When I moved into my condo, the main bathroom had a terrible smell, like backed up drains. No matter how much bleach I poured around the room, the smell rarely went away.

Eventually I got a plumber in to remove the toilet bowl, and he discovered that the condo builder had never connected the toilet outlet directly to the drain because they had misaligned the two, but that the waste flowed into the drain after passing over a few centimetres of the concrete flooring.

So the toilet is now repositioned and connected directly to the drain with a plastic conduit, but the smell remains. Could it be that the concrete has become tainted by the sewage that it was exposed prior to the initial repair? Do I need to lift the concrete and replace it?

Try white vinegar and baking soda. Yes concrete become contaminated.

I probably would. Mold building up underneath the tiles and all that nasty stuff.

Concrete is porus and I rekon its soaked up that crap like a sponge. I doubt you will get rid of the smell unless you get rid of the concrete.

Do you have any drains in the bathroom?

Experienced the same in the past. Bathroom drains didn't have a U bend fitted which allowed smells up into the bathroom. Drain pipes emptied into the sewer pipes.

Short of ripping the floor up to fit U bends I found a cheap solution was to fit a cheap rubber cover over the drains.

I've seen them sold in plumbing shops and small shops or stalls selling that type of stuff.

Do you have any drains in the bathroom?

Experienced the same in the past. Bathroom drains didn't have a U bend fitted which allowed smells up into the bathroom. Drain pipes emptied into the sewer pipes.

Short of ripping the floor up to fit U bends I found a cheap solution was to fit a cheap rubber cover over the drains.

I've seen them sold in plumbing shops and small shops or stalls selling that type of stuff.

I have spent quite some time with nose to drain and never detected a whiff of anything off, so I am certain that it is the concrete floor around the toilet itself :-/

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Concrete is porus and I rekon its soaked up that crap like a sponge. I doubt you will get rid of the smell unless you get rid of the concrete.

I am sure that you are right - have resigned myself to this.

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Farma nailed it. Without U traps before going out to the septic or country drain set up that smell will always backtrack up the toilet. Note The U-trap does not connect from the 4" main pipe to cause an obstruction & needs to act as a vent. Same as a J trap in your sinks.

I doubt the smell is coming from any soaked concrete. What little bit of sewage soaked into the concrete maybe a few millimeters will dry-out quickly. What if the commode and PVC pipe had been connected with a short piece of concrete pipe?...would a person still think that concrete pipe will absorb sewage and stink forever...I don't think so.

I expect your smell is the result of smells finding there way up from much further down due to the lack of U-traps somewhere...like maybe floor or sink drains. When I first moved into my new two story house I would get smells up through the bathroom shower and floor drains all the way up to the second floor sometimes...and very routinely to the first floor bathroom floor drain. Also occasionally some smell from one kitchen sink that did not have a U-trap on it. No smell through the commode but from the floor drains and that one sink drain.

The floor drains cover plates had those smell traps built-in, but for those traps to work there always has to be a little water in the little circular trap...hard to explain without pictures but if you have such drain cover plate traps lift one up and you can see. Since the shower drain would always have some water in its little trap due to my once a week shower, it didn't smell; but the floor drain in the corner of the bathrooms did since unless a person were spraying water on the floor everyday the water in those little drain cover traps would evaporate...no water and the smell can come through. The weather seemed to impact when and how bad the smell got.

Anyway, I found where the PVC drain pipes "from the entire house and septic tanks" emptied into the drain/sewer pipe & boxes running around the perimeter of my yard wall" (which in turned emptied into the soi drain/sewer pipes) and installed some home made U-traps with some of 90 degree connectors connected into a U-trap shape....cheap and easy. Smell gone....never returned...except my wife thinks daily vs weekly showers may reduce my smell...I'm still thinking about that recommendation.