Use gray water drain for new basement bath?

[oasistree]

I am preparing to finish the 800-sq.-ft. basement of my 1925 bungalow. This would include creating a second bathroom, which in addition to the toilet and sink would have a shower and separate bathtub. I also likely will place the washer and dryer in or next to the bathroom. I am further contemplating putting a small wet bar in another portion of the basement.
Before I have plumbing contractors come in and just tell me what I want to hear, I would like to get your views on this: Can the current gray water drain in the basement be used to handle waste water from the washer, sink, shower, bathtub, and bar?

The current plumbing setup includes a large (6-inch?) cast-iron sewage pipe that goes out through the north poured-foundation wall and handles all the plumbing from the first-floor bathroom. The kitchen plumbing in handled by a smaller iron pipe that goes out the south wall of the foundation. The gray water from the washer goes out an even smaller pipe (1.5 to 2 inches), but this pipe exits through the foundation floor by the west wall. All three lines converge in the backyard, so eventually all sewage and waste water from the home exits into the main sewer line via one pipe.

I imagine that waste from the basement toilet would have to be handled by an ejector pump taking it to the large cast-iron sewage pipe on the north wall. Fortunately, I think the layout for the basement bathroom would easily accommodate this.

However, as for the washer, bathroom sink, shower, bathtub and bar sink, can all these be handled by putting in plumbing below the foundation so they all drain to the pipe that currently handles just the gray water? It seems if I can do that, then most waste water from the house will leave by gravity alone, minimizing the wear and tear on the ejector pump for the basement toilet.

All the bathroom and washer plumbing would be placed in a space roughly 10 feet by 14 feet, and the bathroom is planned for the spot where the current gray water line exits the home, so it seems this can be done with a minimum amount of cutting to the concrete foundation floor. The minibar is a different matter, however, since that would involve having to run a pipe all the way the other side of the basement, about another 20 feet.

Any thoughts on this are welcome. Thank you.

[rmw]

Is the gray water a storm water drain for gutters and run off or is other household plumbing going in to it? Also are you on a septic system or municipal sewer?

Normally storm water drains are not allowed tto have any household waste dumped in them. My best guess is that you local codes will make you put in a basin with a ejector pump to remove all plumbing waste to sanitary sewer. This is actually alot easier that it may seem as you can often tie the ejector pump plumbing to a exsisting sewer drain on the level above avoiding cutting foundation walls. With some good planning of plumbing almost all plumbing can be above the slab in the walls to avoid cutting concrete except for the toilet and ejector basin.