What is TDS?

[swimcmp]

Split from here. JasonLion

Well then what does TDS mean. I have been in the pool industry for over thirty years an I reallywant to learn something new so PLEASE explain to me what it is. Why when a pool has over 5000 TDS no matter how much chemical you throw at it no change. But miracle of miracles when you drain some and refill you suddenly have clear water. What the op has I believe is a case of algae from neglect by previous owner. And a byproduct of past care is the high TDS reading that has noting to do with algae.

 

[jblizzle]

Re: Green pool - High TDS - Told to Drain?

From my understanding (and there at many posts that discuss this) TDS is the sum of things in the water like CYA and CH and salt and others that we prefer to measure independently because then we know what needs to be adjusted.

I would think the TDS of a saltwater pool is going to immediately be higher than a non-SWG pool just due to the salt ... so knowing the TDS may not help.

Now if the TDS is high because the CYA is high, then blindly replacing some water might help. But, we would suggest you test the CYA and then KNOW how much water should be changed to get it reasonable so that you then know what is the required amount of chlorine to be effective.

Odd metaphor that popped into my head: To me knowing TDS is like knowing you have a big bag of marbles. While we suggest testing each component so you know how many blue, how many red and how many green marbles you have.

EDIT: I like JL's answer below better

 

[JasonLion]

Re: Green pool - High TDS - Told to Drain?

TDS stands for total dissolved solids, the sum of everything dissolved in the water. The problem with TDS is that it is a total of everything, and only some things cause problems. For example, if all you have dissolved in the water is salt, you can have a TDS in the 30,000 range and still have a totally manageable pool, while if you have say 300 ppm of CYA in the water you can have a TDS of under 1000 and major problems.

Before CYA was understood, TDS was sometimes used as a proxy for the CYA level. People with consistent fill water properties who used trichlor in consistent ways would start to have problems when CYA got high. They couldn't measure CYA, but they could measure TDS, and because they dealt with the pool in consistent ways it was possible to pick a TDS level that corresponded to when you would be having a CYA problem. But this approach completely falls apart when you deal with pools from different regions an/or managed in different ways, where different things contribute to TDS in different ways. We can now measure the things that really matter, so we can stop using TDS.

95% of the time problems attribute to TDS are actually CYA related problems. Very high CYA levels make chlorine much less effective, so you need to use more chlorine. If you aren't aware of that you will never get anywhere. Even if you are aware of that, very high CYA levels mean using so much chlorine that it quickly becomes impractical.

If you are from a specific region, and make some 60-70% true assumptions about what pool care techniques were used, you can mostly still use TDS as a proxy for the CYA level. But you are so much better off using direct measurement of the CYA level, rather than being fooled 30+% of the time (and way more than 30% if you are talking cross region).

 

[Smykowski]

Re: Green pool - High TDS - Told to Drain?

^^This is one of the most comprehensive and clear posts I've ever read on this site.

What the op has I believe is a case of algae from neglect by previous owner.

I agree with this statement. Likely the previous owner threw "powdered shock" or pucks at the problem, but never really understood the chemistry. As a result, the CYA slowly climbed to the point where the chlorine was rendered ineffective. Any additional solid chlorine product made no difference because the CYA was already too high.

 

[swimcmp]

Re: Green pool - High TDS - Told to Drain?

Jason you made the point I was trying to get you guys to reach different regions manage in different ways. What works for one region isn't necessarily going to work in another. I have been in this business for along time and I don't look at new information and say it's bogus. I try to research the information before making a judgment. Some posts get off topic because people replying don't read all the post and information gets dropped. The op has algae I believe, the TDS was a byproduct of the conversation. The pool I mentioned happened to me a couple years ago so I don't remember the specific readings. CYA was in a normal range pH and TA were also normal range. We tried everything even had a chemical rep in to look at it. Crystal clear green water no metal readings not salt and TDS over 5000 drained about half maybe two thirds refilled and blue hue returned

 

[jblizzle]

Re: Green pool - High TDS - Told to Drain?

Except what we teach DOES work in all regions.

No idea what you encountered with that pool though.

 

[Isaac-1]

Re: Green pool - High TDS - Told to Drain?

Swimcmp, you may have experienced something exotic, clear green water suggests a problem with metals, but it may have been something unusual, and not detected on the usual tests for copper or iron.

As to TDS, as others have said it is a nearly useless test, because it does not tell you what is in the water, just how much total stuff is in there. It may have had a place once many years ago when there not effective tests for CYA, CH, etc. and before pools having salt water chlorine generators became popular. But this is the 21st century and using TDS for pool management is like trying to forecast the weather by how the leaves are curling on a tree. Having said that we can't say for sure or not if some water should be drained, if the CYA level is sky high, then yes you probably do need to drain, if not then it can be cleared up through filtering. The thing is a drain and replace is easier and quicker for a pool service, but may not be the cheapest or the right option for you.

 

[onBalance]

There can be "gunk" in pool water (often from bather waste) that does not always show up in a TDS test. Sometimes, draining the water down helps eliminate those unwanted wastes and the water will clear up.

With that said, I have had clear (tint) green water that simply needed doubling of the filter time to clear up the green tint. Other times I have added D.E. or a cellulose product (filter aid) to a sand or cartridge filter, or a clarifier to the water, and that worked very well to clear up the water. And of course, sometimes the pool water needed to be "shocked or slammed" to clear up the water. Sometimes both remedies were needed.

A green tint may not always be a metal problem, and can be from other issues.

 

[chem geek]

TDS can be measured in two different ways. Normally the conductivity of the water is measured and the TDS is estimated by the known parameters such as TA and CH and hopefully CYA though not everyone does that. Whatever is leftover in conductivity is presumed to be sodium chloride salt. So molecules that are not charged ions won't show up in the conductivity test. Quite a few organics fall into this category.

The other way TDS can be measured is by evaporation and weighing the resulting solids. This removes some of the carbonates that gas off as carbon dioxide so will underestimate TDS by that amount, but it will weigh uncharged molecules unlike the conductivity test.

 

[TreeFiter]

TDS can be measured in two different ways. Normally the conductivity of the water is measured and the TDS is estimated by the known parameters such as TA and CH and hopefully CYA though not everyone does that. Whatever is leftover in conductivity is presumed to be sodium chloride salt. So molecules that are not charged ions won't show up in the conductivity test. Quite a few organics fall into this category.

The other way TDS can be measured is by evaporation and weighing the resulting solids. This removes some of the carbonates that gas off as carbon dioxide so will underestimate TDS by that amount, but it will weigh uncharged molecules unlike the conductivity test.

Doesn't Chlorine typically destroy organics in swimming pools? To me it seems like TDS would be mostly metals in pool water along with CYA.

 

[JasonLion]

TDS is usually mostly salt, with CH, TA and/or CYA sometimes making noticeable contributions (depending on levels).

 

[Isaac-1]

Chlorine does not destroy organics, it just breaks down organics

 

[TreeFiter]

Chlorine does not destroy organics, it just breaks down organics

I'm not sure I see the difference.

 

[Isaac-1]

I am not sure how to put this into simple terms, hopefully chem geek can post a reply in here on the subject. Many / most of the organic compounds that we find in pools are long chain organic molicules, think of these like a stack of lego's of a variety of colors, think of the chlorine being special one sided lego's that will go in and replace certain legos in the stack, but can't be stacked on to. This effectively breaks the long chain organics down into shorter chains, that may then in turn be broken down into yet shorter ones. Some of the resulting compounds will be gasses that off gas from the pool, but this will still leave some chlorinated organic compounds in the water that do not easily break down into a gaseous form. This is an over simplification of the process, and it has been over 25 years since I dealt with organic chemestry, so I am a bit rusty on the details, but this illustration should still work.

 

[TreeFiter]

I am not sure how to put this into simple terms, hopefully chem geek can post a reply in here on the subject. Many / most of the organic compounds that we find in pools are long chain organic molicules, think of these like a stack of lego's of a variety of colors, think of the chlorine being special one sided lego's that will go in and replace certain legos in the stack, but can't be stacked on to. This effectively breaks the long chain organics down into shorter chains, that may then in turn be broken down into yet shorter ones. Some of the resulting compounds will be gasses that off gas from the pool, but this will still leave some chlorinated organic compounds in the water that do not easily break down into a gaseous form. This is an over simplification of the process, and it has been over 25 years since I dealt with organic chemestry, so I am a bit rusty on the details, but this illustration should still work.

I think I get what you mean. Your lego description is great. Essentially, the building blocks are still there to allow these compounds to rebuild if the chlorine drops.

 

[onBalance]

You are correct Chem geek, I was only thinking in terms of the conductivity test, not the "evaporation" method. I should have clarified my comment.
As Jason states, TDS is usually comprised mostly of a particular type of salt (sodium and chloride) CH (calcium), TA (bicarbonates), and CYA. But TDS also includes magnesium (which is usually a moderate amount of the total hardness), any iron, copper, sulfates, nitrates, phosphates, bromine, and various organics, etc.

The problem with a conductivity test is that the "meter" has to assumes that water contains a certain percentage of salt and other components in the water and then estimates the total TDS content.
But in regards to CYA, we are seeing that CYA in water actually interferes (resists) in a conductivity test, and therefore under-estimates the actual TDS reading somewhat.

 

[Isaac-1]

I think I get what you mean. Your lego description is great. Essentially, the building blocks are still there to allow these compounds to rebuild if the chlorine drops.

Not exactly those chlorine lego's are more or less permanently stuck on there, but the resulting compounds may be left dissolved in water, the way sugar (an organic compound) can be dissolved in water.

 

[TreeFiter]

Not exactly those chlorine lego's are more or less permanently stuck on there, but the resulting compounds may be left dissolved in water, the way sugar (an organic compound) can be dissolved in water.

So the end result is dissolved organics that are inert?

 

[Smykowski]

So the end result is dissolved organics that are inert?

ChemGeek has posted extensively on this, and here's what I've learned (Richard, please correct me where I'm wrong).

Chlorine is both an oxidizer (it breaks up compounds into smaller parts so the byproducts can be removed) and a sanitizer (kills anything in the pool that is alive).

The sanitation part is much more straight forward. Any living cell in the pool (algae, bacteria, or otherwise) uses water as part of its cellular processes. Hypochlorous acid (the workhorse part of your free chlorine) has an almost identical geometry as a molecule of water. The unsuspecting organism in the pool allows the HOCl through the cell wall thinking it's water, where upon it turns the cell's proteins into a jelled, jumbled mess. This kills the organism. The dead carcass floats around in the pool until the filter traps it, removing it from the pool.

The oxidation part is a little trickier. The HOCl likes to react with anything, so it reacts with some of the more common gross, nonliving compounds in the pool. Pee, sweat, spit, ammonia, etc., are the things that should come to mind. The HOCl reacts with these nitrogen based compounds, changing them into various forms of chloramines. This is what shows up as CCs on the FAS-DPD test.

The iterations are monochloramine, dichloramine, and nitrogen trichloride. In most of our well-maintained pools, any tiny CCs that are present are likely going to be monochloramines, and they will be broken up by the sun before they go through the further reactions. The end result is nitrogen outgassing. I'll see if I can dig up some chemical equations that describe the above from one of Richard's posts.

 

[Smykowski]

Found it. The following comes from Richard's post in this thread.

.....the dominant reactions are the following:

(1) HOCl + NH3 ---> NH2Cl + H2O
Hypochlorous Acid + Ammonia ---> Monochloramine + Water

(2) HOCl + NH2Cl ---> NHCl2 + H2O
Hypochlorous Acid + Monochloramine ---> Dichloramine + Water

(3) HOCl + NHCl2 ---> NCl3 + H2O
Hypochlorous Acid + Dichloramine ---> Nitrogen Trichloride + Water

(4) NHCl2 + NCl3 + 2H2O ---> 2HOCl + N2(g) + 3H+ + 3Cl-
Dichloramine + Nitrogen Trichloride + Water ---> Hypochlorous Acid + Nitrogen Gas + Hydrogen Ion + Chloride Ion

The first reaction producing monochloramine is by far the fastest. It is over 95% complete in one minute when the FC is around 10% of the CYA and the ammonia is much less than the chlorine so that the chlorine level remains fairly constant. With no CYA, the reaction is mostly complete in a couple of seconds. The subsequent reactions are far slower.