Your Fosse is supposed to have a crust on the top , that `s how the system works ,anaerobic digestion in the tank and aerobic in the drainage field .( hope you have not now blocked the latter ) there should be a filter on the out let of the tank pouzzolane or cinder type this is more likely the problem if the inlet is clear...in might even be on the inlet to the drainage field ..have a poke at that with a long stick Dave