Hi Rebecca,
France is one of the few places that I know that actually has a good alternative to the septic tank. There is a company called Aquatiris, which offers reed beds for effluent treatment, and also as a primary filter for effluent that comes direct from the house (possibly for use after a macerating pump). My understanding of the system is that the primary filter works in a similar way to Anna Eday's (www.Solvivia.com) brownfilter (woodchip/compost/soil filter). Have a look at Aquatiris on
https://www.aquatiris.fr/en/reed_beds.aspx for more info on their system.
Feel free to ask any questions in the meantime. Also, I'd be very interested to hear how you get on with them.
:-)
Féidhlim