Hello everybody,

I am new to this forum and not a plumber. I recently moved into a home with a septic system. It is a one story home with three sinks...two at one end of the house and one at the opposite. The two sinks at one end are vented on a 11/2" vent stack. The solo sink on the other end is vented on a 2" vent stack, along with the toilet and shower/tub.

On all 3 sinks, which are all identical, when we turn the faucet on, we get a blast of raw sewage smell coming from the overflow on the given sink for 2 to 3 seconds.

The following is what I have tried to no avail:

Removing the p traps, stopping up the sink drain and allowing bleach to soak in the overflow and sink drains overnight

Replacing the P traps with deeper P traps

Flushing the toilets and removing the P traps to find they still had water inside of them (not suctioned out)

Making sure the vents were unobstructed

Adding additional vent piping to the stacks on the roof to clear the peak of the roofline

sealing the overflows and running the faucet(s) only to find the smell came up the main sink drain instead

The house is located in a deep canyon and was allegedly "finaled" in 2008, but sat vacant for 5 years. It appears to be plumbed correctly, according to every piece of literature I have managed to get my hands on. The solo sink, tub, toilet lines are the closest drains to the septic tank. The sink in that group is the closest drain to the main drain that elbows off to the septic tank. It is on a 2" drain line.

I am at a loss to explain this phenomenon. Every drain in the house has a P trap and drains perfectly. Any suggestions would be greatly appreciated.

Thanks in advance for helping me here.

I've heard of water supply lines giving water an odor, what is the supply lines made of ? Maybe your water has a little sulfur in it, sulfur smells more like rotten eggs, and it usually worse on the hot side.

Agree with NC. If the house sat vacant for 5 years and I assume you are on a well, I'd be eyeing your water supply as the culprit, not the drains.

An easy test would be to fill up a big pot with clean water, and in the morning, dump it into the sink. No stink = it's not the sink or drains.

remove a p-trap under the sink. stick your nose close to the open pipe and have someone flush a toilet. see what happens.
you already said the traps did not syphon when you flushed the toilet bvut this will only take a couple of minutes and will at least become one piece to this puzzle.
don't burn the house down but a little piece of burning paper that is snuffed out to make smoke that is held next to the open pipe will also tell alot about the flow of air inside the pipes when the toilet is flushed.

Was the septic system serviced and the tank emptied before you purchased your house? If not, I would have the cover lifted and see what is growing in your tank. Have you considered a powered vent for your vent system. Living in the saddle or vally of a mountain can create Barometric Pressures, not experienced on normal flat surfaces.
Best Wishes

i had the same sort of problem. the vents were all clear yet at least one sink had a sewer smell anytime water was run.

i explain here in way too much detail since i only have time to type thoughts right now.

here is what i theorized. the sewer gas was under slight positive pressure for a reason explained later. when you run water it produces bubbles of air in the trap. the slight positive pressure of sewer gas backs up through that temporary air bridge and exits in the sink. it is lighter than air so makes its way to the overflow holes.

but now why was there positive pressure at all. if the vent is really open then there should not be pressure in either direction.

even though it looked like the vent was completely open we found a horizontal pipe that serviced both the bathroom sink vent and the kitchen sink on the other side of the wall. there was a T at its center with a vertical section that eventually connected to the main vent pipe.

someone had clogged the drain from the kitchen sink with potato peels. lots of them. they plunged the thing for hours to clear the clog. but that forced peels back into and up that vertical pipe vent section.

the main vent was clear and so was the horizontal section between the two sinks but that vertical pipe section we didn't even know was there. was clogged.

that meant the sink was not really vented. from that, i put together the scenario above.

i know its complicated but in short. we found that the vent from the sink was actually clogged even though it didn't look so, since the main vent and the horizontal section (all that you could test) were fine. its hard to even get to that vertical section without tearing things apart or making a special tool as we did.

at least we solved this, what seemed for a while, impossible problem. and we know what happens when the vent is clogged. the same thing you are seeing (smelling)