Frequently Asked Questions about RO/DI Filters

What is RO/DI?

RO/DI stands for Reverse Osmosis and Deionization. The product is a multi-stage water filter, which takes in ordinary tap water and produces highly purified water.

Why do I need it?

Tap water often contains impurities that can cause problems when added to your aquarium. These may include phosphates, nitrates, chlorine, and various heavy metals. Excessive phosphate and nitrate levels can cause an algae bloom. Copper is often present in tap water due to leaching from pipes and is highly toxic to invertebrates. An RO/DI filter removes practically all of these impurities.

How does it work?

There are typically four stages in a RO/DI filter: sediment filter, carbon block, reverse osmosis membrane, and deionization resin. If there are less than four stages, something was left out (typically, the DI stage). If there are more, something was duplicated.

The sediment filter, typically a foam block, removes particles from the water. Its purpose is to prevent clogging of the carbon block and RO membrane. Good sediment filters will remove particles down to one micron or smaller.

The carbon, typically a block of powdered activated carbon, filters out smaller particles (ideally down to 1/2 micron or smaller), adsorbs some dissolved compounds, and deactivates chlorine. The latter is the most important part: free chlorine in the water will destroy the RO membrane.

The RO membrane is a semi-permeable thin film. Water under pressure is forced through it. Molecules larger/heavier than water (which is very small/light) penetrate the membrane less easily and tend to be left behind.

The DI resin exchanges the remaining ions, removing them from the solution.

What are CTA, TFC, and PVC?

There are three types of RO membrane on the market: Cellulose Triacetate (CTA), Thin Film Composite (TFC), and Poly-Vinyl Chloride. Almost all of the membranes sold for aquarium use in the US are TFC. PVC membranes are currently available only outside the US. The difference between the three concerns how they are effected by chlorine: CTA membranes require chlorine in the water to prevent them from rotting. TFC membranes are damaged by chlorine and must be protected from it. PVC membranes are impervious to both chlorine and bacteria. This FAQ assumes you're buying a TFC membrane.

Do I need a DI stage?

You can save some money by purchasing a three-stage filter lacking the DI stage. Reverse osmosis typically removes 90-98% of all the impurities of significance to the aquarist. If that is good enough for your needs, then you don't need the DI stage. The use of RO by itself is certainly better than plain tap water and, in many cases, is perfectly adequate.

RO by itself might not be adequate if your tap water contains something that you want to reduce by more than 90-98%. For example, if you have 10 PPM of phosphates in your tap water, reducing it by 90% takes it to 1 PPM, which is still too high.

If saving money up front is a major concern, a DI stage can be added later.

Can I use just DI?

A DI stage by itself (without the other filter stages) will produce water that is pretty much free of dissolved solids. However, DI resin is fairly expensive and will last only about 1/20th as long when used without additional filtration. If you're only going to buy either a RO or a DI, it would be best to choose the RO, unless you only need small amounts of purified water.

Where's the value in a 7-stage filter?

Duplicating stages can extend their life and improve their efficiency. For example, if you have two DI stages in series, one can be replaced when it's exhausted without producing any impure water. If you have both a 5-micron sediment filter and a 1-micron filter, they will take longer to clog up. If there are two carbon stages, there will be less chlorine attacking the TFC membrane. Whether the extra stages are worth the extra money is largely a matter of circumstance and opinion (they're more useful if you use a lot of water).

Do I care about GPD?

RO/DI capacities are measured in gallons per day (GPD), and typically fall within the 25-100 GPD range. The main difference between these units is the size of the RO membrane. Other differences are (a) the flow restrictor that determines how much waste water is produced, (b) the water gets less contact time in the carbon and DI stages in high-GPD units than low-GPD units, and (c) units larger than 35 GPD typically have welded-together membranes.

As a result of the membrane welding and the reduced carbon contact time, RO membranes larger than 35 GPD produce water that is slightly less pure. This primarily affects the life of the DI resin.

Most aquarists won't use more than 25 GPD averaged over time. If you have a decent size storage container, that size should be adequate. A higher GPD rating comes in handy, however, when filling a large tank for the first time or in emergencies when you need a lot of water in a hurry.

The advertised GPD values assume ideal conditions, notably optimum water pressure (65 PSI) and temperature (70°F). The purity of your tap water also affects it. In other words, your mileage will vary.

What if I have chloramine in my water?

Some water agencies add chloramine (a mix of ammonia and chlorine) to disinfect drinking water. That's fine, except some carbon blocks are inadequate to neutralize chloramine, so it damages your TFC membrane. This is particularly a problem with high-GPD units. If you have chloramine, you should discuss it with the vendor. The vendor may recommend a second carbon filter or a different type of carbon filter.

What if I have well water?

Well water is free of chlorine, so you don't need to worry about it attacking your RO membrane. Do not buy a CTA membrane if you have well water, as bacteria will be prone to destroy it. A carbon block is not needed for well water, but well water often contains higher levels of particulate matter than treated water. Consider adding a second particulate filter instead of the carbon block. If your well is prone to "red" water problems due to iron bacteria, a back-flush option will help reduce membrane fouling.

Why are there multiple outputs?

An RO filter has two outputs: purified water and wastewater. A well-designed unit will have about 4X as much wastewater as purified water. The idea is that the impurities that don't go through the membrane get flushed out with the wastewater.

There is nothing particularly wrong with the wastewater except for a slightly elevated dissolved solid content. It may actually be cleaner than your tap water because of the sediment and carbon filters. Feel free to water your plants with it.

An RO/DI filter may have separate outputs for the RO stage and the DI stage. You can use the RO output for drinking or other purposes that don't need highly purified water.

How do I hook it up?

The input line is connected to a cold water line. It can be hooked up with a saddle valve that pierces your existing copper pipe, attached to your sink faucet with a special adapter (which is good for those in rental properties or apartments), or to a hose bib.

The wastewater goes down the drain. If you have a PVC drain pipe, the waste line can be connected to it. You just need a drill and a saddle.

The purified outputs go where you want them. A common approach is to feed the DI output through a float valve to a reservoir.

To make sure you have the correct adapters, you should discuss hookup with the vendor when you buy the system.

What is a TDS meter and do I need one?

A TDS (Total Dissolved Solids) meter measures the conductivity of the water, which is an indication of water purity. Without one, it's difficult to tell how well the RO/DI unit is working.

Read your tap water first. Readings in the 50-500 PPM range are typical. The RO output should be less than 10% of the tap water. The DI reading should be 0 or 1. For example, if your tap water reads 200, your RO output should be less than 20 and your DI output should be 0 or 1.

Always let the unit run for a few minutes before measuring TDS on the output. The first half-gallon or so of output will normally have an elevated reading. That's because impurities will tend to equalize across the RO membrane over time when the unit is idle.

New RO/DI units may need to be thoroughly flushed out before reading the TDS values. Let the manufacturer's instructions guide you on that.

Note that TDS is not a good measure of water quality. You can have perfectly good water with a reading of 500 and toxic water with a reading of 50. Also note that some impurities don't register. The purpose of the TDS meter is to measure the efficiency of your RO/DI unit, not cast judgement on your water.

How do I know when the filter needs servicing?

Sediment and carbon stages: If you have city water (with chlorine) you should replace the sediment and carbon stages regularly. The rule of thumb is every six months. This is less critical if you have well water. If you have a pressure gauge, you can tell when the sediment and carbon filters are clogged: the pressure will start to drop.

RO membrane: There are two ways the RO membrane can fail. It can develop holes, allowing impurities through, or it can get clogged up. If your input pressure is OK but you're not getting the expected output, the membrane is probably clogged. If your TDS meter shows RO output above 10% of your tap water, it's developing holes.

DI resin: The TDS reading on your DI output should read 0 or 1. You know the DI resin is exhausted when the reading starts to climb. Some DI resins change color as they are exhausted. Note that the color will probably change well before the DI resin really needs to be replaced.

Do I need a pressure gauge?

The gauge that comes with some RO units measures the pressure on the input side of the membrane. This allows you to tell if you have adequate line pressure and if your sediment and carbon stages are getting clogged. Optimum input pressure is in the 60-80 PSI range. Below about 40 PSI the unit will not operate efficiently. The units are typically not rated to operate above 80-90 PSI.

Do I care about temperature?

The GPD ratings are for room temperature (~70° F). Colder water travels more slowly through the membrane, which reduces the output. If you have a high-GPD unit connected to your cold water line, that can be a problem. Here's a solution (from Marc Levenson):

You want approximately 25' or 30' feet of tubing from the connection at the cold water running to the RO/DI unit.

Fill a 5-gallon bucket with water, and coil the excess tubing in the bucket so it is submerged. Immerse a small aquarium heater and set it to 78° F. As the RO/DI unit kicks on, water in the tubing will be warmed up to 78° as well, since it processes rather slowly, and the membrane will be able to produce maximum output in the dead of winter.
Do I need a flush kit?

A flush kit allows you to periodically flush some water across the RO membrane, removing some of the gunk that sticks to it. Regular use extends the life of the membrane.

Do I need a booster pump?

The RO membrane works best when the input pressure is in the 60-80 PSI range. Lower pressure reduces the output and increases the ratio of waste to purified water. If your input pressure is less than about 40 PSI, you should consider getting a booster pump.

Make sure you have a pressure cutoff switch for the booster pump (connected to the RO output). Otherwise, it will run continuously.

Can I drink the purified water?

The RO output water is excellent for drinking. Most vendors offer a drinking water kit that includes a pressure tank, a small faucet you can attach to your sink, and a post-filter for the drinking water. The post-filter supposedly improves the taste.

A DI stage is not recommended for drinking water.

Do I need a UV Sterilizer?

Some RO drinking water systems include a UV (Ultraviolet) sterilizer. This is appropriate if you are concerned about biological contamination of your drinking water. It is a waste of money if you're just using the water for your aquarium.

Should I buy a premium RO membrane?

Some of the name brands (notably Kent Marine and Spectrapure) offer premium RO membranes that are claimed to remove a higher percentage of impurities. Assuming it's true, that will extend the life of your DI resin. For instance, if a "normal" RO membrane removes an average of 95% and a premium membrane removes 98%, your DI resin should last over twice as long. Whether that's worth the extra up-front cost, you can decide for yourself.

What about silicates?

There's a lot of advertising hype about how some RO membranes are better than others in removing silicates. Before you spend extra money for that you should read Silica in Reef Aquariums by Randy Holmes-Farley in the January 2003 issue of Advanced Aquarist.

How do I choose a vendor?

Several RC sponsors sell RO/DI equipment. You can find opinions on them in the Vendor Experiences forum.