Unless you live in a town and are connected to the mains drainage, you will find that you will probably have a fosse septic tank somewhere in your garden. These tanks filter all the bad stuff out of your water, and then drain the cleaner water away naturally. All of your 'grey water' should go through the septic tank - for example, toilet, dishwasher, washing machine, sinks, baths.
Well, there are three schools of thought when it comes to emptying your fosse septic tank:
From reading about fosse septics on the internet, it appears that the manufacturers instructions for most newly installed tanks state that they should be emptied every 3/4 years depending on the size of the tank. However, real life cases suggest that when people have asked their French neighbours about the procedure to empty their fosse septic's they have received blank expressions and much shrugging of shoulders. If you do want to get your fosse septic cleaned then look in the yellow pages under 'Vidange' and then search for adverts about 'toutes fosses' - costs should be under 200€. Other people have said that their local farmer does the business for them!
In a word - Yes. You can buy the 'bog bugs' in the local supermarket and you put the sachets of bugs down the toilet every week or six months, depending on the instructions. These bugs keep the fosse alive which then eats the solids and allows the cleaner water to drain away. If you only use your house as a holiday home then it is recommended that you add a double dose at the start and end of your stay.
I read a very good forum posting on Living France which simply said "We just tell our visitors to remember the three P's - pee, poo and paper. Nothing else must be put down the loo".