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Deionizing Resins:
Before deciding to invest in a D.I. unit, read this section several times, especially the parts dealing with regenerating the resins. The latter is not simple, and can be dangerous.
Even though I have looked for resins that perform consistently well in a salt water environment, I have not yet been able to find such a resin. What I am looking for is a resin that is a very specific though.
Some resins can, of course, be used, but they exhaust their adsorption ability very quickly and are, therefore, in my mind, a waste of money. What happens is that the exterior of the resin becomes coated with larger ions (e.g. sodium and chloride) and a protein surface coating from the organic load of the water. As a result no other ions can migrate to the inner structure of the resin, where the larger part of the adsorption takes place. This, then, results in a very very poor utilization of an expensive medium.
If you are trying to remove nitrates, phosphates, and heavy metals from the tank itself, disregard using deionizing resins until such time as much more efficient types become available. Beware, of anyone trying to sell you a resin that will do all sorts of wonders for your tank. So far I have not found one, but some will appear, hopefully soon.
Resins should be used to treat the water to which you will be adding salt, before you actually add the salt, to remove impurities, nitrates, phosphates, and so on. Such treated water can then be safely added to the tank. In some cases it will be necessary to aerate the water before using it, because the de-ionized water will be very low in dissolved oxygen. It is a good idea to test for D.O. the first time you use whatever resin you may have acquired. This will tell you whether to expect low D.O. in the effluent water or not, allowing you to determine whether or not to aerate it first.
Many resins that will "purify" tap and well water are available. The key is to get the kind that will do exactly what you are looking for, can be easily recharged, or regenerated, using chemicals that can easily be obtained, and can be used without undue precautions.
There are basically 2 types of resins available to the Hobbyist:
Resins are sold as monobed (one resin) or mixes (e.g. 2 resins). To confuse you a little, the word monobed is sometimes used to refer to a single deionizing column (device), that may contain a mixture of 2 resins (monobed = 1 bed of resins).
In double columns, the ones where 2 resins are used, and are kept separated, the cation exchange resin must be used first. The water flows first through the cation column and then from the cation resin to the anion resin. Flow rates recommended vary greatly; the best results are usually obtained with flow rates of 1-2 gallons of water per minute, per cubic foot of resin. Since most Hobbyists' deionizers only contain about 1/6 of a cubic foot, the flow rates should be adjusted accordingly to about 10 to 20 gallons per hour. If your unit contains even less, e.g. only 1/8 of a cubic foot, the recommended flow rate is 7 GPH. Very slow indeed.
Although recirculating the water through the resin will work, most manufacturers recommend "single pass" operations. This means that the water to be treated is flowed only once through the resin, and as slowly as possible. This is also what I now recommend you do.
Slow flow rates, such as the ones mentioned, are best obtained by gravity feed, without the use of a pump. Just let the water flow slowly through the resin. In two column units, first through the cation and then through the anion. In single mixed bed resin units the principle is the same, let the water flow slowly from the top of the column to the bottom, in a single pass treatment fashion.
The useful life of resins is theoretically indefinite, because, once exhausted, the resins can be recharged, regenerated, and used again as new. Practically, this is however not the case. Some resin will be lost, e.g. when regenerating, and you will need to add resin to the unit at some point. Not a lot, but you may find that after several months (6-7) you may need to add a pound or so.
Hobbyists never have a problem installing a de-ionizing unit. Problems come about when regeneration time comes around. Here are two ways to approach this matter, depending on whether you are using a single or double column unit (remember : regenerating resins can be dangerous if you are not familiar with the use of the chemicals needed):
These units consist of two columns, one filled with a cation exchanger and one filled with an anion exchanger resin. Each of the columns requires a different recharging method.
Prepare a solution of 4 percent acid content. For every pound of resin you have, you will need about 2 gallons of water to treat the resin (water that has been treated with acid). To make a 4 percent acid solution you will need HCl or H2SO4, not something you usually have standing around, unless you are a chemist, a doctor, etc..
Swimming Pool places, and some hardware stores, sell a product called "Muriatic acid". This is usually a 32 percent or a 20 percent solution of HCl (hydrochloric acid). It will say so on the bottle. Muriatic acid can be further diluted to obtain a 4 percent HCl solution, that you can then run slowly through your de-ionizing cation exchange resin. Lower HCl solutions, e.g. 10 % are sold too. They should be preferred as they are less dangerous to use.
Remember to use great caution when working with Muriatic acid. Acids can burn your skin, damage clothing, furniture, carpets etc... Read the instructions very carefully, and if you are not sure about what to do, don't do anything. Ask someone who knows. If you spill any on yourself, you must wash it off immediately. Never add water to the concentrated acid. You must add acid to water and very slowly. Stay away from sulfuric acid, it is a much more dangerous acid chemical.
For example : you buy a 32 percent HCl Muriatic acid. The bottle holds half a gallon. If you now add this half gallon of Muriatic acid very slowly to half a gallon of water, you have a 16 percent solution, and a total of one gallon of water. Adding this to a gallon of water will give you a total of 2 gallons, and an 8 percent solution. Adding this to another 2 gallons of water will give you a total of 4 gallons and a 4 percent solution of HCl. Remember HCl is a very acid chemical and can easily burn your skin, carpets, clothes, furniture....Use care and good judgement, and keep all chemicals away from children.
Four gallons is enough to treat 2 pounds of resin. If you have more resin, you will need to make more 4 percent solution. If you had, for instance, 5 pounds of resin, you will need to make 2.5 times the above quantity. Another example : You buy a one gallon bottle of 20 percent HCl solution Muriatic acid. Add this to one gallon of water and you have a 10 percent solution and 2 gallons total. Add this to 2 gallons of water and you have 4 gallons total of a 5 percent solution.To go from a 5 % solution to a 4 %, you need to reduce the concentration by 20 % because 80 % of 5 = 4.
This may seem complicated at first, but it is'nt really. Granted, you may have to do some calculations, but as long as you know the concentration of the muriatic acid, you should not find it difficult to get to the right dilution quickly.
Remember that you are working with an acid, and that you should follow the precautionary instructions that come with the Muriatic Acid you bought. Especially, keep it away from children. If you do not know how to work with such chemicals stay away from them. Use a different method to purify the water. We highly recommend molecular absorption (see the section on water purification).
Once you have prepared the necessary quantity of treatment water, run it through your de-ionizer slowly, 0.5 gallons per minute is plenty (1 30 gallons per hour). Collect the outflowing water/acid mix, and dispose of it safely. Do not store it. Get rid of it safely right away. You cannot use it a second time ! Best is to add it to a bucket full or two of more water to dilute it even further.
After you have treated the resin, you must flush it with distilled, or previoulsy deionized, water to remove the acid form the column. If you don't do this, the first couple of gallons coming out of the unit will be very acid, and will not be usable unless you first neutralize the acidity with e.g. Calcium or Sodium carbonates or bicarbonates. Flushing is usually much easier and is, therefore, to be recommended.
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