Kitchen Plumbing Drainage Configuration

This is about as optimal as you can get. Everything is draining down. The possible exception is the disposer, the black pipe out of it seems about a 2 cm too long. Remove the connection and hacksaw off a bit and replace the connection.

"@arch It looks like whoever did this wanted only one p-trap but clearly didn’t install it properly. Also since there was no external venting available, they used an internal air admittance valve which is not a problem but it does require you to check it's functionality occasionally as that could also be a culprit (if everything seemed to work well before).

Example of how it works:

Photo provided courtesy of this site

Example of this valve is here: https://www.amazon.ca/gp/product/B0069KD...

Lastly, you know as everyone has told you, that slope is WAY wrong. Here is a sample photo of what it ""should"" look like:"