Running out of POP!

[sosspenc]

Hello all,
I have a 15,000 gallon in ground pool that I opened 3 weeks ago. Holy swamp!!! I have been a longterm lurker at this forum (as in years) and follow the BBB method. I raised my CL to 20 and it 3 days had sky-blue but extremely cloudy water. I couldn't see the first step. I held the Cl and let the filter go for about a week . Now I could see the 1st step but no others. I took apart the filter, and it checked out....I flushed it with a hose and put back together. Sand was replaced last year. I have let the filter run 24/7 for the past 2 weeks while maintaining a free cl of 20. The water is not budging. It is extremely cloudy. For the past week I have been adding about a cup of slurried DE to through the skimmer with minimal results.

The pool is holding CL level overnight, so I think that all the algae is dead.

Current test results
CYA 40
Total cl 20
combined cl 20
ph 7.9
Alk 125

Borates are 15 but are from pool store test, as I don't have a borate test.

I have a SWG but have left it off during this process. I have a 3/4 hp pump on a tagelus 50 (200lbs sand) filter, so I don't think the pump is oversized, but I may be wrong! Is there anything else I can do to speed the process??

Thanks!!!

[AnnaK]

Are you sure those test results are correct? How are you testing?

Because, if CC=20 and TC=20, then FC=0 which means you have NO free chlorine.

[sosspenc]

Oops, sorry. FC and TC are 20 CC has been no more than 0.5 for last 2.5 weeks. pH I use a probe for aquariums...it's been calibrated recently. I test cl- using a FAS-DPD titration method; CYA, alkalinity, and acid demand I usually do at work (university lab). I have also confirmed cl- & pH levels at work. I am beginning to think that I'm missing something, or my filter is sick.

Thanks,
Stella

[PoolDoc]

Yes, if you want to try something, add another dose of bleach and then turn your filter off. I want to leave it off for 2 full days.

Here's what I think may be happening. The Tagelus 50 is a shallow bed 21" sand filter. It's rated at 50gpm, but in reality, should never exceed 35 GPM. I'm not sure what 3/4HP pump you have, but if it's something like a Pentair Challenger 3/4HP . . . you're probably running around 60 GPM, or nearly double the never-exceed rate.

There's a LOT of back-story on this that I can't go into right now. But what I'm beginning to suspect is that blowing dead algae THROUGH a sand filter actually breaks the algae particles into even finer particles . . . and that after they've been broken, they not only cannot be filtered with sand filters, they are difficult to filter even with cartridge or DE filters.

There are two sorts of solutions here: preventative and curative.

+ To prevent this problem in the future, we need to make sure flow through your system NEVER exceeds 35 GPM, except when you are backwashing, and that it never exceeds 45 GPM even then.

+ But, to cure the problem, we've got to find a way to get the algae out BESIDES filtering . . . because if I'm right about all this . . . you can't filter it, now that the high rate flow through the sand has shattered the algae particles.

So, I want to try this
1. Make sure your chlorine is 20 or more this evening, and that there is NO live algae left.
2. If so, turn your pump OFF, and leave it off.
3. Check chlorine levels tomorrow PM, and make sure they are 10+ . If not, add a gallon of plain 6% household bleach.
4. Check Monday am, and see if there has been any change. Let me know what you see.

[TheGoose]

This sounds exactly like what is happening with my pool, except that I have a 1-HP pump and a 300# tagelus sand filter. Water color is good, chlorine levels are holding, all other chemistry looks good, water just won't clear.

Ben do you think turning off the filter will let the particles settle?

[PoolDoc]

Ben do you think turning off the filter will let the particles settle?

I don't know. I'm going to be recommending a 3-stage trial:

1. Try to settle with nothing. Vacuum to waste if it works.
2. Add floc, and try to settle. Again, vacuum to waste if it works.
3. Add clarifier, and try again.

During the recirculation steps (2 & 3) it will be important to bypass the filter. I do know that sand filters can 'break' floc agglomerations.