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Going Back In The Pool After Superchlorination


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Hi,

This is my first post here... We've had a pool for about 5 years, and I have recently taken over the maintenance of the pool after a couple of problems with your pool company (leaving the pump on rinse and draining about a quarter of our pool for example!) Anyway... After adding back the lost water it took our chlorine levels down to very negligible levels, so I decided to shock the pool - it was generally in good shape - the Ph is a little low and there is a lot of stablizer, and normal alkaline levels (according to the aquacheck test strips).

... So after adding roughly a kilo of granular chlorine directly to the pool (100 cubic metres) chlorine levels now appear to be 10ppm - is this a safe level to enter the pool for adults or small children? Any idea how long it will take for the chlorine to dissipate to normal levels? The pump was opne for about 24 hours after shocking.

Thanks in advance - I'm sure I'll have a lot more questions in the future!

Richard

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Hi,

This is my first post here... We've had a pool for about 5 years, and I have recently taken over the maintenance of the pool after a couple of problems with your pool company (leaving the pump on rinse and draining about a quarter of our pool for example!) Anyway... After adding back the lost water it took our chlorine levels down to very negligible levels, so I decided to shock the pool - it was generally in good shape - the Ph is a little low and there is a lot of stablizer, and normal alkaline levels (according to the aquacheck test strips).

... So after adding roughly a kilo of granular chlorine directly to the pool (100 cubic metres) chlorine levels now appear to be 10ppm - is this a safe level to enter the pool for adults or small children? Any idea how long it will take for the chlorine to dissipate to normal levels? The pump was opne for about 24 hours after shocking.

Thanks in advance - I'm sure I'll have a lot more questions in the future!

Richard

The vast majority of Health Departments in the US consider 10 ppm or less safe for a commercial pool to be open when stabilizer levels are in the normal range of 30-50 ppm. If your stabilizer is higher (you did not give any numbers, just said it was high) chlorine levels can be higher that 10 ppm since stabilizer keeps the chlorine chemically 'bound' and not in an active state. As stabilizer levels rise the chlorine level also needs to rise to keep the same amount of active chlorine in the water. For example, a pool with no stabilizer and 2-3 ppm FC (not uncommon in indoor pools) will actually have MUCH more available chlorine in the water than a pool with 50 ppm stabilizer and 5-6 ppm FC (a good place to keep an outdoor pool, BTW)

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Adding 1 kilogram of 73 % calcium hypochlorite into 100,000 liters of water should provide about 7.2 ppm of chlorine.

I have never been comfortable allowing people to swim within 8 hours of adding calcium hypochlorite. This is mostly because cal hypo does not dissolve well.

I think that you would do much better using sodium hypochlorite (liquid chlorine or bleach). I think that it is OK to swim after about 30 to 60 minutes after adding liquid chlorine as long as the level is not too high and there is less than 0.5 ppm of combined chlorine.

What are all of your chemical readings?

Free Chlorine

Combined Chlorine

pH (Current and average)

Total Alkalinity

Calcium Hardness

Cyanuric acid

Salt (If you have a salt chlorinator)

Temperature

What is the granular chlorine that you are using? Calcium hypochlorite, lithium hypochlorite or dichlor?

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Thanks for the replies.

My readings this morning are:

pH 6.8

ppm Free Chlorine 10 (the final color on the test strip) - doesn't seem to be going down after shocking about 48 hours ago.

ppm Total Alkalinity 80

ppm Stabilizer 150-300 ppm - hard to tell from the test strip

- I suppose I could drain part of the pool and top up with fresh water and maybe add some pH+ liquid - Or would you not touch the pH in this case and just wait for it to rise naturally?

The granualar chlorine used is trichlor

I don't have any other readings.

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You should not be using trichlor (tabs or granular) or dichlor for your pool. They lower the pH and they add a lot of cyanuric acid. 1 kilogram of trichlor will raise the chlorine by 9.2 ppm, raise the cyanuric acid by 5.6 ppm and lower the pH by 0.49.

The high cyanuric acid levels are keeping your chlorine from being used up. That is why the levels are not dropping. You should reduce your cyanuric levels to below 90 ppm.

I recommend raising the pH by adding baking soda to raise the alkalinity. Use 5 Kilograms and then retest. If you use pH increaser, you risk triggering a precipitation reaction if you have copper or other minerals in the water.

You really need to get a good test kit, strips are not good enough.

What is the pool surface, vinyl liner, fiberglass or plaster?

Is there a heater?

Has anyone ever added any copper based algaecide?

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cyanuric acid - that's the stabilizer right? I've asked at the local pool shop if they sell chlorine tablets without stabilizer and they don't (I think - my Spanish sucks! : )

I took another reading this morning, chlorine seems to be dropping now, looks more like 5ppm, I'll try to source some chlorine without the stabilizer though.

Thanks for the baking soda tip, I'll try that.

The pool is tiled, and there is no heater - algaecide, we use it, but not sure if it is copper based.

- Actually thinking of switching to salt chlorinator or oxyegen or ionising system, do you or anyone else reading this have any opinions on this? - really want to go for an option which is best for the kids and low maintenance for me.

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cyanuric acid - that's the stabilizer right? I've asked at the local pool shop if they sell chlorine tablets without stabilizer and they don't (I think - my Spanish sucks! : )

I took another reading this morning, chlorine seems to be dropping now, looks more like 5ppm, I'll try to source some chlorine without the stabilizer though.

Thanks for the baking soda tip, I'll try that.

The pool is tiled, and there is no heater - algaecide, we use it, but not sure if it is copper based.

- Actually thinking of switching to salt chlorinator or oxyegen or ionising system, do you or anyone else reading this have any opinions on this? - really want to go for an option which is best for the kids and low maintenance for me.

right, the chlorine tablets (pucks) all have stabilizer (CYA) in them. Use liquid chlorine (store bleach ~ 6%, or liquid bleach from pool store (usually around 12.5%). Then you won't have to worry about adding more CYA.

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For example, a pool with no stabilizer and 2-3 ppm FC (not uncommon in indoor pools) will actually have MUCH more available chlorine in the water than a pool with 50 ppm stabilizer and 5-6 ppm FC (a good place to keep an outdoor pool, BTW)

Huh? Not sure I get this statment. How could a pool without CYA and a FC 2-3ppm have more available chlorine than a pool with CYA =50 and a FC of 5-6? FC is the free chlorine. Isn't that the amount of available (active) chlorine in the pool regadless of the CYA?

Perhpas you mean that the FC in a stabilized pool is less effective than in a non-stabilized pool. That I would agree with.... but I think it is incorrect to say that there is more "available". the FC is what is available (I would say there is more available when the FC is 5-6, but less effective.... maybe just semantics?

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Thanks for that Dave - I'll go to the shop nad ask for some liquid chlorine if the problem persists. There is so much new info for a newbie like me - so thanks to anyone for the replies!

I think we're going to end up going for a salt chlorinator for the lower chlorine levels and less handling of actual chlorine.

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Thanks for that Dave - I'll go to the shop nad ask for some liquid chlorine if the problem persists. There is so much new info for a newbie like me - so thanks to anyone for the replies!

I think we're going to end up going for a salt chlorinator for the lower chlorine levels and less handling of actual chlorine.

Check out http://poolcalculator.com/ to help you know how much bleach/liquid chlorine to use in the meantime

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Huh? Not sure I get this statment. How could a pool without CYA and a FC 2-3ppm have more available chlorine than a pool with CYA =50 and a FC of 5-6? FC is the free chlorine. Isn't that the amount of available (active) chlorine in the pool regadless of the CYA?

For example, a pool with no stabilizer and 2-3 ppm FC (not uncommon in indoor pools) will actually have MUCH more available chlorine in the water than a pool with 50 ppm stabilizer and 5-6 ppm FC (a good place to keep an outdoor pool, BTW)

Perhpas you mean that the FC in a stabilized pool is less effective than in a non-stabilized pool. That I would agree with.... but I think it is incorrect to say that there is more "available". the FC is what is available (I would say there is more available when the FC is 5-6, but less effective.... maybe just semantics?

I said available chlorine, not free chlorine. By this I mean chlorine that is actually available to sanitize and oxizide and not bound chemically to cyanuric acid as a cyanurate. Our tests for free chlorine not only test hypochlorous acid but also chlorinated isocyanurates (chlorine attached to cyanuric acid) which is NOT available to act as a sanitizer or oxidizer. It is in reserve as a 'chlorine buffer' of sorts. I stand by what I said and it is based on the actual chemistry of chlorine in the presence of CYA, which has been known since the 70's as seen by this paper.

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waterbear is, of course, right. Unfortunately, the terminology in this area isn't crystal clear and has changed over time which is why it sometimes gets confusing. At the time of the 1974 O'Brien paper linked to above and as defined in that paper, "Free Chlorine" meant the unbound chlorine not attached to CYA, so the sum of hypochlorous acid and hypochlorite ion. However, as noted in that paper, the actual chlorine tests also include the chlorine bound to CYA, the chlorinated isocyanurates, as part of what they measure and this is now called "Free Chlorine (FC)". So when people now report FC they are reporting what the paper calls the chlorine reserve or reservoir of chlorine -- it is the total amount of chlorine in the pool that CAN be made available or "active" (or is already active), as I like to call it, but is not necessarily active itself. I call hypochlorous acid the "active" chlorine because it is the chemical that actually disinfects, kills algae, and is the primary oxidizer (though for the latter one can consider hypochorite ion to also be an oxidizer). The chlorine bound to CYA is nearly inert in the sense that compared to hypochlorous acid, it has virtually no killing power for pathogens, does not inhibit algae growth, and is not an oxidizer. It is, as the 1974 paper implied, a chlorine buffer or storage facility.

You might say, well since the chlorine can get released from the CYA quickly (in seconds) it must still be active, but that isn't how chemistry works. Reaction rates are determined by the INSTANTANEOUS concentration of the relevant chemical species. The easiest way to think about it is to think of soldiers on the front lines with rifles and many more soldiers in the back in reserve. The rate of killing the enemy is determined by the number of "active" soldiers on the front lines with rifles. Sure, when they are killed, soldiers from the reserve can move to the front line and use their rifle to take their place, but the rate of killing the enemy has nothing to do with how many soldiers you have in reserve. The number of soldiers in reserve tells you how long you can continue to fight, but it does not tell you how quickly you can kill the enemy.

When there is CYA in the water, the vast majority of chlorine is bound to CYA and not active. At a pH near 7.5, with 3.5 ppm FC and 30 ppm CYA 97% of the chlorine is bound to CYA, 1.5% is hypochlorite ion, and 1.5% is hypochlorous acid. Fortunately, it takes a very low level of active chlorine to kill most pathogens though it takes a higher level to kill algae faster than it can grow. Roughly speaking, one can prevent algae from growing by maintaining a minimum FC that is 7.5% of the CYA level -- or in SWG pools a minimum FC that is around 5% of the CYA level.

Richard

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The chlorine atom alternates between three states: hypochlorous acid, hypochlorite ion and chlorinated isocyanurate. The statistical likelihood of finding it in any one of the three states at any particular time is determined by the concentrations and the pH.

As the chlorine atom randomly makes contact with an oxidizable molecule, the reaction is determined by what molecular state the chlorine atom is in. If the chlorine atom is bound to cyanurate or is in the hypochlorite form, then there will be little or no reaction. If the chlorine atom is in the hypochlorous acid form, then there will be a reaction.

The rate of reaction is determined by the frequency of contact and by the likelihood of the chlorine being in a reactive form. The frequency of contact is determined by concentration and temperature.

Cyanuric acid inhibits the reaction rate of the chlorine.

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