Chlorination

Tap water often has chlorine added to prevent the formation of undesirable microbes. Chlorination does not affect the work of commercial yeasts, but has a negative effect on levains (starters). This is because chlorine destroys the friendly bacteria that assist the wild yeast. Chlorine is a gas, so letting the water rest uncovered in the refrigerator for 24 hours will allow the gas to dissipate, making it safe for use in starters.