Persistent Algae on walls of catch tank of wet edge / infinity edge pool

[cbednorz]

Hi,

The catch tank of our wet edge / infinity edge pool just has a plain plain troweled or brushed concrete finish and we are constantly having green algae appear on the walls below the water level.
The main pool is coated in pebblecrete and does not seem to have any problems.
The chemical balance / chlorine levels have been well maintained for the last couple of years thanks to TFP and a good test kit. There is no combined chlorine and free chlorine levels are good at 5 or 6 - sometimes higher - but if the catch tank is not scrubbed weekly algae will inevitably appear, even in the depths of winter (which isn't that cold in Sydney, Australia).
We did have some serious algae problems a few years ago due to a bunch of problems that have all been sorted, I think, and we had a final long SLAM to sort it out.

I wonder if algae has gotten deep into the "raw" concrete and is now impossible to shift?
Do we need to drain the catch tank and attack it some other way. Acid wash? Algecide? Paint the catch tank?
Any thoughts or solutions gratefully accepted.

50,000 liter pool.
oversized SWG
Pebblecrete finish on pool
Plain / brushed concrete in catch tank





Thanks.

Chris

 

[Texas Splash]

Hello Chris and welcome to TFP! As you may already know, we utilize the SLAM Process to remove algae from residential pools. I suspect that catch basis requires additional (mechanical) intervention to keep biofilms from gathering on the wall's surfaces. Perhaps you may also be experiencing some seasonal changes over there, so I'm going to tag @AUSpool and @aussieta who may be able to give you some local advice.

Thanks for posting and stand by for more replies. Enjoy the forum and your weekend.

 

[AUSpool]

Geday Chris and welcome to TFP.
I’m not really sure what’s going on to be honest. You would assume if your FC/CyA ratio is good, you’ve slammed, no algae in the pool and it’s the same water in the pool as the catch tank then the catch tank should be good too, but…

I guess the flow rate over the edge and through the skimmer is adjustable so the water may not be the same. Do you have the FAS/DPD FC kit from CCL? Can you get a FC test from the catch tank and the pool at roughly the same time? What is you CyA currently at? An Overnight Chlorine Loss Test from both the pool and catch tank would help too.

The presence of algae is kind of a long term indication that you've had low FC or high CyA. What are your latest test results and what test kit are you using?

I was kind of surprised that the concrete is not sealed and you can see how the moisture spreads through it. But I don’t think a lack of sealing is a direct cause of algae.

Algae wont grow without sunlight, some sort of cover may help.

I suspect your catch tank has a history of low FC to CyA due in part to the lower flow and UV exposure as the water travels down the wall in a thin layer.

Maybe @JamesW may have some more ideas.

 

[cbednorz]

Geday Chris and welcome to TFP.
I’m not really sure what’s going on to be honest. You would assume if your FC/CyA ratio is good, you’ve slammed, no algae in the pool and it’s the same water in the pool as the catch tank then the catch tank should be good too, but…

I guess the flow rate over the edge and through the skimmer is adjustable so the water may not be the same. Do you have the FAS/DPD FC kit from CCL? Can you get a FC test from the catch tank and the pool at roughly the same time? What is you CyA currently at? An Overnight Chlorine Loss Test from both the pool and catch tank would help too.

The presence of algae is kind of a long term indication that you've had low FC or high CyA. What are your latest test results and what test kit are you using?

I was kind of surprised that the concrete is not sealed and you can see how the moisture spreads through it. But I don’t think a lack of sealing is a direct cause of algae.

Algae wont grow without sunlight, some sort of cover may help.

I suspect your catch tank has a history of low FC to CyA due in part to the lower flow and UV exposure as the water travels down the wall in a thin layer.

Maybe @JamesW may have some more ideas.

Hi,

The catch tank is the inlet to the filter. So unless I’m running the vacuum the filter is drawing from catch tank and outputting into the main pool so the water is basically the same. I run the filter for 4 hours a day and, from memory, that was about enough to turn the whole pool over.
I have a full salt water test kit from CCL and have used that to do a couple of SLAMS over the last few years to get the pool back in shape.
My FC is 4.5 at the moment, 0 combined chlorine and my CYA is 40. I don’t run the CYA very high as I have an oversized SWG and have no difficulty keeping the FC level up.
I will do a separate FC test of the catch tank just in case something weird is going on.

Edit- just tested the water from the catch tank and main pool and they both have 4.5 FC with zero combined chlorine.

Thanks,
Chris

 

[AUSpool]

Hi,

The catch tank is the inlet to the filter. So unless I’m running the vacuum the filter is drawing from catch tank and outputting into the main pool so the water is basically the same. I run the filter for 4 hours a day and, from memory, that was about enough to turn the whole pool over.
I have a full salt water test kit from CCL and have used that to do a couple of SLAMS over the last few years to get the pool back in shape.
My FC is 4.5 at the moment, 0 combined chlorine and my CYA is 40. I don’t run the CYA very high as I have an oversized SWG and have no difficulty keeping the FC level up.
I will do a separate FC test of the catch tank just in case something weird is going on.

Edit- just tested the water from the catch tank and main pool and they both have 4.5 FC with zero combined chlorine.

Thanks,
Chris

Well thats the simplest answer ruled out. The water is the same and your running at ~11% so there is ample sanitising power.

Is it just the overflow wall or are your seeing algae in the catch tank or other walls above the catch tank too?

Do the walls remain damp after the pump is off?

At four hours there are plenty of daylight hours left for the algae survive in a damp environment. It is probably just hanging on and the raw concrete surface would help it survive. It wouldn't take to long for the residual FC to burn off and give the algae some happy grow time.

The simplest solution may be to spray that wall with a 50/50 pool chlorine mix in a plastic garden spray bottle not long after the pump has turned off.

I would avoid any algacides which can cause staining on your pebblecrete. Algacides are really only helpful to prevent an algae outbreak from starting in the even of low sanitiser, they are not match use on an active algae growth.

Four hours run time is fine if your sanitiser levels are good and you’re happy with the water clarity. My SWG is oversized for my pool too and I run it at about 4 hours too. With a FC of 4.5 and CyA of 40 your within the TFP FC/CyA ratio and roughly justify the industry standards. Keep in mind that the current Aussie standard for private swimming pool water quality, AS 3633, gives a CyA range of 30 - 50ppm and states that when CyA is used FC must be maintained at a MINIMUM of 3ppm. No upper limit for FC is given, the 1 -3ppm or 1 - 4ppm FC industry range does not apply. Most pool stores wouldn’t know the standard existed but will quote it sometimes to sell stuff thinking we wouldn't know either.

I run my CyA at 70ppm and FC at 7 - 8ppm which allows me to maintain that level at a lower output and prolong the life of the cell.

 

[JoyfulNoise]

Concrete is not a waterproof surface. Even troweled smooth, concrete is incredibly porous. My guess is that you are never getting rid of the algae because it has the perfect environment to hide in.

I get why people like the infinity edge pools but builders rarely do good work on the overflow basins. That entire basin wall and interior should have been sealed with a waterproofing membrane material and then either plastered or set with tile. Leaving the bare concrete is simply an unfinished job.

 

[JoyfulNoise]

Do you see how the water wicks up above the water line and keeps the cement there damp all the time? That is the perfect breeding ground for mold, bacteria and algae.

The basin needs to be drained, mildly acid washed and scrubbed clean. Then it needs a waterproofing layer applied (a material like Hydroban by Laticrete) and finally a finish layer - either plaster or tile.

 

[cbednorz]

Well thats the simplest answer ruled out. The water is the same and your running at ~11% so there is ample sanitising power.

Is it just the overflow wall or are your seeing algae in the catch tank or other walls above the catch tank too?

Do the walls remain damp after the pump is off?

At four hours there are plenty of daylight hours left for the algae survive in a damp environment. It is probably just hanging on and the raw concrete surface would help it survive. It wouldn't take to long for the residual FC to burn off and give the algae some happy grow time.

The simplest solution may be to spray that wall with a 50/50 pool chlorine mix in a plastic garden spray bottle not long after the pump has turned off.

I would avoid any algacides which can cause staining on your pebblecrete. Algacides are really only helpful to prevent an algae outbreak from starting in the even of low sanitiser, they are not match use on an active algae growth.

Four hours run time is fine if your sanitiser levels are good and you’re happy with the water clarity. My SWG is oversized for my pool too and I run it at about 4 hours too. With a FC of 4.5 and CyA of 40 your within the TFP FC/CyA ratio and roughly justify the industry standards. Keep in mind that the current Aussie standard for private swimming pool water quality, AS 3633, gives a CyA range of 30 - 50ppm and states that when CyA is used FC must be maintained at a MINIMUM of 3ppm. No upper limit for FC is given, the 1 -3ppm or 1 - 4ppm FC industry range does not apply. Most pool stores wouldn’t know the standard existed but will quote it sometimes to sell stuff thinking we wouldn't know either.

I run my CyA at 70ppm and FC at 7 - 8ppm which allows me to maintain that level at a lower output and prolong the life of the cell.

The algae tends to be on all 4 walls of the catch tank below the current water level.
The overflow wall stays pretty wet for a long time. The other three walls of the catch tank are dry except for a 5cm / 2inch damp looking band above the water line where I guess the water is soaking up into?
Having a close look over the weekend I noticed some algae on some of the pool piping and a thermometer tethered in the main pool so I am considering giving it a SLAM.
I'm not sure how long I should SLAM it for - there is no Combined Chlorine to use as a guide, and the FC levels barely move at this time of year anyway. I suppose I could try and gauge if the algae has "gone" or set some arbitrary time period of, say, maybe a week?

 

[cbednorz]

Do you see how the water wicks up above the water line and keeps the cement there damp all the time? That is the perfect breeding ground for mold, bacteria and algae.

The basin needs to be drained, mildly acid washed and scrubbed clean. Then it needs a waterproofing layer applied (a material like Hydroban by Laticrete) and finally a finish layer - either plaster or tile.

Oh dear - that does not sound easy nor cheap.
I will try a SLAM and try keeping the FC levels a bit higher at 7 or 8 as Auspool does above, and brush regularly as an interim measure to try and keep it in check.

I wonder if and acid wash, clean and then painting the catch tank with 2-pack pool paint would be enough?

 

[ajw22]

if the catch tank is not scrubbed weekly algae will inevitably appear,

Scrub the catch tank weekly.

All pool surfaces need to be brushed at least weekly to keep the biofilm from protecting any algae.

Any surfaces that are never scrubbed can eventually become home to algae.

 

[JoyfulNoise]

At the very minimum the tank needs to be dried out and scrubbed clean. Then you could apply a cemetitious water barrier like MasterSeal 581 by BASF (or something equivalent in Australia). Paint is a bad choice as it deteriorates quickly from chlorine exposure. The cement needs to be sealed and waterproofed or else algae and mold will continue to grow.

Also, the water in the basin needs to drain if the overflow isn’t running. Stagnant water loses chlorine quickly. If you can, you need to schedule the infinity edge to run several times a day to increase turnover of fresh water in the basin.

 

[cbednorz]

At the very minimum the tank needs to be dried out and scrubbed clean. Then you could apply a cemetitious water barrier like MasterSeal 581 by BASF (or something equivalent in Australia). Paint is a bad choice as it deteriorates quickly from chlorine exposure. The cement needs to be sealed and waterproofed or else algae and mold will continue to grow.

Also, the water in the basin needs to drain if the overflow isn’t running. Stagnant water loses chlorine quickly. If you can, you need to schedule the infinity edge to run several times a day to increase turnover of fresh water in the basin.

That seems do-able - I will look into that, thanks.

The catch tank / basin is the inlet to the filter - kind of like a really big skimmer box - so the pool is being turned over through it whenever the filter is running.

 

[AUSpool]

The algae tends to be on all 4 walls of the catch tank below the current water level.
The overflow wall stays pretty wet for a long time. The other three walls of the catch tank are dry except for a 5cm / 2inch damp looking band above the water line where I guess the water is soaking up into?
Having a close look over the weekend I noticed some algae on some of the pool piping and a thermometer tethered in the main pool so I am considering giving it a SLAM.
I'm not sure how long I should SLAM it for - there is no Combined Chlorine to use as a guide, and the FC levels barely move at this time of year anyway. I suppose I could try and gauge if the algae has "gone" or set some arbitrary time period of, say, maybe a week?

It sounds like a slam is in order. An Overnight Chlorine Loss Test and the presence of any algae would be your best indicator. It sounds like you’ve got a standby green army reserve ready for action in the event of low FC.
Edit - I would do your Overnight Chlorine Loss Test from the catch tank.

I was a little surprised you don‘t have a weir skimmer and can divert flow between the infinity edge and a skimmer. Infinity edges tend to drive pH up faster then pools without.

From the looks of it I don’t think you see the back side of the infinity edge which is why finishing it properly was not an issue. I don’t have much experience with pool sealers or paints other then I wouldn‘t paint my pool and from what I’ve read the the quality in terms of longevity of the modern paints are not as good as they used to be.

 

[AUSpool]

That seems do-able - I will look into that, thanks.

The catch tank / basin is the inlet to the filter - kind of like a really big skimmer box - so the pool is being turned over through it whenever the filter is running.

In the world of aquaculture or fish tanks they would call it a sump. A water management system, normally hidden out of sight somewhere.

Some form of cover could help. No sunlight, no algae and less FC loss to UV. But out of sight, out of mind.