How much should chlorine level ppm drop?

Firstly to any of you who are able to help thank you.

I am assisting a friend in removing a large amount of black algae from his pool, which he did not wish to drain, we are shocking using 29 ppm chlorine bleach from Wal-Mart.

This arose from very poor maintenance on the part of his pool person 10 CyA, FC 0.5, pH 8.4 and $150 per month!!!! He finally realized that it is better to do it oneself. My feeling is that the pool person deliberately let it go stating that the reason for the black algae was the old delaminating pool and that he knew someone who could re-plaster for less than the going rate! Where I come from this person should be hung, drawn and quartered preferably in reverse order.

If we raise the level to 29 ppm at 6pm by 8am the following morning it drops to about 18 ppm. Does this sound ok or is the drop indicative of algae continuing to eat up the chlorine? Obviously we would like to drop back to normal levels as soon as possible.

We have been at this for 3 days and 98% of the black algae has gone and what remains is very faint. Not sure if we can get completely rid of the remaining algae as it is growing under the sides on some delaminated areas. Please note the pool is 17 – 18 years old and is quite delaminated. He also had the green stuff growing on the walls which has now gone.

The pool looks 98% better, albeit a little cloudy. We are running the pump 14 hours a day and backwashing to clear. He does not like to run it 24/24 so first thing in the morning he manually vacuums the entire pool and backwashes; a little slower method but everyone is happy

Chlorine 29 -18 (will normally be 6)
pH 7.2 (will normally be 7.5)
Alk 85
Calcium 325
CyA 40-45 (Will let it go down to 35)
Salt 3000 (hope to talk him into 3300 – manual states 3000 – 3500)
Borates None (cannot afford it now as he has spent so much on getting chemicals to get it back into shape - in Honululu most chemicals are 1.5 to 2 times more expensive)
20,000 gals Gunite with plaster
Jandy Aquapure 1400 SWG
1 hp Pump
300 lb? Sand Filter
Temp 81 F

Going from 29 ppm FC to 18 ppm FC overnight is a large drop and indicative of chlorine fighting algae or some other organics. A normal drop is 1 ppm FC or less, though at high shock levels you might see something a little higher (perhaps 2 ppm FC), but no where near the 11 ppm drop you are seeing. As frustrating as it may be, keep the chlorine level up high until it stops getting used up so quickly overnight.

Make sure to clean the filter, pump basket and skimmer basket(s) and brush the pool so as to remove as much organic debris as possible.

Richard

Chem Geek,

Oh, I was dreading this answer but not as much as he is as he will have to pay for a lot more chlorine.

Well you might want to starting purchasing Wal-Mart shares as it looks as this is going to be a long chlorine fight.

Thank you for your reply.

This summer, especially in the NE, everyone seems to have been using far more chlorine than usual, myself included. I haven't had a bloom, but I've just bought my 3rd pair of carboys--and it's only mid-july!

Yes, I have used much more chlorine this year too. It seems to be leveling off now though, maybe it was an extra bad year with so much pollen.

I totally agree about the increased chlorine usage this season. I'm in NH and just finished off my 3rd carboy of the season. My daily chlorine usage is always more than 1-2ppm, which isn't typical in my experience. We've had significantly more debris in the pool, too, which I'm sure is making it worse.

Well everyone this pool has a small spa/hot tub (900 gals) attached which until last night looked like the black algae hole of Calcutta. I was going to treat it after I had done the pool. What I did not realize is that there exists a bypass valve which allowed the pool water to enter the tub and spill back into the pool.

So last night we shut off the bypass valve and bombed the spa at 40 ppm chlorine adding acid back to get it to a pH of 7.2. The bottom of the spa was covered in a thick layer of dead brown material. This morning we vacuumed this debris to waste and topped up with water. Today we raised the chlorine to 30 ppm of chlorine, it seems to be holding. We are scrubbing it and will drain and vacuum to waste this afternoon. Hopefully that should be it.

As regards to the pool after we shut down the bypass last night we brought the chlorine back up to 30 and lowered the pH to 7.2. This morning it had only lost 2 ppm to 28 ppm or so. So it seems the pool is done. The reason we could not hold chlorine before was because of the spa water entering the pool.

We will refill the spa with balanced pool water by vacumming pool water to waste into the spa, but not open the bypass valve to make sure it continues to hold chlorine.

The pool is looking like a real poolforum pool, the spa well its clean but so delaminated it looks more like a white swiss cheese.

Poetic justice is about to be served as the ex-pool person left some tools at the home and has to come by and pick them up. Cannot wait till he sees a sparkling clean pool versus the black algae mess he left and looking forward even more to what he has to say. So even though I'm not getting paid this will we worth far more.

Finally without the help for others on this forum and all the education I have received over the past few years this retired fixed income friend of ours would have been stuck with their truely awful looking pool. The sense of relief on the face of his wife was worth every minute of aggravation and time. When the stock market recovers in the next few years they will be able, and plan, to re-plaster; until then they have at least a relatively attractive site to behold.

I hope that this episode helps others of you, or that you may be able to help others. The reason I write this is that try as I may to explain pool chemistry to him he has a hard time following. He is very well educated and had a very complex technical job all his life and rose to the top of his profession but pool chemistry does not seem to be his thing. So it looks as if I am going to have to baby sit him until he can walk, hopefully under nine months.

Thank you all for your help,

Aloha.