Our Rain Water Tank Needs Treatment Again

The last treatment to increase the pH was 29/1/2011 - about 50 days ago.

Our tanks are still full due to the substantial rain we have had.

The pH before treatment was 6.6 which, being below 7, is right for the Blue-Green stain to re-appear indicating that our copper pipes are dissolving again.

I treated it with 1L of lime and the pH rose to 10.6. This will drop as the lime spreads throughout the tank; as we consume the water; and as we collect more water.

In case it is useful, the water meter is on about 104kL.

Blue-green stain in the bath is back

Blue stain is back.


We have had a lot of rain recently and the tanks are full with about 30 kL each.


Tanks read an uncalibrated pH of 5.7 as measured at the kitchen tap.


By uncalibrated I mean that the pH meter has not been used for a while and I should calibrate it before use - but I didn't.

I treated one tank with 500ml lime and instantly it rose to 9.2.

Blue-Green Water Results

After almost 4 weeks, our blue-green stain has not returned.

Treated Tank Water Alkalinity

The addition of 1000 mL of lime raised the pH to nearly 11. Over time this has reduced to below 10 since about 10,000 L of new rain has been collected.

Rain Water Alkalinity

I also noted that the rain water had a pH of about 6. This was a little surprising since our tank water was pH 4.5.

Stale Treated Tank Water Alkalinity

In monitoring the treated tank water I have found that the alkalinity of the water decreases to nearly 7 when left for 24 hours or more. I suspect that this is due to the water absorbing CO2 from the air and this may also explain why rain water has a higher pH compared to our tank water - if left, the pH increases as the water absorbs CO2.

CO2 Absorption of Large and Small Bodies of Water

To help confirm this theory, I left two containers of treated tank water to stand. One was over 300mL (pH=9.94) and the other was less than 60 mL (pH=9.83). Both containers had similar surface areas. The idea is that given the similar surface areas, the two systems would absorb CO2 at the same rate but due to the different amounts of water, the larger body would change pH more slowly. This is what I observed: the smaller decreased pH to 7.17 in less than 24 hours. The larger body had a pH of 7.36. Not much difference, but the temporal reading indicate that the larger body of water lags the smaller.

Note: Although the two samples should have started with the same pH I suspect the smaller sample had already absorbed significant CO2 before I measured it. This was about 15 minutes.

Boiled Water

I also boiled some treated water, let it cool and checked its pH. The water was originally about 9.6 pH. After boiling it dropped to about 8 pH. I have no theory for this at present.