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Old 10-15-2006, 07:40 PM   #1
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TDS in....TDS out of RO well water for reef

Just wondering what some of the users of well water are putting into their RO/DI and what are they getting out of it. Our well water(untreated for now) comes out of the tap at around 440ppm TDS. Out of the RO unit it is around 20-25ppm TDS and out of a fresh DI cartridge, it is 0ppm TDS. How often do you replace your membrane and what do you use as an indication that it is time to replace? TIA
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Old 10-15-2006, 09:24 PM   #2
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about every 4 months, I am on well water as well. The tds meter tells me when it is time to replace.
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Old 10-15-2006, 10:23 PM   #3
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Hara, do you replace the DI cartridge/resin every 4 months or the membrane. I hope the DI cuz my membrane is going on a year with consistent TDS measurements. I learned to keep on top of the DI the hard way...left it go too long and it was putting out 50ppm TDS...fighting the cyano and hair algae bloom back now.
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Old 10-15-2006, 10:35 PM   #4
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The ro filter cartridges and the membrane get changed more often then the di resin
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Old 10-16-2006, 12:51 AM   #5
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Not one to argue, but my filter literature recommends I change the carbon and sediment filters yearly, the DI whenever TDS begins to creep up and the membrane can last from 2-5 years. I'm sure these are all variable based on the water quality being filtered and how heavily you use it.
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Old 10-16-2006, 01:28 PM   #6
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I am not arguing, I just said what my schedule is. I make ro water for 8 tanks, the smallest one being 44 gallons. I go through alot. My situation is such that the ro unit runs 24/7 to accomodate me with top offs and water changes.
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Old 10-16-2006, 01:32 PM   #7
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I don't know if this is a hijack, but what TDS meter do you use? I am looking to get one.
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Old 10-16-2006, 10:38 PM   #8
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Wasn't implying that you were arguing, I was afraid it sounded like I was. But anyways, water for 8 tanks! I only have 2, a 15 and a 20 gallon. This is our first house with a well and we've only been here a year. I'm still getting over the fear that our well will run dry because I use too much water for the tanks lol Probably unfounded but I guess I worry about stupid things...oh and my TDS meter is a handheld from HM digital.
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Old 11-26-2006, 10:59 AM   #9
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Ths should help (from our FAQ's):

When should I replace my filters?
A good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the useable life of these two filters is to use a pressure gauge to identify when pressure reaching the membrane starts to decline. This is your indication one or both of the filters is beginning to clog.

Also be cognizant of the chlorine capacity of the carbon block. The Matrikx+1 (“Chlorine Guzzler”) for example will remove 99% of chlorine from 20,000 gallons of tap water presented at 1 gpm. Original equipment suppliers commonly provide carbon cartridges rated at 2,000 to 6,000 gallons.

Regarding your RO membrane and DI resin, use your TDS meter to measure, record, and track the tds (expressed in parts per million) in three places:
1. Tap water
2. After the RO but before the DI
3. After the DI.

The TDS in your tap water will likely range from about 50 ppm to upwards of 1000 parts per million (ppm). Common readings are 100 to 400 ppm. So for sake of discussion, let's say your tap water reads 400 ppm. That means that for every million parts of water, you have 400 parts of dissolved solids. How do we go about getting that TDS reading down to somewhere near zero?

If you do some experimenting with your TDS meter, you'll note that your sediment filter and carbon block filter (collectively called prefilters) do very little to remove dissolved solids. So with your tap water at 400 ppm, you can measure the water at the “in” port on your RO housing and you'll see its still approximately 400 ppm.

The RO membrane is really the workhorse of the system. It removes most of the TDS, some membranes to a greater extent than others. For instance, 100 gpd Filmtec membranes have a rejection rate of 90% (i.e., they reject 90% of the dissolved solids in feed water). So the purified water coming from your 100 gpd membrane would be about 40 ppm (a 90% reduction). Filmtec 75 gpd (and below) membranes produce less purified water (aka “permeate”), but have a higher rejection rate (96 to 98%). The life span of a RO membrane is dependant upon how much water you run through it, and how dirty the water is. Membranes can function well for a year, two years, or more. To test the membrane, measure the total dissolved solids (TDS) in the water coming in to the membrane, and in the purified water (permeate) produced by the membrane. Compare that to the membrane’s advertised rejection rate, and to the same reading you recorded when the membrane was new. How do you measure TDS? Membranes also commonly produce less water as their function declines.

After the RO membrane, water will flow to your DI housing. DI resin in good condition will reduce the 40 ppm water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1 ppm to 3 ppm, 5 ppm, and higher, you know that your resin needs to be replaced. Sometimes people complain that their DI resin didn't last very long. Often the culprit is a malfunctioning RO membrane sending the DI resin “dirty” water. This will exhaust the resin quicker then would otherwise have been the case. Sometimes the problem is poor quality resin – remember that all resins are not created equal!

Russ @ BFS
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