Factoid of the Day 10/25/06
I thought we might give this a try. Just little educational type posts. I don't claim that I will be able to do one every day, but maybe some of you can jump in and do your own to help me out. If you have any suggestions for one that you are interested in, let me know and if I don't know anything about, I'll find someone that does, or do some research on my own.
HOW DOES AN RO/DI UNIT WORK?
The typical RO/ unit has 4 stages. I'll go through these one by one.
1. Particulate Filter - This serves to remove suspended solids from the water. The Aquifer water that most of us use normally has very low TSS (Total Suspended Solids). However, other things such as bacteria may be removed in this filter. The main purpose is to prevent solid particles from getting to the RO membrane and plugging the pores in it.
2. Carbon Filter - The carbon filter serves to remove Dissolved Organic Carbon (DOC) and chlorine from the water. Again, our aquifer water is very low in DOC. Those of you that may be getting surface water from your water utility may have higher DOC. The chlorine is added to the water by the utility to sanitize the water. It is not sterile, but contains fewer bacteria then unchlorinated water. Chlorination is typically the only thing done to aquifer water. Surface waters may require additional treatment. Organic chlorine compounds can be formed as a by-product of chlorination. Typically, a water utility tries to maintain a residual of .1-.5 ppm of chlorine in the water. This may vary according to how close you live to the point where the chlorine is added (typically at the well head). Most major utilities use chlorine gas. However, some utilities may use other chlorinating compounds that may require a different kind of carbon filter. Check with your utility to see what they use.
The main purpose of the carbon is to remove DOC and chlorine. The chlorine can degrade the plastic used in the RO membrane and cause it "leak" dissolved solids through the membrane. An exhausted carbon filter will allow this to occur, so make sure to change the carbon filter regularly. Some systems have a second carbon filter that acts as a back up to the first to prevent chlorine getting to the membrane.
3. Reverse Osmosis Membrane - The membrane serves as a molecular filter that only allows very small molecules to pass though it. Larger atoms and molecules such as metals (calcium, magnesium, bicarbonate) are too large to make it through the membrane and are flushed out in the units waste water. Basically, a membrane is a very thin piece of plastic with very small holes in it. Think of a piece of saran wrap with tiny holes and you will be close to what a membrane is. Only water molecules and other very small atoms such as sodium and potassium are able to make it through the membrane. This decreases the Total Dissolved Solids (TDS) in the product dramatically. Aquifer water has a TDS of around 317 ppm. The product water from an RO membrane typically has a TDS of 10-20 ppm.
4. Deionization (DI) Resin - The DI unit serves to "polish" the water coming from the RO membrane. It removes the few atoms and ions that managed to make it through the RO membrane along with the water. These are typicall things like potassium and chloride (not chlorine) ions. What we normally use in our units is a "mixed bed" resin. It contains two resin particles. One removes positively charged ions (cations) such as sodium and potassium, and the other removes negatively charged ions (anions) such as chloride. Some more sophisticated RO/DI units may contain two seperate cartridges for cation and anion resins. many of these resins contain a color indicator to show whether they are "used up." DI resin can be obtained in either a sealed, disposable canister, or in bulk that can be used to replace the resin in a refillable canister.
When the water has passed through all of these steps it should contain close to 0 ppm TDS if the RO/DI is working properly. However, it may absorb some gases such as CO2 when stored that can cause the TDS to go back up slightly. The CO2 is coverted into alkalinity in the water and may result in a low reading with a TDS meter. This should not be a concern to the aquarist. The product water from an RO/DI unit should be checked regularly with a TDS meter to see if the RO membrane is leaking or the DI resin is nearly exhausted.
OK, I hope that is useful to you.
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Do we have a MAAST database that we could store articles such as these in?
Originally Posted by cbianco
