About Combined Chlorine

[SupaDave]

I had some traces of green algae on my pool walls last week. I corrected my ph, brushed and vacuumed, and shocked the pool with 12% liquid according to the SLAM tables with a resulting FC of 32 or so the next day. ( CYA of 60 ) I also backwashed and re-charged my DE filter. The following day my FC stayed at approx. 30-32 with a cc of 0.
over the next few days the FC has gradually diminished to 18 today, but I now have a cc of 1.8. My question is.... A FC of 18 is more than double the recommended FC for my cya level, so how can I have cc? This is an outdoor a/g pool completely in the florida sun all day and I do get leaves. But shouldn't a FC level of 18 be adequate for sanitation? Am I to expect episodes of CC for which I have to SLAM, even if I keep my FC levels at recommended levels? Can someone please explain the relationship between FC, CC and the SLAM process a little better for me?
p
h 7.6
TA 150
cya 60

Thanx,

Dave.

 

[kimkats]

Dave, what happened and is happening is the chlorine you put in your pool was killing the algae you could see. That is what the CC is telling you. There is something in your pool that needs to be killed.

SLAM is a process with the M (maintain) is the most important part. You keep up the SLAM until:

-CC is 0.5 or lower;
-You pass an OCLT (ie overnight FC loss test shows a loss of 1.0 ppm or less);
- And the water is clear and there is NO visible algae any where in the pool

During the SLAM you need to make sure to brush the pool at least once a day to get all of the stuff out into the water for the FC to kill.

Does this help? Let me know if you have any other questions.

Kim

 

[GaryT58]

Hi Dave,

Did you perform the Overnight FC Loss Test (OCLT - link here)? That is a critical part of a SLAM process. You must maintain the SLAM level for the FC until

You are done when:
​

• CC is 0.5 or lower;​
• You pass an OCLT (ie overnight FC loss test shows a loss of 1.0 ppm or less);​
• And the water is clear.​

When all three are true, you are done SLAMing and can allow the FC to drift down to normal levels.​

If nothing organic is dropping into the pool at night (i.e. no leaves) then the FC drop should be pretty low. This is done at night since sunlight breaks down FC. Without performing this test, you may miss that there is still active algae in the pool. It can look clear, but still be alive and consuming the chlorine. And once it gets low enough again (below slam levels) it can start taking over more, and why you see the high CC.

Also, follow the SLAM steps completely. A lot of smart people have worked on these over the years (not me, but thank them for coming up with the details). This includes continuing to brush and/or vacuum daily while maintaining the FC at slam levels. This is because you need to clean out all the algae to keep it from coming back. If you brush just once, live algae could still be there or settle elsewhere.

I have been lucky so far to not need to do this, but from reading many threads on this, the key things are to follow the procedure and be patient as it can take some time to fully complete so you do not end up fighting it again in a few days.

 

[SupaDave]

Thank you all for the quick responses. My water is indeed clear, but no, I did not do an OCLT test. I assumed after my initial SLAM and a resulting CC of 0, everything was killed. I guess I assumed that starting out with a CC of 0, and a high FC of 18 after SLAMming, that nothing further would grow as I am advised to keep my FC at only 7-9 with my CYA of 60, which is half of my post-slam FC reading. As I think I understand CC, it is the chlorine that has "absorbed" the organics, etc. If I get a CC of 0 with an accompanying FC of 18, doesent that mean that all the bad stuff has been "absorbed" (cc of o ) and I have more than enough FC ( FC of 18 ) to keep more stuff from growing?

 

[GaryT58]

All tests have a +/- level of accuracy. So for many we consider 0.5 or less as = 0. So you can see CC level at or near 0 and still have algae there ready to grow. It can be enough that it is not registering, but then starts to grow again once the FC level is dropping again. The OCLT will show if something is there by a large FC loss overnight. Since the sunlight is not there to burn it up, if there is a large FC loss, it is due to something else (i.e. organics). If you have leaves in pool overnight, then that could be a cause, but if not, it could indicate your algae is still at work. So that is why SLAM process is pretty specific on all 3 events before you are done. AND why its important to sweep/vacuum daily and check as often as possible to keep FC at SLAM levels.

- - - Updated - - -

Also if your cyn is at 60, then you need to maintain the SLAM level of at least 24. Yes, once the SLAM is complete (all 3 steps pass), then an FC of 18 is more than enough and could keep between 7-9. But until its all really killed, it can still grow at pretty high levels and come back.

 

[duraleigh]

I assumed after my initial SLAM and a resulting CC of 0, everything was killed.

You did not SLAM the pool and you are putting too much emphasis on CC.

1. Repeating what has been posted, a SLAM is not a one time dose of chlorine. Please re-read the SLAM article....you cannot make shortcuts.

2. CC is only an indicator and does not prove or disprove the presence of algae. You can have 0 CC's and still have algae

3. There are specific criteria that indicate you have completed the SLAM. Being repetitive to the posters above they are:

A. Your pool water is sparkling and there is no visible algae (dead or alive)
B. Your CC's are .5ppm or less
C. You can hold your FC overnight without losing more than 1ppm.

So, you apparently passed A and B but not C.