Dechlorination via Aeration

[Aqua Lab Rat]

Watch out chem people, I'm trying to learn again by asking a question. I know that aerating chlorinated water will slowly dechlorinate it. But what I'm trying to understand is how it actually works. You don't have a bunch of chlorine gas bubbling around in the water, so how does aeration get rid of the chlorine residual? Anyone? Bueller?

 

[JoyfulNoise]

... I know that aerating chlorinated water will slowly dechlorinate it...

How do you know this? Have you tested it out?

Sitting a sample of water on the counter and covering it will dechlorinate a water sample as well. Takes a couple of days. Can you show that aerating it achieves the same results faster?

I'm just asking the question....

 

[Aqua Lab Rat]

I haven't done any experiments, though I have observed that if I uncover a spa and run the jets for a long period, the residual decreases more rapidly than it would otherwise. My impression from trawling around aquarium and waste-water related articles and forums is that actively aerating with some type of bubbler will accelerate the process, though it will still be slow. If that is incorrect or a myth (common in aqua industries) I'd like to know. If it isn't, or it is due to some other process like mixing contaminants from the air into the water, I'd like to know that. My hope is that someone on here is more knowledgeable on the subject and can enlighten me. I'd be surprised if it had never come up in this forum.

 

[JoyfulNoise]

So the natural breakdown of chlorine is for the hypochlorite anion (OCl-) to decompose into oxygen gas and chloride ion -

2 OCl- ----> O2(gas) + 2 Cl-

hypochlorite ----> oxygen gas + chloride (salt)

This happens spontaneously in a bottle of bleach and is one reason why breather caps are important otherwise the bottle would swell. When you aerate water, two things happen -

1. You decrease the amount of dissolved gasses in solution (nitrogen, oxygen & CO2); AND
2. The removal of CO2 from water with a measurable amount of carbonate alkalinity causes the pH to rise through the conversion of bicarbonate to CO2

When you increase the pH, the proportion of hypochlorous acid to hypochlorite shifts in favor of hypochlorite (OCl-). So now go back to the reaction above - when aerating water you are decreasing the amount of dissolved O2 in water and increasing the amount of hypochlorite anion.

Based on Le Chatelier's principle, what will happen?

 

[Aqua Lab Rat]

I'm not a chemist, but should I guess that maybe more OCL- would decay to partially counteract the loss of O2? If so, that might support the theory of aeration helping dechlorination. At any rate, it's a better explanation than I've seen before, so even if I'm wrong, thanks.

 

[JoyfulNoise]

Le Chatelier's Principle states, in concise terms, that -

When a system in equilibrium is disturbed the system will adjust itself in such a way that the effect of the change will be nullified.

So, if you increase the concentration of OCl- or decrease the concentration of dissolved O2, the effect will be to push the reaction equilibrium to the right. That will eventually reduce the overall level of FC measured.

But aeration is a slow way to dechlorinate water.

 

[triptyx]

Is your uncovered spa exposed to the sun? That would also cause dechlorination due to the sun's UV rays.

 

[Aqua Lab Rat]

The spas that I observed this in are actually in an indoor lab. But I've seen aquarium forums where they talk about running a bubbler to help speed up dechlorination. Sometimes other people will tell them to just go to a pet shop and buy a chemical to do it.

I mainly wanted to increase my understanding of how it works, and Matt was able to do that. I like to have a better understanding of how water chemistry works, compared to what I can find on pool/spa dealer sites. So it is nice to be able to pick come on here and pick some brains.

 

[AUSpool]

I've been following with interest. The aquaculture setup at the university I went to had an elevated 2000L water tank with an industrial sized aerator. It was always full of tap water via an auto topup and was used to feed the aquaculture system. We were told it was used to de-chlorinate the water. Since it was designed by the Lecturers / professors and installed by the lab techs I had just assumed it worked but never knew how.

 

[JoyfulNoise]

For reefs and tanks, I'd think a small scale RO setup with a GAC prefilter would be a much better option. I'd rather start off with pure water and add back what is needed (salt, carbonates, etc).

For a spa, the fastest way to dechlorinate it is let a bunch of sweaty gross bathers sit in it ??

 

[Aqua Lab Rat]

For a spa, the fastest way to dechlorinate it is let a bunch of sweaty gross bathers sit in it ����

I conduct bather load tests occasionally, using co-workers. There's one I call "Filthy Paul" because I can count on him to really load down the sanitizer. I'm not sure I want to know why.

 

[JoyfulNoise]

Anyone nicknamed "Filthy Paul" should never be allowed in a hot tub....or, he should have to provide you with a container of Ahhh-some to purge the plumbing with ??