perplexed by tds readings

[rick rottet]

I just set up a DI system with inline tds monitors. I can monitor; tap-post sediment-post carbon-post kati-post ani.
tap= 680-760
post sediment= 715-760
post carbon= 630-700
post kati= 1360-1520
post ani=1

Are tds measurements basically equivalent to GH readings?
I was thinking that GH is calcium and magnesium salts only.
Is there a chemical/ionic explanation for the big jump in tds after the kati?

[Randy Holmes-Farley]

TDS measurements measure all conductive ions in solution.

FWIW, your values look fine to me.

What happens in the resins is that H+ and OH- are swapped for all of the other ions present, and then the H+ and OH- combine to form H2O:

So suppose that you start with salty water: Na+ and Cl- in the water.

passing through one bed swaps out the Na+ for H+, changing the Na+ and Cl- for H+ and Cl-. It turns out that H+ is way, way more conductive than is Na+, so the apparent conductivity (TDS) rises.

Then in a second bed, the Cl- is swapped out for OH-. OH- is also more conductive than is Cl-, but it instantly combines with the excess H+ to form H20, which is not conductive, and the conductivity drops way down.

This article details more on TDS:

What is TDS?
http://www.reefkeeping.com/issues/2004-04/rhf/feature/index.htm

[rick rottet]

as always, you shine the light on a muddy world. thanks for your reply and insight.
Rick

[Randy Holmes-Farley]

:lol:

You're welcome. :)

Happy Reefing.

[rick rottet]

heehee. It occured to me after I posted, but the water can still be full of junk. As long as it's non-conductive junk, it won't show up on the monitors. sigh.

[Randy Holmes-Farley]

Yes, that is true. Many organics, for example, or solid particulates that pass through the sediment filter. An RO helps with those.

[rick rottet]

The articles I read from searching were saying that even "pure" water may give a reading of 1 or 2 on a tds monitor. Does that mean we need to recharge at 3 or is there some "magic" number to recharge at or do we simply go by the color change of the media?
After the first 50 or so gallons, I have final product readings of around 6 but has stayed steady at 6 for 24 hours now. This is on an Aquatechnics series 10 which is supposed to process ~ 9,000 gallons before recharging.

[Randy Holmes-Farley]

I'd replace the DI if it was usually 0 or 1 and it rose to 3-6 ppm TDS, yes. I expect it will slowly rise further.

Do you know the TDS of the tap water? Is this an RO/DI system?
Here are my comments from the above article:

"

7. If you are using a TDS or conductivity meter to monitor the performance of an RO membrane, then the measured value should drop by at least a factor of 10 from the starting tap water. So, for example, if the tap water reads 231 ppm, then the RO water should be less than 23 ppm. In many cases, it will drop much more than that. Less of a drop than a factor of 10 indicates a problem with the RO membrane.

8. If you are using a TDS or conductivity meter to monitor the performance of an RO/DI system, then the measured value should drop to near zero. Maybe 0-1 ppm. Higher values indicate that something is not functioning properly, or that the DI resin is becoming saturated and needs replacement. However, that does not necessarily mean that 2 ppm water is not OK to use. But beware that it may begin to rise fairly sharply when the resin becomes saturated. Do not agonize over 1 ppm vs. zero ppm. While pure water has a TDS well below 1 ppm, uncertainties from carbon dioxide in the air (which gets into the water and ionizes to provide some conductivity) and the TDS meter itself may yield results of 1 or 2 ppm even from pure water. "

[rick rottet]

The final product monitor has never read 0. It was at 1 for about the first 50 gallons of output. I never saw any intermediate numbers between 1 and the reading of 6. (i.e., it was at 1 when I ran the first 50 gallons, the next day I turned it on and the reading was at 6. I never saw a reading of 3, 4, 5). I have run a total of ~300 gallons with it staying steady at 6.
Tap measurements have been running between 680 to 760 (see original post).
This system is not an RO/DI. It has 0.5 micron sediment filter, 0.5 micron carbon block, kati, ani.
The sediment filter and the paper around the carbon block are already orange (high rust/ iron water) if that means anything.

Arrgh. So, I go out after posting to turn the system on again and my final product water is reading 19. After the kati it is over 2,000 (compared to 1,300 to 1500 I was getting fron the original 50 gallons).
Surely these units can't need recharging after 350 gallons. They are brand new and are supposed to process ~9,000 gallons between charges. They are 39 1/2" tall x 4" diameter and full of medium. I know my towns' water is bad but not that bad.
Think I should just go ahead and recharge anyway?

[Randy Holmes-Farley]

Is that monitor reading in ppm TDS, or posibly something else?

What is the TDS of the tap water? The reason that folks use RO/DI over DI alone (what you have) is that DI alone is rapidly depleted while the RO can extend the DI lifetime by 20 times.

To deionize 9000 gallons of high TDS water with a DI alone would take a mammoth DI system. That seems an unlikely claim to me based on the size you quote.

[rick rottet]

The monitor reads in ppm.
The tds of the tap water is 680-760 ppm, so I'm starting behind the eight ball already.

I need to produce ~1,000+ gallons per week. According to Spectrapure, to have an RO unit to do that, I would need a pumped unit about the size made for a carwash. Prohibitively expensive. Spectrapure quoted me $5,000+ for an RO for my application. Of course, my response to them was less than polite and, regrettably, I probably made an enemy. I suppose I could set up some sort of manifold with multiple membranes which would then all combine back together to feed the DI. Then I would need a back flow preventer on the lines from each membrane. One of the reasons I wanted to go with a straight DI is the waste involved with an RO-especially making 1,000+ gallons of product water per week. RO units are typically slower as well-but multiple units would have the potential to make water more quickly. There will (predictably) be times when I will need to make water quickly. I guess compared to the time/hassle/cost of recharging the DI every couple hundred gallons...well, it looks like I've got some rethinking to do. I'm just rambling and thinking out loud here.
Surely all of those propagation facilities aren't using monster RO units with their DI systems.
Frustrating-grrrr

[Randy Holmes-Farley]

Yes, that is quite high TDS for the tap water, and it does not surprise me that it is becoming exhausted.

2 x 100 gpd RO/DI units should be able to produce 1000 gallons per week, shouldn't they?

[rick rottet]

2 x 100 gpd membranes could make 1,000 gpw- in an ideal situation (i.e.-tap pressure at 60 psi and 70 degrees F). OK, let me think out loud for a minute.
In my home, I have a 50gpd RO unit with a 3 gallon pressure tank. If I start with a full pressure tank, it still takes about 2 hours for the RO to fill a five gallon jug- if the sediment and carbon filters are new. It gets slower as time passes. That means the RO flows about 1 gallon per hour when in top condition. Translated to a 100 gpd membrane- maybe 2 gph, with two membranes- maybe 4 gph X 160 hours per week =640 gpw.
Our town's water pressure is not good (usually around or less than 40 psi)- old cast iron mains. The water doesn't run out of the tap at 70 degrees, more like 50-60 depending on the time of year.
I suppose 4 membranes (or the two with a pump) might be adequate. Looks like I have some more research to do.
I do appreciate your time and the space here to work on this issue.
Rick

[Randy Holmes-Farley]

1000 gallons per week is a pretty large use of purified water. Good luck finding a suitable solution. :)

[rick rottet]

Well, one thing is for sure, it's too late to turn back now!
20 tanks at 210 gallons each and a 425 gallon live rock vat. Expecting 25% water changes when fully stocked. Only the live rock vat is filled with water at the moment and I just cancelled my order for the first 400 lbs of rock until I get this water situation worked out. I don't want to start with a poor foundation.
If you are interested in seeing the construction process, it can be seen here
http://archive.reefcentral.com/forums/showthread.php?s=&threadid=465933
or just click my little red house, it's my non-commercial subdomain of our local club's web site.

[Randy Holmes-Farley]

Thanks for the info. Good luck with the operation!

[rick rottet]

Thank you.