Repurpose black water line as new gray water line?

Our traditional septic system isn't functioning well at all. We have been told that installing a new leach field won't solve the problem due to our heavy clay soil not allowing percolation. The only traditional solution we've been given is to have a mound system installed where the black water would be pumped up into it. This is by far the most expensive and intrusive and destructive solution.
Rather than blow all of the money on poop management, we are going to begin using a compost toilet. I'm reading Jenkin's Humanure Handbook now.
I'm in a 1.5 bathroom house. The plan I've come up with is to keep the toilet in the small downstairs bathroom connected to the septic as it is already and then drastically reduce the water input to the septic. I'd like to decommission the upstairs toilet and replace with composting toilet. Here's the questionable part. I would like to send the upstairs bathroom sink, tub, and shower to an existing gray water line. My idea is to have it continue to be delivered from the bathroom via the black water line that currently leads to the septic tank. To do this, I would need to divert it before it hits the trap and heads to the septic. I will need to transition from 4" pipe down to 2" pipe in a pretty short span, maybe 3 or 4 feet. So I'm looking for opinions on this idea. Will it work? Is it possible? Can the transition be made from 4" to 2". Do I need another trap there or any special plumbing? Am I missing something?
I will also need to cap the septic line after diverting the upstairs flow to keep the downstairs toilet running to the septic tank.

(reminder: after the upstairs toilet is removed, only gray water will be following this path)

Thanks for any help! I can add more pictures if needed.

Hi C,
I am not a plumbing expert, but I would worry that if the system was designed for a 4" pipe... that it might get backed up pretty easily if you are dropping it to a pipe half the size. I'm imagining taking a shower and the drain not being able to handle the water and it starts filling up the tub. I'm assuming it would drain eventually, but if I were able to put a 4" pipe all the way to whatever gray water system you have... I would.

Thanks Matt, all input is welcome. Yes that is a fear, realistically and probably worse is that if it drains too slow it would back up into the washing machine first. Or maybe it would back up only the the sump pump causing trouble  there. I wonder if there are back flow valve options.
Maybe another approach would be to send the 4 inch out as a new second gray water line and leave the existing gray line as is. This would be quite a bit more work.

I dont think you will have a problem, you are not using solids, and 2 inch pipe is the size grey water normally uses.
In you case the 4 inch is used for solids and the grey is just added.
I dont think an elevated dispersion field would be disprutive if planned properly.

Septic systems are a giant barrel of laughs. I have issues too, and regularly fish in black waters.

I think a 2" drain can handle sink or shower. The tub is best turned into a planter or fish pond. Put low flow heads on everything -- the less water you put in, the less headaches you have to deal with downstream.

EDIT: If you mess with this system, make sure your black water system won't airlock. There's usually a roof vent or two and this ensures there is sufficient whoosh and flow to handle solids without plugging up.

Douglas Alpenstock wrote:Septic systems are a giant barrel of laughs. I have issues too, and regularly fish in black waters.

I think a 2" drain can handle sink or shower. The tub is best turned into a planter or fish pond. Put low flow heads on everything -- the less water you put in, the less headaches you have to deal with downstream.

EDIT: If you mess with this system, make sure your black water system won't airlock. There's usually a roof vent or two and this ensures there is sufficient whoosh and flow to handle solids without plugging up.

Thanks for your input! I hear you on the the tub but we use it for shallow baths for our little one. I do plan to transition away from that soon and low flow heads for sure.
About the possibility of air lock... there are 2 roof vents. One on each existing black line. This has me thinking about where the portion of the old septic line will be removed. Does any remaining pipe here need to be capped as short as possible? This small vertical section will hold air I assume. Can this mess with the flow?

John C Daley wrote:I dont think you will have a problem, you are not using solids, and 2 inch pipe is the size grey water normally uses.
In you case the 4 inch is used for solids and the grey is just added.
I dont think an elevated dispersion field would be disprutive if planned properly.

Thanks John, I didn't know 2" was common practice for gray water. My existing setup transitions to 3" through the wall so could help me assuming it continues at 3" outside.
I've been told the construction of a elevated dispersion field would require dump trucks to enter the property and to do so a road would basically need to be created to drive on. I see this as very disruptive and destructive to what we've developed on the land. It's already an expensive fix but this being needed for access drives up the cost even further.

I can see your problem if trucks are involved?
Can a bobcat carry the material?
Can you load some photos?

I would prefer to reduce to 3" for the new gray water connector, and add a 3" trap where it enters the 3" exit line. 4" to 2" is a 4:1 reduction in cross section which seems really drastic.

I would also make sure the reduction happens in a vertical or steeply sloped section of pipe, so there is no possibility of water and sludge or hair being held up in the bottom of the pipe before the reduction.

As has been mentioned, you will want to pay attention to the vent routing to be sure the remaining black water piping has a clear vent path to the roof.

Plumbing drains are sized so that they never run full; the liquids and any associated solids flow along the bottom of the pipe. In vertical runs, they actually flow down the sides of the pipe leaving open space in the center. This allows air to flow backward to fill the space left by the water, and your capped stub will simply be filled with air.