Pool Suddenly Green/brown

[nogueras]

Our inground pool of 8m x 4m has within a few days turned green/brown. We had five visitors ten days ago and before/during this period kept the pH and chlorine levels optimum. The pool temperature has been more or less constant at 27c to 29c for at least a month.

The water now tests as low ph (less than 6.8) and low in chlorine. I have added at least 1kg of pH positive and loads of chlorine. The pool has not changed, in fact looks worse, I can just see the white drain cover. There is some settling of a green/brown substance on the bottom but it passes straight through the filter (the sand changed about 3 years back).

I cannot understand the total change in pool chemistry. Unfortunately, I have not been able to obtain a 'total alkalinity' test kit, but this I feel would not solve the problem.

Apart from flocking, which means a huge loss of water in a very dry mid-summer, can anyone suggest a cure please?

[chem geek]

It sounds like you have a full-fledged algae bloom. Your CYA may be high so that even adding some chlorine won't be enough. You need to shock the pool with chlorine at a high level appropriate to the CYA level and should consider a partial drain/refill if the CYA is very high. You should get yourself a proper good test kit -- the Taylor K-2006.

[nogueras]

Thanks for your previous help, Wizard, do you mind giving me more guidance on my best course of action now, short of draining the pool?

You may remember that the pool went totally green ie could not see the bottom, within a week of having 7 people using a small 4x8 metre pool, on holiday. I did the usual, shock with about 1.5kg, later added anti-algae, etc., without result, finally flocking it over two days then vacuuming to waste, which left a pool very acceptable, almost shiny.

But, the chemical tests had a different story to tell. The pH was still between 6.2 and 6.8, chlorine non-existent. To raise the pH I added about 3kg sodium carbonate, over two days, to no effect. Adding several more of the large chlorine tablets did not raise the chlorine level. At this point I purchased a test strip kit, which included an alkalinity test. This agreed with the previous pH and chlorine liquid-drop measurement and gave the TA as 40ppm, and stabiliser 100ppm. To raise the TA I added, over three days, 2kg of sodium bicarbonate, which raised the TA to between 80 and 100ppm. None of the other readings were affected.

To compare the readings (all ppm) with tap water, pool, -pH 6.2-6.8/Tap 7.2: -Ch 0-0.5/Tap 0.5: -TA 80/Tap 120: -Stab. 100-150/Tap100. Pool 29c, about 45,000 litres.

Shall I add loads more Sodium Carbonate to try to lift the pH now that the TA is ok? At the moment the pool looks good but feels and smells a bit like a chemical soup!!! I shower immediately afterwards, as well as before of course.

Incidentally, adding about 0.5kg rapid dissolving Chl. granules raises the Chl. to 6 or over within an hour or two but its all gone in the morning. Im mystified, what else is there?

[chem geek]

You can do a partial drain/refill to lower the CYA level in your pool. You can use chlorinating liquid or bleach instead of Trichlor tabs or Dichlor chlorinating granules. I suggest you read the Pool School to learn how to properly maintain your pool. And get a proper test kit since test strips can be next to useless so you are really shooting in the dark.

[nogueras]

Thanks again, Wizard, I see now that our little pool is is in fact a hotbed of advanced chemistry! For ten years I've been thinking Ph and Chlorine levels, with success - until this year. What now figures is that we've had the driest year for nearly 150 years - hence little dilution to reduce CYA levels, aggravated by the fact that we have at least two months every year of full-on, unbroken sunshine, with UV levels around 10 or 11, which didn't help.

I'm now searching round all the pool stores for a decent test kit which I can rely on. Unfortunately the Taylor kit cannot be found here.

[chem geek]

Pool stores sometimes carry the Taylor K-2005, but not the K-2006. You can get the kit online such as at Amazon for $59.99 plus free shipping or from TFTestkits for $49.99 plus shipping. You can instead get the TF-100 from TFTestkits that has more of the reagents you use the most but it does differ from the K-2006 in using a different volume and comparator for the pH test and in not having acid/base demand tests (see Test Kits Compared.