Green Pool With 5ppm Free Chlorine

[tkalla2000]

hello, I started taking care of a pool at a hotel a couple of weeks ago. The pool had been green for about 2 weeks before I took over. With some 10% liquid shock, the pool was looking good 4 days later. The pool stayed nice for a couple days and then turned green again. Now I can't seem to keep it from turning green. Chems always show in range.

FC = 5.0

PH = 7.4

TA = 120

CH = 250

CYA = 70

Temp = 91F

The only thing that caught my attention is the phosphate level was about 300ppb.

I have other pools that are higher on phosphates and I have no problems with them. This is a hotel that I service twice a week. The hotel is out of my normal area so I can only get there on Tuesdays and Thursdays. The filter is a little undersized, it is a 350lb sand filter. The pool is about 40,000 gallons, but it gets slammed on the weekends. We are in south Texas and the heat index has been between 100F and 117F for about the last month. People are using massive amounts of sun block and it really is taking a toll on all of my commercial pools. I have been using Pool Perfect 2X Natural Enzyme and it seems to take most of the oil off of the surface. The only thing I can come up with for this green pool is that the maintenance guy at the hotel is supposed to be backwashing the filter 3 times per day. He says he is backwashing 3 times everyday. The only thing that makes any sense to me is that filter is getting clogged up in the afternoon, when everyone is using the pool, and allowing the chlorine to drop to 0. Then when he sees the green starting he dumps a couple gallons of bleach in the pool. When I get the I get a strong 5.0ppm chlorine reading and can't figure out what is letting the water turn green. If anyone can help I would much appreciate it. I take of many pool and have came across many problems and I very rearly get stumped, but this one has got me going. Also if anyone knows a better product the help breakdown the sun block, that would be a great help.

[chem geek]

My pool has had 2000-3000 ppb phosphates so I know how reactive a pool can get in terms of fairly rapid algae growth if the chlorine level gets too low. I agree with you that the chlorine level probably dropped below 5 ppm and probably below 3 ppm for a time and that let algae grow. However, you keep mentioning green, but a lot of times the water is dull and then cloudy before it gets green from algae, though this depends somewhat on the type of algae. It's possible that this pool is on the edge of algae growth so it doesn't take long for a full bloom. If that is the case, then the chlorine demand will be high and would even by high overnight -- a greater than 1 ppm FC drop overnight. If that is happening, you can shock the pool with at least 20 ppm FC (28 ppm FC is closer to shock level, but 20 will still work though takes a bit longer) until it clears and you have <= 1 ppm FC drop overnight and <= 0.5 ppm Combined Chlorine (CC).

Without an automated dosing system, any algae growth will rapidly consume the chlorine and then this just accelerate the algae growth since the FC level is lower. That's why the overnight test is useful since it tells you if you've cleared the pool even of non-visible algae.

[quantumchromodynamics]

My first suggestion would be to go there at least twice a day for the next seven days to get a better understanding of exactly what is going on there.

Have the maintenance guy test the chlorine every two hours and record the free and combined chlorine levels. Make a chart for him to fill out. A public pool should be tested at least every two hours during the hours that the pool is open.

I will need the following information to better assist you:

01) What are the pump make, model and horsepower?

02) What are the make model and rated flow for the filter?

03) What is the normal flow rate of the pool as measured by an accurate flow meter?

04) What is the flow rate when the filter is dirty?

05) Are you testing for combined chlorine?

06) If you are testing for combined chlorine, what are the readings?

07) List all of the chemicals you have added since taking over the pool. Describe the amounts, method of addition and time of addition.

08) What does the Health Department require as far as chemical testing and records?

09) What is the normal clean pressure of the filter?

10) At what pressure do you backwash?

11) What is your turnover rate?

12) How many skimmers are there?

13) What are the normal ways that chlorine is routinely feed to the pool?

14) How many person-hours are there per day? One person-hour is one person in the pool for one hour.

15) How many main drains are there?

[tkalla2000]

Thanks for the info. The pump is a cheap pool pump from Lowe's, 1.5HP. The filter is a 350lb sand filter. The flow rate after the filter is about 55 Gal/M. Returns are spread out throughout the pool, but the skimmer, and drain concern me. This pool only has one skimmer, one suction outlet and one drain. The drain and suction must be shut off for now due to upgrading for code. The only suction device is the skimmer. The pump primes good and runs out at 55 Gal/M on this single suction line. The chlorinator is a Rainbow In-line that holds 10 tabs. It seems to function fine. It is using about 12 tablets per week and the chlorine is holding. When I took over the pool the CYA was 300 and the phosphates were about 1000. I started backwashing 3 times per day to lower the CYA to 80. After that I treated with a phosphate eliminator, and brought the phosphat level to 50ppb. Since then I have only added 10% liquid shock, and tablets in the chlorinator. I would use a copper algicide but I hate how they stain a white pool, and I never use them anywhere else and I never have algae problems.

My first suggestion would be to go there at least twice a day for the next seven days to get a better understanding of exactly what is going on there.

Have the maintenance guy test the chlorine every two hours and record the free and combined chlorine levels. Make a chart for him to fill out. A public pool should be tested at least every two hours during the hours that the pool is open.

I will need the following information to better assist you:

01) What are the pump make, model and horsepower?

02) What are the make model and rated flow for the filter?

03) What is the normal flow rate of the pool as measured by an accurate flow meter?

04) What is the flow rate when the filter is dirty?

05) Are you testing for combined chlorine?

06) If you are testing for combined chlorine, what are the readings?

07) List all of the chemicals you have added since taking over the pool. Describe the amounts, method of addition and time of addition.

08) What does the Health Department require as far as chemical testing and records?

09) What is the normal clean pressure of the filter?

10) At what pressure do you backwash?

11) What is your turnover rate?

12) How many skimmers are there?

13) What are the normal ways that chlorine is routinely feed to the pool?

14) How many person-hours are there per day? One person-hour is one person in the pool for one hour.

15) How many main drains are there?

[quantumchromodynamics]

Wow, you have a real challenge on your hands. Only one skimmer on a 40,000 gallon pool? Did the Health Department give this pool a certificate to operate? I would not want to be responsible for this pool.

At 40,000 gallons, you need at least 85 GPM for proper circulation. With the specs that you have provided, it is going to be almost impossible to maintain clear water.

I would recommend that you stop using the chlorine tabs and switch to a liquid chlorine feeder. You need to be testing the water at least every two hours while the pool is open, and at least every 8 hours when it is not open. Anything less will not be sufficient to reliably keep the chemicals within acceptable parameters.

The filter should be replaced with a much larger sand or DE filter to give better filtration.

When you test the chlorine, are you getting any combined chlorine?

[PaulR]

You could try polyquat 60 instead of a copper-based algaecide. Regular use of algaecide will also let you tolerate higher CYA levels.

--paulr

[chem geek]

You could try polyquat 60 instead of a copper-based algaecide. Regular use of algaecide will also let you tolerate higher CYA levels.

--paulr

Agreed, though you have to add a maintenance dose of PolyQuat 60 weekly without fail (it breaks down from chlorine and also gets filtered out if there are particles to collect since it's also a clarifier). I used to use this in my own pool when I was using Trichlor pucks/tabs years ago, but still got unusual chlorine demand and dull water (nascent algae growth) after the CYA got to around 150 ppm (with FC around 3 ppm) because I only used the PolyQuat 60 every other week.

Another alternative, with a higher initial cost, but lower maintenance cost, is to use 50 ppm Borates (the EPA allows up to 360 ppm as I describe in this post). However, check your state regs to make sure they are allowed (or more specifically, not prohibited). In the residential market, they are sold in products like ProTeam Supreme Plus (pH balanced) and BioGuard Optimizer Plus (not pH balanced). The pH balanced version is essentially boric acid. The one that is not pH balanced is essentially borax (yes, very similar to 20 Mule Team Borax) so requires acid addition as well in order to get the pH back to normal. More info on borates is here.

[tkalla2000]

Thank you all so much for the help. Sorry it took me so long for an update. The problem was definitely water turnover rate. The hotel is shutting down the pool for the rest of the summer and having a bigger one built. I advised them to run 2" plumbing to 4 skimmers and two sets of drains, along with a 900lb sand filter and 2hp pump. Thanks again for all the info.

[quantumchromodynamics]

I am glad that the pool was shut down. I didn't think that it was a winnable situation. With the specs that you provided, I couldn't see how it would be possible to maintain clear and sanitized water, especially during times of heavy use. I don't think that the pool would pass inspection anyway.

It's never easy to tell someone that you think that their pool is so bad that it should be demolished and rebuilt, but it is sometimes the right thing to do. It's a lot easier with an old pool than a newer one.