established POU system supplier.

As all of this was taking place, entrepreneurs began exploring the feasibility of utilizing the same concept, but retaining the appearance and large storage capacity of the conventional 5-gallon water bottle. This was like Dunbar redux, but this time using modern technologies for plumbed, automatic treatment.

RECENT DEVELOPMENTS

The modern POU cooler still comes in these same two varieties &emdash; flat-top models and self- filling bottles. But the current designs have little more than that in common with their predecessors of just ten years ago.

FLAT-TOP COOLERS

Flat-top coolers arrived on the scene initially as entrepreneurs developed rudimentary float devices for converting existing bottled water coolers. The tanks were small and the float mechanisms often unreliable. But the appeal of saving money and not lifting bottles saw a rapid growth in these do-it-yourself conversions.

Flat-top models have recently experienced numerous changes through product development. Most manufacturers have enhanced the styling of their product lines through the use of plastic cabinets. Plastic has many advantages over metal, from weight reduction to providing a rust-proof exterior.

Flat-top coolers have also experienced a proliferation in reservoir materials, sizes and construction. Typically, reservoirs are made of stainless steel or plastic, and are either fixed or removable (for easier cleaning and sanitizing). Reservoir sizes now range from under one to over six gallons in capacity.

Indeed, larger reservoirs have been a necessity for use with reverse osmosis (RO) purification. The larger reservoir acts as a holding tank to ensure that the cooler is not depleted before the RO process can replenish the reservoir. Recently, at least one manufacturer (Elkay) has introduced a 3 gallon reservoir, in recognition of the faster production rate of certain RO membranes.

Another critical advance in flat-tops has been the development of a variety of reliable float mechanisms for controlling the amount of water in the reservoir. Floats are now available in either mechanical, electrical or combination designs. Many have back-up floats to control the water level if the primary should fail. These floats also incorporate an air filter to prevent airborne contaminants from entering the reservoir.

SELF-FILLING BOTTLES

The initial appeal of the self-filling bottle was exclusively cosmetic. Performing no differently than the reservoir of a flat-top cooler, self-filling bottles addressed primarily the end-user's preference to see the water that he or she was about to consume. Market research would later reveal that the image of a bottle also served as a sort of universal symbol of potable drinking water.

However, the benefits of self-filling bottles rapidly &emdash; albeit accidentally &emdash; transcended appearance. The first of these advantages to be recognized was the large tank size. With a usable capacity equal to conventional bottled water bottles (5 gallons), self-filling bottles were as much as 10 times the size of the original flat-top reservoir. Indeed, the self-filling bottle afforded a capacity double the size of the conventional POU RO bladder tank.

Flat-top models have now matched or even surpassed this capacity. However, self-filling bottles have permitted other unique developments unavailable through flat-top coolers.

Most notably, by performing the last stage of filtration (generally using GAC) at the exit of the bottle, as the water is dispensed, self-filling bottles allow for stored water to be maintained with a chemical disinfectant (most often source water chlorine) in place. This has the obvious attraction of using the residual disinfectant as a preservative, blocking algae growth and other forms of post-treatment recontamination. To abate this problem, flat-tops must be sanitized frequently. Not so with self-filling bottles.

More recently, this concept has been extended not merely to use the residual disinfectant from the source water, but to add chemical disinfectants in the bottle (including chlorine, ozone, and iodine), to permit on site disinfection of biologically contaminated water.

It is also noted that self-filling bottles, like flat-tops, employ air filters. However, since not limited by space, these filters often have a smaller micron rating and longer useful life than the float-mounted filters used with flat-tops.

As a consequence of these developments, self-filling bottles now represent large, flexible storage tanks, with the unique capacity to allow for in situ disinfection and preservation techniques.

THE POU WORLD TODAY

Today, virtually any methodology that can be used to treat drinking water is available in a POU cooler. There are filtration systems of every variety, RO and distillation purification systems, and most recently disinfection products, ranging from UV to chlorination, iodination and even ozonation.

System storage capacities are no longer limited to one or two gallons, and modern system designs can produce hundreds of gallons of treated water per day.

Moreover, and perhaps more significantly, POU is no longer regarded by the bottled water industry as "the enemy." Most of the larger bottled water companies now offer POU alternatives to conventional bottles. POU coolers are common fixtures at IBWA trade shows, as are advertisements for POU coolers in bottled water trade magazines.

Indeed, it is commonly believed that the largest POU cooler purchaser and supplier in the world is the Perrier Group, the Nestlé owned consortium that includes such household bottled water names as Poland Spring, Arrowhead, Ozarka, Zephyr Hills, Deer Park, Great Bear, Calistoga Springs, and many others.

POU coolers have also made great in-roads into the coffee service arena. Coffee companies, under growing competitive pressures from water companies offering coffee service, have used POU as a clever defense.

For the water companies to add coffee service is a simple matter. They already own the trucks and warehouses, and service the routes as often as several times per week. Coffee companies, on the other hand, have smaller trucks and warehouses and less frequent delivery schedules. Moreover, from where are they to source their water &emdash; the water company with whom they seek to compete? POU is a simple, inexpensive alternative which may be serviced fully using their existing infrastructures.

THE FUTURE OF POU

At a recent trade show, one customer described the POU cooler as the refrigerator and bottled water as the ice box. The obvious implication is that, with POU, one can manufacturer water or equal or better quality, for equal or lower cost, with greater convenience.

A colleague countered that this was the wrong metaphor; that the POU cooler was more like the automatic ice maker, and bottled water those ice trays that one still fills by hand. It was this dealer's view that bottled water would not disappear like the icebox, but would endure as an alternative, for example where connecting to running water is not feasible.

Whatever the future may hold for POU coolers, logic and history suggest that they should continue to gain market share. With the demand for treated water of all sorts -- bottled and POU alike &emdash; on the rise, the future for well-made, well-priced POU coolers should be bright.

ABOUT THE AUTHOR:
Steven G. Singer is chairman of SemperPure Systems, Inc., Master Distributors for Pure 1 Systems of Billerica, Mass. He is a graduate of the Harvard Law School and the University of Pennsylvania. Before joining Semper-Pure, Singer spent over six years in the health care industry, most recently as CEO of a NYSE listed conglomerate, and has travelled and conducted business extensively around the globe.


Return to Articles Menu