One of my tenants took a vacation recently (one week long) and upon returning he found his toilet bowl water that was clean and clear before now has a darker stain, and the water smells. He suspect it is some kind of a backup.

There are two other apartments in this single story building.

He flushed the toilet and it flushed fine, mo issue, the problem has not returned. No issue with the adjacent bath tub either. This is an older property from 1955 so most likely cast iron lines but connected to the city sewer in 2008.

He said it happened once before several months ago also went he went on a business trip for several days.

I have a hard time envisioning sewage backing up all the way passed the closet bend passed the wax ring and up to the top of the neck of the bowl...is this possible? If there is such a backup then the whole line would have been clogged bad but everything drains fine.

So I thought may be a slow leak from the tank containing rust stain from the tank metallic parts, but opening the tank cover the inside parts are in good shape with no sign of any corrosion even the tank bolts look fine, and no leak from the flush valve either. Besides, he mentioned a bad sewage odor and he said he is sure it didn't come from the tub.

Any idea what this could be and where I should look? Should I go flush the toilets of other apartments and see if there is any sort of backflow or bubbles introduced into his toilet?

and why would this only exhbit after a few days of non-use? A few days is not enough to have the trap water evaporate enough to break the seal.

Algae? Bacteria? Next time they go away, they could try adding some bleach to the water in the bowl.

If it's bateria would it be from the surface of the bowl? If flushed regularly it's kept under control if stagnant it builds up and causes the stain and stink?

when the chlorine escapes into the air, the tank becomes a breeding ground for any airborne contaminants, especially with the cover open.

I know you have experience with pools sum. With that luke-warm water you have down there, I can see things blooming really quickly. KC makes a good point....add 1/2 cup of bleach in the bowl, and cover it with plastic wrap.

Paul, I thought wraping the bowl opening with plastic wraps was a prank we pull in high schools so that half drunk roommates would take a piss and have the pee rebound into their pants!

Yeah....It's also how you train kitty to use the toilet.

Sum, tell the tenant to fill a bucket with water(siphon out from toilet tank?),set it aside and see if it gets funky.

Steve, thanks I'll do that.

So it seems everyone is thinking this is some bacteria and not some nasty backup from the line below. which makes sense.

Is this not a perfect time to use one of those chlorine tablet products people put into their toilet tank? I am thinking of telling him to do that instead. I know it may damage the plastic flapper over time.

I wouldn't, if it is only after extended lack of use. I still think dealing with the standing water in the bowl, is the way to go.

Verify bacteria problem.

Hand wipe UNDER the bowl rim with a wad of toilet paper --- UGG -- Wash hands good.

Toilet should be cleaned best possible with a brush with a little bleach in the bowl ----- UNDER the bowl rim especially as it gets a lot of "splash back" and may not be cleaned properly.

Of course caution not to splash any bleach water on clothes etc.