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Reverse Osmosis General Info


Most of you have seen Reverse Osmosis (RO) ads in magazines, web sites, and catalogs. Look closely, does the ad give you the proper information to determine what type of system would be correct for your application? Before you purchase, you need to decide what system fits your application and that you meet the certain requirements listed below.

40-80 psi (2.75-5.5 bar)
      MAX HARDNESS: 10 grains (170 ppm as CaCO3)
pH RANGE: 3-11 MAX MANGANESE: less than 0.1 ppm
MAX TURBIDITY: 1.0 NTU LANGLIER SAT. INDEX: LSI must be negative (-)
MAX SILT DENSITY INDEX: 5.0 (based on 15 min. test) MAX IRON: less than 0.1 ppm


In this discussion you will become familiar with each component and what their purpose is. The general design of most RO systems is basically the same but it is the quality of the filters themselves that will make or break a system.

RO systems usually consists of several individual filters placed in series. Each filter is considered a "stage". The individual filter may differ according to the manufacturer but the flow remains the same.




  • SEDIMENT PRE-FILTER (1st Stage Filter)

Sediment filters have specific ratings. They range from 10, 5, 1, 0.5, and 0.2 Microns. Water that comes from rivers can have high levels of particulate matter, so the micron rating of Sediment and Carbon Pre-Filters will affect the unit's performance, and also determines how often you need to replace each Pre-Filter. The Sediment Filter is a fibrous type filter. This filter removes particulate matter that could plug up the Carbon Pre-Filter and RO Membrane.

What is 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.

A Sediment Filter will usually last approx. 4-6 months, depending on micron rating and quality of water. (The life span of the filter is determined by the turbidity, iron content, organics, and total particulate volume in your water source). The best way to determine when your Sediment Pre-Filter needs replacement is to use a Pressure Gauge. When you have a drop in pressure between 15-20% from where your normal house pressure is, replace filter. To check this, run water through the system without the filter in it's housing. If the pressure jumps back to your normal house pressure without the filter, you know the filter you just took out was plugged up.


  • CARBON BLOCK PRE-FILTER (2nd Stage Filter)

A Carbon Filter also has specific ratings that range from 5, 1 and 0.5. Each rating determines how much chlorine is removed in gallons of water. The 5 micron can remove chlorine for up to 6,000 gal, the1 micron for up to 9,000 and the 0.5 micron 20,000. This filter is in the 2nd stage. It removes chlorine, organics, heavy metals, trihalomethanes, pesticides and many other chemical pollutants. It will also break-up chloramines, which is chlorine bonded with ammonia ( removes the chlorine and leaves the ammonia).

(Only the 0.5 micron carbon block filter removes trihalomethanes, pesticides, volatile organic chemicals and 99.95% of giardia and cryptosporidium cysts.)

A Carbon Filter will usually lasts approx. 4-6 months ,depending on micron rating for the filters, usage and the quality of your tap water. The best way to determine when your Carbon Block Pre-Filter needs replacement is to use a chlorine test kit. Any chlorine level above 0.1 ppm will cause damage to the membrane and indicates that the carbon block filter must be changed.


  • DRINKING WATER SYSTEMS (Post-Carbon Filter, after the membrane & tank)

Drinking Water Systems most often have a final Post-Filter. This filter polishes the water as it leaves the Pressurized Storage Tank.


Once the water is Pre-Filtered, it is ready to be processed by the Reverse Osmosis Membrane.



Reverse Osmosis membranes reduce Total Dissolved Solids (TDS) that are present in the water. It also removes arsenic and fluoride. It is capable of rejecting salts, proteins, bacteria, sugars, particles, dyes, and other constituents that have a molecular weight larger than 150-250 daltons. Separation of ions with RO is aided by charged particles (dissolved ions that carry a charge), like salts, are more likely rejected by the membrane than those that are not charged. The larger the charge and particle size, the more likely it will be rejected.

To process the water, the membrane must be under pressure which is provided by your municipal water company or a well pump (i.e. your tap line pressure). If your pressure is below 40 psi, we recommend using a Booster Pump. A membrane should last approx. 2 or more years, depending on the quality of your tap water. (Membrane life is a factor that is determined by the tap water chemistry and total gallons produced per sq. in. of membrane material). The best way to determine when your RO Membrane needs replacement is to use a TDS meter.

An RO membrane naturally produces concentrated brine (waste) water. As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected species away from the membrane, in a concentrated brine water. The process is known as "cross flow" which allows the membrane to continually clean itself. The ratio that you need of waste vs. product water is 4/1. The ratio is achieved by adjusting a capillary tube called the Flow Restrictor.

There are several things that must be taken into account when purchasing RO membranes.

Just because the membrane has a Filmtec label on it does not mean that it is a good membrane. Many of them exhibit low rejection rates right out of the Filmtec box. Filmtec membranes are actually rated with a minimum specification of 96% rejection and -20% of the stated flow rating. (See the Dow / Filmtec website for details).

SpectraPure guarantees that the rejection rates and flow rates will be within Filmtec specs on our standard membrane series and at or greater than 98% rejection on our SpectraSelect Series (-S-) membranes. It is not an easy or inexpensive result to achieve. It requires a high level of diligent effort on our part to provide this service to our customers and ensure that the membranes are what they are advertised to be.

One additional factor to keep in mind is that a 98% rejection membrane will double the life of a downstream DI cartridge vs. that of a 96% rejection membrane. That 2% difference translates into far less cost per gallon for DI water due to the extended life of the DI cartridges. For anyone that demands high quality RO water or has DI cartridges downstream of the RO membrane the benefits received from purchasing our tested membranes exceeds the small additional cost that must be added to the cost of the membrane.

These are some of the most important reasons why our tested and guaranteed Filmtec membranes are worth the extra money. In our opinion, the extra effort that it takes to test and guarantee the membranes is well worth the extra cost and effort. Any customer that has a DI cartridge(s) installed downstream of the RO membrane will save a great deal of money on DI cartridges due to the reduced usage of the cartridges.

RO membranes alone are not capable of removing 100% of the impurities. This is where De-Ionization Cartridges come in.


  • DE-IONIZATION CARTRIDGE(S) (4th and or 5th Stage Filter(s). (Some systems have 2 DI Cartridges)

A DI Cartridge takes the remaining ionic load not removed by the RO Membrane and reduces it down to nearly "pure", which can be measured as 18 megohm or 0.05 micro-siemens. This water is High-Purity grade water used in applications such as Laboratory & Aquarium use. Usually, systems that have 2 DI stages use a back-up method. When the first DI exhausts, the second DI takes over. (This prevents the leaching effect when a single cartridge is exhausted.) Systems like our MaxCap® RO/DI does not use the back-up method. This system has two types of DI cartridges that work together to have a life of 3x that of standard DI systems. The best way to determine when your DI Cartridge(s) needs replacement is to use a TDS/Conductivity meter. We recommend the COM100

A standard DI Cartridge will produce approx. 200-400 gallons of product water. In many applications, the MaxCap® DI Cartridge followed by a Mixed-Bed or Hi-S will process approx. 1200 gallons.

Example: If the RO water entering a standard DI Cartridge has a TDS (Total Dissolved Solid) reading of 20 ppm, the cartridge may only process about 400 gallons of pure DI water. (You will go through 9 cartridges to produce 3600 gallons) The MaxCap® system will go through 4 cartridges to produce the same amount.


  • Be aware of the micron ratings for the Pre-Filters.
  • Choose a good brand of membrane. (we use Dow FilmTec TFC Membranes) The size membrane needed depends on how fast you want to produce the water. If you get a 25 gal per day (GPD) membrane it will only produce approx 1 gal per hour (GPH) v.s. a 90 gpd, which produces 3.5 gph. (This rating is measured with 77 degrees F water temperature and 60psi).



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.



When choosing a system for drinking water purposes always use Revere Osmosis Systems (RO). NOTE: If your hardness level exceeds the systems requirements, we recommend using a Water Softener.



RO (Reverse Osmosis) or RO/DI (Reverse Osmosis / De-Ionization) systems are an investment, so choose wisely.



In order to understand the mechanism of chloramine removal, a little background information on the chemistry of chloramines is necessary. Chloramines are formed by the reaction of ammonia and chlorine gas. Chloramines can exist as three chemical species: monochloramine (the predominant species found in tap water), dichloramines and trichloramines. The chloramine species depends upon the pH of the water and the ratios of chlorine to ammonia. At tap water pH levels of 7 to 8.5, the formation of monochloramines is favored. Of the three species, monochloramine is the most stable and difficult to remove, as well as the most damaging to aquatic life.

A â€oppm-hour” is defined as the exposure of 1 ppm chlorine/chloramine water for 1 hour.
Film-Tec quotes 300,000 ppm-hours (six years at 1 ppm) of chloramine resistance for their TFC polyamide (PA) membrane material, but only 200 to 1000 ppm-hours of free chlorine resistance. This indicates that chloramines will not damage Film-Tec membranes, while free chlorine levels must be held below 0.1 ppm to prevent oxidation damage. The easiest test for chloramine is with a Total Chlorine Test Kit (SpectraPure Part # TK-CL-10). The TK-CL-10 tests for a combination of free chlorine and chloramines. A sample of the wastewater stream from the RO membrane should show no signs of chlorine.

The most important purpose of a sediment filter is to protect the downstream carbon block filters from plugging with sediment. A properly designed sediment filter will have a micron rating smaller or equal to the closest downstream filter element. It will have a gradient density structure such that the outer layers capture the larger particles and the inner layers capture the finer particles. This will maintain a large dirt holding capacity and prevent the finer particles from plugging downstream carbon filters. Using a 5 micron carbon block followed by a 0.5 micron carbon block, maximum chlorine and volatile chemical removal can be achieved without premature filter failure. If a sediment filter is used that passes particles larger than the next downstream filter, that filter will plug, blinding off the active carbon surfaces, reducing its ability to remove chlorine and organic chemicals.

Trade-offs exist in almost any circumstance and carbon filtration is no exception.
The smaller the micron rating, the better the removal capacity due to greater surface area. Carbon block filters made with bituminous carbon are more effective than coconut shell carbon filters for removal of monochloramine. On the other hand, in water supplies with chlorine only, the coconut shell carbon may have higher capacities for the removal of free chlorine and low molecular weight volatile organic compounds such as trihalomethanes (chloroform).

Multi-carbon block pre-filtration is not always necessary, especially in smaller flow rate systems when adequate pre-filtration and sub-micron carbon block filters are used.
Activated carbon will break the chloramine bond and remove the chlorine component leaving free ammonia (NH3+). RO membranes are transparent to dissolved gases that will pass freely through the membrane concentrating in the RO product water.

Generally, reverse osmosis water is slightly acidic, due to the higher ratio of free CO2 to bicarbonate alkalinity. The exception to this rule is the presence of high pH â€osoda-lime softening” used by some municipalities. Free CO2 dissolved in water forms carbonic acid that lowers the pH to the range of 5 to 6 pH. In low pH RO product water, the ammonia is converted to the ionized ammonium ion NH4+. Downstream de-ionizing resins can then easily remove this charged species. It is cationic and removed by strong acid cation resins (in the hydrogen form) in either mixed bed or separate bed systems. Aquarists can be certain that when salt is properly added to RO or RO/DI water, the expected salinity and pH will be realized.