Mustard Algae and Old Plaster

[triptyx]

A quick history:
I purchased this house in April, 2016. The west side of the spillover spa had some plaster discoloration above the step - it was mottled and a bit darker than the rest of the pool. I figured it was just stained in some way, but didn't worry about it. The plaster is potentially original to the pool, I don't know. It's definitely old and quite soft - soft enough that I can see wear from the plastic edge of the pool brush on the side of the spa just above the bench where that edge rubbed it when I was brushing the bench. The wear mark is about 1/16th to 1/8th inch deep. Since taking possession of the house, I've been quite careful with the chem levels using the TFP approach.

Late this spring, I had the pool and spa covered with a solar cover to attempt to get the pool near swim temperature. The pool itself looked fine - I was pulling back the pool solar cover to test and noticed no issues. CC stayed 0, no heightened chlorine demand, nothing out of order. I noticed one day that the color of the cover on the spa looked funky, and pulled it back to see a fine film of green algae on the side of the spa where the discoloration was previously. I SLAMmed, brushed the pool and spa daily with a nylon brush (see soft plaster - I'm not sure a steel brush is a good idea for regular brushing). I saw chlorine demand consistent with killing algae for two days, then passed an OCLT on the third day. All SLAM criteria were passed, so I went back to regular chemical levels. Algae returned in the spa within a few weeks, again, same place. Figured maybe I was dealing with mustard algae, so I did another SLAM (again, about a two day SLAM, then passed all criteria). I then slammed for mustard for 48 hours - all covers, pool toys well doused in chlorine, all towels, suits, etc. washed in hot water. Brushed vigorously. A week later - algae came back. Now I'm also seeing some reddish algae in the shady areas of the main pool near the steps/along waterline grout, etc. I've tested and retested CYA and have been keeping correct FC levels. I basically decided to brush and keep it under mild control, and finish out the summer before attempting to correct the issue.

So, normal mustard SLAMs aren't working. I'm now trying to craft a theory that will explain the issue and help me to take care of it. So here we are:

My theory is that the plaster is old and porous - it's pretty rough in places and very soft. I think the algae has a hold in those pores and my brushing isn't adequately removing the biofilm, hence the algae isn't killed and is able to come back despite a successful SLAM/Mustard SLAM Finish.

Experts - does this sound like a possible theory?

If so, I need to drain to reduce CH levels this winter anyways. My current plan is to drain, and while the water is low, gently sand the affected areas until I see unmottled plaster. Once all areas are addressed, refill the pool and perform a SLAM/Mustard SLAM. The cartridge is due for replacement too, so I'll handle that at the same time. Does this sound like a potential solution or am I missing a trick/not thinking correctly about the problem?

 

[Catanzaro]

It appears you are on the right path, but what about circulation in the areas in question. Chlorine will not hide in those areas. How many skimmers, main drains, and return jets are in the pool? Plaster is somewhat porous and you may be onto something, but I am a big believer that dead spots in circulation causes many problems. This past year, a friend of mine with perfect levels was constantly getting algae in a liner pool because he became a little lazy on brushing. The issue was limited circulation in a large pool with only one return, skimmer and main drain (old pool, long story).

Maybe bumping the thread will have someone with a concrete/gunite pool with a little more experience.

 

[triptyx]

The spa receives enough water from a single non-aerated jet near the bottom during normal filter operation to raise the water level 3 inches onto the spillover. That same jet can fill the spa from half empty to full in about 5-7 minutes, meaning there is significant water mixing turnover during the 4-6 hour daily filtering run. The main "breakout" area with repetitive algae occurrence is on the side of the spa in the upper half of the spa, on the opposite side from the single top step, where you can feel the water moving easily with your hand.

While I might agree that a couple of spots on the side of the pool steps may be slightly low circulation, the algae is re-occurring in extremely high circulation areas.

 

[Catanzaro]

While I might agree that a couple of spots on the side of the pool steps may be slightly low circulation, the algae is re-occurring in extremely high circulation areas

Interesting, as high circulation areas with chlorinated water still suffers from algae. Maybe the lack of sun. I remember that some algae thrives in dark conditions, away from sunlight, but not 100% certain. In the end, what you suggested may be your next step.

- - - Updated - - -

Does Algae like sun or not?

Post # 3 from Jason Lion

 

[triptyx]

Thanks. The spa is close to the north side of the house and gets far less sun than the pool, especially outside of peak summer months.

 

[triptyx]

Alright, this weekend is the date to drain/sand.

A question I've had for some time, especially with mustard algae is: What about the inside of the pipes? The returns, drain, skimmer, etc? We know that mustard forms an aggressive biofilm, and that it likes areas without sun exposure, so how do we clean the pipes (or is there no need)?

Any recommendations on what sandpaper to use? I don't want anything too aggressive on old plaster, but I'm also not wanting to waste extra time on paper that may be too fine and take too much effort to get enough off to be able to access the pores.