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I have a double kitchen sink that the contractors installed several years back. See picture. The main sink is on right side and smaller sink is to left. They installed the disposal to the main sink because space was tight and we have pull out trash drawer in space under shallower left hand sink. Our galley kitchen is small and we were trying to get the most use from the space.

Now when I run the disposal, nasty water shoots back up in to the left hand sink. At first I thought there might be a clog in the wall after the small drain but if there is a blockage, it is short lived because once I run the disposal for less than 30 seconds, the water goes back down in small sink and all is fine with no slow water draining in either small or large sink drains. I just want to eliminate that initial burst of nasty water in the small sink. I think the problem may have to do with air pressure due the design of the connections. There are separate connections to the wall for the sinks as you can see from the attached. And the main stack and vent are located about 4ft to left of double sink.

I am wondering if it would work better to connect the small sink to the same drain as the large sink if I could get fit together in the tight space.... I appreciate suggestions.

PS there is no sewer gas smell coming from the small sink as long as I remember to run a little water to fill that P trap before I run the disposal. But if I don't run water in the Left sink P Trap, I still get the nasty water shooting, but it just comes with the lovely aroma of sewer gas. Double sink layout

  • Can you provide another photo of the drain coming out of the disposal? It appears as though you're trying to get water to go up hill, but it's difficult to tell for sure. – Tester101 May 10 '16 at 14:40
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And the main stack and vent are located about 4ft to left of double sink.

It seems you needed your vent to be to the right of the sinks. Although the installation is apparently within code for the length of a single trap arm, when the disposal forces water into the drain it is backfeeding the small sink trap. Having the vent to the right would allow air pressure to equalize in that direction instead of through the small sink.

A simpler solution might be a backwater valve for the small sink. Although it may be difficult to find room for it but then you were thinking about moving the disposal so maybe there is room.

Good luck!

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  • Thanks for your input. I don't have room to move the GD to other sink so the backwater valve may be a good solution for the small sink drain. I am thinking it should it be placed on the wall side of the p trap so the nasty water doesn't settle in the small sink P trap...? – Eliza May 10 '16 at 11:32
  • Good idea on the backwater valve, especially as the small sink won't be handling any solids. – Daniel Griscom May 10 '16 at 13:40
  • Yeah you may be limited on where you can put the backwater valve. It looks like the glued the P-trap so you may have to cut it out. if you rotated the P-trap it looks like you could gain some distance from the wall and allow enough room to add the backwater valve. Certainly a whole lot cheaper and easier than redoing a lot of plumbing or adding another vent. – ArchonOSX May 10 '16 at 16:16
  • A check valve may alleviate one of the symptoms, but it does not address the root cause. – Tester101 May 11 '16 at 0:37
  • Well you can spend other people's money pretty easy. I would go for the $50 check valve repair. – ArchonOSX May 12 '16 at 16:35
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It's difficult to tell for sure based on the photo, but it looks like you're trying to make water flow up hill. It looks like the wall drain for the disposal is too high, which is causing too much water to be in the "trap".

GD Drain Trap too deep

I've marked the trap weir in the photo with a blue line, and indicated all the water that will be in the "trap" with blue squiggly lines. It's difficult to tell from the photo, but it looks like water might even be sitting in the disposal. That's way too much water in the trap, and it's going to cause problems.

I can't see the plumbing behind the wall, since my x-ray vision does not work on photos. However, I suspect that as the disposal dispels that large volume of water, some of the water is forced back into the second sink drain. Without knowing how the drain lines meet, it's difficult to say if it's done properly.

During normal operation the disposal won't be shooting a ton of water down the drain, so it shouldn't be enough to back up into the other sink. In your situation, the disposal is initially pushing a larger volume of water. The problem goes away after a bit of running, because the large mass of water has been eliminated, and it's back to a normal operating volume.

Eliminating the overly deep "trap" could alleviate the problem, though to do so you might have to modify the plumbing inside the wall. Lowering the drain line for the disposal, will also require you to lower the drain line for the other sink. This could require opening up a large section of wall, as the entire length of drain will likely have to be lowered.

You could check with the manufacturer of the disposal, to see if the elbow coming out of the unit is required. If not, you might be able to modify the plumbing under the sink to reduce the depth of the trap. However, if the disposal outlet is lower than the wall drain inlet, eliminating the elbow isn't going to make much difference.

You may have to move the disposal, or eliminate it altogether.

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  • Good eye. The output arm must be lower than the diposal. If that's the problem, then I think that the only modification that can be done is to lower the drain pipe. I don't think that you can modify the disposal in such a way that this could be corrected (by removing the elbow, for example)... it's still going uphill. But I think you discovered the problem. – Ben Welborn May 10 '16 at 15:26
  • The camera is somewhat above the top of the disposal, which may be magnifying the apparent problem. However, unless the rear wall is a significant distance behind the disposal it does look like there'll be standing water in the disposal. – Daniel Griscom May 11 '16 at 2:24
  • Thanks for all the helpful comments. I will snake the wall drain first and also see if I can figure out how they are configured behind the cabinet back. Once the snake is done and I know if it does/doesn't improve the situation, I may decide to cut out the back of the cabinet from the inside to see if I can do something about the inside wall drain height for the main sink with disposal. – Eliza May 11 '16 at 13:02

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