Kitchen sink waste line question

Hello everyone! I’m replacing my current two-basin sink with a single basin (and single disposal/drain) and I’m confused at how my current sink’s waste lines are configured. The pvc on the right side of the image is connected to the second basin drain trap and also the dishwasher drain.

1) Why are there two waste lines draining through the floor of the cabinet?

2) What is the purpose of the tall upside-down U pipe that connects the two waste lines?

3) Since I will need to saw this out and fit pipes for a new sink, what’s the ideal configuration for a single basin drain with disposal and a dishwasher?

I think this is an island vent, so I should likely recreate this after I replace the kitchen cabinet.

Does it matter if I run the dishwasher into the disposal vs. a separate pipe into the primary waste line (left-side drain going into floor)

I think the U is giving you venting from somewhere below.
Because you have an island, no adjoining wall to tie into a vent.
One leg with the trap is the drain, other leg provides vent from below.

I have usually seen a “cheater” vent used instead, not sure which is best, ask the pros on here.

Run the dishwasher to air gap if you have one then towards the disposal

BC33 said:

I think this is an island vent, so I should likely recreate this after I replace the kitchen cabinet.

Does it matter if I run the dishwasher into the disposal vs. a separate pipe into the primary waste line (left-side drain going into floor)

Click to expand...

Except for the fact that you would likely be moving your new single sink drain to a different location, you would functionally just be eliminating one trap and probably rerouting the dishwasher drain.
The dishwasher drain hose can go to either the disposal connection or to a connection on the drain piping upstream of the trap. The hose, of course, should be hooked as high as possible, under the counter, prior to being connected to the drain line or disposal.
If you have had no issues with you existing setup, I would assume that island vent system is working properly. Have you already determined that the island vent system must be rearranged to clear the new sink drain/disposal location?
I hesitate to suggest looking into the possibility of using an AAV(Air Admittance Valve) but that may be option that you could look into.

Diehard said:

Except for the fact that you would likely be moving your new single sink drain to a different location, you would functionally just be eliminating one trap and probably rerouting the dishwasher drain.
The dishwasher drain hose can go to either the disposal connection or to a connection on the drain piping upstream of the trap. The hose, of course, should be hooked as high as possible, under the counter, prior to being connected to the drain line or disposal.
If you have had no issues with you existing setup, I would assume that island vent system is working properly. Have you already determined that the island vent system must be rearranged to clear the new sink drain/disposal location?
I hesitate to suggest looking into the possibility of using an AAV(Air Admittance Valve) but that may be option that you could look into.

Click to expand...

My existing setup seems to work properly with the exception of loud gurgles from the drain anytime the dishwasher is running. I haven’t determined that the vent system needs to rearranged, but I will need to disassemble it (and cut it) when I replace the kitchen cabinet. There will also be a RO filtration system that I’ll need to work into the setup as well.

Current plan is to completely remove all plumbing, replace cabinets, recreate the island vent pipes under the sink similar to the existing setup. Finally I will have the dishwasher drain into the main waste line upstream from the disposal trap, with the filtration system waste line also draining into the main waste upstream from the dishwasher (and trap). Hopefully this makes the setup simple and minimizes drain noise.

Thanks for the replies! This has been a lot of help.