The answer to 'why is my water softener foul smell' is that a smelly water softener is often caused by bacterial growth or mineral buildup inside the tank. This can result in a rotten egg or sewage-like odor coming from the softener.
Common causes include not regenerating the softener often enough, using low-quality salt, or letting the brine tank run dry. The odor may also be due to trapped air, a problem with the brine line, or issues with the resin beads inside the tank. To fix a smelly water softener, you'll need to clean and maintain it regularly, replace the salt, and potentially have a professional service it if the problem persists.