A little bit of an odd scenario

[JohnT]

Just a post to maybe let us expand our understanding of algae control.

Last week was really hectic with two grandkids having baseball tournaments and trying to get some things done at home, so I neglected the pool. Wednesday evening, I had a little time to spare so I went out to check the skimmer basket. Nothing there, but I was a little surprised to see that the pool was cloudy enough that I couldn't see the drain 8.5' down.

I immediately raised my chlorine to shock level after figuring out that rain water and backwashing/draining had dropped my CYA by more than 10ppm. Took the dog off the timer so the pump would run all night. Went back out right before bed and replaced about 3ppm of chlorine and hit the sack.

In the morning, I was only down a couple of ppm, but what was really surprising is that water clarity was much better. I could see the slots in the drain cover in about the same light where it was invisible the night before. Even more surprising is that the water was almost clear by 4:30 that afternoon, and was back to very near sparkling by Friday morning. Less than 24 hours from cloudy to clear, and 36 hours from cloudy to sparkling seems almost impossible to me.

I believe that I happened to catch an algae bloom at a very early stage, and I killed it off with an immediate shock. What is surprising is that the water was back to clear so quickly. It almost seems that the chlorination somehow directly cleared the water, as I would have expected 4 or 5 days to get back to sparkling from the stage of cloudiness I had with my filter.

Anybody have any ideas what actually happened?

 

[duraleigh]

I have observed somewhat identical conditions and results. Embarrassingly, my chlorine applications are not nearly as even as they should be. I can often "sense" the need to add chlorine because the water just has sort of a dull look and I know I have gone beyond when I should've added.

That "dull" looked is often wiped out in a matter of hours as a somewhat heavy (10 ppm CYA 60) dose of chlorine is added. I, too, believe that just the dose of chlorine often clears the pool from this slight cloudiness over and above the filtration although the filtration probably plays some part.

One of the reasons for my skepticism with OP's who have "shocked for 6 weeks straight" is because the algae that occurs in my pool seems so darn easy to get rid of.

 

[sarahspins]

One of the reasons for my skepticism with OP's who have "shocked for 6 weeks straight" is because the algae that occurs in my pool seems so darn easy to get rid of.

I agree.. my impression is that people who struggle long-term with water quality issues are either not shocking at a high enough chlorine level, they're not holding it there long enough (common from the people who think shock is a "product" rather than a process), or they have some kind of filtration issues (typically user error) that prevent the water from fully clearing. A good example would be my dad's pool - I don't think that it has ever stayed clear and algae free for more than a couple of week at a time because they don't EVER test their water (certainly not on a regular basis), they dump pool store goo into their pool, and my dad considers "shocking" to be a one time thing that is only a last resort when nothing else is working. They rely on an auto chlorinator that uses trichlor pucks and it's always 'malfunctioning' (or rather, running empty because no one bothers to check it). I look at his pool and it's so disappointing to see how hard they make it to care for because it would be relatively easy to whip it into shape with BBB and it would really require only minimal daily maintenance rather than being the constant struggle to clean up that it is now.

Last summer when I didn't know anything my intex pool turned into a swamp.. I was trying to chlorinate it using pucks in a floater (which wasn't working well, obviously) and when I read about BBB and finally took charge of my pool, I had it completely clear and sparkling in about 48 hours, and that was using the lousy cartridge filter the pool came with (I did have a fair amount of dead algae and dirt the kids keep tracking into the pool that settled on the bottom, which we did eventually upgrade our pump to get rid of, but the water was clear as long as the kids weren't swimming). I was amazed and so was my husband... he thought for sure we'd need to drain and refill the pool. It stayed clear and beautiful for the rest of the summer, but developed a leak in the liner last September so we just took it down in anticipation of putting in a larger pool this year rather than repair it.

 

[chem geek]

I've had the same thing occur a couple of times when I was lax with chlorine dosing. What is happening is a combination of two things. First is that the algae has only gotten started so there isn't a whole lot of it, but enough to make the water dull to slightly cloudy. This makes any combination of oxidation with chlorine and filtration of fine particles go faster. Either process works by percentages so if you are closer to being clear because there isn't that much material to get rid of, then it doesn't take as long to get to clarity. The other factor is that shocking with chlorine when there isn't very much algae is able to oxidize more of it faster because it hasn't clumped up into large groups that take a lot longer for the chlorine to get to (though for filtration such clumps are easier to remove, unless they are just sitting at the bottom of the pool).

When there is a lot of algae in a pool to remove, then it will take longer to do so, especially if one has a sand filter since that is less efficient in clearing the pool as it removes less on each turnover, especially for smaller particles. That's why adding DE to a sand filter often helps.

Chlorine does not oxidize everything and the bulk of chemicals in algae will not get oxidized by chlorine. It won't touch the cellulose (or sugars in general) that makes up the structure of the cell walls, for example. It won't oxidize the amino acids in the proteins that don't have nitrogenous amine or sulfur sites. This paper shows that chlorine does not react with steroids (e.g. progesterone), alcohols (e.g. ribose), is slow to react with most amides (though relatively fast with amines), and slow with phenolic compounds. Even the hydroxyl radicals that are produced when chlorine breaks down from sunlight don't oxidize everything, but mostly because their quantity is relatively small and very short-lived in pool water (the bicarbonate buffer tends to extinguish these radicals so only if they are formed right near something to oxidize will they be effective).

If it takes weeks to clear a pool then it means one of two things: 1) they are not shocking consistently to kill off all the algae (usually the pool doesn't get to the cloudy state when this is the case and the overnight FC drop is usually still rather high) or 2) they have poor circulation or filtration. Poor bottom circulation is quite common in some above-ground pools with only one return and no floor drains. There is a third possibility that is far less common and that is that the cloudiness is due to a very fine particulate that won't get filtered out, much like a colloidal suspension. This can happen if one uses a phosphate remover, for example, since the lanthanum phosphate precipitate does not normally clump or precipitate as growing crystals and can at least somewhat flow through most filters unless one consolidates such particles with a clarifier or flocculant.

There is also the possibility that I've brought up about how shocking significantly raises the pH in pools with CYA and that this could result in over-saturation with calcium carbonate causing additional cloudiness that might get resolved when one lowers back to normal chlorine levels and pH. With a CYA of 50 ppm and TA of 80 ppm (and no borates), going from an FC of 4 ppm to a shock level of 20 ppm raises the pH from 7.5 to 8.5 and increases the saturation index by +0.95 units. If one shocks to a higher level due to a higher CYA level or for yellow/mustard algae shocking or if the starting pH is even higher, then the effect is even more extreme. I'd normally expect such calcium carbonate cloudiness to be able to be filtered out, though that adds some to the total time.

 

[JohnT]

Just to add a data point, I had a very similar situation again this week. Due to crazy work hours, I hadn't even looked at my pool since last Sunday. Saturday around 11am I went out and found it difficult to find the drain from the opposite end of the pool. I immediately went to shock level and brushed the pool, which made it even cloudier. I was amazed to find the pool very clear by 4 or 5pm. Around 9:30 as the grandkids were getting out of the pool I turned the light on to help find all the toys, and the water showed no signs of cloudiness at all.

 

[Patrick_B]

I've seen it. Just when the water gets that dull look, up to quite hazy. When you are on the edge, or just before a bloom, it's pretty easy to knock out and get crystal clear again. That said, I've had haze that took near 24 hours to get back to perfect, but this was with less than SLAM value dosing.

 

[pooldv]

I am just the opposite. Water never clouds up, ever. It has been perfectly clear for 3 straight years. I had algae spots 3 years ago due to low FC and again a month ago also due to low FC. I will see a dozen or so little green fuzzy algae spots, mostly on benches or waterfall rocks below and at the water line.

Yesterday I found two algae spots on a bench, 1/4" or so. After brushing Saturday. FC at 25ish, cya 70, for 2 weeks during SLAM a month ago and then FC at 7-9 every afternoon for the last two weeks, higher in the AM. Water is completely transparent, not a hint of haziness. All tests "in range". lol!

Algae is a strange bird! Or flock of birds.

 

[stev32k]

I see the same thing with my pool. After 17 years of watching it every day I know that if it is not as clear as clean glass I have a potential algae bloom starting. Bringing it up to shock level will normally clear it up in 4 - 8 hours. Sometimes it takes a full 24 hours but that is unusual.