What is the right chlorine dosage to prevent algae in the pool?

This has been a frequently asked question on our facebook page.

The ANSI/APSP (American Standard) states the chlorine level should be between 2ppm-4ppm. Most pool owners who have test kits maintain the chlorine level within this range. But most of them still have problems with the pool, especially the growth of algae. Why?

That's because the range is recommended for every pool owner. Regardless of the pool problem (sanitation or algae prevention), the range remains. That's why there are so many answers on the internet to this question.

We've helped hundreds of pool owners with their chlorine dosage, so I'll do my best in answering this question through my experience.

But to better answer this one question, I'd like to ask a better and multiple questions:

1. When should I maintain 2ppm and when should I maintain 4ppm?

2. Should I always maintain the chlorine level at 4ppm?

3. And is there any way I can be any higher than 4ppm? If yes, when?

4. What if my pool is at 3ppm of chlorine? Should I leave it as is?

Since there are many factors involved in the right dosage of chlorine, I'll focus on only two: indoor/outdoor pool, and the weather. (It is noted that we're only talking about pools with only a filter. No advanced oxidation, no UV disinfection systems are installed. If you have any of these systems, these answers may or may not be of benefit to you.)

First Factor: Is the pool indoor or outdoor?

Because indoor pools usually don't have any direct sunlight on the pool, chlorine will not be destroyed. That's the first benefit.

Second benefit is that you'll also have very little to no algae growth, since algae needs sunlight for photosynthesis.

With algae out of the way, we can now focus on disinfection of the pool (bacteria control). For us, 1ppm has worked quite well. We've tested the pool water in the lab and no coliform, or E Coli was detected. No 2ppm of dosage required. Third benefit is definitely less chlorine dose required.

So if you go for 2ppm-4ppm range, your pool will be over-chlorinated. Is it a good thing? Definitely not. Over-dosage of chlorine will lead to higher toxic disinfection-byproduct (also called THMs), which is dangerous to humans, especially children below 2 years of age. THMs also lead to corrosion, and make the air quality around the pool dangerous (chlorine odor).

So if your pool is indoor, stick to 0.5-1ppm.

Now since the pool is indoor, should you use 70% chlorine, 90% chlorine or liquid chlorine?

My answer: 70% High Dissolving Chlorine. Use liquid chlorine if the total hardness in the water is high already. Stay away from 90% Chlorine since it contains cyanuric acid and you don't need the protection of the sunlight.

Additional information: Ensure the pH of the pool is 7.2. It makes chlorine work like magic. Keep the indoor pool area well ventilated. If possible, install a UV Disinfection System and lower the chlorine usage even more. Over time, the pool will have bacteria film in many areas. To remove it, shock it using chlorine dioxide (not chlorine) once every 3 months. If the pool is commercial, shock every month.

Second Factor: What's the weather in your place?

I've made a table for our clients to follow.

As you can see, we don't recommend 2ppm. And the dosages fluctuate based on the weather. 

During sunny days, the sunlight will destroy chlorine. So we recommend to use a high enough dosage so that you'll have enough chlorine to burn throughout the day. 

During rainy, and heavy rainy days, there will be a lot of nutrients from surrounding areas that will enter the pool. If you have strong winds, expect the same thing to happen, whether it's raining or not. If that's the case, you should add more chlorine to stop the algae proliferation. Better action to take would be to use a good nutrient binder (like IwNite Hybrid Algae Preventer), or a good clarifier (like Clear-Up Pool Clarifier). They bind high amount of nutrients in the pool, which highly reduces the chance of uncontrolled algae growth in the pool. 

Normal Day means not much sunlight and rains throughout the day. There are also no strong winds in the area. In this weather, you can simply go with any normal dosage. We still stick to the high dosage.

Additional Notes: 90% High Dissolving Chlorine is the best for outdoor pools. Unless you know how to use Cyanuric acid properly, stay away from 70% and liquid chlorine.

Disclaimer: I've been very conservative with the dosages. If you have a very good filtration system and also use Cyanuric acid, you can lower the each dosage by 1ppm. This table was made with an assumption that filtration speed is high, and is not suitable for small particle filtration. 

Last Question: When is the best time to add chlorine?

Our recommendation is during sunset. You give chlorine the best chance to disinfect the pool. If you dose in the morning, or in the afternoon, the sunlight will deplete it very quickly.

Thank you for your attention!

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