Algae control in winter

[mgtfp]

I am following your Forum for quite a while now, but this is actually my first post. After constantly having algae problems when following standard advice on FC levels, I was very happy to learn about TFP's approach in adjusting the FC-level to the CYA-level. Having a physics background, I quite enjoyed diving into the deep end and going through some of the papers that Richard was referring to. After SLAMming, no more algae by sticking to the recommended FC-level.

Now that we are heading into winter here in Australia, the pool is cooling down. Melbourne is at the cool end of Australia. We don't get snow, but water temperatures will go down to 5-7°C (41-45°F). The pool is now at about 11°C (42°F) and I noticed some algae returning. Then I thought about the temperature dependency of the CYA / Cl equilibrium and turned the temperature dependency in Richard's pool equations spreadsheet on, and calculated the HOCl concentration at different temperatures.

At FC=4ppm, CYA=60ppm and pH=7.7, we get HOCl=0.025ppm at 25°C(77°K). At 20°C (68°F) this goes down to 0.013ppm, at 10°C (50°F) we are at 0.0032ppm, and at 5°C (41°F) we reach 0.0015ppm. This is a massive drop and I am wondering if I should increase the FC-levels to keep HOCl constant at about 0.025ppm. This would require FC=18ppm at 10°C (50°F) and FC=23ppm at 5°C (41°F).

This would be quite a high FC-level to be maintained through the peak of winter. The standard advice usually is that high FC will damage the pool (I have a plaster pool). You hear reasons like high FC will result in low pH. I don't really believe in this, I am running an SWG, sometimes topping up with Cal-Hypo, both increase pH rather than decrease. I am watching the CSI and keep the pH now more towards 7.8 to compensate for the colder temperatures, so plaster wise I should be OK, I think. But what about other equipment like pump, filter, pool cleaner, hoses and pipes? And are there other effects on plaster by high FC, rather than just CSI effects? I am not really concerned about swimmer safety, the pool is not being used over winter anyway (apart from a few ducks from time to time - cute visitors, but very messy).

I guess, step 1 is to not fight the CYA decrease over the winter season with increasing rainfalls diluting the water. At CYA=30ppm and pH=7.7, we'd only need about FC=12ppm to maintain HOCL=0.025ppm at 5°C (41°F), which sounds a lot more manageable.

Or I could add an algaecide. I did that last year before I learned about adjusting FC to CYA, but I don't really like the smell these algaecides are adding to the pool, so I'd rather go without them.

What are your thoughts, does that make sense to maintain very high FC over winter, keeping HOCl constant according to Richard's spreadsheet? You would think that algae gets less active in winter. But at least now (at water temperatures around 10°C (50°F)), algae seems to be happy to grow at FC-levels that were fine above 20°C (68°F).

What are your experiences with algae at lower temperatures?

 

[mknauss]

Welcome to the forum!
I have to admit, you put a lot of thought in to your discussion. In most areas of the US, the pools are either closed for the winter and covered. As long as you close late and open early, both times with water temperature below 60F, algae is minimal. In other areas, like my pool, I keep the pool open year around. The water temperature gets to mid 50F, and I just maintain normal FC/CYA levels with out issue.
Maybe @JoyfulNoise Matt could provide you some guidance on your thoughts.
I suggest you read ABC's of Pool Water Chemistry.

 

[JoyfulNoise]

What are your phosphate levels?

Typically the algae that finds its way into pools is the standard blue-green type algae which is both highly susceptible to chlorine sanitation AND doesn’t reproduce well below 50F. But you appear to have some healthy algae and I’m wondering if you have a good load of nutrients in the water. Nitrates are common from human bather waste and will always be high. Potassium is also highly available. Phosphates are the only nutrient that is easily controlled and removable.

You can certainly SLAM your pool in the winter and it should go quickly but your going to have to deal with cold water and that’s not always fun.

What types of algaecides have you added?

 

[wireform]

I believe if you close the pool with a slam level FC that should keep the algae out while the water is cooling down to the lower temps. Most likely you water was too warm still with below minimum FC so by the time the temperature in the water is where it's now you have algae growing. Leave it this way and open the pool next season before the water temp reaches 60° and it'll slam out really fast. I've opened several pools this season, some were clear and some had a tinge green which slammed out very fast.

 

[mgtfp]

Wow, thanks for these very quick replies. Must be late for you. I did remove phosphates a couple of weeks ago, at that time I hadn't noticed any algae yet. Haven't tested for phosphates since, have to do that at the pool shop, don't have a test kit for that. For other tests, I found a supply for Taylor-like kits here in Australia. I don't have a pool cover, so I certainly have the problem in feeding algae with light. But the pool is very close to the house (closer than I would have built it myself, but you don't move a pool by a few meters when you buy a house), and the reflected light from the water is actually quite crucial in getting enough light into the house in winter. Last year, I controlled algae over winter with a Benzalkonium Chloride based algaecide. It worked, but as mentioned before, I didn't really like the odour. So I was hoping to control it this winter with chlorine alone. Currently, I upped the Chlorine to a level equivalent to HOCl=0.027ppm (according to pool equations), and the algae seems to have gone for now. To really get rid of it, I'll have to SLAM, but this will have to wait for warmer temperatures. Generally, I am happy with maintaining higher FC-levels, but I was worried if I could damage some pool equipment in keeping an FC around 15ppm permanently. Once the water goes significantly below 50°F and algae gets less active, I can probably start reducing again, so I won't have to go towards 20ppm to keep the HOCl constant at 41°F water temperature. The pool doesn't need to be perfect over winter, but it should look at least look clean.

 

[mgtfp]

It hit me that when I SLAMed at the end of summer before going into winter mode, I didn't open the return line to the jets and the pump line from the spa during the SLAM. There might have been some spores hiding which could explain why some algae returned. I had my FC around fresh water target for 77°F. I usually think that in autumn, when the SWG is not running much, it is better to stick to fresh water than SWG target. But with sinking temperatures my HOCl went down and seemed to have these buggers allowed to grow again. After my new CYA test arrived (I first thought the order could wait until spring, but I decided that I need it now) I did another CYA test, and realized that my CYA level was already lower than I thought (we had some very heavy rainfalls in the last weeks to the point where I had to drain water a couple of times). That made it a lot more feasible to go to shock levels at low temperature . At very low CYA (below 10-20), the HOCl rises with decreasing temperature. I am now at CYA=30 where the FC required for shock at 50°F is basically the FC required for mustard shock at 77°C. I decided that is manageable and started to SLAM again. Now the water is clear (always was, I had only noticed some algae growing on the side walls before the SLAM), CC=0 and I just passed the OCLT. Allowing the FC to drop now. My pool stays open over winter, I will aim for an FC around 10 for now. And once temperatures go down towards 41°F, algae will hopefully not be very active anyway.