Late summer algae - happens every year

[poolguy575]

Can anyone guess what might be happening here? Every summer about early August I start to see cloudiness develop. My pool is 24000 gallons. I pump 60 gallons per minute for 12 hours a day. I keep my chlorine (SWG) at 5ppm or higher. My CYA is 50 ppm, pH is 7.5, CH is 110 ppm, Alk is 70 ppm.

Three things change with my pool around the beginning of August: The pool is mostly in the shade almost all day long, (just two hours of late afternoon sun). I put the cover on the pool at night to keep the temperature up (78 is the best I can do with the solar heater going). And the pool is used much less, (maybe an hour or two on the weekends).

I have to vacuum the sides of the pool at least twice a week because the south side, (the shady side), develops a green film, (presumably algae). The neighbor's very large tree to the south of the pool provides the shade as well as periodic droppings, (very nice).

I thought that keeping the chlorine at 5ppm or higher would keep the cloudiness from happening, but I was wrong. Is there some type of algae that thrives in the shade that I should specifically be treating? The sunny side has no green film.

 

[linen]

Since the algae is mainly in the shade including on the sides of the pool, it may be Mustard Algae. See: http://www.troublefreepool.com/pool-school/mustard_algae

Basically you do the normal shocking process until you pass the three criteria, then you bring the pool up to Mustard Algae shock level for 24 hours.

 

[jamyers]

chemgeeks's CYA chart indicates a target FC of 5.7 with a CYA of 50, you may be "riding a bit low" at 5.0...

After you do what linen suggested, with a CYA of I'd let the FC drop down and keep it at 7.5 or 8 couple of days and watch those trouble spots. Easier to back off on the FC than play catch-up.

 

[chem geek]

chemgeeks's CYA chart indicates a target FC of 5.7 with a CYA of 50, you may be "riding a bit low" at 5.0...

He said this was an SWG so the 5 ppm FC with 50 ppm CYA should normally be OK. It could be yellow/mustard algae if it's dust-like, but it would only look green if the pool had a blue color, such as blue vinyl. Also, the general cloudiness of the pool doesn't sound right if one is circulating for 12 hours/day. Is the 5 ppm FC measured or just your estimate of where you intended to keep the FC level but that it could have dropped with higher chlorine demand? You should measure the FC just before your SWG comes back on since the FC should then be at its lowest (given that there isn't much sun on the pool).

 

[ping]

You might have had a nascent algae problem that the SWG was keeping in check but has now bloomed somewhat. What size of cell do you have and what % did you have it set to?

You should follow linen's advice.