Not a great subject but something we must do when we live in the countryside in Brittany and other remote parts of France!
Every 5 years it is advisable to have your septic tank emptied. It is now regulated so you can’t ask the local farmer to come and do it for you. You must use a registered company and the sewage gets sent to be recycled ‘properly’ rather than being spread on nearby fields.
The company will come and empty the septic tank, clean the tank and fill it again with water.
The price depends on the size of the tank. Expect to pay around 200 euros.