Are you using french pipe?
The easiest thing is too throw the waste in the bin & get another.
I think they would actually fit uk pipe (well some of it as there is several sizes of 40mm)
Also if it's acryilic (spellin?) silicon won't stay stuck to it & the shower trays crack after 3 - 5 years
but otherwise bonne chance
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.