What is this weird pipe loop under a single basin kitchen sink?

That loop is a textbook example of a DIY mess that needs to go immediately. It’s likely creating a double trap or a weird **S-trap** situation, which explains why the sink smells like a swamp. When water flows through a setup like that, it often creates a siphon that pulls the water out of the trap, allowing sewer gases to vent right back into the kitchen.

Since you're adding a disposal, you need to simplify the whole run. Get rid of that flexible rubber coupler while you're at it. Those things are prone to slipping and the internal ridges collect slime and food particles that add to the odor. You want a smooth, hard-piped connection from the disposal discharge to the trap. Just make sure the disposal outlet sits higher than the wall drain entry, or you'll have standing water inside the disposal chamber constantly.

If that loop was some failed attempt at venting, you shold absolutely replace it with an **AAV (Air Admittance Valve)**. Tuck it as high as you can get it under the cabinet, ideally just below the sink rim. This allows air into the system so the water drains properly without siphoning the trap dry every time the sink drains.

Just cut all that junk out back to the main waste line and start fresh with a standard P-trap kit. If the waste line in the wall is at a good height, it’s a quick fix. If the wall drain is too high for the disposal outlet, you might have to open the wall to lower the sanitary tee, but usually, you'll have enought clearance with a single basin sink. And definatly don't forget to knock out the dishwasher plug on the new disposal if there's a dishwasher line connecting to it.

I'm curious about the AAV suggestion—are those things allowed in every state? I’ve heard some inspectors are relly picky about them. Also, that tip about the dishwasher plug is huge. My cousin totally missed that when he put his in and thougth the new disposal was broken beacuse the dishwasher wouldn't drain.

If the OP replaces that loop with a standard P-trap, does the pipe going into the wall need to be a certain size? I’m trying to learn this before I mess with my own sink, and I'm worried about getting the wrong parts at the hardware store.

About those AAVs, check your local codes first becasue some jurisdictions still require a dedicated vent thru the roof. If you go the AAV route, make sure you buy a decent brand like Oatey or Studor; the cheap ones fail and you'll be smelling that swamp again in six months.

Also, since you're adding a disposal, check the slope on the horizontal arm going into the wall. If it’s too flat or pitched backward, even the best disposal will just push sludge into a standing pool of water. Aim for a quarter-inch per foot drop. It makes a massive difference in keeping those pipes clear of ground-up food waste.

One thing I haven't seen mentioned yet is the power. If there wasn't a disposal there before, make sure you've actualy got an outlet under the sink and that it’s on a GFCI. I’ve seen people finish the whole plumbing job just to realize they’re stuck running an extension cord because there’s no juice down there. Also, since you’re cleaning out that 'swamp water' mess, give the sink flange a good scrub before you seat the new disposal. If there’s old gunk or dried putty on the rim, the new seal won't sit right and you’ll be chasing a slow drip for a week.

While you've got everything disconnected, run a small snake down the waste line in the wall. If that loop has been trapping gunk for years, there's a good chance the horizontal run behind the wall is half-clogged with thick black sludge. No point putting in a new disposal if it just backs up immediately. Also, when you pick up the P-trap, grab a 'LA trap' with a union connection if your local code allows it. It makes it way easier to pull apart later if the client drops a ring or something down the disposal. It makes future maintanence way less of a headache.