The Skinny on Reverse Osmosis RO/DI Systems

[Myka]

First things first, let’s learn some vocabulary.

RO - reverse osmosis
DI - deionization
GPD - gallons per day
TDS - total dissolved solids
Tap water - the water coming out of the tap
Product water - the filtered water coming out of the RO/DI system
Waste water - the unusable water that flushes the RO membrane

In order to figure out which RO/DI system you should buy you need to know what is in your tap water. First, check your city’s water supply website to find out if they use chloramines. Second, go buy yourself a handheld TDS meter. Buy a TDS meter that comes with a mini screw driver for calibration. A uncalibrated measuring device is as useless as not having one at all. A TDS meter measures the amount of organic and inorganic substances contained in the water.

A basic RO/DI system is a 4-stage system. Those stages, in order, are: sediment filter, carbon filter, RO membrane, DI resin. If you will be dealing with chloramines then a 5-stage system with two carbon filters is a good idea. If you have a large amount of particulate matter in the water, like some well water has, you may want a 5-stage system with two sediment filters. If your tap water measured over 400 ppm of TDS then you may need additional stages on the RO/DI system to deal with this. Ideally, we like to be dealing with 250 ppm of TDS or less. An RO/DI system will be given a “GPD” rating which refers to the flow speed of the RO membrane.

When you hook up a RO/DI system you will have three lines. One line goes from your tap to the first stage of the system - the sediment filter. One line comes out of the RO membrane housing. The RO housing is the smaller housing that is usually placed horizontally on the top of the system. The line that comes out of the RO housing is waste water. That water is bypassing the RO membrane and flushing it to help keep it from clogging up. You don’t keep that water - it goes down the drain (or you can collect it to water your lawn, etc). The waste line MUST be free-flowing - it cannot be restricted. The third line is coming out of the DI resin housing. That is your product water, also known as RO/DI water.

Some RO/DI systems waste or flush more water than other systems. Typical systems will waste 4 parts of water for every 1 part of product water. There are many systems on the market these days that will produce 1:1 ratio of product and waste. This is a green choice! The more efficient systems tend to run at a slightly higher cost, but not significant.

The sediment filter catches undissolved particulate matter and helps to prevent the carbon filter from clogging up. I prefer to use 1 micron sediment filters, but if you have two sediment filters as described above you may want to use a 5 micron first and a 1 micron second. Sediment filters should be changed when you can visibly see they are discoloring or when you notice the product water production is slowing down.

The carbon filter breaks down chlorine and chloramines. Chlorine will damage the RO membrane if allowed into it. Typical carbon filters will range from 5 microns down to 0.6 microns. The finer micron carbon will filter out much more chlorine, but it will clog much quicker. If you find your sediment filter needs to be changed within 6 months, then a 5 micron carbon filter would probably be a better idea than the 0.6 micron. Carbon filters are rated for anywhere from about 5,000 gallons to over 20,000 gallons. This rating includes the waste water from the RO membrane! Take a look at the carbon filter you are purchasing so you know when to replace it. Keep a log of how many gallons of product water you make and you can calculate the amount of waste water depending if you have a 1:4 system or maybe you buy a 1:1 system. You can buy filter monitors with flow meters that will measure the amount of water that goes through the system for you and helps you determine when to change the filters.

Chloramine is chlorine bonded to ammonia. The carbon filter breaks the bond between chlorine and ammonia, then neutralizes the chlorine. The left over ammonia continues on through the carbon. The extra step carbon has to take to break the chlorine and ammonia bond takes extra time. This is why a person dealing with chloramine may want to use two carbon filters. There are also special carbon filters that use catalytic carbon which is a particularly aggressive carbon that can deal with chloramine faster. Catalytic carbon may be a good choice for someone who only has one carbon filter and has chloramine in the tap water. In my own experience, if I can get away with a 0.6 micron carbon filter without it clogging too fast, the finer micron carbon is able to deal with the chloramine. I would not trust a 5 micron carbon filter to deal with chloramine.

The next stage will be the RO membrane. Filmtec brand RO membranes are industry standard and are typically the brand you will see. Personally, I only buy Filmtec brand. They have served me well for many, many years. There are some smaller and many larger membranes, but typically you see 75 GPD, 100 GPD, and 150 GPD for aquarium purposes. This is the amount of water the new RO membrane will produce in 24 hours. You may also see RO/DI systems rated for 200 GPD or 300 GPD which use two or more RO membranes combined for even faster RO/DI filtering. RO membranes also come with an efficiency rating expressed as “rejection rate”. This is the amount of TDS a RO membrane will filter out. This means that if your tap water has 200 ppm of TDS then the membrane will filter out 98% of that and water leaving the RO membrane will contain about 4 ppm of TDS when it is brand new. As an RO membrane gets older, the TDS of the water leaving the membrane will slowly increase. The RO membrane should be replaced when the water leaving the membrane increases to about 15-20 ppm.

75 GPD membranes are certainly functional for smaller aquariums - say 120 gallons or less. A bigger aquarium may need a faster RO membrane simply so that the system doesn’t have to run all the time in order to keep up to the demands of the aquarium for top offs and water changes. A 75 GPD Filmtec membrane will have a 98% rejection rate. The Filmtec 100 GPD membrane has a 92% rejection rate which means it is less efficient, and not a good choice if your tap water has higher TDS. A Filmtec 150 GPD membrane has 98% rejection, but the membrane requires 65 psi water pressure to operate properly. Most homes will not have 65 psi out of the tap so the use of this membrane will usually require a booster pump. You can buy RO/DI systems with booster pumps already installed on them.

A final stage of the RO/DI is the DI resin. The DI resin catches what the RO membrane cannot filter out - like residual ammonia from chloramine. So this means if you have chloramine in the tap water you will probably go through DI resin faster. DI resin has the maximum ability to remove 20 ppm of TDS. So if there is more than 20 ppm leaving the RO membrane, the DI resin will not be able to completely remove it. The DI resin ideally should be replaced as soon as the product water shows 1 ppm of TDS, but many people allow the DI resin to go a bit longer and change it when the TDS gets to 2 or 5 ppm.



Further reading:

Chloramine and the Reef Aquarium by Randy Holmes-Farley (2003)
Reverse Osmosis/Deionization Systems by Randy Holmes-Farley (2005)