Bring sugar and 1 l water to the boil in a large pot. Peel and slice the ginger. Cut lemon grass in half lengthwise and remove the outer leaves. Add the sticks and ginger slices to the sugar water and simmer for about 10 minutes.
Then allow to cool and chill for about 1 hour.
Wash the limes hot, dry them and dice them roughly. Pour mineral water over the ginger-sugar water and add the limes. Serve in glasses with ice cubes.