edited: This was posted on another forum but better explained here:
I've had a basement rough-in put in place in 1994 (refer to drawing below). It consists of a sink drain (also connected to a vent), which is connected to a toilet drain. The toilet drain is connected to a sewage tank of ~26" dia. x 3' deep, which is also vented. This sewage tank is for a future lift station (there's no sewage pump yet). The actual septic tank is outside the house but higher than the basement. All the drains are under the basement concrete floor.
This rough-in passed inspection.
Note that although the picture shows the toilet and the sink, these are not present. Everything is roughed-in i.e. there's no walls yet, no ceiling, therefore the 2 existing vents are easily accessible.
I would like to add a shower which was not part of the original rough-in. This will require breaking the cement floor and adding a pipe with ‘P’ trap.
Can I bring the pipe directly from the shower to the sewage tank ?
I will need to drill a hole on the side of the sewage tank, and connect and seal the pipe around it.
If so, do I need a vent since the pipe length will be only ~25" between both ‘open-air’ ends and since this will be a stand-alone pipe (no other pipes will be connected to it, therefore no siphon problems cause by the other drain).
Or, do I need to connect it to the existing drain (assuming that no more than one drain can go to the sewage tank) ? In this case, I do understand that I would need to vent it, to prevent siphon problems when flushing the toilet.