Sewage Backing up in Kitchen Sink

[Bushi54]

Help! The shed in my backyard has a bathroom which is connected to some kind of underground tank. The tank has an electric cord going to it (electric pump?) and ocassionally it sends sewage through an underground 2.5 inch PVC pipe that goes across my yard into my slab foundation near my kitchen. When that happens, raw sewage comes up in my kitchen sink (but not in any other sinks or even my floor level drains in the shower or bathtubs). The sewage drains out of the sink in a few seconds. This happens whenever I use the facilities in the shed and sometimes when it rains (even if no one has used the facilities in the shed for days...) Why is this happening? Is there an easy solution? Can I put a backflow preventer or check valve below the p-trap of the kitchen sink where the drain comes out of the slab foundation? Or is my only solution to dig up my yard and try to hook the shed up to the main sewer?

Thanks,
George

[prashster]

I speculate that your shed's sewage ejector joins your house drain via the sink drain branch. If that line is only 1.5", then the ejector volume might be too much for the drain. Having nowhere to go, it backs up in the sink. As the 1.5" drain clears, your sink clears too. BTW, I think your pump's discharge is 2" - not 2.5". Correct?

Can you identify how the 2" sewage pump line joins your main drain?

A backflow preventer is NOT the way to go. If the problem is a small drain, then the sewage - having no outlet anymore, will back up into the pump itself, and quickly burn it out.

[Bushi54]

Thank you for taking the time to reply! You are correct - the pump discharge is 2" PVC instead of 2.5". That pump discharge pipe goes about 50 feet underground from the shed and comes up next to the house where it turns into the slab foundation. At that point, the PVC elbow connects to a horizontal black metal pipe that is approx. 3.5" (outside diameter). That metal pipe appears to be about 4-5" below the floor of the house inside the concrete slab. The kitchen sink is about 15 feet from where the PVC elbow connects to the black metal pipe. Under the kitchen sink is a similar black metal pipe (approx. 3.5 OD) that comes vertically out of the slab about 1" and is connected to a greyish metal "Y" pipe that is approx 2" (outside diameter). One arm of that "Y" goes to a vent stack (correct term?) about 8 feet away and the other arm goes to the P-trap (correct term?) and then the sink/disposal/dishwasher. I don't know what is going on under/in the slab, but I suspect that the horizontal black metal pipe is connected to most of the drains in the house and is eventually connected to the sewer. I was hoping that if I put a backflow preventer just after the "Y" but before the P-trap that the problem would be solved - either the extra sewage would be forced through the horizontal black metal pipe quicker or would go up the vent stack arm of the "Y" instead of the kitchen sink arm. I may just have an obstruction in the horizontal black metal pipe after the kitchen sink connection that needs to be cleared. Do you think I'm understanind the system correctly? Do you think the backflow preventer will work and not burn out the pump? Is the connection outside the house that I described (pump discharge pipe connected to the horizontal black metal pipe) sound like someone hooked up the shed to my house's clean-out access?

Thanks again for any advice that you can provide.

[prashster]

Important note, I'm not a pro. All my opinions are speculation...

What you described leads me to believe it's NOT improper drain sizing. I suspect there's a blockage where the ejector enters the main line. Ironically, the cleanout is where you'd go to inspect this. Can you disconnect the pump line and inspect in there?

Did you always have this problem or only recently? Also, describe how the pump line enters the main line. Draining sewage through the cleanout into the main line seems like a bad idea, since the turn is a sharp 90 degree turn. If you've added another 90 degree elbow to aim towards your pump then the flow of solids could be greatly reduced, causing a block there or even further in the line.

Again, I wouldn't put a backflow preventer on your trap. You don't want water or sewage to back up into the vent, the pump or anywhere for that matter. The vents have to be clear for air to allow proper draining. You might be pushing the problem to another portion of the system that you can't predict.