A sewage or musty smell when a tap runs is alarming and, more often than not, has nothing to do with the water. Bacterial growth in the drain releases odour as running water disturbs it, and that reaches your nose at exactly the moment the tap opens, which makes it seem like the water. One test separates the two in under a minute.
The glass test
Fill a glass from the tap, then carry it to another room or outside and smell it there, away from the sink. If the water smells fine in the other room, the odour was coming from the drain rather than the water, and no treatment system will help. If the glass still smells, the odour is in the water and the investigation moves upstream. Do this before anything else, because it costs nothing and it resolves the majority of these complaints.
Cleaning a smelly drain
The growth lives in three places: the tailpiece below the plughole, the basin's overflow channel, and any waste disposer. The overflow is the one most often missed, which is why a smell returns after a basin has apparently been cleaned thoroughly, and it needs a brush and a suitable cleaner rather than pouring something down the plughole. Flush with hot water, clean the disposer per its instructions, and repeat if the smell has been established for a long time.
If the water itself smells
Distinguish it from the rotten egg smell of hydrogen sulfide, which is sulfur and has separate causes including the water heater. A genuinely musty or sewage like smell in the water suggests bacterial growth in the plumbing or, on a well, septic influence reaching the source. On a municipal supply it can follow work on the mains or water standing in a little used branch. The EPA notes every community water system publishes an annual report and the utility should be told either way.
What to test
On a well, coliform bacteria and nitrate, alongside an inspection of the casing, the sanitary cap and the ground slope around the wellhead, since septic influence usually means surface water or effluent is reaching the well. On a municipal supply, report it and ask whether there has been work nearby. In both cases test before buying treatment, because the equipment that addresses bacterial contamination is different from anything that addresses an odour. Our softener installation guide covers the treatment train.
Questions people ask about water smells like sewage
How do I tell whether the smell is the drain?
Fill a glass and smell it in another room. If it smells fine away from the sink, the odour came from the drain.
Where does drain odour come from?
Bacterial growth in the tailpiece, the basin overflow and any waste disposer. The overflow is the part most often missed.
Is a sewage smell the same as rotten eggs?
No. Rotten eggs is hydrogen sulfide, a sulfur problem. A musty or sewage smell suggests growth or septic influence.
What should I test for on a well?
Coliform bacteria and nitrate, along with an inspection of the casing, cap and the ground around the wellhead.