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Maximize Performance - Minimize Cost
Maximizing Performance and Minimizing
Cost of Ownership of RO/DI Systems
by Charles Mitsis, President, SpectraPure, Inc.
for ReefChat - 10/21/98 - Updated 6/21/13
Let's discuss the topic "Optimizing the performance of your reverse osmosis (RO) or reverse osmosis/de-ionization (RO/DI) system and minimizing the cost of ownership." This discussion will focus more on the practical aspects of water treatment rather than on theoretical.
Benefits of Using Pure Water in Reef Tanks
The benefits of pure water to the reef hobbyist are well documented: elimination of nuisance algae and enhanced health of corals, invertebrates and other marine wildlife. Currently the best technology for producing this ultra-pure water for reef tank use is the RO/DI system. The RO/DI system is a set of discreet technologies which must be properly integrated to optimize pure water production and minimize operation costs.
Basic System Design Considerations
In a properly designed RO/DI system the most vulnerable and expensive components are placed nearest the point of use and are protected by upstream water treatment components which are less vulnerable and expensive. For example, the DI cartridge is the last component in the chain of purification and is one of the most vulnerable and expensive. It is protected by the preceding reverse osmosis membrane which in turn is protected by the carbon block filter which is preceded by the sediment filter. If any one of these critical components is not properly selected or is of sub-standard quality premature failure of downstream components can result.
First, let's talk about sediment pre-filters and the difference between absolute and nominal micron ratings. In an absolute rated filter all particles larger than the stated micron rating are removed by the filter. In a nominal rated filter a certain percentage of particles larger than the nominal rating will actually pass through the filter. The % efficiency rating of the nominal rated filter tells you how many of the larger particles will pass through. Most sediment filters are of the nominal rating variety. String wound and spun filters have a low % efficiency rating and offer little protection for downstream components. Resin bonded sediment filters offer a very high nominal % efficiency rating and offer good protection. New micro-sediment filters are absolute filters and provide superior protection of downstream carbon filters and membranes.
Carbon Block Filters
Next in line are carbon block filters. The primary function of these filters is to remove chlorine and further filter out sediment. They are available in three ratings: 10 micron (removes chlorine for up to 3000 gallons), 5 micron (6000 gallons) and 0.5 micron (20000 gallons) which is also effective in removing organic chemicals such as pesticides, herbicides and trihalomethanes. Since carbon block filters are absolute filters and are prone to plugging with sediment it is essential that they are preceded by a very high efficiency sediment filter. The best means of determining when to replace the filter is to test the waste water stream for presence of chlorine. Any chlorine level above 0.1 ppm will cause damage to the membrane and indicates that the carbon block filter must be changed.
The RO membrane is the workhorse of the water purification system. It uses a thin film membrane technology to "electronically" reject impurities. RO membranes are especially good at rejecting large atoms and molecules and those species which exhibit relatively strong electronic charge. It is less effective in rejecting small atoms and molecules (smaller than water) and those with relatively weak electronic charge. RO membranes will allow all dissolved gases (such as ammonia and carbon dioxide) to pass through. The % rejection rating for a membrane is an indication of how much of these impurities will pass through the membrane and on to the DI cartridge. A membrane which has a 98% rejection rating will pass only half as many of these impurities as one with a rating of 96%. The direct result of using a 98% rejection membrane as opposed to a 96% membrane is that you will double your DI cartridge lifetime. Lifetime is extended even further with a 99% rejection membrane! It is important to note that all membrane manufacturers adhere to a lower % rejection limit for small membranes (2" x 10") of 96% at 60 psi and that the only way to ensure that you are using a higher % rejection membrane is to purchase it from a source which screens for higher % rejection membranes.
The DI cartridge serves as a final polish of the water before it sees the actual application. The cartridge contains a mixed bed of anion and cation resins which uses an ion exchange mechanism to remove impurities that pass through the RO membrane such as silica, phosphates, nitrates, ammonium and other trace impuities. Like a filter, the DI cartridge must be monitored for performance and replaced when it becomes exhausted. The best way to accomplish this is through use of a resistivity monitor. Another way is through the use of color change resins which give a visual indication of when the resins are exhausted. It is difficult to determine the exact replacement point of color change cartridges which often results in dumping of silica and other contaminants back into the water.
Applications for Reef Tank Enthusiasts
So how does a reef tank enthusiast go about selecting the right water purification equipment and components? Selection starts with understanding the feed water you will be using and the water purity requirements of the wildlife you intend to care for. Water is a very complex medium. In fact, it is technically a solvent and readily dissolves many materials which we refer to as contaminants. The method of removal for these contaminants determines what equipment and components must be employed in the design of the water purification system. An aquarist who lives in an area which has relatively problem free water and whose wildlife water purity requirements are minimal may only need a stand alone RO system. More sensitive wildlife such as corals and invertebrates will require the added water purification performance of a DI cartridge. An aquarist in an area with problem water may have to resort to difficult, relatively expensive solutions, especially if the wildlife water purity requirements are high. Some of the more serious problems encountered with feed water include:
Chloramine - chloramine is used by some municipalities to control bacteria growth. It is a stabilized compound of chlorine and ammonia. Chloramine is extremely detrimental to many wildlife species. Our 0.5 Carbon Block Pre-Filter effectivly removes the chlorine component but the ammonia passes through the membrane. Employing a single DI cartridge will effectively remove the chloramine but the high concentrations typically seen will rapidly exhaust the cartridge making this a very expensive solution. Our dual cartridge combination of DI-SF-CI-10 followed by DI-AR-CI-10 is an effective and more economical way to remove the ammonia. An alternative method often recommended when soda-lime softening is used by the municipality is injection of acid into the feed water prior to purification. The reduced pH allows the chloramine to be effectively removed by the membrane.
Carbon Dioxide - some areas have water which contains high levels of carbon dioxide. While this is often not a big problem for wildlife it is a very big problem for RO/DI systems. Carbon dioxide is a gas and readily passes through the RO membrane and on to the DI cartridge where it is removed, once again at great cost. The solution we most often recommend is direct removal of the carbon dioxide through degassification of the RO product water prior to introduction into the DI cartridge.
Understanding what is in your water and how it affects your wildlife is crucial in determining the correct water purification equipment and component design. No one system or combination of systems will work for all water compositions and wildlife requirements. The best approach to take is to first obtain compositional data on your feed water and your wildlife. Water composition data is often available at no charge from your municipal water treatment authorities. Wildlife requirement information is available from a variety of sources including the internet, books, or your local aquarium store. Many times your local aquarium store can also give a recommendation for water treatment. Of course, you are always welcome to contact the water purification specialists at SpectraPure who will be very happy to assist you with designing a system which will meet your needs and those of your wildlife.