Monday, February 7, 2011

Scale build-up on your pool

I am a chemist by career so that helps to know a bit about the way these systems work.

What is scale?
It is a rough deposit (white, cream or light brown) that forms on the walls of your pool and makes a nice surface for algae to grow on - so it may take on the colour of your latest algae.

Causes of scale:
Firstly calcium carbonate is the main culprit - it is basically insoluble and comes out of water as it warms up.

The cause of calcium coming out of water to form scale include:
1. High pH - the higher the pH the more likely it is to come out.
2. High alkalinity - alkalinity is a form of carbonate and promotes the scale build-up.
3. Hard water i.e. lots of dissolve calcium and magnesium.

1 and 2 above can easily be tested and controlled - acid lowers both. Marbelite and chiptile tiled pools prefer to run at a pH of 7.4 -7.6 while fiberglass pools can be run a little lower 7.0 - 7.4. Going to lower pH levels causes the cement to dissolve slowly and can cause weak or recessed grouting.
3. Hardness is more complex to test but some test kits are available for this - you can take a sample of water to your local poolshop where they may be able to test for harness. The hardness should be between 200 and 300 ppm. If you have have water to top up your pool check its hardness too especially if it is from a borehole - it is no good draining some water out of your pool only top up up with water that is as hard or harder.

I once visited a heavily scaled pool with a hardness of 1567 ppm, alkalinity at 230 ppm and a pH of 8.2+. Due to the drought they were topping up with borehole water that had a hardness of 800 ppm. Sure 800 is only about half of 1567 but it is still 2-3 times higher than the desired range. As you can expect this pool had severe scale as the pH and alkalinity were high along with very hard water.
I calculated that they can keep the scale dissolved in the water if they run at a lower pH and lower alkalinity despite the excessive hardness.

My next step was to dissolve the scale on the pool walls to get it back into the water - this I did over two weeks using acid and it worked like a charm. I got at least 95% of the scale off the walls - there was still a bit in places but I suspect it was gypsum (Magnesium sulphate) which cannot respond to acid treatment. Once this was done I rebalanced the pool water to a lower pH and alkalinity. The customers were delighted. I saw them last week in the shopping center and they were still very happy with the state of the pool. The trick is of course not to let the pH and alkalinity rise.

Here is a good tip for testing for calcium carbonate build-up. Pour a little pool acid into a cup and add about 3 time that volume of water (roughly).
Pour that onto the build-up on you pool. If it bubbles or fizzes it is a carbonate - most likely calcium. You can wet a sponge with dilute acid and hold it onto the build-up (USE GLOVES!) and it will dissolve.

If your pool is getting such a calcium build-up run it at a lower pH - the lowest allowable for your type of pool - it will reduce the chance of calcium build-up. Also run your alkalinity at the lower end of the range 80 -120 ppm - I run my tiled pool at about 60ppm alkalinity and at pH7.2 - this prevents the calcium build-up during the drought we are experiencing. a rough deposit that fouls the walls of your pool and makes a nice surface for algae to grow on


Daryn said...

I have a severely scaled vinyl pool. My alkalinity and ph were very high. Will lowering my ph really low for a few days or weeks dissolve the scale built up on the walls and floor of my pool?

Russell said...

I have a pool that was resurfaced after 10 yrs late last Fall (plaster and pebble mix). I closed up the pool for the Winter and I'm pretty sure the fresh surface drove the PH too high. When I opened it up in the Spring the walls were covered in white-ish crystals that when scraped off look like salt. They dissolve rapidly in muratic acid (producing bubbles). I've lowered the PH quite a bit in the pool, but after several weeks there's not much improvement. The pool store tested the water (TA=120, CYA=0, CH=220, TDS=800, NACL=0, PH=<<7) with the comments that they can't see anything wrong with the test results (other than my low PH which I did on purpose).

Got any suggestions other than a drain and acid wash ?

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