P-trap plays an important part in keeping the foul smell of sewage and drainage out of bathrooms. Since wall hung WC has an inbuilt p-trap (or any other recommended trap), we have observed many homes are not installing p-trap in the bathroom. We believe it is not right as there is a possibility of foul smell and insects coming from the drainage line of the bathroom also. There generally a nahani trap is being installed, however water seal provided by the nahani trap is weak compared to p-trap. So, we recommend - and follow - use of p-trap in the bathroom as recommended by good practices and hygiene standards