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From YourSITE.com Membrane technology Advance in membrane technology
Activated Carbon: Beverage Water Treatment: Filtration: Membrane Technology: Reverse Osmosis INTRODUCTION Reverse osmosis (RO), ultrafiltration (UF) and nanofiltration (NF) are relatively new words to the beverage industry, describing a series of filtration processes that have recently found their way into quite a number of production facilities. While all three tend to sound as if they describe new high-tech, very difficult to understand processes, in fact, the concepts are really quite easy to understand. All three simply describe forms of very fine filtration that are capable of producing a final water product of a purity unattainable through the more conventional means of water treatment. FILTRATION CHARACTERISTICS While the terms RO, NF and UF tend to set each other apart, it is important to underscore just how physically similar they are. All three are "membranes" created by coating a thin layer of a very porous polymer, or plastic, onto a backing material. The end result is the finest form of filtration presently known, with reverse osmosis being the smallest, nanofiltration being a slight step larger and ultrafiltration being a bit larger again. The actual size of the pores in any of these polymeric type membranes is measured in terms of Angstroms, with one Angstrom being one 10 billionth of a meter, which is, of course, very small. To put this in an easier to understand perspective: consider if one square foot of membrane were the size of the entire Pacific Ocean, a reverse osmosis pore would be roughly the size of a dime. Nanofiltration would be about the size of a half-dollar and ultrafiltration would be similar in size to an old silver dollar. Or perhaps more graphically: if you had the opportunity to observe an RO membrane under a Scanning Electron Microscope (SEM) at 8000 x magnification, the pores would be undetectable (Figure 1). This is in contrast to the pores of a submicron filter (0.2 ), which are very distinguishable (Figure 2).
While each of the membrane types are very similar physically, the reasoning behind the different labels is each of the three performs very different functions. Reverse osmosis, for example, "filters" most inorganic salts from water, allowing only the water to pass through the pores. This membrane will also reject some forms of non-ionic organic compounds such as a fructose molecule (molecular weight = 180). However, it will pass, quite readily, smaller organics such as ethyl alcohol (mw = 46). As the pore size is increased, larger and larger organic compounds pass through, as does more of the salt. At a point where all the salt passes, the membrane is then referred to as ultrafiltration. At that point only organics of greater than 1000 mw are rejected. Nanofiltration covers an area in-between with organic rejections of 300 -1000 mw and salt passages of 15 - 90%. Several different types of nanofiltration cover the entire spectrum of the area between RO and UF. Because UF has larger pores, it is the least costly to operate, requiring fewer membrane elements and lower operating pressure (and thus less horsepower) than the other two processes. As the pores get smaller, as they do for NF and RO (in that order), the cost of operation increases somewhat. However, it is important to note that the capital costs tend to be relatively similar since a large portion of these costs is associated with piping, frames, controls, etc., all of which are related to the quantity of water to be processed-- not the extent of filtration. Generally, UF is used when the only issues are fine filtration and removal of most organic contaminants. RO is used when the reduction in inorganic salts (e.g. sodium, alkalinity) is added to this list; and nanofiltration is used when the interest is only a partial reduction in the inorganic salt levels, along with fine filtration and organic removal.
A good example of the different membrane processes abilities can be created by simply filtering a sample of typical surface water that is fairly heavily laden with organic contamination. In a trial a sample of Mississippi River water was taken from a point near downtown Minneapolis, and the product water quality after processing through RO and UF membranes was examined. It was observed that the RO removed all of the color-contributing organic contamination while the UF appeared to remove most of the larger molecular weight organic compounds (most likely lignans and humic acid compounds). While this does not offer assurance that low molecular weight compounds such as trihalomethane compounds (THMs) are affected, it does show that most of the organic "precursors" are removed, thereby reducing the THM formation potential with organic contamination being removed to a degree well beyond more conventional means of water treatment. Such filtration has proved to be a savior in cases where the raw water supply becomes unexpectedly contaminated, such as occurred in Milwaukee with a Cyrptosporidium parasite coming from Lake Michigan. In this particular case, the local soft drink plant using an NF system to treat their water was able to continue production while other food plants were threatened by loss of production. SYSTEM DESIGN Because all three-membrane processes are "dissolved" solids processes, the system must be designed in a manner so that the contaminants are continuously removed before they become "insoluble" solids. This sets the processes apart from other forms of "normal" filtration where the intent is to remove contaminants either in the form of a solid sludge or impinged within a disposable type of filter media. Thus, an inherent aspect of a cross flow membrane system design is the discharge of a small percentage of water carrying away the contaminants.
Naturally, as water is filtered, the remaining contaminants become increasingly concentrated. The degree of concentration is dependent on the ratio of water purified versus the water discharged. If, for example, one sets this ratio at 50/50, the concentration factor is 2x (meaning that all contaminants that do not go through the membrane are concentrated by a factor of two). If the ratio is 75/25, the concentration factor is 4x; at 90/10, 10x and so forth. The challenge is to set this discharge as low as possible, while avoiding the concentration of any constituent to a point where it precipitates, or changes from a dissolved solid to an insoluble solid. Because each chemical species is different, the limits of solubility for each compound must be studied separately. In some cases, such as calcium carbonate, it is possible to push the system beyond the limits of precipitation by adjusting the pH to the more acidic range, converting the calcium carbonate to calcium bicarbonate which has a higher level of solubility. In a typical water supply, the limiting factors are usually calcium sulfate, which precipitates at 2000 ppm; calcium carbonate, which will still precipitate at levels in the 500-1000 ppm range even after pH adjustment and silica precipitating in the neighborhood of 120 ppm.
Table 1: St. Louis AC Operation
Other factors such as barium (which has a solubility limit of 2.0 ppm when in the presence of sulfate) and even general dirt and debris found in the water must also be considered since almost all contaminants are concentrated in the process. On the other hand, in cases where all or a portion of the material does pass through the membrane, as salts do with UF and non-ionized inorganic compounds (such as silica) will do with NF, the concentration effect will be lower. The upshot is that when one is processing cleaner water supplies, such as water from the Great Lakes or from East Coast surface supplies, it is not unusual to operate a membrane system with a low percentage of water discharge (10% for example). But, when operating on water supplies from the Great Plains, for example, one may need to discharge as much as 25 % in order to avoid precipitation. A case in point is the comparison of the system in Milwaukee, operating on Lake Michigan water (TDS = 147 ppm) versus a second system in suburban Chicago (DuPage County) operating on 664 ppm TDS water. In the DuPage case, the system was set at 25% wastewater discharge, yet the Milwaukee system was able to be set in the range of 10 - 15% wastewater discharge and still have a TDS in the wastewater discharge not much greater than the DuPage feedwater (Table 2). Table 2: Milwaukee System
(All figures. except pH, as CaCo3) While this discharge is a "waste water," it is important to keep in mind that it is still good quality water-- the dissolved material is simply concentrated to a higher degree. Thus, it is very worthwhile to consider using this water within the plant for container rinse, can warming and even in some cases as cooling water. And, any water that does ultimately need to be discharged from the plant can often be sent to the storm sewer system as has been demonstrated by a Minnesota bottler. ACTIVATED CARBON While membrane systems have found their way into many production facilities, so far they have generally been limited to replacing the more conventional means of alkalinity reduction and filtration portions only; most plants continue to use activated carbon (AC) after the membrane system as a means of removing chlorine. Thus, many of these installations have combined a more conventional means (AC) with the newer technologies (RO, UF and NF) which has created some new design considerations for the activated carbon portion of the system. Putting it simply: one must "throw away the book" on AC when it comes to designing a system with activated carbon operating downstream of a membrane system. Most of the old "rules of thumb" relating to AC simply no longer apply. This is because so many of the AC operating conditions are linked to the eventual fouling with dirt and organic compounds-- all of which are absent in the water treated by a membrane system. Taking the fouling by organic compounds and other foreign materials out of the equation sets up a condition where the AC will last for a long time. Under these circumstances, the limiting life factor likely becomes the simple breakdown of the AC itself to the point where the pressure drop becomes intolerable. And, one must keep in mind, the AC need not be backwashed nearly as often because the membrane filtered water is cleaner. This minimizes the physical damage to the AC particles. Tab. 3: Milwaukee AC Operation
This is in contrast to the relatively high particle loading from a conventional system which has been shown to send almost 260 million particles per liter to the activated carbon treatment step. A case in point is a bottling plant in St. Louis that has processed almost several million gallons of water over the past seven years through an RO machine and then through the same charge of AC, never witnessing a chlorine breakthrough. And, the feed to the system contains 2.0 ppm chloramines, not simply a small amount of chlorine. In fact, had the city water contained simple chlorine, the loading to the AC would have actually been less since the RO rejects about 30% of the chlorine, yet passes 100% of the chloramines. Part of the improved performance of AC on chlorine compounds is related to the lower pH inherent to the membrane system product water. Chlorine is more susceptible to reduction to chloride in more acidic conditions. However, one must also be cautious to use a "low ash" type carbon to avoid the dissolving of carbonaceous ash contaminants when operating on acidic water. The sizing for the St. Louis AC unit was a very conventional one gpm per cubic foot of AC; however, testing shows that AC can be very effective for very long periods of time operating at flow rates as great as 3 gpm per cubic foot. Meanwhile, in Milwaukee, an AC filter of similar design (one gallon per minute per cubic foot) has operated for more than one year downstream of an NF system, reducing 20 ppb THM levels to "non-detectable" on a routine basis. Had the same water supply been subjected to the chlorination processes of a conventional system, the THM levels would have a very strong potential of increasing significantly. FUTURE Long term, the impact membrane systems will have on soft drink water treatment will very likely go beyond the simple reduction of sodium levels or other inorganic contaminants. No doubt, the value of using a membrane system will also include a more highly filtered water, THM reduction, greater control for a more consistent water supply and the ability to take on opportunities presented by "New Age" drinks that may be the products of the future. © Copyright 2004 by YourSITE.com | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
