Saturday, 25 September 2010

Blue-Green Water

We collect rainwater from our roof into two large 30,000 litre tanks. A pump takes the water from near the bottom of one tank and feeds the house.

From the time all the down-pipes drained into the tanks, we have not run out of water. Previously less than one third of the roof was connected to the tanks and we ran out of water three times in about three years (all in the August - October months).

The Stain

At some point we noticed that our bath would become stained with a blue-green film. Over time the thickness of the film increases. The stain was on the taps, tiles, bath-base, shower curtain, and anything left in the bath for a long time. Being white, it was most noticeable on the bath-base.

The Experiment

Late last year, we ran an experiment: we cleaned the blue-green film from the bath and switched to town water.

The stain did not return.

I had a number of theories.

1. It was a bio-slim similar to that which would grow in a sand filter. I had experimented with filtering washing machine water through a sand filter and it develops a blue-grey slim on the surface of the sand.

2. It was some other mould.

I initially thought that the town water - being high in chorine - was keeping the mould/bio-film under control. But bleach/exit mould would not shift it so these theories did not make sense.

After a recent bath cleaning, I did some more research. The only mention of blue-green stains was in the context of copper stains - but we had rainwater which was 'pure' water I thought.

Rainwater is Acidic

It turns out that rainwater is acidic due to CO2 - carbonic acid.

Acidic water dissolves the protective copper oxide layer inside the pipes and in an alkaline environment, copper hydroxide will come out of solution.

The bath is probably an alkaline environment due to the soaps and shampoos.

I checked the toilet cistern - no blue-green stain. Probably because it is not an alkaline environment.
I checked the washing machine - no blue-green stain, but there is a yellow-brown stain (could this be some other copper product?).

Data Collection

I needed to measure pH. So I purchased a pH meter, buffer solutions and de-ionised water.
I also purchased some universal paper, but this does not seem to work very well.

All samples were taken after running the taps for a period long enough to ensure all stagnant water was flushed.

Tank Water

Water in tanks: 4.5 pH - acidic
Water from kitchen tap: 4.9-5.0 pH
Water from bath tap: 4.89 pH
Hot water from kitchen tap: 5.35 pH at 50 degrees C
Hot water from bath tap: 5.4 pH at 47 degrees C

Our rainwater is very acidic, but the more copper pipe it travels through the less acidic it gets.
Also, the hot water is less acidic than the cold water.

In both cases it seems that the acidity is being reduced as copper is being dissolved and this process is accelerated by high temperature.

Town Water

At meter: 7.75 pH

Water from kitchen tap: 7.75 pH

Town water is alkaline.

Stagnant Water

0 min: 5.11 pH
11 min: 5.56
13 min: 5.7
15 min: 5.7
17 min: 5.73
20 min: 5.82

It again looks like the acidity drops when the water is sitting in the pipe. I suspect that this is because the acidity is dissolving the copper pipe and it seems to happen quickly.


I found that blue-green water is a mystery. There does not seem to be a single cause, but acidity, O2, CO2 and temperature are all suspects.

The CSIRO has done some research.

They suggested that Microbiologically Induced Corrosion (MIC) should be researched but they did not seem to offer a cause.

A massive Thesis by Owais E. Farooqi might be interesting to others as it contains a lot of data, statistical analysis and mathematical modelling of various forms of copper corrosion.

I did find one paper on copper corrosion in particle accelerators.

The paper shows that:

  1. Increasing temperature increases copper solubility.
  2. Very low O2 (less than 30 ppb) and very high O2 concentrations ( greater than 1000 ppb) decrease copper corrosion, but lower than 30 ppb is best.
  3. pH less than 7 (caused by CO2) increases copper corrosion.

So, I need to increase pH to 7.5 or higher. A pH of 9 seems ideal. Town water seems to be above 7.5, so this indicates that the town water minimises copper corrosion.

At pH greater than 7, the corrosion due to temperature is minimised.

At pH greater than 8.5 and less than 9.5 minimises corrosion.


My inlet is very low in the tank. This is supposed to be anaerobic so it should be low in O2 - but how low? I lowered the inlet pipe some time ago. Could this be contributing to copper corrosion by inadvertently ending up with a O2 concentration between 30 and 1000 ppb?

According to wikipedia, fresh water has 6mL per litre or 6,000,000 ppb.

After some experimentation I decided that 500ml of lime power added to one tank would be my first step.

This seemed to take the pH from about 5 to 5.8 almost immediately. But I needed the pH to be over 7.5 and ideally 9. So I added more lime and the pH went to 10.8.

I can not take the lime out, so I will monitor the pH over the next few weeks and see what it does.


The bath is virtually clear of the blue-green stain so time will tell if the stain re-appears. If it does not return I may have solved our problem.