The answer to 'water softener foul smell' is that a rotten egg or sulfur-like odor coming from your water softener is usually caused by a buildup of bacteria in the system. This is a common issue that can occur when the softener isn't maintained properly.
Some common causes include infrequent regeneration, low salt levels, and stagnant water. The bacteria feed on the salt and produce hydrogen sulfide gas, which creates the unpleasant smell. If left untreated, it can also lead to corrosion of metal components in the softener.