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Total Dissolved Solids (TDS) Chemistry

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Just how much do swimming pool chemicals add to the total dissolved solids (TDS) content of the water? Do all chemicals of equal amounts (by weight) add identical amounts of TDS to swimming pool water?

When one pound of a chemical is added to 20,000 gallons of water, the usual amount of TDS being added to the water is 6 parts per million (ppm) of TDS. While this is true with most chemicals, it is not true with all chemicals. After being added, some chemicals will escape from the water and not contribute to TDS. Another possibility is that portions of some chemicals will convert into water and thus will no longer be part of TDS.

We will first look at adding a pure chemical that provides the full increase of its weight to the total dissolved solids content of the water. Chlorine gas is a good example. At levels of 1000 parts per million or below, chlorine gas is nearly 100 percent soluble at a pH of 5.0 or above. Therefore, if one pound of pure chlorine is dissolved into 20,000 gallons of water, the result will be 6 ppm of chloride (which is TDS).

One way of calculating this is as follows: 20,000 gallons of water weighs about 166,700 pounds, which is one-sixth of a million. Multiply the one pound of chlorine by six to get the 6 ppm of TDS.

As with all acidic sanitizers, after chlorine gas is added, it is usually necessary to add sodium carbonate (soda ash) or sodium bicarbonate (bicarb) to adjust the pH and alkalinity upward. Interestingly however, adding these alkaline chemicals does not ultimately add the full amount of TDS per pound added.

When soda ash and bicarb are added to pools, these chemicals will react with acid that has been added to form carbon dioxide, which, over a period of time, will release into the atmosphere. Only the sodium element from these two chemicals remains in the water and increases the TDS.

The following equations will help illustrate:
Na2CO3 + 2HCl = 2NaCl + H2CO3
H2CO3 = CO2 + H2O

In the first reaction, soda ash (Na2CO3) reacts with acid (HCl) from acidic sanitizers, muriatic acid, or dry acid, to form sodium chloride (NaCl) and carbonic acid (H2CO3). Sodium chloride remains in the water as salt, which does contribute to TDS, while carbonic acid will dissociate into carbon dioxide (CO2) and water (H2O).
The pure water that is formed does not contribute to TDS, and carbon dioxide will eventually escape into the atmosphere – thereby not contributing to the TDS content. Sodium bicarbonate also reacts with acid and forms the same compounds – salt and carbonic acid.

To determine the actual amount of TDS that is being added by these chemicals, a look at their molecular weight is needed. The (rounded off) molecular weight of soda ash is 106, and sodium bicarbonate is 84. Sodium by itself has a molecular weight of 23, and there are two sodium molecules in soda ash, only one in sodium bicarbonate. The sodium element (which adds to the TDS content) comprises about 43.4 percent of the soda ash, and about 27.4 percent of the sodium bicarbonate. Therefore, when adding one pound of soda ash, only 43.4 percent would contribute to TDS, and with sodium bicarbonate, 27.4 percent contributes to TDS.

The calculations are as follows: if one full pound of a material in 20,000 gallons of water would normally equate to 6 ppm of TDS, then one pound of soda ash, with only 43.4 percent of its total weight (sodium) remains as TDS, contributes about 2.6 ppm of TDS. Likewise, one pound of sodium bicarbonate (with 27.4 percent as sodium) adds about 1.65 ppm. (Note: Until these chemicals actually react with an acid, the entire contents of the various compounds would be present as TDS).

Another type of chemical which does not introduce the full TDS of its weight are compounds which adds hydroxide or which forms hydroxide when added to water. Hydroxide (OH) will likely combine with a hydrogen ion to form pure water (H2O) and no longer be TDS. Sodium hypochlorite (often referred to as liquid chlorine or bleach) is an example of this type of compound.

In every gallon of sodium hypochlorite (in this example, 15% trade or 12.5% by weight), there is about 1.25 pounds of chlorine and about 1.6 pounds of sodium hydroxide, for a total of 2.85 pounds of potential TDS. This would normally equate to about 17 ppm of TDS for every gallon added to a 20,000 gallon swimming pool. However, since hydroxide accounts for about 42.5 percent by weight of sodium hydroxide (and since hydroxide will eventually react with hydrogen and convert to water), this component is subtracted from the compound, so that, together with the chlorine (contained in bleach), about 2.2 pounds in a gallon of sodium hypochlorite adds to the TDS content of a solution. Thus one gallon of sodium hypochlorite adds about 13 ppm of TDS (rather than 17 ppm) in 20,000 gallons of water. (Bleach also adds a small amount of alkalinity to the water).

With 10 % (weight) bleach, about 10.5 ppm is added, and with 5.5% (weight) household bleach, about 5 ppm TDS is added, per gallon to 20,000 gallons of water.

Also, chemicals that include oxygen or nitrogen will also likely lose portion of their weight, thus a lower overall TDS contribution. Calcium hypochlorite (65%) is an example of a compound that adds oxygen. The percentage of the calcium hypochlorite contribution that ends up as oxygen is about 22 percent. Cal Hypo also contains a small amount of water. Therefore, its contribution to TDS is only about 75 percent of its total weight. Lower strength Cal Hypo will result in a slightly lower percentage of TDS contribution per pound. (Of course, part of the TDS contributed by calcium hypochlorite is calcium, which is added to the water and increases the hardness).

Trichlor, sodium dichlor, cyanuric acid, and regular salt, added to swimming pool water will result in the full increase of their weight to TDS.

Muriatic acid (hydrochloric acid) is product that, when added to pool water, will contribute significantly to the TDS content. Muriatic Acid at 31.45 percent strength adds about 3 pounds of TDS per gallon. Although the hydrogen from this acid will react with hydroxide to form water, it is such a small percentage of the total weight of the acid that it will not significantly effect the contribution of this acid to TDS. Therefore, one gallon of muriatic acid in 20,000 gallons of water will increase the TDS 18 ppm.

It is recognized that this information on TDS isn’t critical to the understanding and maintenance of water balance, but perhaps some will find it interesting.

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Comment by Thomas Jarvis on February 21, 2018 at 8:26pm

RIchard  thank you so much for you invaluable input!

I will look into the calcium chloride to improve the SI.

As I mentioned I am a new CPO, my first and only class was last fall.  I have found patience to be my best friend but now that I am a few months along I want to make a plan.

There is no CYA in these pools, they are both indoors.  I do chemical testing to measure my pH and Cl2 levels. A coworker tests them in the morning and I test them in the afternoon/evening.   I try to compare them with the numbers on the controllers and even started recording the ORP number when I do my chemical testing to try and glean some insight into how ORP relates to my chemical test results.

I believe that the controller must be faulty - or the probes.   I have not yet been taught how to clean the probes or test them, it was last done by another CPO - so I'll add that to my 'to learn' list.  This will be a priority to fix or replace for sure.

My target pH is the setpoint on my controllers which is 7.4.  From your comments I may want to look into increasing that setpoint to 7.5 or 7.6 once I have reliable controllers. The pH has been lower than this setpoint since the spike last Friday which is confusing.

It is interesting that you ask about heavy activity spikes when the bounce occurs, I had this thought also because the spikes tend to be over the weekend but when the activity is lower.  Monday-Friday the pools are quite stable, but (not as a rule) my problems occur from Friday-Sunday when there is significantly lower traffic.

Thank you for your information,


Comment by Richard A. Falk on February 21, 2018 at 5:45pm

You can increase your Calcium Hardness (CH) by adding calcium chloride to help balancer the saturation index.

The ORP controller will react not only to the chlorine level but also to pH unless it has specific compensation for that.  The sodium bicarbonate will slightly raise the pH unless your pH is pretty low in which case it will raise it more.

So that is good that you don't try and push the TA up to 80 or more since you'd just be adding even more acid and bicarb in that case.  However, you never mentioned your pH target.  What is your pH target?

As for a bounce in chlorine and pH, your ORP sensor is likely raising the chlorine level when the pH rises.  Is there any Cyanuric Acid (CYA) in this pool?  At any rate, this does not explain the source of the bounce which I suspect is due to the pH rising from your pH controller not working properly to add acid to compensate consistently.  Basically, it is failing to add enough acid at some point so the pH rises and then the ORP sensor is having the controller raise the FC level.

Is there any sort of heavy activity spike when this bounce happens, such as a significant increase in aeration from bather splashing, waterfalls, spillovers, fountains, etc.?  It may be that there is some increased aeration that has the pH rise faster than the controller can add acid, but that seems doubtful since aeration doesn't usually have the pH rise that fast.  I suspect it's a pH sensor problem.  When you see the pH spiking, is that from your manual measurement or is the pH sensor saying the pH is high yet it isn't adding acid?  It's not something simple like an empty acid tank, is it?

Comment by Thomas Jarvis on February 21, 2018 at 5:23pm

Richard - Thank you so much for the concise reply!  I work at a facility that has 6 pools including a 50m, leisure, spa, and therapy pool built after 1999 and equipped with acid and CO2.  The other pools are managed by 2 other CPOs.  The two pools I have been assigned were built in the 60's and have gone through various restorations including maindrain re-routing and maintdrain sealing (with a plastic like liner)  consequently making them the more difficult pools in the complex.  They also handle a LOT of bathers on a daily basis.  I have 200-300 school children for lessons every school day, private lessons, high school teams and MLA teams every day from 8am to 8pm.   I am just learning how the chemistry and controllers work together and it can be overwhelming.  My incoming water has an alkalinity of about 120 and calcium hardness around 200 but the pools themselves are significantly lower.  My saturation indexes are on the corrosive side (-.3-/-.8) and I am dealing with this yo-yo effect with the acid and sodium bicarbonate.  I don't quite understand how the controller reacts via ORP/HRR when I add the sodium bicarbonate.  Directly after I add the sodium bicarbonate I turn off the acid pumps for about 2 hours - but inevitably (sometimes 1 day, sometimes 3 days later) I will have a bounce where the chlorine spikes, the pH spikes and I have to turn off the pumps manually until my numbers settle back down and then I go through the same thing again when I add the bicarb 2-3 weeks later.  The pools can stay pretty stable at 60 for TA - I've been waiting until the numbers drop to about 40 for TA before I add the bicarb which is every 2-3 weeks.  I can stay within safety parameters by testing and watching the pumps/chemicals but I am looking for a long term solution.  Do you have any advice concerning this bounce?  My probes are clean and my pumps are new - I am wondering if it is the controller itself?  I use a Strantrrol System 5f for the dive well and a ProMinent DCMS for the lap pool.


Comment by Richard A. Falk on February 21, 2018 at 4:27pm


You are facing the acid/bicarb cycle that is well-known.  Basically, you have carbon dioxide outgassing from the pool and that has the pH rise.  When you then add acid to the pool to lower the pH, that also lowers the TA.  If you then add bicarb to raise the TA, then you are in a vicious cycle of adding acid and bicarbonate to compensate for the carbon dioxide outgassing.

Ironically, a higher TA level increases the rate of carbon dioxide outgassing, as does a lower pH value and higher aeration/splashing.  The easiest way for you to reduce the amount of acid and bicarb you have to add is to target a lower TA level and higher pH level which will reduce the CO2 outgassing rate.  Some facilities switch to adding CO2 directly since it is generally less expensive than acid/bicarb, but the same principle applies of having a lower TA and higher pH target to reduce the amount of CO2 that needs to be added.

So target the lowest TA level allowed by your regs (60 ppm?) and a higher pH target that is allowed (7.8?) and you should find you use a lot less acid to maintain the pH and bicarb to maintain the TA.

Richard (chem geek)

Comment by Thomas Jarvis on February 21, 2018 at 4:15pm

Hello! I am a relatively new CPO charged with two of the older tile pools at the Holland Aquatic Center in Holland Michigan. I need some help to understand why I need to add so much bicarb to maintain my Alkalinity over 50. I have an Instructional Lap pool, 75' x 45', 3375 sqft, 113,902 gal, hybrid sand filter, turnover 5.4 hrs, recirculation rate 320 gpm, and a surge capacity of 2,047 gal. The dive well is 45' x 25', 1125 sqft, 105,465 gal, hybrid sand filter, turnover 7.6 hrs, recirculation rate 280 gpm and a surge capacity of 1075 gal. I use liquid sodium hypochlorite and liquid muriatic acid fed via pump and controled by prominent (lap pool) and siemens (dive) controllers. My raw incoming water has an alkalinity of 120ppm but I can't keep the alkalinity of the dive and lap pools over 70 without adding 50lb bags of Sodium Bicarbonate ever 2-3 weeks. None of the other pools in my complex have this issue and I would like to find the root of the problem. Please contact me if you have any advice!

Comment by Clemente Rivera on January 7, 2011 at 9:50am

On another note:


On Mr. Faulk's comment about nitrates and phosphates and algae growth...I once ran into a pool that was completely green from algae, with a phosphate level above 2.0 ppm.  the chlorine levels were over 10ppm, but the pool was still green; also, no CYA was present.  I added an emergency dose of phosphate remover, and in 24 hours, the algae was gone.  How did so much algae get there...Lets just say that somones "propietary scale inhibitor" was the culprit.  Not mentioning which system it was, but Al and I have had a few laughs on that one.


Lamotte specifies a high limit of 1.0ppm for phosphates.


Just wanted to put that out there.

Comment by Clemente Rivera on January 7, 2011 at 9:44am

Hi Everyone:


While TDS has been a stickler for Pool Operators concern for many years, I have recently decided to look at a solution for the Pool Dumping Problem in the United States.  Did you know that states such as New Jersey, and parts of New York State require a pool be dumped at least SIX TIMES PER YEAR!!!.  Suffolk County, NY actually requests a timetable from CPO's every year...


I firmly believe that TDS is not a factor to be concerned about, as I have worked on pools that have fill water with 4000ppm TDS, and have been able to keep them running VERY well.  The issue is whats in the water, as what was previously mentioned.


back to the TDS issue with respects to dumping...I find that this REQUIREMENT to dump millions of gallons of water is attrocious, so I recently started to have a look at what can be done about it.


The answer: RO (Reverse Osmosis Filtration).


I am working on a Utility Patent to create a Service that can come to a Facility, connect to the circlulation system, and reduce the TDS of the pool via a portable RO system (it fits in a semi truck, so its not a small device).  After about 12 hours, the TDS can be reduced to as low as 200PPM (which means you will have to re-balance your pool).  There is still a 25% loss of pool water from the process, but dumping 1/4th of the pool is better than the entire thing.  Also, the service would only be needed when the pool reaches a specified limit 9some codes want pools dumped at 2000ppm TDS, for example).


Im working out some of the issues that arise with RO, especially the issue with the superconcentrated flush water that will be expelled into waste (the EPA restricts how much TDS can be present in that waste - an issue that industrial processes go through every day), but Im finding that there is a comprimise on the horizon.


Just wanted to let you know what people in the industry are doing to solve these operational concerns.


Comment by Richard A. Falk on December 24, 2010 at 11:30pm

Merry Christmas to all as well.


Good info Al.  There is very little that is black and white with pool systems.  There are pros and cons with all, but full disclosure and discussion isn't usually done (unfortunately).


My only counter-comment to what you wrote is with respect to nitrate levels and algae growth.  There isn't a magic number where either phosphates or nitrates get to a point where you can't kill algae with chlorine alone.  Even with plentiful nutrients, algae growth is still limited by sunlight and temperature -- under ideal conditions with only sunlight at its normal limit and at an ideal temperature for maximum growth, algae only double in population in 3-8 hours.  They simply won't grow any faster than that no matter how many nutrients you give to them -- they can only photosynthesize so fast and only reproduce so quickly.


Now it is true that at high phosphate and nitrate levels you really need to stay on top of the sanitizer level because letting it get too low results in a big jump in chlorine demand and ultimately (if chlorine levels continue to get lower) to dull, cloudy, and then green water in a full bloom.  I've had 3000+ ppb phosphate levels in my own pool and can attest to this, but maintaining the appropriate minimum FC/CYA ratio had no unusual chlorine demand nor no other signs of algae in the pool.  The same is true for many other pool owners where high phosphates and/or nitrates were measured.  This is why I say that phosphate removers should be seen in the same vein as algaecides in that they take the edge off of rapid algae growth so are like insurance, but are not necessary if one maintains the appropriate chlorine level relative to CYA.

Comment by Al Neumann on December 24, 2010 at 8:13pm

Merry Christmas Guys.

Just waiting for Santa to come. But while I’m waiting, I might as well put my 2 cents in here now.

I think maybe some clarification is needed on the nitrate/nitrite issue, and why I asked the question on if anyone knew if swimming pools per se, had a recommended maximum level, and if so, what effects are likely to be seen with levles that were considered high. It was initially asked as a TDS type of question, in that I saw a potential for build-ups in some pools, and was wondering what the over-all effect of that would be.


Now I did not expect nitrates to be a health issue in swimming pools, as they are in drinking water for children under 6 months of age due to “blue baby syndrome”. I did suspect there would be some issue with chlorine demand for couple of reasons, as it is well documented as being linked to algae growth due to nitrates being part of its food chain, similar to phosphorus. As Richard stated, nitrates are algae food. A link for the algae connection, and for one of the reasons for chlorine demand, is found here. It also states a limit of 25 ppm’s, although, I don’t yet know where that is coming from. At least it is a start.


As I stated before, the other factor of chlorine demand was what Chip Blatchley found out in one of his studies which showed, and I’ll quote what I wrote before so you won’t have to look back,


“Reason why I ask this is that in one of Chip Blatchley’s papers on DPB’s and UV, he mentions a sort of “nitrogen cycle”, (my words) with medium pressure UV. It appears that the photolysis of chloramines end up as nitrites and nitrates. When nitrates pass though the UV chamber,  exposed to wavelengths under 240 nm, converts the nitrate to nitrite. In pools with residual chlorine, this nitrite is then converted back to nitrate. It becomes a source of chlorine demand not really thought of before, and is a continous cycle. It keeps on repeating, with nitrates increasing by chloramines and I suppose some other DPB’s. Was wondering what effect nitrirates has on the pool, at what levels are a concern, etc. I should note that this is just a concern with medium pressure UV, as low pressure UV  has only the wavelength of 254nm”.


Medium pressure Uvc created a chlorine demand issue by the repetitive conversion of nitrates to nitrites, whch are then oxidized by chlorine back into nitrates. Constantly repeating, constantly increasing in numbers due to the the continued photolysis of new chloramines added by bather load, (which is a good thing). Since, in a way it is like cyanuric acid, in that build-ups can only be reduced by drain and fills, or by RO treatments, then it would be nice to know when enough is enough.


Now do I think that this newly recognized form of chlorine demand is significant. I don’t know, but it may be fuel to feed the controversy between Low Pressure and Medium Pressure UVc manufacturers are going through these days.  MP claims as being far superior to LP technology is not as strong as it used to be. Does the nitrate /nitrite issue play a role in their repective role on chlorine savings claims? Would be nice to know. This particular study by Chip Bl;atchley has been out for a couple of years now, yet no one has talked about it. Kind of wondering why.


My thoughts, as good as MP is, it appears it is not living up to all the exagerated claims that many of their manufacturers/dealers like to make. Like any other technology, it’s a tool to be carefully used. It’s not the panacea that our industry is trying to make it. There are trade-off differences besides the initial cost, and maintenance costs associated with bulb replacement and electrical usage. All you have to do is take a closer look at some of Blatchley’s studies to see that the 99% reductions in chloramines and DBP’s, without any detrimental side effects, isn’t quite substantiated.  You will find that they do quite a good job on some, but also in the process, creates some DBP’s all their own, that are just as bad…yet nobody talks about it. Why??

 My apologies to Kim, in that I didn't intend to make this a commentary on UV. This thread is on TDS. And please, don't get me wrong. I'm not against UV. I just like to know the good along with the bad. All technologies have trade-offs. I try to be objective, in my journey in gaining knowledge, and in how I re-act with my customers. I just feel it is up to me, along with my customers, as to decide which trade-offs are acceptable, and which ones are not. 

Comment by Richard A. Falk on December 24, 2010 at 12:56pm
Nitrates are far more innocuous than nitrites.  You won't find nitrites in a chlorinated pool since the chlorine will oxidize them to nitrate.  The EPA maximum contaminant level (MCL) for nitrate in drinking water is 10 mg/L (ppm).  The EPA Integrated Risk Information System (IRIS) describes how this limit was set based on the effects of higher nitrates levels in infant formulas.  People do not normally drink large quantities of pool water so the risk is much lower.  This link gives more info on nitrates and nitrites where there is no information on dermal absorption of nitrate.

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