Yes, there are several possible avenues for help when it comes to installing or changing a septic tank.
France’s six regional Agences de l’Eau are a good first port of call as they may be able to provide some funding, as well as helping you to obtain other grants.
You can obtain details on the one covering your area by clicking on the map here: tinyurl.com/o75zuca
You should also contact le Service Public d’Assainissement Non Collectif (SPANC), an official local body attached to the mairie, which inspects septic tanks and can offer advice.
Ask your mairie for its details.
You may be able to benefit from a means-tested grant from the Agence Nationale pour l’Amélioration de l’Habitat (anah.fr) of up to 50% of the price of the work.
An éco-prêt (interest-free loan) of up to €10,000 is also available from banks under certain conditions.
Note that VAT for septic tank work is at a reduced rate of 10%.