TDS and RODI Unit
- Thread starter SaltySecrets
- Start date
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If you have a new RO/DI you should be running 0, I just got mine and that's what mine is testing. No idea what acceptable is tbh im too new but if I am paying for it, nothing but 0 is for me at this point. just my opinion. Rise out your buckets with RODI before you fill them up with RODI water, i found this will drop you down to 0 TDS if you hitting 1 or 2. Whatever was in the bucket, i.e. moisture or saltwater before might still be lingering.
ritter6788
Coral Fraud Private Eye
Is that the reading as soon as you set it up? You usually have to run a few gallons of water through a new unit before the reading hits 0.
DaveMorris
MACNA2016 Co-Chair
I wouldn't worry about it reading 2 too much. Keep in mind that there can also be some range of error on your tester. And a TDS of 2 is a heck of a lot better than 400 or whatever is going in! 
What is your tap water TDS, RO only TDS and final or RO/DI TDS?
What are you testing with and how and has it been calibrated?
What is your water temperature and water pressure?
What is your exact measured waste ratio?
All of the above can have something to do with TDS.
Normally a RO/DI should produce 0 TDS but there are several factors such as the quality of the filters and RO membrane, the quality and age of the DI resin and whether the DI is a true 10" vertical canister properly packed or a little horizontal hollow tubewith a little bit or resin bobbing around in it. If the system is not designed and constructed properly and it uses low end components you may never see 0 TDS. If its a quality system with a good name brand membrane such as a Dow Filmtec 75 GPD and the DI resin is nuclear or semiconductor grade and most importantly fresh and properly packed then 0 TDS should not be a problem.
I worry about any TDS for a couple reasons. For one, TDS is not a measurement of anything specific so you have no idea what it is, only that it is present. The other is many things are weakly ionized so are not removed well by lower end DI resin or horizontal configurations with little contact time and many do not register well on a TDS meter so the levels can be much higher than you see on the meter since it does not pick them up. Some of those weakly ionized substances are nitrates, phosphates and silicates.
0 TDS is the only goal.
What are you testing with and how and has it been calibrated?
What is your water temperature and water pressure?
What is your exact measured waste ratio?
All of the above can have something to do with TDS.
Normally a RO/DI should produce 0 TDS but there are several factors such as the quality of the filters and RO membrane, the quality and age of the DI resin and whether the DI is a true 10" vertical canister properly packed or a little horizontal hollow tubewith a little bit or resin bobbing around in it. If the system is not designed and constructed properly and it uses low end components you may never see 0 TDS. If its a quality system with a good name brand membrane such as a Dow Filmtec 75 GPD and the DI resin is nuclear or semiconductor grade and most importantly fresh and properly packed then 0 TDS should not be a problem.
I worry about any TDS for a couple reasons. For one, TDS is not a measurement of anything specific so you have no idea what it is, only that it is present. The other is many things are weakly ionized so are not removed well by lower end DI resin or horizontal configurations with little contact time and many do not register well on a TDS meter so the levels can be much higher than you see on the meter since it does not pick them up. Some of those weakly ionized substances are nitrates, phosphates and silicates.
0 TDS is the only goal.
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mcarroll
10K Club member
Mostly ditto what AZD said above...
TDS is a non-specific measurement so is nearly meaningless by itself. Really only good as a comparison with other TDS readings to gauge overall filter system efficacy over time. About the most you can say is that "high is bad; low is good...when it goes from low to high, the filters need to be replaced".
Having said that, 0 TDS is a good and usually quite attainable goal. Considering the cost of your filter system and all the water (esp. waste) you'll be sending through it, it's also a worthwhile goal purely from that standpoint. I will also add that many people do have water over 0 TDS going into their reefs - even plain, city tap water in some cases - without issue. (It's easy to get hysterical in chasing numbers like TDS...and similarly pH...so while >0 TDS is not your goal, nor even recommended, I do feel this is worth pointing out. Not the end of the world if you ultimately can't reach 0. If you were using water of unknown quality (untreated pond, lake or aquifer, collected rainwater, etc...) then you may have cause for a bit more concern over TDS >0.
Knowing your tap water's TDS, the RO membrane's output (not waste) TDS as well as the DI stage's output TDS would paint a more complete picture of how well your system is working/if there is actually a problem with any stage. Depending on how your system is built, more than likely this will require that you unhook those output hoses one at a time and manually collect each water sample to test. (Which might be a little time consuming, but shouldn't be difficult.) To get the best readings, make sure the system has been on and processing water for at least 30min before you start unhooking anything to collect samples.
Also, not sure if you're on Tallahassee city water or not, but if so this link is to their official water quality report. TDS doesn't seem to be listed. Some localities (mine, for example) do provide auxiliary water testing reports which are more comprehensive, but I guess they are not mandated and maybe Tallahassee is only doing the bare minimum as dictated by the Federal EPA? (A bit sad, if true...esp since Tal. is the state capitol.) It may be worth calling into their office (number is listed on that link, bottom of the page, again assuming this is really your water source) to ask if there is more info available. Just tell them you have an aquarium and more than likely they'll know what you want and why you're asking. In my locale this aux report is called the "Lab Analytical Report" and provides test results on source water (all sources) as well as treated water. This should include TDS, chlorine and everything else they test for - 350 different test param's on mine, with test results posted for every month. If you don't have some luck with the local water authorities, it looks like the USGS does do some testing of your state's waters - here's an old USGS report I found. The figure on page 16 indicates that there are many areas that you might be getting your water from which are pristine in terms of TDS, but it's hard to be sure without knowing specifically which pond/aquifer and treatment plant where your tap water is coming from. Also, the Florida EPA's Water Management site has a "Drinking Water" site that seems as though it "should" contain the test data, but it seems to have everything but...maybe you'll have better luck browsing (or as suggested...calling in) for it.
If possible, let us know the answers to as many of AZD's questions as you can and we can help you know if this is worth worrying about (i.e. looking for a solution) or not and maybe point out any obvious problems. If so, there are several potential solutions, including correcting a bad setup, replacing stale filter media, adding one or more additional carbon, RO and/or DI stages to what you already have....and/or reducing your flow rate through the membrane, increasing contact time in all your chambers - great for carbon and DI particularly, but RO also is typically a bit more efficient as well. It's also possible that if your source water is truly bad enough that 2 TDS will be as good as it gets without undue expense. (This is pretty unlikely if you are on a municipal water system.)
-Matt
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Adding more stages is not usually the best option, replacing them with better quality filters and DI resins is almost alkways a much better, and often less expensive choice.
Reducing the flow through the membrane isn't advised either as that usually involves lowering the entrance or operating pressure which in turn lowers the removal efficiency. Membranes like the opposite, higher pressure not only means higher flow or GPD but also it improves the removal efficiency or rejection rate. Better carbon is an option but is designed to protect the RO membrane and not really lower the TDS any since dissolved solids are in the 0.0001 micron range and carbon blocks are measured in the 0.5, 0.6, 1.0, 5.0 and 10.0 ranges so many many times larger and TDS passes right through, with teh exception of a few ppm of chlorine and volatile organics. Improving the DI resin and the configuration, ie vertical bottom fill versus horizontal flow, gives you bang for your buck.
Improving the RO membrane itself provides the best bang for your buck though. Since it does the most work, improving what it does makes the most sense and can be done for as low as $35 for a high quality, high rejection rate membrane.
Reducing the flow through the membrane isn't advised either as that usually involves lowering the entrance or operating pressure which in turn lowers the removal efficiency. Membranes like the opposite, higher pressure not only means higher flow or GPD but also it improves the removal efficiency or rejection rate. Better carbon is an option but is designed to protect the RO membrane and not really lower the TDS any since dissolved solids are in the 0.0001 micron range and carbon blocks are measured in the 0.5, 0.6, 1.0, 5.0 and 10.0 ranges so many many times larger and TDS passes right through, with teh exception of a few ppm of chlorine and volatile organics. Improving the DI resin and the configuration, ie vertical bottom fill versus horizontal flow, gives you bang for your buck.
Improving the RO membrane itself provides the best bang for your buck though. Since it does the most work, improving what it does makes the most sense and can be done for as low as $35 for a high quality, high rejection rate membrane.
mcarroll
10K Club member
e.g. 35gpd vs 75gpd vs 100 gpd membrane - the higher flowrate the lower the quality of output water. 75 gpd is the de facto standard in our field cuz it's a nice, acceptable compromise between quality and speed of output. While still small, the difference in quality between 75 and 100 is bigger than 35 vs 75 from my understanding. (Never had a 100 gpd, personally.) No gimmicks - basic selection of membrane flow rate (and matching flow restrictor) is the way one would reduce flow (also increasing contact time, improving the efficiency of the user's choice of carbon and DI media) if that turned out to be a sensible solution, not by reducing what is probably already barely passable pressure...which would also be a useful number to know*. ~40 psi is fairly typical (but not to be assumed), and arguably worth boosting especially if he's got a 60 psi membrane. FWIW, Dow Filmtec (tapwater) membranes are rated at 50 psi and claim identical 96-98% rejection across flow rates, as well as a variance of +/- 20% of the rated flow (e.g. 75 gpd = 60-90 gpd) on any given membrane**. But it would be guessing to say any of that is the OP's problem.
It's hard to say if anything on the system would need to be added or even changed at this point. Without more specifics from the OP I think we can only speculate...which unfortunately doesn't help much.
-Matt
* For those without a gauge, the Watts IWTG is a convenient way to find out your actual house water pressure. If you're on city water, they should be able to tell you what PSI they're giving to your house...not as good as measuring at the tap, but still a baseline and better than knowing no number at all.
** While people certainly use other brands with success, all the specs and ratings for the DOW membranes add up to a good reason to seek out Filmtec membranes. Another reason: Try finding those stats for most other brands of membrane. Most don't seem to publish the deep numbers like that.
It's hard to say if anything on the system would need to be added or even changed at this point. Without more specifics from the OP I think we can only speculate...which unfortunately doesn't help much.
-Matt
* For those without a gauge, the Watts IWTG is a convenient way to find out your actual house water pressure. If you're on city water, they should be able to tell you what PSI they're giving to your house...not as good as measuring at the tap, but still a baseline and better than knowing no number at all.
** While people certainly use other brands with success, all the specs and ratings for the DOW membranes add up to a good reason to seek out Filmtec membranes. Another reason: Try finding those stats for most other brands of membrane. Most don't seem to publish the deep numbers like that.
The rejection rate on a 15 GPD to 75 GPD is identical, it makes zero difference.
The 100 GPD you are referring to is NOT, I repeat is not a reverse osmosis (RO) membrane. The Dow 100 is a NF or nano filter and is not and never was intended for drinking water use, it carries an ANSI/NSF rating of "Pool and Spa Use". If anyone trys to sell you this membrane, look elsewhere as they don't have your interest in mind.
Dow Filmtec membranes are tested to an operating pressure in excess of 150 psi and the rejection rate will improve tremendously. I run my 75 GPD membrane at 100 psi with a booster pump and the rejection rate improves to 99.43% removal efficiency. The 50 psi is not whet you should run it at, its a standard number picked out of thimn air, actually its about the average tap water pressure nationwide for most municipal water systems, used to come up with a way to standardize membranes. Other manufacturers use 60 or 65 psi but its really no different, you can lay one graph on the other and they line up pretty closely.
You can get a hose bib presure gauge like in mcarrols post above at any Lowes, HD or hardware store in the lawn irrigation section for $8-$12.
For what its worth, my DI resin and configuration has been tested at 10 GPM or 600 GPH with no change in effectiveness, the proper resin, way it is packed and vertical bottom up configuration makes all the difference in the world. And its just standard 10" refillable cartridges, nothing unusual.
The 100 GPD you are referring to is NOT, I repeat is not a reverse osmosis (RO) membrane. The Dow 100 is a NF or nano filter and is not and never was intended for drinking water use, it carries an ANSI/NSF rating of "Pool and Spa Use". If anyone trys to sell you this membrane, look elsewhere as they don't have your interest in mind.
Dow Filmtec membranes are tested to an operating pressure in excess of 150 psi and the rejection rate will improve tremendously. I run my 75 GPD membrane at 100 psi with a booster pump and the rejection rate improves to 99.43% removal efficiency. The 50 psi is not whet you should run it at, its a standard number picked out of thimn air, actually its about the average tap water pressure nationwide for most municipal water systems, used to come up with a way to standardize membranes. Other manufacturers use 60 or 65 psi but its really no different, you can lay one graph on the other and they line up pretty closely.
You can get a hose bib presure gauge like in mcarrols post above at any Lowes, HD or hardware store in the lawn irrigation section for $8-$12.
For what its worth, my DI resin and configuration has been tested at 10 GPM or 600 GPH with no change in effectiveness, the proper resin, way it is packed and vertical bottom up configuration makes all the difference in the world. And its just standard 10" refillable cartridges, nothing unusual.
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