2016 Rio Olympics: How to turn a green pool into a blue pool

With a pool of green water 10 m below, Meaghan Benfeito and Rosie Filion jumped in to win a bronze for Canada in synchronized diving. It was an odd sight for all the competitors, but Rio officials promised the water was perfectly safe—and that the discolouration would be fixed in time for the next day’s competition. Despite their best efforts, not only is the diving pool still green at the Maria Lenk Aquatics Centre, but so too is another. On Thursday, water polo players in the adjacent pool complained not only of poor visibility underwater, but some even struggled with burning eyes due to excessive chlorine.

Which begs the question: is it really all that hard to get the pool blue?

To get some answers, we turned to Sean Healy, the Vancouver Board of Parks and Recreation supervisor of aquatic services.

Q: I take it you’ve noticed the green pool in Rio.

A: As the park board swim guy, you can appreciate I have a competitive swimming background. I’ve been glued to the Olympics, so I did notice the pool colour.

I know you’re not in Rio, but say one of your pools in Vancouver turns green. How do you fix it?

A: When a pool turns green, there’s something out of balance in the pool water. You could check for algae, but if you have good chlorination, algae is not going to be the issue. If there is copper in the water, you can use what’s called a metal sequestering agent that binds the metal and it gets filtered out. So your pool shifts from being green to blue.

RELATED: Why did the Rio Olympics pool turn green?

Rio officials were saying the water became alkaline, which means what exactly?

A: They’re speaking about the pH balance. The pH scale runs from 0 to 14, with the lower half being dedicated to acids and the top half for bases. When the pool pH gets high—or more alkaline—chlorine becomes less effective. With a pH of about 8, your chlorine is about 20 per cent effective. If you were at a pH of about 7, your chlorine is about 80 per cent effect.

You typically want it roughly between 7.2 and 7.8 because you want the chlorine to be effective, but you also don’t want the pool to be too acidic and attack the structural components. So the answer to correct the pH problem is make the pool more acidic and the most common way is to add acid, typically muriatic or hydrochloric acid.

The Rio officials also said that the filtration system ran out of chemicals, which caused the discolouration. What chemicals could they have run out of?

A: I don’t know which chemicals they’re using to maintain their pool balance. When we commission our Vancouver pools, both indoor and outdoor, there are a core group of chemicals we use. Calcium chloride is one. Sodium bicarbonate—plain old baking soda—is another. We may use chlorine in a variety of different forms. You can buy it as bleach, but some pools use gas chlorine or those little pucks that you see in backyard hot tubs. There are a lot of different chemicals that can be used.

The pool in Rio for the divers turned green on Tuesday, and an Olympic spokesperson said they’d have it blue the following day. But not only was it still green on Wednesday, the water polo pool next to it also turned green. Now it’s Thursday and neither is blue. Is it so hard to get a pool blue again?

A: The pool has a circulation system. Think of the pump like a heart. Think of the pool filter like your liver or kidneys. The filters take out the particulate and the pump keeps pumping around the pool. It sometimes takes a pool about six to eight hours to do what we call a turnover—for all the water to pass through the system. If it’s turned green, it may take awhile for the pool to circulate and the problem to be fixed because it requires good circulation.

Wouldn’t shocking the Rio pool with chlorine be a quick fix?

A: I can’t speak to Rio, but in our situation if we had algae in our pool, shocking would kill algae. If it takes six to eight hours for a pool to turn over once, you might get four or five turnovers per day. So it’ll take awhile to take effect. But if algae were the problem, then a superchlorination is the answer.

So long as people stay out of the pool for the duration.

A: Right. If you think of superchlorination as a medicinal approach, the stronger the dosage, the faster the result would be. But it would also require you neutralize the extra chlorine after you shock the pool.

Here’s a quick fix. Couldn’t Rio just empty the pool and refill it?

A: No. The water has to be heated, treated and chemically balanced. We have some fairly large pools here in Vancouver—50-m competitive pools are in the range of 800,000 to a million gallons. They take anywhere from four to seven days to fill, heat, treat and get them clear. It wouldn’t be possible to drain and have it refilled overnight. It’s simply too big.

Did you ever swim in a green pool in your career?

A: I’ve been in many lakes that were green.

But would it freak you out swimming in a green pool?

A: If a lake is green, I’m comfortable. I always think of pools as being blue.

This interview has been edited and condensed for clarity.