Postby AusFastLife » Thu 10 Mar, 2022 19:19
This is a really useful discussion for anyone having this problem, so even though the original post was from 2007 (15 years ago for me) I just wanted to add my experience with this.
I have a 40,000L (~10,500 gallons) inground concrete pool, lines with pebblecrete. The back of our property is a big reserve which slopes down to our house. Every time it rains all the run off from the reserve comes through our yard and if it is heavy enough into our pool. So every time it rains we get a fierce algal bloom. This year has been full on El Nina here in Sydney Australia.
When the rain is really heavy it fills the pool up to overflowing and the water turns a thick murky brown (not just green from algae). I found that when the water gets to its highest point it seems to wash tannins from the soil into the pool turning it brown. When it is that dark you can't see the green form the algae, but the algae is there too.
So I have a lot of experience in clearing extremely heavily loaded pool water. I wanted to make a couple of notes here about the types of issues others have been posting in this discussion, and also describe the problem I am currently having, which is what brought me here.
1) Alot of folks describe how they have shocked and flocked and their pool is still green. Others have asked what your pool numbers are. Remember the only thing that matters in clearing your pool is Free Chlorine. All the other parameters are all about optimising the conditions for free chlorine to exist and to do its job. When you shock your pool you are pouring an excess of chlorine into your pool to get free chrlorine levels up above 30 PPM.
If you have a very large pool with a very high load of organic matter, you are likely going to need ALOT of chlorine. The chlorine will interact with organic matter in the pool as soon as your pour it in. This will bind up all the free chlorine preventing your free chlorine levels from reaching as high as you need. In a heavily affected pool you can add the recommended amount of chlorine to shock the pool, and effectively still have no free chlorine as soon as you treated it since the free chlorine has been bound up. To shock a pool you need to keep adding chlorine until free chlorine is above 30 PPM. This will kill everything.
If there is no free chlorine, and there is algae left, algal growth will always re-populate your pool. Remember the growth rate is exponential, the more algae there is the faster it generatres more algae. If you have Cyanuric acid in your pool higher than 50PPM it will also bind up your free chlorine and make it difficult to shock your pool. It is best to use unstabilised chlorine when shocking your pool because of how much you have to add.
2) My suggestion for folks who's flocking isn't working is you are not shocking your pool hard enough. All of your free chlorine is being used up by the organic load in your pool so your chlorine levels are not getting high enough to shock the pool.
3) My brown water is from the tannins. Flocking does not get rid of that. I have to shock to bleach it out. That lets me see the turbidity from the algae. I need to completely kill the algae before flocking. If there is any algae left the water will remain murky as the flock gets used up and can't bind up all the particles that are generated as more algae reproduces. Normally one 15L of 125g/L Bleach is enough to shock my pool. But I think my organic load is so high at the moment I will need 30L. My pH is going to be messed up so I will need to correct it after the bleach has had a chance to circulate for about an hour. I have a tonne of trees over the back of my house so I think my phosphates will be high so I will add some anti-phosphate to bring that down.
(I tend to add treatments based on symptoms since I buy my pool chemicals from Bunnings a big hardware store chain that sells the chemicals but does not do testing, stores that do proper testing sell the chemicals but they are very expensive. I feel bad going to the pool stores to get testing done, only to then go to a cheaper store to buy the stuff.)