What happens if I stop using chlorine

[AUSpool]

Moderator note: Moved from this thread will Liquid Chlorine Raise pH?

[edit]The title probably should be 'pH, alkalinity and chlorine, what happens if I stop using chlorine[edit]


Just a thought but what would happen if you stopped chlorinating and add a bio filter to prevent an algae bloom?


I recall a post in an unrelated forum that talked about a natural pool where the water parameters where given as;


Ph is always 7.5.
Total alkalinity, 17mg/l as CaCo3.
Turbidity/clarity, 0.5 NTU.
Total dissolved solids, 58ppm.
No ammonia, no nitrite, no nitrate.

 

[tim5055]

Chlorine is not only there to prevent prevent algae growth, but it is also there to kill pathogens, especially preventing person-to-person transmission of disease. The pool would be unsafe, especially if children who have "issues" in the pool at times.

 

[JoyfulNoise]

See these posts for discussions of "natural pools" -

Natural Swimming Pools (NSPs)

Natural pools. I think they deserve another look.

 

[duraleigh]

Any discussion of natural pools always ends the same way. The OP goes away.

I think that's because they simply don't work out. It would be wonderful if they did, but my logic tells me they simply are not practical in the least bit.

 

[AUSpool]

Sorry everyone I didn't mean to start a thread about natural pools. Some of the natural pools I've seen look good from the pictures but I've also seen a lot of ponds and dams that I wouldn't let my kid swim in. I wouldn't have one or convert my pool to one. Can you imagine the drama trying to convert one back to chlorine.

I was mulling over the chlorine and pH question and kept thinking about what whould happen if we stopped chlorinating. What would happen to the pH in the absence of a massive algae bloom. It's a bit of an ask, or very subjective as natural water bodies have a broad pH range (~6-9) but I was thinking that the pH would drop a little, particularly if it was up arround 8 to start with. And it would likely remain quite stable, at least for a while. So by default does that imply that chlorine had applied upward pressure on pH?

 

[tim5055]

Well, the use of chlorine is basically a pH neutral operation. When you add chlorine to the water you will have an increase in pH which is offset by a pH decrease when the chlorine acts on organic matter. Absent the addition of chlorine I would expect your pool to act as it normally does, either remaining pH stable or as mine does, a slow pH ride due to the areation of my sheer decent waterfall. Chlorine in and of itself is not the only thing in your pool that effects the rise/fall of pH.

 

[MarianParoo]

Yup, Auspool, your musings got you in the deepend!

 

[AUSpool]

Very likely.

The more I think about it it was just too much of an assumption to assume the pH would go down. The options should be up, down or no change and given chlorine has no net affect on pH I assume it would remain the same with no change.

 

[JoyfulNoise]

It difficult to make the comparison between pool waters and "natural " bodies of water because they are not really the same. What drives the pH rise or fall depends on what is going into the water. Natural water bodies have inflows, outflows and evaporation. Pools generally just have inflow and evaporation.

If nothing was added to pool water (acid, alkalinity or chlorine), then at the typical TA found in most pools, the equilibrium pH is about 8.4 or so. So, adding nothing and assuming no algae blooms, the pH of a pool would simply rise to the equilibrium pH of the water as CO2 outgassed from it since that would be the main pH driver.


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006

 

[Stackbarrel]

It will get green I guarantee it if you stop with the chlorine.

 

[AUSpool]

It difficult to make the comparison between pool waters and "natural " bodies of water because they are not really the same. What drives the pH rise or fall depends on what is going into the water. Natural water bodies have inflows, outflows and evaporation. Pools generally just have inflow and evaporation.

If nothing was added to pool water (acid, alkalinity or chlorine), then at the typical TA found in most pools, the equilibrium pH is about 8.4 or so. So, adding nothing and assuming no algae blooms, the pH of a pool would simply rise to the equilibrium pH of the water as CO2 outgassed from it since that would be the main pH driver.


Sent from my iPhone using Tapatalk,16k gal SWG pool (All Pentair), QuadDE100 Filter, Taylor K-2006

I run a SWCG and with five years of history the pH has always slowly increased and I guess this is quite typical of a SWCG pool but, and another big assumption on my part, with all the pH up products on the market lots of pools must have a pH that slowly decreases. This simple lodgic would seem to suggest that chlorine is present in both it dosen't have a significant effect on pH. Whilst it's important to know the trend, knowing the mechchanics that drives it can be used to mitigate it, like lowering the alkalinity.

I must admit that until recently I had always thought that chlorine drives the pH up and with a pH of 12-13 from the msds's it's easy to see where the assumption comes from. It's a bit embarrassing and I should have known better, its was told to me once and it stuck, and I've heard it repeated in the last few weeks.

I get the CO2 degassing an it's effect on the CO2, bicarbonate, carbonate equilibrium. And where, or why a pH of ~8.4 represents the point where no more CO2 is left to degass from the system. I assume everyone has seen the equilibrium distribution graph.



I can't help thinking that there is more to it and have been going through my old text books pondering the issue. Does pH drive the alkalinity equilibrium or does the alkalinity equilibrium drive the pH. And what of the atmospheric partial pressure of CO2. My old environmental chem text book mentions atmospheric CO2 but does not imply any effect from the partial preasure. It does however suggest that there is an equilibrium between the CO2(g) in the air and the aqueous acid H2CO3(aq). CO2(g) + H2O(aq) <--> H2CO3(aq) where most of the dissolved CO2 remains as CO2(aq) rather than H2CO3(aq). Which implies to me that we could never get to a pH of 8.4 driven solely be CO2 degassing as CO2 would remain in equilibrium and be re-dissolved into the water at a rate equal to the atmospheric partial presure of CO2(g).

 

[JoyfulNoise]

That Bjerrum Plot is not correct for pool water. It is not corrected for the effects of salinity on the equilibrium constants of the water-carbonate system. If you're interested in the detailed chemistry of CO2 in water, see the discussions in this thread -

Some general Bicarbonate/CO2 related questions

You can also see this post which has a table of the equilibrium pH values for a given amount of carbonate alkalinity -

Why does pH rise with salt systems?

The fact is, CO2 outgassing drives pH rise in pools. Outside of a freshly plastered pool, the process of driving off CO2 and then having bicarbonate convert to CO2 by consumption of a proton (in order to attempt to restore the equilibrium) is the biggest driver of pH change.

 

[AUSpool]

It will get green I guarantee it if you stop with the chlorine.

Yes indeed, an oversized micro biology multi culture experiment. A good example of the microbial growth curve in action. But it was really hypothetical and I added the natural pool in an attempt to show where the physical parameters might go and these in theory should prevent an algal bloom.

 

[AUSpool]

Thanks Matt, the old text books don't really answer any specific questions. :study:

 

[Leebo]

I run a SWCG and with five years of history the pH has always slowly increased and I guess this is quite typical of a SWCG pool but, and another big assumption on my part, with all the pH up products on the market lots of pools must have a pH that slowly decreases.

These pH Up products find their way into pool care when a user chlorinates with Trichlor or Dichlor, both which are acidic and drive the pH down. This is such a very common form of chlorinating pools and many people never know of this side effect.

 

[AUSpool]

Thanks Leebo, the obvious last question is which way does pH tend to go in liquid chlorinated pool - just nice to know.

And again, thanks again Matt, and although I quoted Matt I wasn't questioning Matt's response but using his answer and info to direct my thoughts and put my question out to everyone.

And I finally know what the alkalinity concentration graph is called, the Bjerrum Plot, that's so cool, my old $100+ text books didn't know that.

Steve.

 

[AUSpool]

See these posts for discussions of "natural pools" -

Natural Swimming Pools (NSPs)

Natural pools. I think they deserve another look.

I'm sure these are going to be a good read too, when I get the time. Thank you.

I didn't intend this to be a thread about natural pools but a simple thought that I've always had is that they are often an oversized, low bio load, fish tank but often under size the flow rate. And what about a pump and filter to vacuum the pool to keep it clean. And if they do provide a pump and filter to vacuum then they are just one appliance away from having a properly sanitised pool.

 

[Patrick_B]

No problem. We are all here to learn and talk pools.

 

[MarianParoo]

Any discussion of natural pools always ends the same way. The OP goes away...

Hahaha! I don't think AUSpool is going anywhere. He is a believer!