Source Water: Choosing and Maintaining your RO/DI Filter

Tenji is pleased to present a series of exclusive articles for Reef2Reef members. We will be delving into the various aspects of reef keeping,...
By Tenji, Apr 2, 2018 | |
  1. Tenji is pleased to present a series of exclusive articles for Reef2Reef members. We will be delving into the various aspects of reef keeping, focusing on tried and true methods that can be implemented by aquarists of all levels.

    Our first installments will be concentrating on the most basic aspect that is frequently overlooked; your source water.

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    The most important start to a reef system is quality source water. As most of you know this begins with using a reverse osmosis deionizing system (RO/DI). It is important not to get caught up in the number of stages, or any aesthetic nonsense. You want something simple and affordable to maintain while providing water with 0 ppm total dissolved solids (TDS).

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    Remember that TDS measures every solid dissolved in your water, so while a level of 1 ppm TDS might not seem bad, we don’t know what that 1 ppm of dissolved solids is without additional testing. It could be copper which is toxic to invertebrates in high enough levels. It could be the source of your never-ending algae problem.

    Basic filters for an RO/DI system consist of (in order): sediment, carbon block, membrane, and deionizing resin (DI). Note that every filter is used to protect the next filter in line. You could run your tap water directly through DI resin and produce 0 ppm TDS water, although you would likely be replacing the DI very frequently which can get expensive quickly. Most of us will have source water with chlorine added, which a good carbon block will remove. However, many municipalities are using chloramines nowadays, which will sneak through and damage a standard carbon block. Specialty carbon blocks for chloramine removal should be used in this case.​

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    As previously mentioned, we measure the quality of our freshwater based on total dissolved solids (TDS). Inline TDS meters are commonly sold for RO/DI units, but it’s important to have a secondary tool to ensure our TDS readings are accurate. Entry level inline meters can skew over time providing false measurements, which can send us down a wild goose chase. Adding a handheld meter to your arsenal is a great idea for long-term reef keepers. Handheld meters are affordable, and since they are not constantly submerged in water tend to remain accurate for longer periods of time. Most handheld meters measure temperature as well, which you can use to double check your reef or quarantine systems.


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    Maintaining your RO/DI system is just as crucial as finding the best filters suited for your application. A properly fit RO/DI system should not require much maintenance, but be sure to keep up on your filter replacements. As soon as TDS creeps above 0 ppm, change out your DI resin. If your unit is fitted with a pressure gauge prior to your membrane, a drop in pressure is indicative of your prefilters (sediment and carbon) requiring replacement. If you do not have a gauge, it's best practice to prophylactically change your prefilters every six months. Once your DI replacements become more frequent it's time to look at your membranes efficiency. Test your the water's TDS pre and post-membrane. Compare the results to the manufacturer's claimed rejection rate and if drastically askew, it's time to replace. Generally, membranes can just be prophylactically replaced yearly. Be sure to flush new filters with downstream filters removed, or bypass them entirely. The picture below shows what comes out of a carbon-block after flushing; this could be prematurely clogging your membrane!

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    Everyone’s source water is different. The water coming out of your tap might even be different than what comes out of your neighbors. This is especially true in homes with older plumbing. Many off-the-shelf units will work fine for hobbyists; however, you might be replacing filters significantly more frequently than you have to. Try working with a company dedicated to selling RO/DI systems since they’re more likely to sell units best suited to their client’s source water. Make sure your supplier is asking you about your source water and pulling your township’s water report from the internet.

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    Finally, we all need somewhere to store our RO/DI water. Keep a tight-fitting lid on your water, as it’s so pure it will literally grab compounds out of the air which can raise your TDS prior to even being used. Brute trash cans are an easy and affordable vessel for your precious water, but of course, you can source other shapes and sizes from various suppliers. Any reservoir intended for potable water use is fair game.

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    In our next article, we’ll take a look at the second most important thing for your reef’s source water, your salt!

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