I am starting a complete remodel of my basement bathroom. My home has cast iron pipe and already had a shower, toilet, and sink in the basement. When I demo'ed the existing bathroom, I noticed there was no vent present for any of those fixtures. I need to know if I need to place a vent or not. There is no stack nearby, although the main 4" sewer line is approx 5' from the furthest fixture. Everything drained fine prior to demo...no gurgling. So I'm confused. I understand the theory of a vent and don't understand how these fixtures were draining like they were.
I attached a photo of the drains exposed under the slab. In the bottom right you can see the main 4" line that goes through the wall and out to the public sewer. It has a 4" tee which runs to the toilet, then reduces to a 2" shower drain. The toilet is a tee off of that 4" line. There was a 1.5" sink drain plumbed into the toilet drain as well...if you look close you can see the blue rag stuffed where that was plumbed in to.
Essentially I am going to keep the drains in similar order, with the shower in the furthest corner of the picture, then a toilet, and then a sink. Should I consider an air-aspirating valve kit in the sink drain?
It's a lot of questions. I appreciate the professional advice. Thanks guys!