Help with green debris

Jenrudolph1

Bronze Supporter
Sep 26, 2020
15
Crystal Lake, IL
First time poster - I apologize in advance for the lengthy post but wanted to give sufficient detail.

Stats:
20 x 40 inground vinyl pool we moved into last year
Very old equipment - 24” sand filter, single speed pump (runs 24/7), Hayward heater

Our plan is to get a new filter and pump at start of next season so we need to limp along with what we have.

Fighting high pH (gets to 8.0+ every few days and I treat with dry acid)
CYA 50
Use liquid chlorine (12.5%) - between a half gallon and full daily. We only shock when indicated - not on a weekly schedule.
Free Chlorine stays between 5-7
Zero Combined Chlorine

Since a huge storm passed through over a month ago, we’ve been fighting a green powder in our pool. I don’t believe it’s algae anymore since our FC levels have been consistently between 5-7. Left to it’s own devices, the water does become a greenish tint, although still easy to see through. The powder seems to accumulate near the jets (photo of it in the stair crevices below). We have been brushing and backwashing. We use the robot vac very 2-3 days and it picks almost all of it up (see photo of what washed off the filters into a mason jar).

I did shock with Cal Hypo and saw it clear up for a day or so. The pool store also convinced me to use a clarifier - we have no idea how old our sand is and they suggested the clarifier might help the particles get trapped. It did help for a few days.

I did an overnight chlorine loss test and we only lost 0.3 so it doesn’t seem to point to an algae. Also, I collected some pool water in a mason jar and it’s stayed completely clear for over 24 hours (not sure that means anything…)

Edited to add: we are in IL and back up to an extremely large preserve with wildflowers and prairie. That leads me to think pollen could be what I’m dealing with but don’t want to assume that and have it actually be an algae. We close in 3-4 weeks so I need to get this resolved STAT.

Any ideas? The only thing I can’t keep under control is our pH - could that be a contributing cause? Or is it likely a crapped out sand filter? Or am I totally off base and it is algae?

Thanks for your help
 

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mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,931
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Regardless, you need to follow the SLAM Process. Green is algae or some other organic debris.

Sand filters, unless you use clarifiers and floc, do not go bad. You may need to deep clean it. See Deep Cleaning a Sand Filter
A rising pH is due to elevated TA with aeration unless chemicals (soda ash, baking soda) are being added to raise the pH. Do not use dry acid. It is sodium bisulfate. The sulfates build up in your water and will corrode metals (heater) and destroy concrete.

 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,931
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Mknauss, you are catching me rushing and multi tasking at work. You are right. Using the 25mL sample I miscalculated. It was actually 31 drops so would have been 6.2
Great. See my post above.

Future FC testing use a 10 ml water sample. One heaping scoop of R0870. And each drop of R0871 to clear is 0.5 ppm FC.
 

Jenrudolph1

Bronze Supporter
Sep 26, 2020
15
Crystal Lake, IL
Regardless, you need to follow the SLAM Process. Green is algae or some other organic debris.

Sand filters, unless you use clarifiers and floc, do not go bad. You may need to deep clean it. See Deep Cleaning a Sand Filter
A rising pH is due to elevated TA with aeration unless chemicals (soda ash, baking soda) are being added to raise the pH. Do not use dry acid. It is sodium bisulfate. The sulfates build up in your water and will corrode metals (heater) and destroy concrete.

Thank you for your advice.
I was under the impression that sand needs to be replaced every few years and we do not know when this sand may have been put into use by the last owners.
So because it’s green it must be organic and alive? That makes it easy to determine what it is then. If algae is dead, does it change color and if so, what color does it become?
Thank you for your help.
 

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Jenrudolph1

Bronze Supporter
Sep 26, 2020
15
Crystal Lake, IL
Great. See my post above.

Future FC testing use a 10 ml water sample. One heaping scoop of R0870. And each drop of R0871 to clear is 0.5 ppm FC.
I normally do that proportion but wanted it accurate to the 0.2 for the OCLT. Even if I transcribed it wrong!

I thought I’d this was organic matter that I would see a >1.0 FC loss overnight. With only a 0.4 loss, it still could be organic?
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,931
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
With only a 0.4 loss, it still could be organic?
OCLT is one indicator. Green water is a bigger indicator.

that sand needs to be replaced every few years
Nope. If no pool store potions are added to the pool it will last the lifetime of the filter housing.

So because it’s green it must be organic and alive? That makes it easy to determine what it is then. If algae is dead, does it change color and if so, what color does it become?
Green is chlorophyll. So what you have is alive. Dead algae is light brown/tan color.
 
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mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,931
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
A deep clean of the sand filter will tell you if the sand needs to be changed. If it is clumpy and sticky, then change it. If you can clean it as described in the article I linked, all good.

Be sure to switch to muriatic acid.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,931
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
You can use the robot. Best to clean it after every use.

Vac to waste if you have large areas of clumpy algae. But if just dispersed dead algae, use the robot.
 
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