Cause

• Sulfate-reducing bacteria produce hydrogen sulfide, a gas that smells like sulfur or rotten eggs. They can live inside your hot water system, including your water softener. When enough bacteria build up inside your water softener, the water inside and the unit itself can smell like rotten eggs.
  
  

Solution

• You need to get rid of the bacteria to get rid of the egg smell. Your water softener can be drained and disinfected. A strong chlorine solution, or shock chlorination, can kill sulfate-reducing bacteria. Before attempting to clean your water softener, you should contact its manufacturer or a plumbing professional.
  
  

Considerations

• You should not ignore the problem of an egg smell in your water softener. Over time you could get used to the smell, but the bacteria causing it could damage your water softener. They can damage the resin bed in an ion exchange unit or produce a black slime that builds up inside of it.