Returning to swimmable FC levels after SLAM

Welcome to TFP.

This an indoor pool or outdoor pool?

What is the CYA level of the pool?

What is the current FC level?

What is the FC level you intend to raise it to?

What FC levels are acceptable for swimming in your jurisdiction?

Terebin said:

I am a director at a public pool. We had a fecal incident that requires slamming. I took CPO about 3 years ago, but my current job doesn't have me doing much of the pool maintenance, so I'm quite rusty on several of the details.

I feel comfortable raising FC to necessary levels and keeping it there sufficiently long to kill any chance of crypto. But how do I reduce FC to swimmable levels? In a public pool, uptime is a more pressing concern than a home pool that can take a couple days off. A number of factors prevented me from starting to slam until the morning instead of overnight.

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Safe swim levels are anything following the FC/CYA chart up to and including SLAM level for the current CYA measurement. This is TFP guidance.

For a commercial/public pool there are sometimes laws governing what those levels are so you have to follow those.

My skin feels itchy and eyes burning already.

Terebin said:

Outdoor pool.
My guards checked chemicals last night and got under 20 for CYA.
Last known FC was 1.4
My training and CDC guidelines say CYA under 15, FC to 40 and maintain for 8.5 hours. No mandated maximum, I'd probably feel comfortable letting swimmers return under 20 FC and I'd be happier about under 15ppm

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That is enough outside of our guidelines to say that we are not the right place to advise you.

We follow the FC/CYA Levels and say that it si safe to swim up to SLAM FC for your CYA level.

For a residential pool we would say that with CYA 20 the sun will rapidly lower the FC level.

From Free Chlorine - Further Reading

How Can Free Chlorine be Lowered in Pool Water?​

In outdoor pools free chlorine will naturally decline 2-4 ppm a day due to the sun.

There are chlorine neutralizers most likely contain sodium thiosulfate.

• Add Sodium Thiosulfate, aka Chlorine Neutralizer for instant reduction.
• Add Ascorbic Acid, aka Vitamin C to remove chlorine.
• Add Hydrogen Peroxide, aka H2O2, to breakdown chlorine.
• Remove chlorine naturally with sunlight, aeration and agitation of the water.

Sodium Thiosulfate, add 2 oz per 10,000 gallons of pool water to lower chlorine level by 1.0 ppm.Probably the cheapest and most reliable method to lower chlorine levels, in pools or spas.[1]

Ascorbic Acid – yes, granular Vit C! Add 32 oz per 10,000 gallons of pool water to lower chlorine level by about 9 ppm.

Hydrogen Peroxide – use 1 gallon of Aqua Silk Oxidizer per 10,000 gallons of pool water to lower chlorine level by 55 ppm. For spas, use 3% drug store peroxide, 2 ounces per 100 gallons of spa water, to lower chlorine (or bromine) level by about 10 ppm.

For Sodium Thiosulfate, add about a cup (8 oz) of the dry white crystals per 5,000 gallons of pool water, or 1 oz per 100 gallons – to lower chlorine by 10 ppm. Check your pool water pH and adjust to within label instructions, and add directly to the pool or spa.


Sodium thiosulfate dosage In pools and spas, you want to be careful not to overshoot the mark, or overdose the pool with Sodium Thio, or you may have difficulty adding new chlorine, temporarily at least. Sodium Thiosulfate will dissipate from most pools within a few days. Follow the label instructions carefully when using a chlorine remover, for recommended water balance and dosages.

Another good resource is this sodium thiosulfate calculator on Koiphen.com – enter your pool size in gallons (or liters) and the current chlorine level in ppm, and it tells you how many ounces or grams of Sodium Thiosulfate to add to reduce chlorine to zero.