3/4 c. Very hot water
1/2 c. White vinegar
1 tbsp. Hydrogen peroxide (omit for dark colored carpets)
1 tbsp. Liquid castile soap
1 tbsp. Powdered citric acid
20 drops Grapefruit seed extract
Combine all ingredients in a bowl and blend well.
Pour formula into your steam cleaner and clean carpet.
Source: Natural Home, July/August 2010
Don't want to make your own? Here are some safe alternatives: