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Project: A Reverse Osmosis - Deionization System

One of the most basic problems that arise in our hobby is the difficulty in assuring a constant source of pure fresh water. We need fresh water for mixing our saltwater and for our daily replenishment of the water that has evaporated from our tanks. When I started keeping reef tanks, more years ago than I'd like to remember, we knew that we needed clean water for our systems. We weren't aware, though, that we actually required pure water, free of chlorine, nitrates, phosphates, ammonia, etc. The result was that, even after aquarists had figured out that proper light, sufficient flow, and stable levels of things like Alk, Ca, and Mg, and pH were necessary to sustain SPS and many LPS corals, we still had problems. Our fresh water just wasn't pure enough.

The great majority of aquarists used tap water when mixing up their salts or topping off their tanks. The tap water in some homes comes from municipal sources, others access residential wells. Each source has it's own problems when it comes to pure water. Municipal water is usually dosed with chlorine and chloramines. Depending on the type of pipes installed in the home, metals such as copper came become a problem. Silica, chromium, nitrate, and phosphate are other possible "contaminants". Depending on the geographic region, well water can contain even larger amounts of nitrate and phosphate than city water due to heavy agricultural based fertilization, etc.

With the event of home sized reverse osmosis and deionization (RO/DI) systems, it has become a simple exercise for everyone to have pure fresh water for their reef systems.

What is reverse osmosis?

We all learned in school that osmosis is the diffusion of molecules through a semipermeable membrane from a place of higher concentration to a place of lower concentration. Sounds good, huh? The point, of course, is that osmotic pressure tends to equalize the molecular concentration of two liquids which are separated by a semipermeable membrane. In our RO systems the membrane has been designed with holes that are only large enough to let water molecules through. Anything larger (the bad stuff) can't get through the membrane.

So we have a situation where "dirty" water from the tap passes through a membrane and all the gunk stays on one side and only pure water gets to the other side. Something's not right, though. We said osmosis is where a liquid of lower concentration (pure water) passes through to a solution of higher concentration (gunky water). What we need to achieve is the opposite - uh, the reverse. Oh yeah, reverse osmosis. How do we reverse osmosis? By applying pressure to the gunky water and pushing it through the membrane! The pressure, of course, comes from the pressure in your water lines. If we can apply enough pressure, we get nice pure water on one side of the membrane and really gunky water, on the other side, which gets eliminated down the drain.

So, that was my totally unscientific attempt at defining reverse osmosis. The pressure thing, by the way, is very important. If your water lines don't carry enough pressure (usually defined as a 40 PSI minimum) your reverse osmosis system will be very slow at supplying water and the quality of the water that is supplied will not be optimal. In cases of insufficient pressure there are booster pumps available which can raise the pressure of the water reaching the membrane up to 80 PSI.

Pre-filters are required

Before the tap water can be allowed to reach the RO membrane, we need to take care of a couple of things. First of all - sediment. Tap water, and especially well water, often carries significant amounts of sediment. Not good for our RO membrane! The answer is a sediment filter. Sediment filters come in various pore sizes. Normally a size between 1mm and 5mm is used. The sediment filter is normally placed as the first pre-filter in an RO system.

I get all my water from a well. Chemically the water is fairly pure, but it is loaded with sediment. For this reason I have a total of three sediment filters in my system. A 10mm, a 1mm, and a 0.5mm.

Activated carbon (block) filters are placed next in line. Among other things, the activated carbon removes chlorine and chloramine from the water. Both these substances can damage the RO membrane. Depending on the chlorine and chloramine levels in the water to be treated, a second carbon filter may be advisable.

The RO membrane

Not all membranes are created equal. Aside from the material from which membranes are manufactured, there are also differences in the rated daily amount of pure water production (in gallons per day). With membranes suited for our needs, the range goes from 24gpd to 150gpd. Experience has shown that for reefkeeping purposes, the Dow FilmTec 75gpd membrane delivers the optimal results.

It should be noted that the amount (and quality of the product produced) depends on the temperature of the water being treated and the source water pressure. A water temperature, as it comes from the tap, of at least 70 degrees is advantageous. As for water pressure, most RO system manufacturers recommend that water pressure no be allowed to fall below 40 PSI. If you find that you are not able to supply this pressure, it's possible to buy booster pumps to increase the pressure to around 80 PSI.

Under optimal conditions of temperature and pressure, the Dow FilmTec 75gpd membrane can remove 94% to 98% of impurities from your tap water. We judge our success in removing impurities from the water by measuring TDS - Total Dissolved Solids. We try to shoot for the lowest TDS value possible. To use my water as an example - the TDS of water at my tap is around 190. After the water has passed through my pre-filters and RO membrane, the TDS has dropped to 3 or 4. That means I'm getting optimal results from my unit. Later, we'll discuss how to bring that 3 or 4 down to zero.

The automatic shut-off valve (ASOV)

The automatic shut-off valve enables your RO unit to turn off the source water supply. This is accomplished using pressure from the pure water side of the unit. The ASOV turns off the raw water supply, whenever the pressure on the pure water side of your system reaches approx. 60% of the raw water pressure. This happens, for example, when a float valve shuts off the flow to a storage tank as the water level reaches the top of the tank. In sort, anything that raises the pressure sufficiently, on the pure water side of the RO unit, will cause the ASOV to block raw water from entering the RO unit.

Booster pumps

The best known, and probably most used booster pump is the Aquatec 8800:

This pump raises the incoming water pressure to around 80 PSI. If needed, the pressure can be adjusted using a small flat head screwdriver. The pump comes with it's own power adapter and, if ordered in a kit, a pressure switch is available. The pump is installed between the tap and the RO unit. The switch is installed between the RO unit and the DI cartridge(s). The pressure switch has a built-in switching point of approx. 40 PSI. If pressure falls below that, the switch allows power to reach the pump - the pressure is raised to approx. 80 PSI. When the flow of water is stopped, for instance by a float valve closing as an RO storage container fills up, the switch stops the flow of electricity and the booster pump shuts off.


We said that RO, by itself, will only bring down TDS to a value of 3 or 4. But, optimally, we need to reach zero. That can be accomplished by allowing the product water, leaving the RO membrane, to flow through a deionization resin. These resins have microscopic pores that serve to trap any charged (negative=anion, positive=cation) molecules/atoms that come into contact with them.  Molecules/atoms with no charge pass right through and are not held back.

DI resins will not work indefinitely. When the resin reaches a point were it is saturated with ions it becomes exhausted. It is possible to regenerate DI resins. The regeneration process  essentially cleans the resin beads of the charged ions (replaces ions) it has collected.

There are resins designed to catch cations (plus + charge) as well as resins designed to catch anions (minus - charge). In this country most aquarists use mixed-bed deionization cartridges where both types of resins are mixed together. This has its negative and positive (no pun intended) aspects. If you intend to regenerate your resin after it has become exhausted, mixed-bed cartridges cause problems. Each type (cation or anion) resin must be regenerated separately. This means, in the case of a mixed-bed, the resins must be separated before they can be regenerated. This can be done, but is a time consuming process. Better is to use the resins separately, if you intend to regenerate.

DI resins can be purchased with a color indicator that changes color when the resin has been exhausted. The price is a little higher than resins without the indicator. I don't find it particularly useful as I use the TDS value of the product water to indicate when it's time to change resin.

DI resin tends to exhaust completely, all at once. One minute your product water has a TDS of zero and next it's already 3 or 4. For this reason, I use two DI cartridges in my system, run in series. For the sake of this discussion, let's call them DI position one (just after the RO membrane) and DI position two (just after the cartridge in position one). As soon as TDS drops below zero, I know that the cartridge in position one is exhausted, so I remove both cartridges and refill the first (totally exhausted) one with fresh resin. I place the cartridge from position two (which isn't exhausted yet, position one always exhausts itself first) in position one. Then I place the newly filled cartridge in position two. In this way my product water is always at zero TDS.

Useful accessories

There are a few items that can enhance the functionality of a RO?DI system:

  • booster pumps - as already mentioned above, booster pumps can greatly increase the amount of product water produced in a given time in situations where water pressure, at the tap, is below 40 PSI.

  • pressure gage - placed just before the RO membrane, a pressure gage can aid in determining when one or more of the pre-filter are clogging. This is indicated if the pressure drops from the normal value.

  • dual TDS meters are available which allow permanent mounting to an RO/DI system. Measurements can then be taken directly after the RO membrane and after the DI cartridge(s). This tells us when the RO membrane is no longer producing RO product water in the 3 to 5 TDS range. We can also see when the DI product water raises above zeroTDS.

The nuts and bolts

So let's take a look at some of the components that go into making up an RO/DI system. Here's a pic of my current installation:

The system is mounted on a 24" X 48" piece of plywood that I covered with some old Formica I had laying around. Starting from the left and moving right, I'll describe the individual parts:

  1. 10mm sediment filter. This is places before the booster pump to eliminate any danger of sediment (read sand) damaging the pump.

  2. the Aquatec 8800 booster pump. This pump turns on whenever the the pressure falls below around 45PSI, which is almost always. The pump turns off automatically when the water movement stops due to a float valve closing when the water level in a storage tank has reached the top.

  3. the power adapter for the booster pump.

  4. a pressure gage. The gage is connected to the input side of the RO membrane, which is output side of the pre-filters. In this position the gage can tell us when pressure falls, indicating that one, or all, of the pre-filters is clogged or clogging.

  5. 1mm sediment filter.

  6. first activated carbon filter. Gets rid of chlorine chloramine and some organic chemicals

  7. 2nd activated carbon filter. This pic is a few months old. I have since decided my system doesn't really require two carbon filters so I replaced it with a 0.5mm sediment filter.

  8. the 1st of two DI filters.

  9. I have two 75gpd FilmTec RO membranes installed in my system. Two membranes greatly increases the amount of product water produced every 24 hours. You can see one membrane cartridge. The other is below the one that is visible.

  10. the 2nd of two DI filters.

  11. a DIY RO/DI ATO (automatic top-off) unit. It takes care of controlling when the RO/DI turns on so that the fresh water storage tank is filled.

  12. the dual TDS meter. The meter reads TDS at two locations: after the RO membrane - indicates the condition of the membrane, and after the first DI filter - lets me know when the resin is exhausted.




James says...
Excellent article.
17th June 2016 5:50am
Adam Michaels says...
This is a great read, and will help the many who have question regarding reverse osmosis and ro/di. I personally got my system form Aquasafe Systems and have been using it for years without any issues.
GlassReef: Hi Adam, thanks for the kind words!
9th August 2012 8:56pm
Jim Colley says...
I like the idea of the 10 micron filter, but where do you pick those up?
GlassReef: carries them - I think they run around $3.00. They fit into any 10" cartridge. (see: "Material Sources" above)
3rd December 2011 8:42pm
Scott Conner says...
Great write up. I have really enjoyed this site you have built, very helpful and informative. One question about this setup, where does the pressure switch that shuts off your Aqutec 8800 booster pump go?


GlassReef: Hi Scott. The pressure switch is located (in-line) between the last RO cartridge and the DI section. Works perfectly.
11th April 2011 9:43am
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