# Part 2 - Frequently Asked Questions about RO/DI Filters



## sig (Dec 13, 2010)

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 (~70Â° 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):

Quote:
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 78Â° F. As the RO/DI unit kicks on, water in the tubing will be warmed up to 78Â° 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.

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