Ongoing Green Algae Problem

[aGrandma]

I have an inground gunite pool 7 years old with a salt chlorine system. For the past 2 years I have been fighting a losing battle with green algae. When I get it clear, it only stays clear for a few days then the green begins to develop again. I keep my chemicals balanced year-round. I take a bottle of water to my pool store and they sell me chemicals which do clear up the green but only for a few days before it builds up again. I have never drained my pool, never washed the plaster, and have never bought new filter cartridges for the filter. I clean my filters once a month but lately much more often in trying to kill the algae. My filters look completely clean, but once the green starts in the pool on the walls or the tile grout, the filters are very mossy green. When I take the water into the pool store they tell me either they can't test for phosphates because there is already algae growing (because I tell them there is green in the pool), or they test for phosphates and they tell me there are NONE. I have bought and used their bottles of phosphate killer too, and it doesn't seem to help any more than anything else. Usually they tell me to shock the pool and it will kill the algae. This week I have added 4 bags of shock (normally I add 2). I can still see some green tint on walls and tile grout. My pool is about 15,000 gallons and I run my filter 5 hours per day.

Does anyone know what I am doing wrong? Could it be that I just need to buy new filter cartridges - that maybe my filter is not working well enough? (My filters look clean but the bands have broken because I used a pressure washer to clean them a few times. There are no holes in the filters. Of course I know now not to use a pressure washer again.) Or should I drain the pool and acid wash the plaster? Or could it be that there is a tiny amount of algae in the tile grout that is re-invading the pool every few days? Should I run my filter longer than 5 hours? Help!

[aGrandma]

p.s. - I have not been sweeping the walls with a broom. Instead I have been washing them with a hose with a nozzle on the end for higher pressure. When I do, there is a cloud that comes off the wall into the water. I thought this was enough - but do you think I really need to be brushing it with a broom instead? Could this be my problem? If so, what kind of brush should I buy?

[chem geek]

No doubt your Free Chlorine (FC) level is too low relative to your Cyanuric Acid (CYA) level. First, shock the pool using chlorinating liquid or bleach as described in Defeating Algae. Then, balance your pool as described in Water Balance for SWGs. Note that 1-3 ppm FC with 80 ppm CYA is NOT sufficient chlorine (after you get rid of your algae). The FC should be at least around 5% of the CYA level to prevent algae growth in SWG pools, so 4 ppm FC minimum if the CYA is 80 ppm.

[aGrandma]

Is it ok for me to use liquid chlorine in my salt water pool? My pool store always tells me to use the shock that is a powder that comes in small bags (this is Leslie's Pool and they sell Genesis shock.)

[polyvue]

Is it ok for me to use liquid chlorine in my salt water pool? My pool store always tells me to use the shock that is a powder that comes in small bags (this is Leslie's Pool and they sell Genesis shock.)

I have a salt pool and have used Genesis shock (powdered Trichlor) on occasion, but if I had an algae problem (or high CYA) I would cease using it and use only liquid chlorine (10-12% or 6% bleach) to combat the green monster. Read the articles linked by chem geek (above) and prepare to buy a good amount of bleach or liquid chlorine. Do you have a good test kit? To defeat algae while relying on a pool store to conduct your tests is difficult because you'll be needing test results several times a day.... Recommend purchase of the Taylor K-2006 Complete FAS-DPD Chlorine kit, available from a number of vendors for less than $60.00:

Amatoind.com

Poolcenter.com

Spspools-spas.com

For whatever reason, brick and mortar pool stores don't carry this kit.

Once you have your test kit, test for Cyanuric Acid (CYA, stabilizer) and then determine the right level of chlorine for shocking*. You can use the Pool Calculator to determine how much chlorine to add. The main idea (and this is explained in the links provided by chem geek) is to quickly escalate the level of chlorine in the pool and then HOLD it there, pumping 24/7 until the water clears or an overnight chlorine loss test shows less than 1 PPM chlorine lost from dusk to dawn. You will be testing several times a day and adding chlorine as needed to maintain this high level.

* If CYA > 80 PPM the sensible course is to drain some of the pool water and refill so you can start at a reasonable baseline.

[aGrandma]

Ok thanks so much, I feel like I am ready to try the bleach. I read the post about killing algae, and used the chemical calculator. I understand it better now. But I am still afraid to add gallons of bleach to my pool! I've never done it before....When will the pool be safe to swim in again? After I get the chlorine level way up (shock), will I then have to add a bunch of additional chemicals to balance the water again - will it get all out-of-whack or will it be easy to get it under control again? What I'm wondering is, how much is this going to cost me....in trips to the pool store, in chemicals, etc. Also, how do I add the bleach? Do I walk around the perimeter of the pool and slowly pour evenly or does it even matter? Do I add it near the filter input?

[polyvue]

But I am still afraid to add gallons of bleach to my pool! I've never done it before....When will the pool be safe to swim in again?

There's a bit of a trade-off regarding swimming. I wouldn't want to swim in a pool that is being shocked for algae but once it is cleared up (usually in a few days) you can swim all summer -- assuming, of course, that you continue to maintain sufficient chlorine levels. Don't be worried about the gallons of bleach needed to reach and hold shock level; once it is in the water it becomes Hypochlorous acid - the same stuff that's generated by your SWG.

After I get the chlorine level way up (shock), will I then have to add a bunch of additional chemicals to balance the water again - will it get all out-of-whack or will it be easy to get it under control again?

There may be an initial rise in pH --- that's one reason why you will want to adjust pH down to 7.0-7.5 before shocking -- but it will come down again as the chlorine does its work. You won't have to add a bunch of chemicals to balance the water again if the water started off balanced.

What I'm wondering is, how much is this going to cost me....in trips to the pool store, in chemicals, etc.

Invest in a good test kit (one-time purchase) and you will not have to keep visiting the pool store for testing. If your pool store carries 12.5% liquid chlorine ("pool chlorine") at a reasonable price you can use this to shock -- or just buy unscented regular or ultra bleach (active ingredient: 6% sodium hypochlorite). So..... $60 or less for a good test kit and however many jugs of chlorine it takes to clear the pool. If you compare this expenditure against the dollars you'd spend buying unneeded chemicals and powdered shock over the summer, you'll be saving money. If you need to lower pH, don't use pool store products such as "pH down" -- save money by purchasing Muriatic Acid 33.45% -- many pool stores carry this, as do hardware stores. In my state, even grocery stores carry muriatic acid.

Also, how do I add the bleach? Do I walk around the perimeter of the pool and slowly pour evenly or does it even matter? Do I add it near the filter input?

Best way to add bleach (first, determine how much you want to add -- x number of gallons or jugs etc.) and then pour each jug in slowly away from the pool wall and in front of a return (the jets that return filtered water to your pool). If you have a return in the deep end, that's preferable. Keep the pool pump/filter running the entire time you're adding any chemical and for at least an hour afterward. In my pool I divert suction to 100% main drain to accomplish this quickly, but it's not a requirement. I don't recommend adding liquid chlorine/bleach to a skimmer.

[aGrandma]

Thanks so much - I did it! Wish me luck. Hope my pets have the sense not to drink any of the pool water...!!

[jkusmier]

Per the numbers you posted in your other thread, your levels look great (you could bump your CYA up to 75-80 but 60 is fine, that's where I usually keep mine). The algae may keep coming back because you haven't completely killed it off when shocking previously. Keep shocking for 2-3 days, including one more day after the pool appears to have cleared up completely. It's important to brush the sides and bottom each day.

You can swim the day after you stop shocking - worst case, the elevated FC levels may be a mild irritant, and it may fade bathing suits a bit (so wear an older one the first day or two). My experience is that it will take a couple of days for your FC to drop from shock levels (40% of your CYA level of 60ppm = 24ppm) to normal level (as others have noted, you should maintain your FC at 5% of your CYA level - that's 3ppm for your CYA level of 60ppm, but you can aim for 4-6ppm w/o worry. As I said, my CYA is also at 60ppm and I maintain my FC at 5-6ppm).

[chem geek]

Even with an FC that is 40% of the CYA level, this has the same active chlorine (hypochlorous acid) as a pool with an FC of 0.6 ppm and no CYA so is less than most indoor pools with no CYA. While I wouldn't recommend regular swimming in this level of chlorine, my wife has to to swim in an indoor pool over the winter and her swimsuits only last one season while in our own outdoor pool they last for many years. The difference is that the indoor pool has no CYA so the 1-2 ppm FC is 10-20 times higher in active chlorine than our outdoor pool that has around 3 ppm FC with 30 ppm CYA. So the shock level of chlorine will oxidize swimsuits, skin and hair about 6 times faster, but you won't have any immediate fading from something like that -- it would take months before you noticed anything different.

[jkusmier]

Richard, is their a chart or formula that one can use to calculate active chlorine based upon FC and CYA? Just curious - and thanks, as always.

[chem geek]

This chart shows the active chlorine (hypochlorous acid, HOCl) level at various Free Chlorine (FC) and Cyanuric Acid (CYA) combinations. The equivalent FC with no CYA is around double the HOCl level (at a pH near 7.5). This chart is the inverse showing the FC level at various HOCl and CYA levels. The color coding on the chart is old where roughly red was at risk for bacterial (and algae) growth while green was where algae could grow and blue was clear blue water. This was overly conservative since an HOCl level of 0.03 still prevents algae growth in manually dosed pools, though is a minimum recommended level -- roughly an FC that is 7.5% of the CYA level. For SWG pools, they can usually prevent algae with an FC that is 5% of the CYA level. The recommended shock level of chlorine is an HOCl level of around 0.3 which is roughly an FC that is 40% of the CYA level.

The above info is summarized in the FC/CYA recommendation chart I made here which was simplified into the charts here. The original algae inhibition levels were determined by Ben Powell at The Pool Forum in this chart, but you can see how that chart compares to the HOCl concentration by looking at this chart.

The rough rule-of-thumb is that the equivalent FC with no CYA is approximately the FC/CYA ratio. A somewhat more accurate pair of formulas is

(HOCl as ppm Cl2) = (FC as ppm Cl2) / ( 2.7*(ppm CYA) - 4.9*(FC as ppm Cl2) + 5 )

(FC as ppm Cl2) = ( 2.7*(ppm CYA) + 5 ) / ( 4.9 + 1/(ppm HOCl) )

These rules works OK up to a ratio of 30% (i.e. 0.3) and are for a pH near 7.5. As far as all FC/CYA combinations with the same ratio having the same HOCl, that works reasonably well up to a ratio of 50% (i.e. 0.5).

[Oceansol]

Also consider having your water checked for nitrates. I couldn't keep chlorine in the pool, had green algae, and the local national pool supply company doesn't check for nitrates. I suspected a bad cell, but a friend recommended that I have the water checked by a specialty pool outfit, small, local, and passionate, who also checks for nitrates. It was off the chart. Not sure how, but it was. Had to drain and refil