Having a strange problem with rain water…

[An_Outlier]

So, I collected a large quantity of rainwater recently, around 90-100 gallons (about 300 liters).

I normally do not have a problem with rainwater. I collect it from my roof and then run it through filter floss/physical filtration media to remove small pieces of organic debris. Then I usually run power filters with activated carbon to remove any potential chemical contaminants, and store it for use later.

I was testing the pH of the water I collected out of benign curiosity, and found that to my dismay, the water that had not yet been filtered chemically had a pH 0f 6.4 while the water I was filtering with the carbon now had a pH of well above 7 (it seemed to be around 7.5.

I never had this problem when I was collecting it in smaller quantities with plastic containers laying on the ground, but for some reason running the carbon raised the pH. I don’t understand why this could be happening, and after tossing the rainwater with the increased pH, using new activated carbon and rinsing the filters with dilute phosphoric acid (and rinsing again with reverse-osmosis water) and even rinsing the activated carbon with more reverse-osmosis water, it happened again, and the liquid pH test came out to approximately the same value.

I also ran replicate tests the next day and the same results appeared. I don’t want to have to dump all that rainwater, but I also feel a little nervous about using it straight from the barrels. I don’t live in an area with much heavy industry, but I’m not one to make assumptions about things like this.

Could it be the carbon doing something to alter the pH? It seems far-fetched to me, since if that’s what is happening it means that the carbon really isn’t suitable for aquaria that are meant to have more acidic water, but I’ve never had any trouble with the brand before.

Does anyone have any ideas? I’d rather not throw all this free water away…

[Arno Beißner]

In water without carbonate hardness (rainwater) of PH is very labile.
Smallest amounts of acidic or alkaline substances cause quite a change.
How high is the electrical conductivity? pre- u. after filtering?
So slight variations are usually harmless.
PH measurements in water with low conductivity are anyway difficult –
and usually wrong.

[An_Outlier]

Well, I think I may have answered my own question.

After searching for other instances of this problem, numerous threads and online conversations came up; other people have had problems with this as well, and it is indeed linked to the activated carbon.

Finally, one thread on a marine fishkeeping forum detailed that activated carbon can raise the pH of the water it is used on if it was not acid-washed first. At some point during the production process, ash is produced or acquired as a contaminant. Some brands/grades of activated carbon are treated with an acid wash, which dissolves the ash (ash is often composed of metal oxides), but it seems that there was also some backlash against this practice, since it could alter the pH in the “wrong direction” for certain applications and since phosphoric acid was often used, which meant that phosphates were left in the carbon and contributed to algae problems in aquaria.

The only real solution appears to be finding a brand of activated carbon that is acid-washed, or to use an acid/acidic solution to wash/rinse the carbon with prior to using it on the rainwater.

[An_Outlier]

I know this is old now, but I wanted to update it with something that might be valuable to other people.

First of all, I have since stopped using activated carbon in all of my tanks, paros and otherwise. It’s too difficult to find a brand that actually will not raise pH and sponge filtration seems to be working better all around anyway.

Second, the rainwater was dirty for a few reasons. We had a very dry and hot summer, which meant that more debris and air pollution ended up sticking to the roof, and then the rain washed it down; I have since found that keeping the roof clean (or at least keeping the surfaces and gutters that contribute to one’s collection apparatus clean) by simply rinsing them with tap water from a garden hose (i.e. standing on the roof and spraying it down) goes a long way towards keep the collected water clean. I usually wait to set up the collection apparatus until the rain has had a chance to rinse away any minerals from the tap, but this does not take long under sustained moderate to heavy rainfall.

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