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How water purifiers use the reverse osmosis technology

Water Purifiers are commonly used those days, to treat drinking water efficiently. Of the various technologies used for water purifiers, the reverse osmosis systems purify water by passing it through a series of filters to remove large contaminants and harmful chemicals/pollutants and then by forcing it through a special plastic membrane to remove very small contaminants.

In the filtration stage, tap or well water first passes through a sediment filter where silt, sediment and particles (like sand and clay) are removed. Water is then forced through a high efficiency carbon block filter where micro-pulverized carbon efficiently adsorbs chlorine, chloramines, pesticides and other harmful organic chemicals and pollutants. The pre-filtered water, now stripped of membrane-damaging particles and chemicals, flows into the RO membrane module where pure water molecules are forced through the RO membrane leaving salts, hardness, bacteria, viruses, pyrogens and other impurities to be flushed from the system.

The unit's ''waste water'' is a drawback, typically wasting 2-4 gallons or more of water for every 1 gallon of product water. For most areas of the world, R/O is the only reasonable alternative. They will produce water for about .5 cents per gallon and the membranes will last for 5,000 to 10,000 gallons of product water if properly cared for!

R/O daily water production ratings are nominal. A 10 gallon per day unit will produce about 10 gallons of filtered water per day when supplied with a pressure of 65 PSI, maybe 70F, and water containing average TDS (total dissolved solids). If your TDS is high (i.e. you have very hard water), water temperature is too low, or your pressure is low, your R/O unit will produce less water per day. This is a fact of life. So, if you need exactly 24 gallons of water every day, and you know you have hard water, cold water or low pressure, DON'T buy a 24 gpd unit. Go to the next larger size.

CTA Membrane / TFC Membrane

There are two types of membranes-CTA or Cellulose Triacetate, and TFC or Thin Film Composite. Which do you need?

The CTA membrane is less expensive. It will remove about 90% of most pollutants, and together with an activated carbon post filter, will provide all the filtration required by people and most aquariums. Water must be chlorinated to use the CTA membrane. That is, don't use it on well water systems unless they are chlorinated regularly.

The TFC membrane is more expensive. It must have carbon filtration ahead of the membrane, to remove the chlorine. Chlorine will ruin the membrane. This requires all water, both product and waste to flow through the carbon filter, exhausting it much quicker and requiring frequent changes (5 times as often). Forgetting to change the carbon filter will cause the membrane to self-destruct. The TFC membrane removes a little higher percentage of most pollutants. Note: If you use a 4 stage (or more) Reverse Osmosis system, the water is filtered from chlorine BEFORE reaching the membrane. It's usually: 1. Sediment Pre Filter; 2. Carbon (chlorine) Pre Filter; 3. Membrane (may be TFC); 4. Carbon Post Filter.

The main difference between CTA and TFC membranes for aquarists is that nitrate removal for the CTA is 50-70% and it is about 95% for the TFC. So the question is: do you have high nitrates in your tap water, and do they cause problems in your tank. If the answer is yes, then purchase the TFC membrane. If the answer is no, then purchase the CTA membrane.

In about 75% of cases, the TFC membrane makes the most sense. So if you just can't decide, go with the TFC.


Chloramine - chloramine is used by some municipalities to control bacteria growth. Chloramine is extremely detrimental to many wildlife species and readily passes through RO membranes. Employing a DI cartridge will effectively remove the chloramine but the high concentrations typically seen will rapidly exhaust the cartridge making this a very expensive solution. An alternative method often recommended is injection of acid into the feed water prior to purification so that the chloramine is 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.

High levels of iron, manganese, phosphates, chlorine, hydrogen, sulfide, or chloramine, etc. can cause problems for your water treatment system. You may need to contact one of the manufactures for help when these problems occur. In some cases, special pre-filtration may be required ahead of the RO or DI Unit.

If you live in an area with exceptionally high phosphates, say over 1 ppm and/or you have a reef tank, you should consider an RO Unit followed by a DI to get the phosphates low enough. Water temperature below 50 deg F will severely limit R/O output. If your water is that cold, purchase a larger unit than normally required. You can also construct a simple heat exchanger with an aquarium heater, a water bath, and a coil of tubing to raise the temperature.

If you have a well, you'll most likely have low pressure and high total dissolved solids. Buy an R/O Unit that is at least twice as big as you would normally need.

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