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by Timothy A. Hovanec, Ph.D.

For a variety of reasons a hobbyist may wish to have pure or purer water at his disposal than what normally comes out via the garden hose, kitchen faucet or water well. The desire for pure water can range from a discus breeder who wants soft water (water with little calcium and magnesium) to the reef hobbyist who wishes to remove as much nitrate and phosphate as possible from the top-off water periodically used to compensate for daily evaporation. The purpose of this article is to review the fundamentals of the two most common types of water purification units available to the hobbyist: deionization (DI) and reverse osmosis (RO). Both systems will remove impurities from water but they operate on different principles and each removes some substances better than the other.

The Basics: Before describing the processes and equipment used to make water purer some basic concepts must be understood. Pure water is often confused with good quality water. In fact, the terms 'pure water' and good water quality are often used interchangeably in the hobby literature. This usage is wrong. Good water quality means that the water has no pollutants or toxicants and has the correct chemical make up for culturing the fish or invertebrates of interest to the hobbyist. But good water quality is a subjective term while pure water is not. What is a good quality water for one fish can be poor quality water for another. For example, good quality water for African cichlids from Lake Tanganyika would mean hard water with a high alkalinity, a high level of dissolved solids, and a pH above 8.0. This same water composition would be considered relatively poor quality water for discus, another cichlid but one which prefers soft water with a low pH.

Pure water, by definition, contains nothing but hydrogen (H+) and hydroxyl (OH-) ions. It contains no dissolved gases such as oxygen or carbon dioxide nor does it contain bacteria, trace elements, minerals, or other dissolved substances. Essentially pure water contains nothing except water. It takes relatively expensive devices to make pure water. The operating costs of this equipment is high, and] furthermore, water this pure is not required to have a successful aquarium. In fact, the culture of aquatic animals in pure water is not possible because of, among other things, the lack of oxygen and at least the presence of some dissolved minerals. A commonly used phase in aquariology is 'pure seawater'. This is an oxymoron because, as the definition above states, it is impossible to have 'pure seawater' since seawater has a high level of dissolved salts, and if salts are in the water, the water cannot be pure.

While these points may seem academic they are important because the water produced by DI and RO systems is different. In a strict sense neither system alone can produce "pure" water. As will be explained in greater detail in the following sections both DI and RO units need mechanical filtration and activated carbon to work efficiently. Even with these two accessories RO systems cannot remove all the dissolved ionized substances and gases. On the other hand, a DI unit with these additions can remove all of the chemicals in the water including bacteria and particulates producing pure water.

The Water Source: Probably the most common source of water for the home aquarium in the U.S. is tap water. This water reaches your house after treatment from a municipal water authority. The water is treated to make it safe for human consumption which means it can be high in chlorine, chloramines or other disinfection products. Further, in cities with very old water delivery system problems can be caused by the pipes leaching metals or other substances into the water after the treatment plant, rendering it less than healthy for aquarium use. One example: due to recent lead problems and changes in the drinking water regulations some cities add a form of phosphate to their water supply to coat the inside of old pipes to keep lead from leaching into the water. This has increased the phosphate levels in the tap water to the worry of some hobbyists. In addition, most water companies have more than one source of water and they change from one to another depending on many factors without informing their customers. Thus, it is easy to see that not all tap water is equal in quality and there are cases where further treatment by the hobbyist would be beneficial.

There are a couple of ways to find out what is in your tap water before purchasing a filtration system without spending a lot of money on laboratory fees. Telephone your local water district and ask for a copy of their Annual Water Quality Report. If this is not available ask to speak to a chemist or someone in their water quality laboratory. The report or the chemist should be able to tell you about the water. The water should be free of organic chemicals and other toxins. Ask for data on the pH, ammonia, nitrite, nitrate, hardness, alkalinity, phosphate, and total dissolved solid levels. Ask if there are any substances with unusual values or if there are wide seasonal variations in values.

Even after reviewing the information from your water source it is a good idea to analyze the water from your tap for the substances you are most concerned about to get baseline data. The standard test kits available to the hobbyist are generally fine for this purpose. A logbook of the values should be maintained so you can catch any changes that occur in the water over time. The chemical composition in the water can change during the trip through the pipes to your house. Don't assume the water will be the same as that which leaves the treatment plant. Further, don't assume it will stay the same year-round. The treatment regimes at water plants vary through-out the year based on run-off, algae blooms and other factors so regular testing of the tap water on the part of the hobbyist can eliminate surprises.

Why Buy a Unit? An obvious question to consider before purchasing either a RO or DI unit is: what do I want to remove from the water? Among the most common answer to this question is water hardness. Calcium (Ca2+) and magnesium (Mg2+) are the two cations (positive charged ions) most responsible for water hardness. Two other chemicals, sometimes found in tap water, that aquarists may wish to remove are phosphate (an anion, usually in the form of orthophosphate PO43-) and nitrate (another anion, NO3-). Maintaining very low concentrations of these chemicals may help control unwanted algae blooms. Alkalinity is another parameter that you may wish to remove or reduce. Alkalinity is the acid-neutralizing or buffering ability of a water and is caused mostly by bicarbonate (HCO3-) or carbonate (CO32-) anions. Normally, alkalinity is a good thing to have as it prevents rapid decreases in pH which can negatively affect the fish and nitrifying bacteria. But for certain fish, like Discus, many hobbyists try to maintain the pH near 6. If your water has high alkalinity you will have to add a lot of acid to get the pH down below 7. Thus it may be easier to mix tap water and pure water to get the pH, alkalinity and hardness you are looking for. Having decided that you want a water purification system the question then becomes which one; reverse osmosis or deionization?

Deionization

Deionization (DI) is an ion exchange process utilizing synthetic resins or natural media to remove substances which are ionized from water. Ionized means that an element or compound (which is a group of elements) has a charge. The charge is either positive and called a cation, or negative (called an anion).

While natural media, such as clinoptilolite can perform ion exchange they are too inefficient to be used in DI systems. The resins are specially man-made beads which are chemically charged with a large number of bonding or exchanges sites when manufactured. The resins remove the impurities by exchanging ions on the resin for ones in the water. Deionization resins come in two basic types; anion and cation. Anion resins remove negatively charged ions and exchange them with other anions such as chloride (Cl-) or hydroxyl (OH-) ions depending upon the resin. Cation resins remove positively charged ions and replace them with another cation such as sodium (Na+) or hydrogen (H+). In order to produce pure water a unit will usually combine an hydroxyl producing anion resin with a hydrogen producing cation resin in what is called a mixed bed resin. As ions are removed these two ions combine to form water (Figure 1).

There are several types of DI systems so be sure to ask exactly what the capabilities are of the unit you are considering purchasing. Beware of buying a 'water softener' type DI unit as these will remove hardness from your water but replace it with sodium which is not healthy for some fish such as South American tetras and characoids.

A professional laboratory grade deionization unit which will produce Type 1 water consists of three cartridges with different types of ion exchange resins plus a fourth cartridge with activated carbon and a 0.22 µm bacteria filter and is pictured in Figure 2. Type 1 water is a national designation for water that can be used for laboratory procedures of the maximum accuracy and precision. This type of unit is much too expensive for the home aquarist, at least one on a budget, but the same theory of operation occurs with less expensive DI systems available at the local hardware store or a national water company such as Culligan (Figure 3).

Deionization is basically a chemical exchange process that works very well on dissolved ionized substances and gases. DI resins will not, however, remove dissolved organics, particulates or bacteria from the water. Therefore, a DI system uses activated carbon to first remove organic and then has mechanical filtration to remove bacteria and particulates.

The advantages of a DI unit are that no water is wasted. The water coming out of the unit is ready to use as long as the resins are not exhausted. The units can produce water on demand at a fairly high flow rate so that gallons can be produced in hour(s) rather than a whole day. DI units are easy to install with some having adapters that hook-up to a kitchen or bathroom faucet.

The disadvantages of a DI unit are that the resins become exhausted and need replacing. The resins can be recharged but only with special chemicals which are dangerous and that a hobbyist or other non-qualified person will not have the expertise to use. Some units tell you when the resins are no longer working because the resins will change color. The system seller should be able to estimate how many gallons of pure water the resins will produce if you give them the results of your water analysis. If your goal is to have the purest water possible then a DI unit is your choice.

Reverse Osmosis

Reverse osmosis (RO) can be thought of as essentially an extremely fine mechanically filter. The heart of the RO system is a membrane that is impermeable (or nearly so) to a number of substances but not to water. To understand reverse osmosis one must first understand osmosis. Figure 4a shows a beaker which is divided in half by a special membrane. On one side is pure water and on the other side water containing dissolved salts. Over time water from side A will move, without applying any external force, through the membrane to side B in an attempt to equalize what is termed the osmotic pressure on both sides of the membrane (Figure 4b). This movement is called osmosis. Reverse osmosis is, just as the name implies, the reversal of osmosis. A force is applied on the more concentrated side of the membrane that is greater than the osmotic pressure causing the water to go against the natural osmotic pressure. This is accomplished by using high pressure which forces water from the salty side to go through the membrane but the dissolved salts are left behind as they cannot pass through the membrane (Figure 4c).

The RO systems available to the hobbyist use city water pressure to operate efficiently so they need to have the pressure for which they are rated. The tap water, which is called the feedwater, enters the unit and may pass through activated carbon and then flows to the membrane and the water pressure forces a certain amount through the membrane leaving the salts behind (Figure 5). The remaining feedwater becomes more concentrated with salts and is flushed out of the system. Reverse osmosis is called a percent removal technology because it normally will remove only a percentage of most substances. Typically, an RO unit will remove 85 to 95% of the impurities in the feedwater. RO units remove polyvalent ions (ions with a multiple charge such as calcium Ca2+ and magnesium Mg2+) better then monovalent ions. While dissolved gases will pass through the membrane the relatively large size of bacteria means that essentially 100% of them are removed.

RO units are designed to work all the time so your water requirements should be determined beforehand and a unit that produces the amount of water you desire purchased. Units are sized according to how many gallons of water they produce in a day. Beware, the rating for a unit is under the best most controlled situation and seldom will you get the water production in a day for which a unit is rated. If in doubt, buy the next larger unit.

From this description, it is evident that there are many things that determine the effectiveness of a RO unit. These will be discussed individually.

Pressure: The water pressure on the membrane is the most critical factor in determining the efficiency of the RO unit. Most units are rated at a pressure of 60 to 70 psi. If your pressure is lower the unit will produce lower quality water. If your pressure is below 50 psi be sure to check with the RO manufacturer to determine what you can expect in terms of percent removal of minerals. Lower pressure also results in less water being produced. Before selecting a unit it is best to determine your water pressure so you can have that information for the manufacturer who can then tell you how the unit should perform. Many people are surprised when their newly hooked-up RO unit doesn't produce the water quality or quantity that was advertised and the reason can often be traced to low pressure.

Membranes: There are several types of membranes used reverse osmosis units. The most common are cellulose acetate, cellulose triacetate, polyamide and thin film composite membrane. These membranes are affected differently by the various substances in the water. Most units available to hobbyist use either the cellulose triacetate (CTA) or the thin film composite membranes (TFC). RO units with a CTA membrane are less expensive and the water must be chlorinated because this membrane is adversely affected by bacteria. Cellulose based membranes also have a narrower optimum pH range (5 to 6) compared to other membranes. The CTA units can be an economical choice as they will remove 80 to 95% of most minerals. The exception is for nitrate and borate. These types of units will only remove about 50% of the nitrate and even less of the borate. Thus, if these are of concern to you do not purchase a RO unit with a CTA membrane.

A more expensive option is a unit with a thin film composite membrane. These will remove far more nitrate than a CTA unit and a little bit more borate (note: the chances of borate being in one's water is rare unless you live in an arid region that was, at one time, an ocean. But again it pays to determine what is in your water source before selecting any type of water purification device). TFC membranes are adversely affected by chlorine in the water so they must have activated carbon pre-filtration which should be changed as required to ensure that no chlorine contacts the membrane.

Water temperature: The temperature of the source water greatly affects the production rate of RO units. The stated performance of a membrane is based on a water temperature of 77°F (25°C). If your water is colder than 77°F (25°C) the membrane produces less water. The basic rule is that a 3% reduction in the amount of water produced is experienced for every 1.8°F (1°C) drop in temperature from 77°F (25°C). Hot water, on the other hand, will damage membranes so water with a temperature above 95°F (35°C) should not be used.

Reject water: All RO units produce more waste water than pure water. Most units produce 4 to 6 gallons of waste water (or reject water) for every gallon of purer water produced. Some units may produce as little as 2.5 gallons while others as much as 10 gallons per gallon of purer water. The reject water will be much higher in minerals than the feedwater. If hardness and alkalinity are the major properties being removed by the RO unit than the reject water will be high in these and, thus, possibly a good water for fish such as African cichlids from Lake Tanganyika. The cost of water should be considered when you are evaluating the economics of using RO In some areas of the U.S. water is expensive and the continuous discharge of water down the drain can be costly.

Other Factors: There are a few other properties that affect the efficiency of RO units. Very hard water (high in calcium and magnesium) has the potential for scaling depending upon the water's pH and alkalinity. Scaling is the coating of the membrane with minerals mainly calcium carbonate. Scaling will cause pre-mature membrane failure and results in increase maintenance costs. If the feedwater is very hard with high alkalinity and pH scaling is a good possibility pretreatment of the feedwater should be considered. A common pretreatment is to soften the water by exchanging the calcium with sodium (a deionization process). Sodium does not have nearly the potential for scaling as calcium because it is more soluble.

Large amounts of iron in the water will also adversely affect membranes. Water drawn from wells can sometimes have a high iron concentration. If this applies to your situation consider an iron pretreatment system. If your water is turbid or contains suspended solids then it should be mechanically pre-filtered as these will clog the membrane.

The advantages of a RO system are that the cost per gallon of water produced will, in most cases, be less expensive than DI. Also, once installed there is little maintenance to do. But the systems do not last forever, the membrane should produce about 10,000 gallons depending upon you local conditions.

The disadvantages of a RO system are that they waste water. They also only produce a limited amount of water each day so if you need a lot of water for a water change you have to stockpile the water in another vessel somewhere. Further, the water they produce is not as pure as that of a DI system. If you don't mind wasting some water and don't need the purist water available a RO system may be your best choice.

Deionization or Reverse Osmosis - Which One?

The best way to decide whether to purchase a DI or RO system is to first answer the questions about the costs and your needs detailed in this article. This might take a little time and effort but it will eliminate headaches and possible disappointment. If your water needs are small the simplest option might be a small DI system that would hook up to the bathroom or kitchen faucet when you need water. If you let the resins sit for long periods of time between uses than the first few quarts out of the unit should be discarded. Or you can install an under-the-sink RO unit which will produce a few gallons a day and use it as needed. There is no absolutely correct answer, everyone has to evaluate their situation and decide for themselves.

In all cases, using the points that are described in this article can help you make an informed decision. If unsure, ask questions of the distributor of the system you are thinking of purchasing to make sure you're getting what you want and get any guarantees in writing.

©1995, Timothy A. Hovanec, Ph.D.
Originally published in Aquarium Frontiers, Dec. 1995


__________
"To steal ideas from one person is plagiarism; to steal from many is research."
researched from Steven Wright

That is very important info for any one thinking along those lines but I think the overall cost and to keep it fully maintained could be rather costly and way beyond the average fish keeper.
I also notice it is ten years old I am wondering if there has been any major changes to those systems today.

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Keith

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I didn't see anything really different from when I went shopping for my RO system a few months ago. I've never seen a DI system by itself but it's usually an add on option to RO. Not necessary in my case but it came with the system so I'm using it. It claims it can make 99.99% pure water and without the DI most were claiming around 95-98% pure depending on membrane. Buying and maintaining an RO system is considerably cheaper than refilling containers with RO water from the grocery store or walmart. I have the exact cost breakdown on my laptop which is dead for now but I didn't figure in water costs because I'm hooking it up to a system with well water. Also you don't know how well stores maintain their system so if you are really concerned about getting as pure of water as possible you're better off with your own. It is easier though to just pick it up at the store instead of researching, buying, and setting up your own. Drilling pipes and fixing leaks isn't all that fun.

Hi,
At our home in the mountians. We have a 752 ft well and
the tests show that the water has 0.083 iron in it, which
is 0.003 too high for our EPA standards. I went to Home
Depot to look at their DIY treatment systems and neither
the DI or the RO systems said specifically, that they would
remove the excess iron.
So,I'm still looking.
Frank


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