Understanding "Shock" levels and killing algae

[TreeFiter]

I've been thinking about just how the process of killing algae through superchlorination works lately. We raise the concetration of chlorine so that we can shorten the amount of time necessary to kill algae. The goal is to kill algae at a rate faster than it reproduces. So is the "Shock" level essentially the concentration at which the kill rate will exceed the growth rate of the algae?

 

[chem geek]

No. The "Minimum FC" is the level that kills algae a little bit faster than it can reproduce under ideal conditions of sunlight, temperature, and nutrients. That's how the minimum FC level is able to prevent algae.

The "Shock FC" level is designed to kill algae more quickly in order to clear a pool faster, but also it is designed to kill algae even when it is clumped as it often is from an algae bloom. Clumped algae takes higher chlorine levels to kill because it is thicker so the chlorine must penetrate deeper so has to essentially kill and react with the surface algae and then more chlorine comes in to go deeper. The higher concentration makes this process go faster so that chlorine can get to the interior to kill the algae inside. If you just used a Minimum FC level and maintained it, you'd kill the surface algae, but the algae inside the clumps would continue to reproduce faster than chlorine would kill it.

Unlike the "Minimum FC" level that has an absolute number based on worst-case fastest possible green and black algae growth, the "Shock FC" level is somewhat arbitrary and is basically set high enough to handle most situations with a speed that most people can tolerate. One could probably use a 20-25% FC/CYA ratio and still kill off an algae bloom but it could take some time. One could use a 60% FC/CYA ratio as with the yellow/mustard algae shock level and kill off green/black algae faster, but of course would lose more chlorine if not to algae than to sunlight. So the "Shock FC" is somewhat of an arbitrary balance between reasonable speed and moderate chlorine loss.

The "Yellow/mustard shock FC" level has somewhat more than twice the active chlorine level as regular "Shock FC" and this is because yellow/mustard algae has a heartier cell wall and takes about double the active chlorine level to kill it at the same rate.

 

[TreeFiter]

So how is "Shock FC" determined? It seems to me that higher should be better until we reach a point of bleaching liners or damaging equipment.

Along the lines of the "Minimum FC", I've been wrestling with the idea of a SWG pool that seems fine for several weeks, and suddenly shows a drop in FC, and the following week has no FC, and is starting to become cloudy. I had this very situation happen in one of my pools yesterday, and I can't figure out how or why it happens, other than the algae was there all along. Its a new build, only a few months old now, and I have been maintaining it from the start, so I know the chemistry has been kept right. Usually when I see a sudden drop like this it is because the CYA is low, and after a hot sunny day, the SWG can't keep up, causing the FC to drop below the minimum and allow algae to start to grow. I have found that as long as the CYA is kept high enough, (I usually aim for 60ppm) and the SWG is set appropriately, my pools will have plenty of FC on any given day. This seemed to be the case in this pool, but all of a sudden it went cloudy with 0ppm FC.

So is it possible that there has been algae growing just a tiny bit faster than it was being killed for a few weeks, or even months? I suspect that there was likely a small amount of algae in the fill water that was trucked in, and since my boss refuses to shock new pools most of the time, the algae was allowed to establish just enough of a population to not be killed off by the FC levels being maintained by the SWG, and eventually grew enough to bloom.

Is this more or less the same idea as when someone "shocks" a pool, the water clears up, things are great for a few days, and then all of a sudden the algae comes back?

 

[chem geek]

There are diminishing returns to "Shock FC" because after chlorine kills the algae there isn't much point trying to continue to oxidize it. The idea is to filter the algae out to remove it. It's cheaper to physically remove the algae than to continue to oxidize it and it doesn't completely oxidize anyway. Also, chlorine has a cost so why waste it oxidizing already dead algae that is going to be filtered out to remove? And yes, you could have a higher shock FC to try and do things more quickly and then dechlorinate to not continue to waste so much chlorine, but that's more work. And finally, during the day the chlorine loss is related to the FC level and in a non-linear way so you'd definitely waste more absolute chlorine loss to sunlight with a higher shock FC level.

As for the SWG pool or even a manually dosed pool where things seem OK and then seemingly out of the blue things go south quickly, such pools are likely to be on the edge of an algae bloom which initially may be nascent algae growth that isn't yet visible because the algae hasn't clumped enough to even make the water dull yet. So yes, it does sound like you had a situation "on the edge" and it tipped over either from more algae or organics (e.g. pollen) getting into the pool to use up chlorine faster than it was being generated or the algae growth rate could have picked up due to more sunlight, warmer temperature, or increased nutrient levels. You should not try and operate the pools on the edge -- was this pool at an FC/CYA ratio of 5% as the minimum FC throughout the day so that means it was at that FC when the SWCG first turned on during the day?

Yes, what you describe is similar to how someone "shocks" the pool where things are OK for a while and then algae comes back seemingly quickly, but in reality it was just slowly building up because one had enough chlorine to slow down its growth but not enough to kill it faster than it could reproduce. Once enough has grown that it starts consuming chlorine noticeably quickly, then it's a very fast downward spiral because the FC level drops and now growth picks up as a result -- a very non-linear event. Remember that it takes algae from 3-7 hours to double in population so you don't go from no algae to visible algae in even a single day. By the time you see algae, it had been growing for at least a couple of days if not more.

 

[Pogueld]

Great information here. The one time my FC got real low this season, there was a dead rabbit in the skimmer. The FC had been working super hard on those organics! I'm interested in continuing to follow this conversation.


Sent from my iPhone using Tapatalk

 

[TreeFiter]

In this specific pool, I'm reasonably confident that the FC was at or above the minimum based on the FC/CYA ratio. Since I can only visit these pools once per week, I maintain roughly 10% FC/CYA to allow for the low points in the week. I'll admit I am using test strips to determine how much FC I have, but I'm not relying on them to tell me I'm at a 2 vs a 3, its more like am I closer to 5 or 10. I keep my pools at roughly 60ppm CYA, and I aim for FC of greater than 5 at all times. So on a test strip, anything between 5 and 10 should be plenty. When I see a FC result at or below 5, I consider it a red flag. At that point I start testing salt level and CYA, and checking to make sure the SWG is functioning. In the past I would turn up the SWG, but as I've realized over the last few years, its probably a waste of cell life. So these days, I typically add some sort of chlorine to try to get ahead of any algae growth, usually 4lb of trichlor, sometimes cal-hypo when I have it available.

The vast majority of my pools are perfect and never have issues, but there are always a few that will have issues every season. For the most part, I have been able to keep the SWGs running at 30% this season without a problem, running for 12 hours per day. Most of them I suspect had algae from the start, and when the weather was right for supporting algae growth, they suddenly showed no FC, or got cloudy. As I mentioned, the pool I mentioned above was a brand new pool this year. I had the chemistry balanced within two days of filling it, yet somehow it ended up with algae. It had to be there all along.

My boss keeps trying to claim that there can't be algae in the fill water because it comes from underground lakes, where the water is about 55 degrees. I'm not convinced that cold water will completely prevent algae. Maybe slow it down, but I don't think it will prevent it. I suspect the inside of the truck tanks is a likely source of algae as well.

 

[cowboycasey]

Treefitter, I know your "in the industry" but you have been on this site and read about this... You are most likely getting outbreaks in those pools because...

1. your using strips to test "using the ballpark system" it could be 8fc or it could be 3fc...
2. 60 CYA in saltwater pools should be 70 to 80 CYA a fc never under 4 to 6 (I am happy you know and understand CYA as a lot of pro's do not )

is the "boss" not allowing you to get a tf100 or k2006? how are you slam'ing the pool if you don't have 1?

wouldn't it be nice to have a truck with a 300 to 500 gallon metal tank filled with liquid chlorine

 

[tim5055]

My boss keeps trying to claim that there can't be algae in the fill water because it comes from underground lakes, where the water is about 55 degrees. I'm not convinced that cold water will completely prevent algae. Maybe slow it down, but I don't think it will prevent it. I suspect the inside of the truck tanks is a likely source of algae as well.

You are correct, 55 degree water will not kill algae, it just slows it down.

 

[Patrick_B]

Once that water comes out of the ground and goes into anything to hold it, it is a short matter of time before it has Algae spores in it. They are virtually everywhere, and there is no stopping them from getting in.

 

[ping]

Do you start your new fills with a sequestrant?

 

[TreeFiter]

Treefitter, I know your "in the industry" but you have been on this site and read about this... You are most likely getting outbreaks in those pools because...

1. your using strips to test "using the ballpark system" it could be 8fc or it could be 3fc...
2. 60 CYA in saltwater pools should be 70 to 80 CYA a fc never under 4 to 6 (I am happy you know and understand CYA as a lot of pro's do not )

is the "boss" not allowing you to get a tf100 or k2006? how are you slam'ing the pool if you don't have 1?

wouldn't it be nice to have a truck with a 300 to 500 gallon metal tank filled with liquid chlorine

A little bit of clarification might help here. I use test strips for typical maintenance situations. The vast majority of my pools are in great shape, and because I maintain a higher FC than is necessary, as long as I'm in the ballpark, I'm in pretty good shape. This has been working well for several years now, and I'm reasonably confident that I'm working with safe FC levels.

I do own a TF-100, and I use it whenever something seems out of the ordinary. In other words, if a pool normally maintains FC around 7 or 8, and all of a sudden I see a 3, its time to start testing. Whenever I take on a pool that is cloudy or green, or has visible algae on the surface, I test the CYA with the TF-100 in order to determine just how much chlorine will be effective, rather than blindly wasting chlorine week after week.

Switching from test strips to the TF-100 would add a significant amount of time to each pool, which in the course of a day would add up. Lets say it takes an extra 10 minutes to run all of the necessary tests. After 10-15 pools in a day, that could add over 2 hours to an already long day (I've been working 12+ hours, 7 days a week since mid April).

So I certainly understand where you are coming from with the TF-100, and it is a very valid point, but unfortunately its just not practical in my situation. However, it has been an important part of how I arrived at my current methodology.

As for the CYA and FC levels, I realize that my numbers aren't quite in line with TFP reccomendations. I have built in a bit of a safety net for myself by running higher FC levels, and since I do see a fair amount of problem pools, I try to maintain the CYA on the lower side when I can so that if I ever need to, I can bring things back with more reasonable FC levels.

This is why I try to keep posts like this here in the Deep End, so that I don't lead others to believe that what I'm doing is the preferred method.

 

[chem geek]

As for the rare problem pool, most likely something happened that created a higher than normal chlorine demand in the pool (e.g. blown-in pollen or debris, a pool party especially with urinating kids), and lowered the FC enough to have algae growth overcome the SWCGs ability to keep up. It's also possible that something happened with the SWCG so that it's output was reduced (lower salt level if current is not auto-compensated, scaling in the salt cell, older cell that is worn). Maybe the homeowner did something to the equipment turning off the pump (and therefore SWCG) mid-week for some reason and then turned it back on later (or there was a power outage).

If the problem seems to always happen with the same pool, check on the phosphates level and if high you can either lower them or use an algaecide in that pool since it is "on the edge" and gets some sort of occasional mid-week bump in chlorine demand or may have poor circulation in some areas. Or you can target an even higher FC level for that particular pool. If instead you see this occur in different pools over time, then it's more likely to just be unusual events that deplete FC. If you notice that this occurs in a group of pools around the same time of year, then check for pollen or other droppings from trees and when that time of year occurs either boost the FC level or use a phosphate remover or algaecide in those pools.

As a pool service visiting only weekly, you can't rely solely on the FC/CYA levels since you aren't there mid-week to check for any unusual chlorine demand. You've got some buffer, but for some pools it might not be enough. As you've seen from the thousands of reports here at TFP, homeowners do occasionally report unusual events that cause FC depletion, but because they are checking regularly they get on top of it before it becomes a problem.

 

[TreeFiter]

As for the rare problem pool, most likely something happened that created a higher than normal chlorine demand in the pool (e.g. blown-in pollen or debris, a pool party especially with urinating kids), and lowered the FC enough to have algae growth overcome the SWCGs ability to keep up. It's also possible that something happened with the SWCG so that it's output was reduced (lower salt level if current is not auto-compensated, scaling in the salt cell, older cell that is worn). Maybe the homeowner did something to the equipment turning off the pump (and therefore SWCG) mid-week for some reason and then turned it back on later (or there was a power outage).

If the problem seems to always happen with the same pool, check on the phosphates level and if high you can either lower them or use an algaecide in that pool since it is "on the edge" and gets some sort of occasional mid-week bump in chlorine demand or may have poor circulation in some areas. Or you can target an even higher FC level for that particular pool. If instead you see this occur in different pools over time, then it's more likely to just be unusual events that deplete FC. If you notice that this occurs in a group of pools around the same time of year, then check for pollen or other droppings from trees and when that time of year occurs either boost the FC level or use a phosphate remover or algaecide in those pools.

As a pool service visiting only weekly, you can't rely solely on the FC/CYA levels since you aren't there mid-week to check for any unusual chlorine demand. You've got some buffer, but for some pools it might not be enough. As you've seen from the thousands of reports here at TFP, homeowners do occasionally report unusual events that cause FC depletion, but because they are checking regularly they get on top of it before it becomes a problem.

What you are saying makes perfect sense, but I 'm not sure my ego is ready to admit that some things may just be beyond my control. I have actually fought pretty hard to stop my boss from turning to phosphate removers whenever these pools pop up. Most of the time, its cheaper and causes less of an interruption to superchlorinate a pool than to attempt to remove phosphates. The vast majority of these problems pop up because my coworkers have no clue what they are doing. They are either too dumb or too lazy to figure out that what they are doing just isn't working. Phosphates have always been a convenient boogie man to place the blame on.

Within the last two or three years, we started using Polyquat. Up until then, we had been told we couldn't use algaecides in salt water pools. Nobody ever bothered to read the active ingredients and see they were not copper based, but we were banned from using them until we started using Polyquat. When we got the OK to use Polyquat, all of a sudden it was the secret weapon everybody relied on to keep their pools from turning. We keep hopping from crutch to crutch instead of figuring out what we are doing wrong. Needless to say, its a bit frustrating.

I've been trying to move in a direction where we pay attention to what is going on in a pool based on the chemistry, and reduce or eliminate the need for these crutches. Its not only the right thing to do for our customers, but it would be great for the business in the long run. After all, there is a reason many TFP members don't trust the swimming pool industry. If only there was a pool company that knew what they were doing, and could help straighten out your pool quickly, and could be trusted not to rip you off and sell you all kinds of unnecessary chemicals. It amazes me that this is such a rare thing, but it is.

 

[chem geek]

You've got a good strategy. Working on first principles is always best. I was just laying out different options for you given that you aren't there when pools experience a rare event that would need some additional chlorine to maintain the FC level.

I don't see the use of other chemicals as being a problem -- it's a matter of balance between costs. You could clearly avoid any of the problems in these rare pools by visiting them every day or two, but obviously that would cost you (and the homeowner) a lot more. I get your point about not wanting to use other chemicals or techniques as a crutch, but you have to recognize that you simply aren't there on-site every day so you have to do something to address the situation. If you simply raised the FC target higher to increase your "buffer" to handle the rare event, then if this is common policy then all other pools have higher FC levels than their pool owners would otherwise need to experience and ultimately pay for (one way or another, including shorter SWCG cell life).

What you really need is a monitoring system that would let you know when the FC drops in these pools so that you could schedule a mid-week visit to catch the problem early.

 

[cowboycasey]

A little bit of clarification might help here. I use test strips for typical maintenance situations. The vast majority of my pools are in great shape, and because I maintain a higher FC than is necessary, as long as I'm in the ballpark, I'm in pretty good shape. This has been working well for several years now, and I'm reasonably confident that I'm working with safe FC levels.

I do own a TF-100, and I use it whenever something seems out of the ordinary. In other words, if a pool normally maintains FC around 7 or 8, and all of a sudden I see a 3, its time to start testing. Whenever I take on a pool that is cloudy or green, or has visible algae on the surface, I test the CYA with the TF-100 in order to determine just how much chlorine will be effective, rather than blindly wasting chlorine week after week.

Switching from test strips to the TF-100 would add a significant amount of time to each pool, which in the course of a day would add up. Lets say it takes an extra 10 minutes to run all of the necessary tests. After 10-15 pools in a day, that could add over 2 hours to an already long day (I've been working 12+ hours, 7 days a week since mid April).

So I certainly understand where you are coming from with the TF-100, and it is a very valid point, but unfortunately its just not practical in my situation. However, it has been an important part of how I arrived at my current methodology.

As for the CYA and FC levels, I realize that my numbers aren't quite in line with TFP reccomendations. I have built in a bit of a safety net for myself by running higher FC levels, and since I do see a fair amount of problem pools, I try to maintain the CYA on the lower side when I can so that if I ever need to, I can bring things back with more reasonable FC levels.

This is why I try to keep posts like this here in the Deep End, so that I don't lead others to believe that what I'm doing is the preferred method.

I see, that makes perfect sense... Thanks for the explanation...

 

[TreeFiter]

You've got a good strategy. Working on first principles is always best. I was just laying out different options for you given that you aren't there when pools experience a rare event that would need some additional chlorine to maintain the FC level.

I don't see the use of other chemicals as being a problem -- it's a matter of balance between costs. You could clearly avoid any of the problems in these rare pools by visiting them every day or two, but obviously that would cost you (and the homeowner) a lot more. I get your point about not wanting to use other chemicals or techniques as a crutch, but you have to recognize that you simply aren't there on-site every day so you have to do something to address the situation. If you simply raised the FC target higher to increase your "buffer" to handle the rare event, then if this is common policy then all other pools have higher FC levels than their pool owners would otherwise need to experience and ultimately pay for (one way or another, including shorter SWCG cell life).

What you really need is a monitoring system that would let you know when the FC drops in these pools so that you could schedule a mid-week visit to catch the problem early.

I think you are steering me in the right direction here. I just need to find some middle ground between proper preventive measures like FC/CYA ratio, and running higher FC levels to accommodate for increased loads, and the remediation methods like superchlorination or phosphate treatments. The culture at the company I work for is stuck on the side of remediation and reactive behaviors, so I'm really trying to push in the extreme opposite direction hoping I can get us to somewhere near the middle.

As always chemgeek, you have been very helpful, as have the others that have offered their input. Thank you all.

 

[billy0952]

I'm not an expert by any stretch but I did have a AGP with SWG for about three years. I believe some of my former problems were the result of not cleaning the SWG plates often enough. I'm assuming its production of chlorine drops off exponentially as scale builds up on the plates. I had an Intex SWG and if I were to use that system again I'd plan on cleaning the plates at least every two weeks. I'd venture a guess that your pools chlorine level dropped off due to a drop off of chlorine production by the SWG.

I realize this post may not apply to a lot of SWG's as they might be entirely different from my "former" Intex SWG...

So how is "Shock FC" determined? It seems to me that higher should be better until we reach a point of bleaching liners or damaging equipment.

Along the lines of the "Minimum FC", I've been wrestling with the idea of a SWG pool that seems fine for several weeks, and suddenly shows a drop in FC, and the following week has no FC, and is starting to become cloudy. I had this very situation happen in one of my pools yesterday, and I can't figure out how or why it happens, other than the algae was there all along. Its a new build, only a few months old now, and I have been maintaining it from the start, so I know the chemistry has been kept right. Usually when I see a sudden drop like this it is because the CYA is low, and after a hot sunny day, the SWG can't keep up, causing the FC to drop below the minimum and allow algae to start to grow. I have found that as long as the CYA is kept high enough, (I usually aim for 60ppm) and the SWG is set appropriately, my pools will have plenty of FC on any given day. This seemed to be the case in this pool, but all of a sudden it went cloudy with 0ppm FC.

So is it possible that there has been algae growing just a tiny bit faster than it was being killed for a few weeks, or even months? I suspect that there was likely a small amount of algae in the fill water that was trucked in, and since my boss refuses to shock new pools most of the time, the algae was allowed to establish just enough of a population to not be killed off by the FC levels being maintained by the SWG, and eventually grew enough to bloom.

Is this more or less the same idea as when someone "shocks" a pool, the water clears up, things are great for a few days, and then all of a sudden the algae comes back?

 

[chem geek]

You bring up a good point with regard to scaling in the SWCG cell. Prevention is better than frequent cleaning so the pools should run with a slightly negative calcite saturation index and ideally use 50 ppm Borates (if possible) as that significantly cuts down scaling in the SWCG cell. Cleaning should be done as needed, but one doesn't want to do that frequently since each cleaning removes a little more of the special coatings on the plate and therefore shortens the life of those plates.