Sewer-gas smell under very specific conditions

August 14, 2023 9:37 AM

I have a dishwasher that drains into a garbage disposal in the sink, which in turn drains into the wastewater line. When I start the dishwasher, it begins by pumping out any standing water that might be at the bottom of the tub. A new development I've noticed is that during this pumping process, I can smell sewer gas for a few seconds. I'd like to know why this is happening and how to make it stop.

One possibility that crossed my mind is that what I am smelling is from water being pumped out of the dishwasher, not the sewer line. But we run the dishwasher every couple of days, there's generally not any standing water in it, and the dishwasher itself doesn't stink. So I don't think that's it. The dishwasher is about 10 years old and has a couple of known problems, but I'm not sure how to account for this. I can't imagine that the dishwasher is somehow creating a vacuum that is sucking all the water out of the drain trap.

posted by adamrice to Home & Garden (12 answers total)

Have you tried cleaning the dishwasher's filter(s) and vent(s)?
posted by Iris Gambol at 9:49 AM on August 14, 2023 [1 favorite]

What kind of venting does that drain stack have? Do you have an air admittance valve (AAV) under the sink? (example image)
posted by JoeZydeco at 9:54 AM on August 14, 2023

Is it pumping out of the dishwasher into your garbage disposal? Once a year or so I usually unhook the drain hose between the dishwasher and the disposal and snake it out with a long handled wire brush and warm water with dish soap.
posted by cmm at 9:55 AM on August 14, 2023

I can't quite tell from your description whether this is the same issue we had, but it was an easy fix, and something I'd never heard of before. So I thought I'd share! The dishwasher drain line is supposed to have a "high loop" (YMMV, check your manual). When we didn't have that set up properly, we were getting a smell that wasn't quite what I would describe as "sewer," but was what I would describe as rotting food/stagnant water.
posted by neutralhydrogen at 9:55 AM on August 14, 2023 [4 favorites]

As a test, you can put a splash of bleach in the bottom of the dishwasher after it has been run. If this cures the smell problem then you have identified the standing water or the drain line as the source.
posted by Midnight Skulker at 11:46 AM on August 14, 2023

ours always smells like rotten eggs when it starts even when everything is clean so I will also be trying out these suggestions!
posted by brilliantine at 12:21 PM on August 14, 2023

Bleach corrodes stainless steel. If your d/w has a steel tub liner, please don't do this. OP has reported the dishwasher itself not smelling, the drain water in the sump is not the source of the smell.
posted by JoeZydeco at 12:28 PM on August 14, 2023

My money is on a grody filter on the Dishwasher
posted by rockindata at 12:43 PM on August 14, 2023

Response by poster: Some responses:
- We have what I think can be considered a high loop. It could be a few inches higher.
- The tub and filter are very clean. Not sure the dishwasher has a vent, or if it does, where it is.
- No air admittance valve.
- The line draining from the dishwasher to disposal is translucent and visibly has some gunk in it.
- There actually is standing water below the filter, but like I said, it doesn't smell.

Nothing has really changed in the setup but the smell is new, so something has changed out of sight.
posted by adamrice at 2:54 PM on August 14, 2023

Is there a p-trap installed?
posted by Saxon Kane at 3:42 PM on August 14, 2023

The drain pipe is required to hold water for the one-way drain valve to function. That's where you say you see visible gunk (which is normal.) It can get stinky and because there's a one-way valve, you won't usually smell it inside the dishwasher. You could pour some cleaner down the drain pipe and let it soak, or manually clean it out.
posted by flimflam at 5:58 PM on August 14, 2023 [1 favorite]

"Fine" filter & steam vent cleaning; afterward, run an empty machine cycle with "a lime, rust and calcium deposit cleaning product."
posted by Iris Gambol at 6:58 PM on August 14, 2023