What you should have in a current, to the norms system, is a 100mm pipe above roof line with a mushroom on it where this is at the end of the pipework upstream of any WC. Then there is the extractor that is connected after the fosse, also in 100mm pipe and should be higher and more exposed to the wind. (see attached if it attaches)
You cannot use an air admittance valve, durgo, in place of a mushroom.
The principle is that air is extracted from the, er, extractor and at the same time air enters the mushroom, as this air goes through the pipes it passes over the fosse & takes any odours up where the air is clear (I feel a song coming on).
If you only have one then the air can't flow freely, if you have both then putting something on the the end is restricting or stopping the flow & the positioning of the extractor needs to be improved.
Minimum size for a septic tank (fosse septique) is 3000 litres. All installations must conform to DTU64.1 The S.P.A.N.C. technician will verify this. A principle piece is a room without water larger than 7 metres square.