I am not sure in which country this septic tank is, but we have one at our house in France (entirely ours, we do not have to share) and the instructions we give all visitors, and obey ourselves, is not to use any household cleaning product, whether for washing dishes, clothes, paintwork or any other purpose unless it clearly states on the bottle that it can be used sagely of you have a septic tank.
I have recently been using mould resistant paint on the walls of an unventilated cloakroom and I always wash the brushes in a bowl and empty it on the gravel courtyard, because, by definition, it must contain anti bacterial/microbial ingredients.
Our installer told us that if there was a problem with the cesspit it would smell. Because, as a holiday home the house is only used intermittently we use the six monthly product Tarax topped up with a packet for Eparcyl at the start of every visit
So many of us are recommending Eparcyl, do you think Gransnet could get some extra money from them by charging for these recommendations?