|toxicity to marine organisms, and is about a hundred times higher than I found in my aquarium the last time I tested it for copper (about 10-15 ppb copper).|
other articles, but some important points are:
CTA membranes are inexpensive and resistant to oxidation by chlorine.
TFC membranes are more costly, but have high impurity rejection. They must be protected from chlorine and chloramine.
SPSF membranes are generally optimal only in special situations, such as very soft source water.
If the membrane’s pore sizes are made just a bit larger than water molecules, then water can pass through them, but larger compounds cannot. Size in this case is a somewhat simplified idea. Many ions are smaller than a water molecule (Figure 3), but it turns out that charged ions (such as sodium, Na+) in solution contain several very tightly bound water molecules. Removing all of these attached water molecules requires a lot of energy, so when passing through a porous membrane, they act as if they are as large as the whole hydrated assembly (Figure 4). These larger assemblies cannot pass through an RO membrane as readily as they could without the tightly bound water molecules (Figure 5).
[img src=”http://reefkeeping.com/issues/2005-05/rhf/images/Figure3.jpg” border=”0″>
Figure 3. Comparative sizes of a water molecule (H2O; right)
and a bare sodium ion (Na+; left).
[img src=”http://reefkeeping.com/issues/2005-05/rhf/images/Figure4.jpg” border=”0″>
Figure 4. Comparative sizes of a water molecule (H2O; right) and a sodium
ion with tightly bound water molecules (Na+; left).
[img src=”http://reefkeeping.com/issues/2005-05/rhf/images/Figure5.jpg” border=”0″>
Figure 5. A schematic representation of an RO membrane, showing pores
large enough for water molecules to pass through, but not large enough for
assemblies of sodium ions with their tightly bound water molecules.
The more charges an ion has, the more water molecules are attached and the harder they are to remove. It has recently been suggested that the ratio of the hydrated volumes of two ions approximates the ratio of the square of the charges of the same two ions. So, for any simple inorganic X, Y, and Z, X+ is one-quarter the size of Y++, and X+ is one-ninth the size of Z+++. The same holds true for negatively charged ions.1 For these reasons, the relative order of rejection by RO membranes is typically trivalent > divalent > monovalent, as shown below.
Table 7. Typical Rejection Rates of Ions From RO Membranes
|Typical monovalent ions (Na+, K+, Cl–, F–, I–, NO3–)|
|Typical divalent ions (Ca++, Mg++, Cu++, SO4—, CO3—)|
|Typical trivalent ions (Fe+++, Al+++)|
Since RO membranes purify based on size, they are subject to some obvious limitations. Certainly, anything that is very large cannot pass though them. In this category would be bacteria (although they may colonize both sides of the filter, they cannot pass through it), viruses, large organic molecules such as proteins, and inorganic mineral particulates that were small enough to pass through the sediment and carbon filters (often called colloids).
Also, in order to get a sufficiently fast flow of water through the membrane, membrane pores are actually significantly larger than a water molecule. For this reason, some of the molecules of compounds that are somewhat larger than a water molecule can still get through (sodium ion, for example, is not perfectly rejected).
However, at the small end of the spectrum a number of compounds can pass through a reverse osmosis membrane to some extent and are, therefore, of concern to reef aquarists. These include carbon dioxide (CO2), ammonia (NH3), hydrogen sulfide (H2S, especially a concern with well water) andsilicic acid (Si(OH)4, which is the uncharged and predominate form of silicate at pH values below 9.5). All of these should be trapped by a functioning DI resin (discussed below), but can still be a concern.
In the case of CO2, for example, there can be a lot of it in certain well waters, and DI resins may become rapidly depleted because the CO2 so readily passes through RO membranes (how to deal with this is discussed later in this article). As another example, ammonia that comes from chloramine in the water can be significant, and is one reason that RO/DI is greatly preferred to RO alone in those situations where chloramine is added to the tap water.
In the case of silicic acid, some types of RO membranes can be better than others at excluding it, even before it gets to the DI resins. For example, a thin-film polyamide membrane might let only 0.3% of the silicic acid pass, while a similar cellulose acetate membrane might let 12.7% of it pass.
In order to function properly, the RO membrane is coupled with a flow restrictor that allows pressure to build on the upstream side of the membrane, rather than letting the water simply run out of the unit and down the drain. This pressure helps force the water molecules (and other small molecules) through the membrane. After passing through the pores, the water then continues on to the DI resin.
Many systems will include a pressure gauge that measures the line pressure ahead of the RO membrane. It is the pressure across the RO membrane that forces water through. At low pressure, the water may simply run past the restrictor and down the drain. Most membranes need at least 40 PSI or so to get reasonable flow and purification. In my system, the pressure drops over time as sediment clogs the sediment and carbon filters. I use this gauge as an indicator that the filters before the membrane need to be replaced. Some RO/DI manufacturers (e.g., Spectrapure and Kent) sell kits that allow the membrane to be flushed with water, permitting loose sediments and calcium/magnesium carbonate that can clog it to be washed away.
Various factors, such as temperature and pressure, impact not only the flow rate through the membrane but also the purity of the resulting water. Lower temperatures make the water more viscous and less likely to flow through the small pores, reducing the production of purified water. The effect of temperature on purity is much smaller, with purity decreasing slightly at higher temperatures. Higher line pressure across the RO membrane results in higher rates of production and quality, although a pressure that is too high can damage the membrane. Any backpressure on the effluent will degrade performance. Very high TDS (total dissolved solids) in the source water also leads to higher osmotic backpressure, reducing the membrane’s effectiveness. As a rough guide, every 100 ppm of TDS produces 1 psi of osmotic backpressure.
For those interested in a great many more details on reverse osmosis membranes and their engineering applications, a big library of articles is available online at General Electric’s website: “What is Reverse Osmosis.”
inline conductivity meter to alert users when ions are starting to appear, indicating that the resin needs to be replaced. Without such an inline meter, users need to periodically monitor the effluent’s conductivity (in mS/cm or ppm TDS; details are given in the tips section on what conductivity to target for resin replacement).
[img src=”http://reefkeeping.com/issues/2005-05/rhf/images/Figure10.gif” border=”0″>
Figure 10. A cation-binding resin shown nearly depleted of H+, and
allowing sodium ions to pass through.
Some DI resins incorporate color changes to indicate when the DI is depleted. Such indicators are typically pH indicating dyes that change color when the pH in the interior of the beads shifts from the very high or low pH values when OH– or H+ are the dominant counterions, to more neutral values when other ions dominate (such as Na+ or Cl–). Such color changes may be less effective than measuring the effluent’s conductivity for indicating early breakthrough of ions. The color change may not indicate that some beads or parts of beads may become depleted before others due to channeling of the ion flow. Consequently, I would not rely exclusively on such color changes unless they have proven to accurately predict the rise in conductivity of the effluent for a given brand of DI filter and bead.
Several issues arise relating to the depletion of the DI resins that aquarists need to be aware of. Primary among these is that when a DI resin becomes depleted, that does not simply mean that the water passes through just as it came from the RO effluent. It may actually be much worse from an aquarist’s perspective. The reason for this is that while the DI resin is functioning properly, all ions will be caught. But when it is depleted, not only the new ions are coming through and might show up in the product water, but so are all the ions that ever got into the DI resin in the first place. The total concentration of ions coming out of the exhausted DI resin will not be raised as compared to the RO’s effluent, but which ions are released may be very different.
In the DI descriptions above, I did not address the fact that some ions will show a greater preference for attachment to the resin than will others. When the resins are not depleted, it does not matter what the ions’ affinity is, as all are bound. But in a depleted scenario, when there are more ions than ion binding sites, those with a higher affinity for the resin will be retained, and those with a lower affinity will be released. It turns out that silicate is found at the lower end of affinity for anion resins. Consequently, if the DI resin has been collecting silicate for a long period and is then depleted, a large burst of silicate may be released.
Perhaps even more of a concern is ammonia. In a system with chloramine in the tap water, the DI resin will serve the important function of removing much of the ammonia produced by the chloramine breakdown. Ammonia has a poorer affinity for many cation-binding resins than do many other cations (e.g., calcium or magnesium). Consequently, when the DI resin first becomes depleted, a big release of ammonia from and through the DI resin is likely. I recently had a DI resin become depleted, and the effluent contained so much ammonia that I could easily smell it.
Other complications can also impact resin depletion. One potentially important issue is that the anion and cation-binding sites may not become depleted at the same time. Figure 10 shows this scenario when both types become depleted together, with sodium and chloride in the effluent. But, it is possible for one to become depleted first, and in that case, the pH of the effluent can swing far from neutral. Figures 11 and 12 show what happens when a lot of carbon dioxide is present, as is the case with some well waters. Initially, it is mostly bound as bicarbonate, and the effluent is essentially pure water. Note, however, that as the bicarbonate is removed, the anion binding resin is being taken up with bicarbonate, while the cation-binding resin is unchanged and is therefore not being depleted.
[img src=”http://reefkeeping.com/issues/2005-05/rhf/images/Figure11.gif” border=”0″>
Figure 11. A DI resin, shown ready to bind carbon dioxide that has dissociated
into H+ and bicarbonate (HCO3–).
[img src=”http://reefkeeping.com/issues/2005-05/rhf/images/Figure12.gif” border=”0″>
Figure 12. A DI resin, shown binding carbon dioxide as bicarbonate.
Eventually, the anion-binding sites become fully occupied (Figure 13). At that point, additional ions coming through (such as sodium and chloride) are no longer equally swapped out to produce pure water. The sodium is swapped for H+, but the chloride does nothing, potentially leaving the effluent water with a very low pH.
[img src=”http://reefkeeping.com/issues/2005-05/rhf/images/Figure13.gif” border=”0″>
Figure 13. A DI resin that has been depleted by carbon dioxide (Figure 12), shown
binding sodium but not chloride, resulting in highly acidic product water.
A similar effect can be hypothesized for silicic acid in the RO permeate:
Si(OH)4 à H+ + Si(OH)3O–
The effect on pH of the DI resin’s initial depletion would be similar here to the effect of carbon dioxide in the tap water.
The same can happen in the opposite sense with ammonia. If a lot of ammonia gets through the RO membrane (as is the case when chloramine is present), the ammonia will be bound in the DI resin as ammonium:
NH3 + H2O –> NH4+ + OH–
The ammonium depletes the cation-binding resin, while the OH– does not impact the anion-binding resin. Eventually, then, the cation-binding capacity can become depleted before the anion binding is depleted, and Na+/Cl– passing through is converted into Na+ and OH–, with a potentially high pH.
[[As an aside, my RO/DI effluent always seems to have a high pH (9-10) even before its conductivity rises significantly. While there are many complications to measuring pH in pure water, where pH kits and meters do not function well, I cannot so easily dismiss these readings as being purely artifact, although they may be. I have wondered for years what might be causing it, and have not yet found any clear answer. However, if the above process is happening in my system even on a small scale, it might explain the results (my tap water contains chloramine). A sodium hydroxide solution with a pH of 9 has only 10-5 moles/L of sodium hydroxide, or 0.4 ppm sodium hydroxide by weight. Is that all or part of the high pH that I observe in my effluent? I’m not sure.>]
this is not a cause of low pH nor is it something to be generally concerned about, for the following reasons:
1. The pH of totally pure water is around 7 (with the exact value depending on temperature). As carbon dioxide from the atmosphere enters the water, the pH drops into the 6’s and even into the 5’s, depending on the amount of CO2. At saturation with the level of CO2 in normal (outside) air, the pH would be about 5.66. Indoor air often has even more CO2, and the pH can drop a bit lower, into the 5’s. Consequently, the pH of highly purified water coming from an RO/DI unit is expected to be in the pH 5-7 range.
2. The pH of highly purified water is not accurately measured by test kits, or by pH meters. There are several different reasons for this, including the fact that highly purified water has very little buffering capacity, so its pH is easily changed. Even the acidity or basicity of a pH test kit’s indicator dye is enough to alter pure water’s measured pH. As for pH meters, the probes themselves do not function well in the very low ionic strength of pure freshwater, and trace impurities on them can swing the pH around quite a bit.
3. The pH of the combination of two solutions does not necessarily reflect the average (not even a weighted average) of their two pH values. The final pH of a mixture may actually not even be between the pH’s of the two solutions when combined. Consequently, adding pH 7 pure water to pH 8.2 seawater may not even result in a pH below 8.2, but rather might be higher than 8.2 (for complex reasons relating to the acidity of bicarbonate in seawater vs. freshwater).
ppm TDS) should drop by a factor of more than 10 across it (to as much as 100), relative to the tap’s water. If the drop is less than a factor of 10, it is not working properly, and may have holes in it.
Monitor the DI resins by measuring the effluent’s conductivity, either with an inline meter (set to its most sensitive level), or by measuring the effluent manually. If you are using a TDS or conductivity meter, then the measured value should drop to near zero, or maybe 0-1 ppm TDS or 0-1 mS/cm. Higher values indicate that something is not functioning properly, or that the DI resin is becoming saturated and needs replacement. That does not necessarily mean, however, that 2 ppm TDS water is not OK to use. But beware that the flow of impurities and the conductivity may begin to rise fairly sharply when the resin becomes saturated. Do not agonize over 1 ppm versus zero ppm. While pure water has a TDS well below 1 ppm, uncertainties from carbon dioxide in the air (which gets into the water and ionizes to provide some conductivity; about 0.7 mS/cm for saturation with normal levels of CO2, possibly higher indoors) and the conductivity/TDS meter itself may yield results of 1 or 2 ppm even from totally pure water by not being exactly zeroed properly. Also note that the first impurities to leave the DI resin as it becomes saturated may be things that you are particularly concerned with (such as ammonia if your water supply uses chloramine or silica if there is a lot in the source water).
If you recharge your DI resins yourself, be very careful with the acid and base used, as they can be dangerous.
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