New Salt System, algae is recurring regularly

[SunnyOptimism]

You need to check your water chemistry more frequently in the beginning of the process. Once you add the bleach, check it again in 1 hour to see if you're at the correct level. Even in clear water with a possible (but not visible) algae bloom, your FC will drop very rapidly in the beginning as the chlorine oxidizes biologic and organic matter. I'd be willing to bet a bottle of bleach that your FC will not be 20ppm in 1hour.

Other than that, good plan. You can actually keep running the IC40 during the process, the high FC level won't hurt it. You only need to shut it off when doing an overnight chlorine loss test.

Oh, and only check FC at this point. The phenol red pH test doesn't work at FC levels greater than 10ppm.

[jhk49]

As to my "guesstimate" of an FC of 20, it was based on the internet pool calculator that I have used numerous times with good results. Based on what I added 484 fl. oz. of 8.25% bleach and a starting FC of 8.2 the new level should have been 21 ppm. I did this at about 11:30 am, it is now 3:28 pm and I rechecked the FC as you recommended .... it came out to be 22.5 ppm. Instead of the bleach, how about a another bottle of chlorine titrating reagent (R-0871).

Be that as it may, it has been sunny but relatively cool down here in south Georgia, maybe that has something to do with it. I will continue to check and thanks for your confirmation of my approach and the tip that I can leave my SWCG system on.

[SunnyOptimism]

Happy to be wrong

And yes, when cleaning up the pool water, the chlorine titrating reagent goes FAST! I always keep a spare 2oz bottle in reserve. I hope you're using the 10mL (0.5ppm/drop) water sample size? That will save you on reagent.

Cooler weather is a big help. The algae, if present is less active and doesn't reproduce as fast.

One other point which I hope your aware of - all pumps/water features should be running while you do this. You want to make sure chlorinated water is reaching every part of your system and you should leave the pumps running for 24 hours while you wait to test again. In fact, you should dose again this evening after sundown to get your water back to 20ppm and then measure again in the morning before direct sun is hitting the pool to see what your overnight chlorine loss is. If, by morning, your water is clear, your CC's are 0.5ppm or less and your overnight loss of FC is less than 1ppm, then you are good to let the water drift back to normal FC.

Be sure to keep an eye on your filter pressure as well. After all is said and done you'd probably want to give it a good backwash to clean out any accumulated dead junk.

[jhk49]

Shall I leave my SWCG "on" during the overnight test?

[SunnyOptimism]

No.

Since the overnight test is attempting to determine the loss of chlorine due to the organics and biologicals in the water (no UV degradation at night), you do not want a source of chlorine adding FC to your water. Just get the water up to shock level, leave the pumps running and turn off the IC40.

Good luck.

[Watermom]

If you lose no more than 1ppm of FC overnight and have less than 0.5pm of CC, we typically recommend one additional day of leaving the chlorine level high for added insurance and then you can let it drift down.

[SunnyOptimism]

If you lose no more than 1ppm of FC overnight and have less than 0.5pm of CC, we typically recommend one additional day of leaving the chlorine level high for added insurance and then you can let it drift down.

Totally agree with that! Thanks for the clarification.

[SunnyOptimism]

Shall I leave my SWCG "on" during the overnight test?

So what's up? How did your overnight tests go?

[jhk49]

As previously discussed, after adding the bleach to the pool at about 11:30am on Thursday and checking the FC at about 3:30pm, the level was 22.5 ...... then again checking about 7:00pm, the FC was about 22.0 (Thursday, 10/16/14). The pump ran all night and the recheck in the morning (about 8:30 am) the FC level was 19.5 (Friday, 10/17/14). I was concerned that the chlorine was not eliminating the organics, and the pool had somewhat of a green cast to it. I let the pool pump to continue to run until about 6:00pm and rechecked the FC which came in at 19.5 ppm. The pump did not continue to run last night but did kick on in the morning according to schedule, I checked the FC again (about 9:30 am on Saturday, 10/18/14) and it recorded as 18.5 ppm. And, the pool looked much better than previously, the green cast is gone.

The weather has been sunny but not real warm, mid 70's for the high and mid 50's for the lows. I am hoping that we have taken care of the problem. However, I am concerned with the differences in the FC during the overnight of the first comparison (between Thursday and Friday). Could the testing procedure at 10 ml sample provide a greater opportunity for error when using the titrate liquid and multiplying the result by 0.5 ppm? What do you all think?

[SunnyOptimism]

The 10mL test will have a +\- 0.5ppm error (Taylor error is basically +\- 1 drop)

However, you had an OCL (overnight chlorine loss) of 2.5ppm. Therefore, proper procedure is to immediately readjust your pool water back up to shock level and hold it there all day. If no one is able to test and adjust during the day, then you could go over the shock level by a few ppm's and then check again in the evening.

Basically you need to maintain shock chlorine levels until your OCL is 1ppm or less. I know it sounds tedious, and it is. Typically, the algae kill rate is high in the beginning and then the process tails off towards the end as your dealing with organic oxidation. It sometimes helps to backwash or clean the filter during this process to remove a lot of the dead algae (think of it as algae corpses rotting in your filter medium and emitting lots of organics into your water). Filter backwashing speeds up the tail end of the process which is the oxidation of organic compounds into volatile chloramines.