hydrogen peroxide caused green pool water

Conflicting Chemicals: Hydrogen Peroxide in the #RioOlympics diving pool

This article is a follow-up to our last article about the Rio Olympic diving pool turning green overnight. Regardless of what happened in Rio, we at Orenda looked at the situation as a great opportunity to share knowledge, and apply it to a current event. What we have discussed are known, established practices in the aquatics industry. Our objective has been to simplify the conversation for everyone reading our website. It’s a lot to absorb, and we encourage the discussion.


For the mystery of the green Olympic diving pool in Rio, officials announced that a maintenance person mistakenly dumped over 160 liters of hydrogen peroxide in the pool, thinking it was the right thing to do. It remains unclear if they thought it was a different chemical, or if they genuinely believed hydrogen peroxide belonged in a chlorine pool. It is also unclear if hydrogen peroxide in the pool is the only reason for it turning green, given the myriad of other reasons given already; reasons that also could have contributed to the problem.

Let’s put this new information in context, so we can try to make sense of what happened in Rio. If this latest information given to the public is true, one thing is for certain: those pools were not safe for people to be using.

What is Hydrogen Peroxide?

hydrogen peroxide

Hydrogen Peroxide molecule (H2O2)

Hydrogen peroxide is a chemical compound—H2O2—which is an oxidizer and weak sanitizer. It is typically diluted so it is safe for human contact, because pure hydrogen peroxide is so harsh, it would burn us. While it is most commonly used around the household for first aid, such as cleaning cuts and disinfecting wounds, such household products are heavily diluted (usually about 3-6% in solution). When you put hydrogen peroxide in the pool, however, the most common concentration is 35%.

For use in pools, hydrogen peroxide is an alternative sanitizer and oxidizer to chlorine. It also effectively wipes out free chlorine. They are conflicting chemicals.

Chemical incompatibility

shutterstock_236819650When chemicals conflict, the reaction and results are what we need to pay attention to. In the case of putting hydrogen peroxide in a chlorine pool, the reaction is

Cl2 + H2O2 => O2 + 2HCl

Translation: hydrogen peroxide neutralizes chlorine. Without chlorine (or active hydrogen peroxide acting as a sanitizer), bacteria and other contaminants can pollute a pool in short order. The initial response from the Rio event staff was ‘algae’, which is starting to sound more plausible now. It’s really hard for algae to grow so fast with proper levels of chlorine.

This is not the only example of conflicting chemicals in the pool business. Here are some others to look out for:

Bromine and chlorine. Both of them are effective sanitizers for pools, but when mixed together, the pool can turn brown under certain circumstances. You may have heard that it is possible to convert a chlorine pool into a bromine pool, but not vice versa. This is because adding chlorine to a bromine pool converts the used-up bromine (bromide) back into free bromine. When you add bromine to a chlorine pool, on the other hand, the bromine does not go away but builds up over time, creating a chlorine demand. Therefore, when you shock the pool with chlorine, the residual bromine will convert the chlorine into, you guessed it, bromine. So when professionals test the water, it will indicate that there’s no chlorine, when there’s actually bromine.

Acid directly in chlorine. Pouring acid into liquid chlorine can cause a dangerous reaction. That said, when properly introduced, chlorine can be stabilized with cyanuric acid. Just don’t mix them directly in high concentrations.

Trichlor and cal-hypo or bleach. This can cause a dangerous reaction. Yes, they are all forms of chlorine, but they are incompatible to use together.

Acid and sodium bicarbonate. When adjusting alkalinity with sodium bicarb, do so before adjusting the pH with acid. Acid and sodium bicarb accomplish opposite goals. While sodium bicarb raises alkalinity and pH, acid drives them down.  If you pour them both in the same location, acid can destroy alkalinity and defeat your chemical goal.

Undiluted muriatic acid. Before adding muriatic acid to the water, dilute it. Dilution raises the pH of the acid, which reduces its alkalinity-lowering impact. Dilute muriatic acid to preserve your alkalinity, which is a buffer for pH.

orenda phosphate control, orenda phosphate, phosphate removerStain and metal sequestering agents and phosphate remover. Most stain and metal sequests contain phosphonic acid (phosphate-base) for stain protection, even if the label doesn’t say so. It is counterproductive to use these products with phosphate removers. The phosphate remover diminishes the efficacy of the stain/metal sequestering product.

It is important to know that these byproduct/side reactions are not regulated by the EPA or NSF. There is currently no requirement to label products in a way that discloses such interactive risks or byproducts.

Lessons Learned

It appears the staff in Rio allowed a mistake to get out of control. Their chemicals conflicted, and as a result, the pool turned green. The lessons here are pretty obvious.

  • Educate yourself before you pour something in your water, and learn how it reacts with everything else before you add it.
  • If you aren’t sure, ask. The pool community is readily available for advice and there are plenty of forums online to learn more.
  • If things go really wrong (like a suddenly green pool), close it down until you figure out what’s going on. Letting people swim in a questionable body of water is irresponsible, and subjects swimmers to unknown bacteria and contaminants.
  • Continue to learn. The moment you think you ‘know it all’ is the moment your guard goes down. With multiple operators, it’s virtually impossible to control all variables in a pool. So pay attention and speak up if there’s an issue.
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