Problem

A smell of sewage is noticeable near the washer/dryer in your unit.

Root Cause

There’s no P-Trap in the wall, which means no water seal, which means sewer gases make their way back up and get into your unit.

Solution

You need to make an external P-Trap, with a standpipe because a washer can discharge a lot of water in a short time.

Parts needed

• 3 feet of 2-inch diameter PVC pipe.
• 3 90-degree elbow pieces.
• PVC cement and primer.
• Some sandpaper to rough up the pipe pieces prior to gluing them together.
• A saw to cut the PVC pipe. A regular hand saw works fine.

This can be obtained at Ace hardware for around $40.

Process

A schematic can be found here.

• (Optional) verify that your unit indeed does not have a P-Trap in the wall.
• Cut some of the pipe to use to join the 90-degree elbows to each other and to the wall.
• Dry-assemble the whole thing and make sure it fits. You can try making a water seal at this point but the pipe will probably leak.
• Sand+Prime+Glue the pieces together. I was able to avoid gluing the final assembly into the wall because the seal was watertight.
• Give it 24 hours to fully set.

Photos

This is the resulting assembly:

Note that the standpipe part is very long (almost 36" above the water seal line). This isn’t strictly necessary: In my testing the water only backs up by a few inches so it could be half the length and still have plenty of margin.