Hi all. Can anyone recommend a method for removing a build up of limescale on a toilet bowl that doesn’t have an adverse effect on a septic tank? I have tried (several times) emptying the bowl, leaving a mixture of bicarbonate of soda and vinegar for several hours then scrubbing but it hasn’t really shifted at all. Thanks for any advice.
UPDATE: Thank you everyone for your words of wisdom - I will give some of them a try 
Have you tried boiling water with the bicarb? Or just cold? I find heat really helps.
Citric acid crystals? Or somehow stop the flow of water down the u bend and fill with industrial strength vinegar, leave it overnight at least.
It has to be an acid to counter the limescale. Acids dissolve alkali and limescale is strongly alkali. The acid will be neutralised dissolving the limescale so should have hardly any effect on the sceptic.
Our local Super U sells Dr Beckmann Détartrant Intensif which contains sulphuric acid which should do the job. They also sell 1 litre bottles of Hydrocloric Acid if you need even more oomph. Whichever you chose, just use it in small quantities on a cut and come again basis until the desired result is achieved. That way you will have neutralised the acid with the limescale that you need to remove.
I find the best way using the minimum amount of product is to use an old paintbrush.
Whatever you do, remember that it is an acid product, so please wear gloves and eye protection when using it.
Also an activated charcoal mask as the vapour is nasty as well. For those not used to very strong acids, I prefer Vero’s recommendation of high strength white vinegar.
Use the toilet brush to push as much water round the U bend to reduce the dilution of the acid.
Perhaps try using a bottle of cola?
Pour it it in the bowl, leave over night, brush the bowl next morning and flush.
Way back when, we used cola to make copper pennies shine 
I thought you used HP sauce for that 
Nope, never. Coka cola was what we teenagers used.
(must confess, never even heard of doing anything with HP sauce, other than using it with food, but I prefer tomato sauce anyway.)
EDIT: this has made me realise how many years it is since I tasted tomato sauce (or HP for that matter) we seem to have migrated to aioli and similar stuff (often homemade).
Got a feeling that using sulphuric acid and or caustic soda will kill all the bacteria in the tank and hence not be the best idea for the environment?
You need a phosphoric acid based descaler. They work very well and they are safe for a fosse if used in small amounts.
Sounds like you don’t know, as caustic is at the top end of pH it could add to the limescale but won’t remove it. Sulphuric or hydrochloric will be neutralised by the limescale so having very little effect if any on the septic. If any residual low pH exists it will be diluted pretty quickly after a couple of flushes.
I carefully (gloves and goggles, old clothes) use hydrochloric acid and a small paint brush - it takes the limescale off quite well. We have a fosse and i think the amount used will not have a terrible effect on the system.
I had a horrible ‘line’ where the toilet had been running which would not come off. I ended up trying dissolved citric acid crystals in a container - I think it was 2 tbps in a mug size of water as I wanted it quite strong, then dipping kitchen roll strips in it and leaving it on the offending limescale. I ended up doing it twice but it completely cleared it and no harm to the septic tank. I left it for about an hour each time (it was quite bad) and it then came off quite cleanly. Just one small bit at the top that’s tenacious that I want to do again. It depends where your limescale is!
The important thing is only to use the minimum amount of product necessary to do the job. I use an old paintbrush to apply it (as others have mentioned), and above the water line in the toilet bowl one can use sheets of toilet paper to stop the product from just running down into the water, thus increasing its effectiveness.
The Dr Beckmann’s is excellent when brushed onto chrome finish taps, though it is a good idea to open a window to disperse the fumes given off.
Other malic acid/citric acid descalers are available.
use with caution,
Hello Brandon, Welcome to SurviveFrance.
I do use with caution, anything in particular I need to be aware of ?
Don’t use boilng water, it will crack the WC bowl.