High TDS

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
Eddieboy

High TDS

Postby Eddieboy » Wed 03 Dec, 2008 16:11

Hi, I'm new to all this, with a new 'water park' pool to look after. It's 1,000,000 litres with flumes and rides. Bare concrete balance tanks and tiles finish. We get a lot of airation dut to water jets and rides. I have been told to keep my Total alkalinity at around 150ppm with calcium at 200ppm ph is 7.4. My problem is my TDS keeps rising currently 1900. I dilute at about 50 cubic metres a day and add 40 litres of Sodium Hypochlorite as disinfectant ( Free 1.5 ) 100 kilo of Sodium bicarbonate (to maintain TA), 15 kilo of calcium (to maintain hardness) and 175 kilo of dry acid (Sodium bisulphate) to maintain ph at 7.4. I have been told by our users (average 800 a day, max 2000) that the pool is great and the water is fantastic but I wonder why I must dilute so much to keep the TDS below 2000? The structure is 7 months old. Temp 30 degree celcius. Help???


chem geek
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High TDS

Postby chem geek » Wed 03 Dec, 2008 17:18

First of all, some of your numbers don't sound correct. 40 liters of Sodium Hypochlorite in 1,000,000 liters would raise the FC by 5 ppm if the strength were 12.5% as is pretty typical with chlorinating liquid. Even if it were only like 6% bleach, this would still be 2.5 ppm instead of the "Free 1.5" you quoted. The calculation is very simple as (12.5/100)*40 = 5 ppm since your pool is 1 million liters.

So my comments below are based on the quantities you said are added daily and not based on the "1.5 ppm". Basically, your TDS is rising quickly because you have your TA way too high and that is accelerating the outgassing of carbon dioxide (the aeration also accelerates this, as does low pH, but I assume you can't slow down the aeration and don't want to have the pH too much higher). This causes the pH to rise with no change in TA. Then, you have to add acid to lower the pH and that causes both the pH and TA to drop. Then, you have to add sodium bicarbonate to raise the TA (with little change in pH). The net of all of this causes the TDS to rise by about 200 ppm per day. Even worse, is that you are using dry acid (sodium bisulfate) instead of Muriatic Acid (31.45% Hydrochloric Acid) so your sulfates are increasing by 140 ppm per day and will register on a conductivity type of TDS test as an even greater rise in TDS than the 200 ppm per day that I quoted. I'm not counting the approximately 75 ppm TDS drop from dilution.

Even with all of the numbers you gave me, I show TA dropping by 8 ppm per day so something isn't quite right with the numbers. Also, the dilution of 50 cubic meters, which is 50,000 liters, is 5% per day so the CH should drop by about 10 ppm per day while you are adding about 13.5 ppm per day (if I assume you are adding anhydrous Calcium Chloride; if you are adding Calcium Chloride Dihydrate, then this is 10.2 ppm per day so would be about right).

Also, my calculations show that the pH would be rising to almost 9.9 in a day if you weren't adding the acid (and the baking soda) so are the quantities you are describing of chemical additions something that is being done several times through the day? Are automatic dosing systems being used? Technically, the pH wouldn't actually rise to 9.9 since the rate of rise slows down as the pH rises and would get no higher than 8.6 and would probably stop even around 8.0 or so.

If you were to stop adding the sodium bicarbonate for one day, then the TA should drop by almost 70 ppm (if you added the amount of acid you are currently adding). If you also target a pH of 7.5 or 7.6 instead of 7.4, then that would also help. You should then find that the pH doesn't rise as much and that should let you use less acid. The TA also shouldn't drop as fast (because you add less acid) and that will let you use less sodium bicarbonate. You will, however, have to raise your your Calcium Hardness (CH) to at least 300 ppm and possibly a little higher to keep the saturation index near zero.

The higher TDS is salt which is not terrible if it were sodium chloride unless it got really high (saltwater chlorine generator pools have around 3000 ppm salt), but since yours is both sodium chloride and sodium sulfate, that may not be so good. High sulfates could potentially cause some problems, but I don't have data on exactly when that starts to occur.

I also assume, since this is an outdoor pool, that you are using Cyanuric Acid (CYA) but you did not give that level nor how much you are adding in order to keep it stable from dilution. Without CYA, the chlorine will degrade too quickly from sunlight. You don't want too much CYA, however, since it lowers the disinfection and oxidation rate of chlorine (by lowering the hypochlorous acid concentration). 30 ppm should be more than enough since a high bather load will tend to swamp chlorine consumption and be greater than the loss from sunlight at that CYA level (even 20 ppm might be OK). With CYA, however, you should maintain a higher FC level than you normally would. At 30 ppm CYA, you should have at least 3 ppm FC. That gives you more buffer against local chlorine loss anyway. It sounds like you may be using 5 ppm FC per day anyway in which case you may swing from 8 ppm FC to 3 ppm FC each day (with 30 ppm CYA) unless you are adding it more frequently. Note that the EPA says the maximum allowed FC is 4 ppm but as should be pretty obvious, any pool adding 5 ppm FC only once per day isn't going to be able to stay under 4 ppm (and I won't get into here why it's not a problem if it's higher, as long as there's CYA in the water).

Pools are essentially over-carbonated both to provide a pH buffer and to saturate the water with calcium carbonate to prevent dissolving plaster. TA is mostly a measure of that over-carbonation. So if you target 80 ppm as your desired TA and perhaps 7.6 as your desired pH, then you should find the pool to be much more stable. With less acid addition and bicarbonate addition, the TDS rise will slow down significantly. If you got to the point where the pH rise was stopped completely, then you'd only have to add 6.5 kilos of sodium bicarbonate and not need to add any acid. If you weren't using CYA in the water, then the CYA should lower your chlorine consumption considerably and that will also lower the rate of TDS rise.

Normally when using a hypochlorite source of chlorine, you don't have the TA be any higher than 80 ppm. It's only when using acidic sources of chlorine, such as Trichlor pucks/tabs, that you have the TA be higher such as 120 ppm or higher.

You can learn much more by reading up at the at Trouble Free Pool and can use The Pool Calculator for calculating some dosages and the saturation index.

Richard
chem geek
Pool Industry Leader
Pool Industry Leader
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Joined: Thu 21 Jun, 2007 21:27
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High TDS

Postby chem geek » Sat 06 Dec, 2008 16:38

Any updates about your situation?
Tim Bullard

High TDS

Postby Tim Bullard » Sat 13 Jun, 2009 12:06

My pool will not clear up. I have found out that the total TDS is very high. I have read up on this and it said that I needed to drain out the water down until about a foot of water is left in the pool. I did that and refilled the pool with my city water. After doing this it turned a dark green again. I shocked the pool with a Super Zappit with 2 bags that treats 16500 gallons per bag. I have only 16500 gallons of water in the pool. I also added 32 ounces of Regal Algaecide 60 for all algea growth and the water is still as green as it was when I first started. I also have no reading of total chlorine. I have the test strips for TDS and read my city water from the tap and it also reads over the 5000 level. What should I try next? I have been a pool owner for over 20 years and this is a first for me. Please help!

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