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RO/DI (reverse osmosis)

Posted on Thursday, September 10th, 2009 at 8:57 pm by
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What is RO/DI?
RO/DI stands for Reverse Osmosis and Deionization which is a multi-stage water filter used by most hobbyists to convert tap water into the purified water used in reef tanks.

Why should I use RO/DI in my reef tank?
Ordinary tap water may contain various impurities such as nitrates, phosphates, chlorine, as well as various heavy metals (including copper). The RO/DI water filter will remove essentially all of these impurities and will help with many of the problems that may occur when using tap water (high nitrates and phosphates, green algae, cyanobacteria, etc).

How does an RO/DI filter work?
The basic four stage filter uses a sediment filter, carbon block, reverse osmosis membrane, and deionization resin. A three stage RO filter, will simply be one that leaves out the Deionization stage.

Typically a foam block, the sediment filter will physically remove small particles from the water. It is the first “line of defense” and it’s main purpose is to prevent these particles from clogging or interfering with the carbon and RO membrane. Sediment filters are rated by their micron sizes; the smaller the micron rating, the more effective the filter will be.

The carbon filter, typically comprised of activated carbon, will filter out smaller particles, and absorb several dissolved compounds.  Additionally, the carbon will deactivate the chlorine present in our tap water. These is essential as chlorine will destroy the RO membrane in the next stage.

The RO membrane is comprised of a thin semi-permeable film, through which water is forced under pressure. Molecules which are larger and heavier,  do not pass through the membrane as easily as the smaller and lighter water molecules, and are typically left behind.

The optional DeIonization stage, uses the DI resin to exchange the remaining ions, removing them from the solution.

Do I really need the DI stage for my filter?
We want to start by saying that anything you do is better than using regular tap water. Reverse osmosis will typically remove between 90-98% of the impurities existing in your tap water. The true answer to the “what is enough” question is the amount of impurities existing in your tap water. Use a TDS meter to measure the amount of dissolved solids remaining in your system after the RO stage. If you are happy with the results, then that is all you need. If you need to further purify the water, than the DI stage is for you.

Which Gallons Per Day (GPD) model should I get?
RO/DI capacities are typically listed in gallons per day. Most units range between 25-100 GPD, where the most common differences are either the permeability or size of the membrane. Units that product higher GPD are generally more expensive and are useful when initially setting up larger tanks, or in emergencies where large amounts or water are needed. Typically, hobbyists will not need more than 25 gallons per day in the long run, as long as an appropriately sized reservoir is used for the storage of purified water.

Keep in mind that listed RO/DI capacities are projected given “ideal conditions”, such as a water temperature of 70°F and water pressure of 65 PSI.

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