If the water contains just chlorine then standing overnight will usually allow the chlorine to escape. If the volume of water is huge (commercial brewing scale) then some agitation or sparging with air may be necessary. If the utility uses chloramine (chlorine chemically bound with ammonia) then standing still works but it takes much longer (a week perhaps) for it all to dissipate. When Campden tablets are used both free chlorine and chloramine (some free chlorine is usually found where chloramine is used) are converted to chloride ion which is not harmful to beer. In fact, it is beneficial in most cases. The conversion to chloride takes place quickly.