We had a new kitchen fitted four years ago. The sink is a double drainer (small and larger sink), and the dishwasher and washing machine are plumbed into the same waste pipe below the sink, which has caused a gurgling sound ever since (a reliable plumber told us that the way it has been plumbed is the cause of the gurgling).

Recently, there has been a really bad smell (like rotten cabbage) coming from the kitchen sink drain, so we used this as an opportunity to take all of the plumbing apart underneath the kitchen sink to clean it and fit an anti-syphon trap, hoping that this would fix the gurgling and the smell.

When we took the plumbing apart, the pipes were all reasonably clean and there was plenty of water in the U-bend, which was not what we were expecting to find, given the bad smell.

Now the anti-syphon trap has been fitted, there is no more gurgling; however, the bad smell it still happening!

For reference, there are no pipes or drains on the outside of the property.

Any ideas what is causing the smell please and how we can fix it?

 

1 Check the garbage disposal (if there is one).
2) Check the dishwasher's internal debris filter and if you can, put a downward loop the dishwasher drain line just before it connects to the sink drain.
(The output from a clothes washer is soapy water which shouldn't cause an odor issue)

So, the sink drain pipes are clean, and the traps are holding water to block sewer line gasses,
That leaves 2 remaining source of bad smells - a gargage disposal OR a dishwasher drain line.
My guess would be that the dishwasher's internal gunk filter is clogging up, so it's not pushing water fast enough to blow all of the food and gunk out of the dishwasher discharge tube. Since most dishwasher discharge lines are open to the air, you are probably smelling the gunk that has built up on the inside of the dishwasher discharge tube.

Cleaning out the basket at the bottom of the dishwasher should help.
If you have some play or extra length in the dishwasher discharge tube, you can take the extra length to make a downward loop in the discharge tube around just before it heads into the drain ( hold the top of the loop together with a bread twist tie or zip-tie). This achieves the same result as a "U bend" in a steel drain. Standing water blocks gasses and smells from lower down in the pipes.

 

Need a picture or diagram of how it is plumbed.

 

Thanks Hal_S but we do not have a waste disposal unit and the dishwasher is clean.

manden image attached.

 

I assume that the U trap is under the cutout so we cannot see it.
Is the corrugated tubing on the left the washing machine?
Is the black tubing at the rar right the dishwasher?
Do you get the smell under the sinks?
What type of dishwasher soap do you use? The reason I ask is that liquid and pods are not very strong so stuff can grow in the drain hose.

Often a dishwasher drain hose lays on the floor and water can collect there.
Many units have a flapper valve which keeps the drain from draining back into the dishwasher so you Would not smell anything in the dishwasher itself.
Run a cycle without any dishes and add a couple cups of vinegar during the wash cycle.
If you now smell vinegar that is where the smell is coming from.

 

Yes, the U bend is under the cutout
The corrugated tubing is the overflow from the small sink. Been taken off and cleaned. Not the culprit.
Yes, the black tubing is the washng machine. The dishwasher tubing is just underneath it as part of a T-junction. When this was disconnected we checked for odours from both - nothing!
There are no smells under the sink at all, only when you sniff the sink plug hole. When we took the U-bend off, we anticipated that the water in it would smell, but it didn't.

 

Well then I am also stumped.

You could try pouring some bleach down the drain to see if that clears it up.

 

Hi Manden

We are a bit stumped too. However, the smell is worse when the sink hasn't been used for a bit, and we didn't take to U-bend off when it was like this, only after some water had gone through it.

We will now experiment with taking the U-bend off again when the smell is there to see if the water in it smells!

If that is the case, what could be causing the water in the U-bend to smell, given that we have taken all the plumbing pipes and bends out and given them a thorough clean?

 

Try re-routing the dishwasher drain to go under the drain and then up - to create a "U trap" and see if it makes a difference...

 

Thanks both.

We are going to let the plug hole smell develop overnight tonight and then take everything apart again tomorrow to try and isolate where the smell originates.

I will reply again once we have done that.

 

I think we have solved it!

We have taken all the pipes apart and cleaned them again, only to find that the horizontal pipe from the smaller sink to the main outlet was the culprit - very stinky! No matter how well we have cleaned this, there is still a lingering odour that seems to have penetrated the plastic!

We will now replace this piece of the pipework, but are surprised that the odour has managed to impregnate the plastic after only four years of being installed!

 

There is more....

After we cleaned everything, the smell was still there, so we took all of the pipes off again.

On further inspection, the smell was still coming from the now disconnected plug hole of the smaller sink. This is the one at the left of the photo and you will see that there is a capped off pipe on the 'cup' where an overflow pip would fit. Only one of the sinks has an overflow.

We removed the capping from the end of this outlet. Bingo! You would not believe the stuff that was in there. YUCK!

Needless to say that it has all now been thoroughly cleaned and the problem has now been sorted.

I just thought I would let you guys know of this potential cause of smelly sinks :-)