Reverse Osmosis frequently asked questions:

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What is osmosis?
Osmosis refers to the passage of water thru a thin membrane from the side with low salt concentration to the side with higher salt concentration. This can happen even when the water level is higher on the high salt side and the water must move against a pressure difference. The bottom line is that osmosis refers to a concentration difference manifesting itself as a pressure difference.

What is reverse osmosis (RO)?
The other side of the coin is reverse osmosis. A difference in pressure is used to cause a difference in salt concentration. It is as though the pressure is being used to force the water molecules through the membrane while retaining the larger salt molecules (sort of like a screen). Salt means any inorganic compound dissolved in water. When water is processed by reverse osmosis a large fraction of dissolved material is removed. The cleaner the input water the cleaner the output water will be. Conversely, if your input water is clean enough, you may not need an RO unit.

How does RO equipment work?
After preliminary filtering for suspended particles and carbon filtering for organic substances that are readily adsorbed on carbon particles, RO equipment uses the pressure of the water line (usually 50-100psi) to force water through its membrane, producing as product relatively pure water and producing as waste product water with a higher concentration of salt than the raw input water has. Some systems let you adjust how pure the product is (less pure product produces less waste water product).

What about membranes?
There are two basic types of membranes: TFC and CTA. CTA membranes must have chlorinated water (such as from a water company system). If they are fed well water, microbes will build up and clog the membrane. TFC membranes are used with well water as they resist the clogging. But they cost more to use.

How much does RO equipment cost?
Simple systems start at about $80 for CTA systems or about $150 for TFC systems. Small systems produce about 10 gallons of water a day. Larger systems can produce fifty or even 100 gallons per day, at about 2-3 times the cost of a simple system.

What can you use RO water for?
RO water has a lot less calcium and magnesium in it than the input water. This makes it much better suited for applications requiring soft water. For the rest of this FAQ I will focus on aquarium applications. Depending on your water change scheme, the best use of an RO unit is usually on the water-saver flow (high output, not quite as clean) and using the water for water changes. In this case, since the RO water is being mixed into the remaining aquarium water that is NOT being changed out, it is not necessary to add any chemicals to the RO water. If you need to use straight RO water in some application, it may be desirable to mix in a little unprocessed tap water. If the pH of your RO water is too high, sphagnum peat can lower it. To raise the pH, a very weak solution of a strong base like lye would be a good choice. Use caution with concentrated solutions of strong bases and acids.

What are the pros of RO usage?
RO water is cheap (if a unit lasts four years and requires 8 membranes during that time, and costs $180 to buy and $500 for supplies and produces 50 gallons per day, that is only about a penny a gallon).
RO water is simple to understand. It is just tap water most of whose salts have been removed.
RO water is simple to manage. By testing for hardness and pH you can become aware of membrane and filter exhaustion easily.