I want to brew malt + extract beer. I will boil about 10 liters of wort and then I will mix it with extra 10 liters of cold water. I don't want to boil that extra 10 liters if water the day before brew, so I wonder could I use campden to kill all yeast that water might have. How many grams of campden should I use per liter of water? How long should I let it stay before mixing it with wort and pitching yeast?