Where to tap into drain line?

The city has run new sanitary sewers to my area, and I am about to abandon my septic tank. There are now 2 lines exiting the house and going to the septic tank. One from the kitchen, and one from the bathroom. There are 2 separate vent stacks going to these two drains. I have had a lot of slow drainage trouble with the kitchen drain backing up, so I want to abandon that line outside the house, and run a new line inside the house to tap into the main drain servicing the
bathroom. Key points:

- the new line will be 2" PVC sloped down to the 4" main drain
- the new line will continue to be vented as before for the existing kitchen, through the roof (both vents will still be in use)
- the new line will be about 20 feet long, run along the basement ceiling
- by running the new line inside the house, there will only be the one main 4" drain exiting the house, pretty much on a straight shot to the lateral

My question is how to tap into the existing bathroom plumbing with this new line. My options are to:

1. put a wye into the main line going from the bathroom fixtures (bath and shower) to the 4 " main drain, and run the kitchen line into that.
2. tap into an existing plug that is in the structure (see photo) where the bathroom line now taps into the 4" main drain. But, is that plug intended as a clean-out? This option is a much straighter shot than option 1.

I'm attaching two photos. One shows the main 4" drain, which has the clean-out readily accessible. The other is looking up at the structure where the bathroom fixtures tap into the main drain, and the plug.

Thanks for any advice.

Ideally, the new building sewer will come in below the basement floor so that the basement could one day have gravity drained plumbing fixtures. It can also come further into the interior, to allow more room for additional connection points.

How the connection can be made will depend on the location of the main and direction of pitch as it leaves the home.

If you can make the connection to the new sewer outside the house then either option is viable if done properly.

Usually when the city runs a new sewer main they require that a licensed plumber obtain the plumbing permit and perform the work to connect the houses and abandon the septic tank. And it would be up to HIM to decide which is the best way to run the new line.

I did not make the questions clear. I am having a licensed plumber attach the existing 4" main drain to the lateral. There is no issue with that. But I am running the new line inside the house. My question is where to tap into the main drain with my new line from the kitchen. Can I use the plug in the photo, or is that reserved as a clean-out?

Since in 20', the line would have to drop at least 5", consider that. No, you should not use the cleanout. As mentioned, it might be better to run it outside instead of through the basement, and connect to the line outside. You could still only have one line connected to the new sewer.

The OP wants to connect to the 2 " next to the closet bend not the 4" C/O plug.
Look at the 1st pic I would not consider that a C/O,but for future use.

As expected trying to remove that plug is a SOB. I have read about using chisel to remove brass plug. But please see photo of plug. Is that brass? Does not look like it, but has been there 60 years.

What if I remove the plug but threads are mangled? Is there a "plug in" adapter that will seal the hole and mate to the 2" PVC line?

I would cut in a wye between the foundation and the first fitting.

Thanks for the advice Terry. I assume you would completely cut out a section of the cast iron, then connect in the wye. How do you guys connect the PVC wye to the cast iron? Fernco couplings?

Inside, you'd use banded couplings. Fernco makes some, but don't confuse these with the all rubber ones with just hose clamps at the end...it needs to be supported by a metal sleeve (and the rubber is generally thinner). Fernco is a brand, like Kleenex...sometimes people use the brand name in place of the actual product, but that can lead to confusion as, for example, Kleenex branded stuff is more than just tissues.