you are talking crap pal some houses do have a trap after the 4" plastic (modern type) (not the 6" cast iron (older type)) pipe leaves the house, i have installed them, there is a manhole inspection and drain clearing channel after the trap and then the pipe changes to a 6 inch, go to a drain centre and ask for a 110mm drainage trap, i bet they pop one on the counter, these traps are used when there is a sewage pumping station to pump sewage up a hill from your housing area, if you live at the bottom of a hill on the outskirts of town then the sewage gets pumped to the top of the hill before gravity takes over and it heads for the sewage works and these pumps cannot pump the gasses from the sewer, and any vent pipe in the house is to control the pressure in the house drainage pipes, if you empty your bath into a closed pipe without a vent the water would not run away without an air intake, it would just gurgle, to vent smells into a sewer is possibler but not recommended. people do it i know they do. but its not the long term answer when you can buy 10 kilos of carbon refill for 60 gbp every 6 months, and if i opened an inspection manhole and the sewer was reeking of grass, and i was an honest workman, then you know what would happen next. if it was me i would say nothing but thats me.