Are living algae eating the chlorine?

[rock]

This is my first post so please be gentle with me...

My pool is green so I googled and found this wonderful thread, which tells me I have to hit hard with free chlorine (FC).

After reading the first post of that thread a half-dozen times, I realize that the time the free chlorine lasts is (somehow) an indicator of how much algae is left alive.

If the FC remained stable (within 0.5 of the same reading) overnight, and the CC level is 0.5 or lower then all of the algae is gone.

But I'm confused about how living algae do that. What is it that makes the free chlorine drop when there is living algae, but, when the algae are dead, the free chlorine doesn't drop?

Q: Are the living algae 'eating' the chlorine?
(or is it something else that makes the chlorine drop when the algae are still living?)

 

[zea3]

Hi, welcome to TFP! Actually the chlorine is killing the algae and gets depleted while it is oxidizing the organic material, much like gas in your car is depleted when it is used to fuel the engine. While there is a lot of algae in the water the chlorine will be depleted fairly quickly and must be replenished often in order to kill algae faster than it grows. As the algae dies off, there is less demand on the chlorine and it will last for a longer period of time.

There are other factors that influence how long the chlorine lasts, and if you keep reading the different threads on the site you will see all of them! For now, lets focus on getting your pool under control. The pool school articles shocking your pool and defeating algae are a good place to start.

A good test kit is also highly recommended and will be invaluable as you learn to take control of your pool. The TF 100 is a high quality, FAS-DPD based kit that will allow you to test chlorine levels greater than 5ppm. As you will find during your reading, knowing your shock level for the shock process is essential and this kit will allow you to test chlorine at levels 50 ppm or higher with accuracy.

Please read through pool school (upper right hand corner) and ask anything you need to know. We're here to help!

 

[rock]

the chlorine is killing the algae and gets depleted while it is oxidizing the organic material

Ah. I see. Thanks for the advice.

I had 'shocked' the 38,500 gallon gunnite pool with 5 pounds of dichlor and the very next day the chlorine level was zero! It was I who was shocked! That's what got me googling. I didn't know where all that chlorine went in just one day!

There are other factors that influence how long the chlorine lasts, and if you keep reading the different threads on the site you will see all of them!

My pH was 7.2 (yes, I know, a bit low). I'll be reading the quoted references! And, I'll do the detailed pool analysis at Leslie's pool tomorrow and report what they find.

 

[rock]

If it matters, I did a local survey of the least expensive way to get chlorine into my pool over here:
- How to calculate cost per effective chlorine liquid v powder

At Leslies, they ran an analysis which I reproduce here:
5 ppm Free available chlorine (HIGH, should be 1-4ppm)
5 ppm Total available chlorine (HIGH, should be 0.2 difference)
140 ppm Calcium hardness (LOW, should be 200 to 400 ppm) <== add 28 pounds 4 ounces of Calcium Chloride
55 ppm Cyanuric acid (OK, should be 30 to 99 ppm)
90 ppm Total Alkalinity (OK, should be 80 to 120 ppm)
7.2 pH (OK, should be 7.2 to 7.8)
1 Base Demand (no test results) <== add 1 pound 4 ounces of Soda Ash
0 ppm Copper (OK, should be 0 ppm)
0 ppm Iron (OK, should be 0 ppm)
500 ppm Total dissolved solids (OK, should be less than 2,500)
100 ppb Phosphates (OK, should be below 100 ppb)

Right now, the pool is a cloudy white-blue color. I can now see the first step, but not the second or third, and certainly nothing on the bottom. So I'll head on over to the recommended articles for details on how to fix this.

 

[JasonLion]

As Leslie's recommended, you do need to raise the CH level with Calcium Chloride (or calcium increaser, or any similar stuff). However, their suggestion to add soda ash isn't worth following (it won't hurt, but it won't help, the PH is fine where it is).

The cloudy water is caused by dead algae. The filter should remove the cloudiness, though it may take it a couple of days. If everything is working correctly you should see a visible improvement in the water each day. If you don't see an improvement you need to try something else.

By the by shocking is a process, not something you dump in the pool and are then done with. You need to continue adding chlorine until you can maintain an FC level. Also, dichlor isn't usually the best thing to use when shocking. Dichlor will rapidly raise your CYA level. As it turns out your CYA level appears to be alright, but that won't continue to be true if you keep adding dichlor.

 

[rock]

their suggestion to add soda ash isn't worth following (it won't hurt, but it won't help, the PH is fine where it is).

I 'was' confused by their suggestion because the pH was within range so I didn't understand 'why' they wanted me to raise it with the soda ash.

Looking at the report from Leslies, it looks like they couldn't run the "Base" test because they needed the pH to be in the range of 7.4 to 7.6. It seems strange to add 1 pound 4 ounces of soda ash 'just' so they could run their test. What is the 'Base' test anyway?

The cloudy water is caused by dead algae.

That's interesting as it's quite cloudy. And blueish white. Very pretty. Like blue milk.

You need to continue adding chlorine until you can maintain a FC level.

Thanks for the reminder. I dumped 3 gallons of 12% (nominal) chlorine liquid in last night and this morning there was about 5 ppm chlorine based on the basic test kit (although there are also 3" trichlor tabs in the three floaters). I'll check it again tomorrow morning to see what's left without me adding any more chlorine than what the floaters add.