Each tetrahedron has four triangular faces. I export those triangular faces with vertex information into a file and you get the triangular soup (the OBJ meshes). What I want to recover back is the tetrahedra, so I can cull away the inner triangles and do more interesting things with the volumetric mesh.
sorry for the delay of the first reply.
The Obj format manages only surface meshes. If you want to handle a volumetric mesh (with volumetric elements, like tetra or hexa, and not only surface element, like triangles or quads), you shall use another format like the VTK format.