What Causes a Bad Smell from a Shower Drain?
There are a number of causes of a bad smell from a shower drain. Clogs inside a drain typically composed of hair, soap film, and oils can amass and begin to decay inside the shower drain, resulting in a foul odor. Biofilms or a sheet of bacteria and mold that clings to the pipes may also produce a bad smell. A nasty drain smell may also be caused by sewer gas leaking back up the tub drain. When a bad drain smell is not caused by one of these problems, the offensive smell may only seem to be coming from the drain, but could actually be a separate bathroom issue.
Clogs form gradually over time as particles that pass through the shower strainer collect along the sides of the pipe. The organic matter in the clog will then begin to rot, giving off a foul smell from a shower drain that will become increasingly pungent. If there is a bad smell coming from the drain and the water takes long to run from the tub, then a clog is more than likely the cause of the bad smell. Removing the clog should eliminate the smell.
Biofilms can also be the cause of a awful smell from a shower drain. Composed of layers of bacteria, bacterial waste and mold, biofilms can grow in any aqueous environments that provide adequate nutrients, such as shower drains. When water has a small quantity of sulfate in it and some of the bacteria in the biofilm are capable of reducing sulfur, the biofilm will produce hydrogen sulfide. This chemical has a powerful and distinct odor of rotten eggs. Flushing bleach down the drain may destroy the noxious odor-producing bacteria.
Another cause of a bad smell from a shower drain can be the back flow of sewer gas into the drain. Below the shower drain is a pipe that is curved. This bend, know as the trap, is supposed to be completely filled with water to prevent sewer gas smell from passing up into the house. When a shower is not used, the trap can dry out and sewer gas can pass up through the drain. Running water to refill the trap can eliminate this smell.
Sewer gas smell emanating from a shower drain may also be caused by a block bathroom vent. When water passes into a drain, air is forced out of the pipe. The air in the pipe is resupplied by the roof plumbing vent rather than through the drain. If the roof vent gets clogged by a bird nest or something else, the air refilling the pipes can force the water from the trap, allowing the sewer gas to run back up the drain.
Occasionally, the drain can be mistakenly blamed for a smell that is actually coming from somewhere else in the bathroom. A leak in the shower or tub tile, allowing water to flow behind the tile, can create a warm moist environment for mold to grow. Mold can have an overpowering odor and is sometimes mistakenly traced to the shower drain. A dead animal decaying in between walls can also create a very overpowering smell that can be difficult to trace.
What can I do to flush the smell out?
Some people just have bad pipes. Particularly if they are old pipes or it's an old house. They just seem to have a tendency to get backed up all the time, or to have problems with other systems in the house (like the sewer system).
If you are continually having to deal with problems, the easiest solution in the long run will probably be to replace the pipes altogether.
@Iluviaporos - It is important to try and make sure you don't get mold growing in the house, but most of the time it will be harmless, to the family at least.
And it usually doesn't cause a bad smell in the bathroom. In my experience it smells musty rather than sulfurous. Usually if there is a bad smell, it's because there is something wrong with the drains.
You really have to remove hair regularly from shower drains so they don't clog up, particularly if anyone in the house has long hair. Most of the time it won't cause that much of a problem, because soap kills any bacteria that might happen to get caught up in it, but eventually, if it forms a plug, you can get all kinds of nasty issues with the drains.
It's really important to track any kind bad smell in the bathroom down, because it can be dangerous. Any mold growing in human houses can be extremely dangerous and cause all kinds of long term problems, particularly for children. I've known kids who suffered terribly from asthma who basically were completely cured when their parents moved house and it turned out to be mold was causing the breathing difficulties.
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