As I had to drain the house down when we were away over Xmas,
all the toilets and cisterns were loaded with anti freeze. One of the
toilets which grew mould at 10 day intervals has now been mould free for
28 days. Just experience.
If you used one of the cheaper antifreezes which has a high
concentration of methanol then that would probably be the explanation.
Methanol isn't as good as ethanol or isopropanol as a disinfectant, but
it will probably deal with moulds quite well if left in contact with
them for several days, even as a dilute solution.
The mould grew at the water air interface and spread to the
underside of the toilet rim, presumably by splash contamination. I've
not seen it before, it's only one toilet of 4, and purging the tank and
bowl over a couple of years failed to deter it. The anti freeze was
Poundland and one litre? was used per toilet, after draining down.
Any mould in the cistern when you added the a/f? Otherwise I would have
expected pretty clean water entering the pan after the flush and be
unlikely to support mould growth.
If the cistern was supporting mould, each flush would dilute the content
by 4 or 5 to 1 and might not be enough to kill it.
The antifreeze has possibly done you the favour.
If it comes back then one of those disinfectant blocks in the cistern
might be a solution?
Both toilet blocks and bleach in the cistern had been tried. A
block and bleach in the pan was also tried. This toilet was in use
daily, yet still grew black mould. No mould was apparently present in
the cistern, just iron staining which comes from our water supply.
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