Thanks for the info.
Yes, you're right : water from town's public network is already treated and should not need much more bleach to keep it clean for a year.
I store/rotate/use 5liters mineral water bottles for everyday drinking purpose.
I just want to use the empties to store tap water, mainly for nondrinking uses, but just in case I ever have to drink it ...
I guess I will add 1 tsp of bleach to 50cl of water and divide this in 5 x 10cl, to treat five bottles.
+ Anyway I will rotate it once a year.