How to fix a poor plumbing installation resulting in severe odor problems?

Question

After reading this related question and checking that the wax seal is OK, I assume I have a problem with the trap, but where's the trap? The outline of the plumbing in my home (a bit simplified) is like this:

Legend:

• Blue: the main pipe that comes from the bathroom to the city wastewater system through the house basement.
• Yellow: City wastewater (I can't touch it).
• Red: The problem.

Chronology:

• When I bought the house, there was a bad smell in the bathroom. I assumed it was because nobody lived there for months.
• After some weeks, the bad smell hadn't dissappeared, so I checked the bathroom (everything right), then I opened a manhole I found in my garden (the red part). I found it dry and had a strong bad smell.
• After asking to a professional plumber, we decided to make a trap there with a pipe. Something like this:

• This got rid of the bad smell for 3 months, but after it appeared again. Then we cleaned the strap and the bad smell dissappeared again for another 3 months.

Of course, that's not a good solution for 2 reasons:

1. We have to clean the trap (at least) 4 times every year.
2. Worst: after a couple of month the main pipe is full of water because it doesn't have enough slope. I'm afraid a day it can explode and flood the basement.

So, the question is: how can I fix this? There are no other traps in the house, should I make one? Or there is a simpler way to fix it?

Answer 1

On top of looking for a dried out trap I would check your vents. I had a squirrel build a nest in a vent pipe and plug it so when you flushed it pulled the water out of the other traps and let the smell in. I had to make a cap with vent holes to keep the little bugger out.

Answer 2

That's a very odd setup & I'm surprised by the Plumber's actions. If I were the Plumber, I would've piped a trap in the manhole hole & dropped a pipe to even just make loose contact or within 25 to 50mm of the city's pipe. This would either eliminate the quarterly cleanouts or drastically reduce the need for them.

And definitely, put traps under your sinks & even under the toilet if it's a straight shot situation now. But yes, the red box is your main & initial problem.

If you're not allowed to do any of the above, then your manhole cover needs to be turned into a manhole grate (this should be a requirement & figured out VERY long ago), not only for ventilation but also so rainwater can clean out the red box for you. After or along with that, put a trap absolutely wherever you want (just 1) in the blue line.