Water hardness is defined as the amount of dissolved calcium and magnesium in the water. Hard water has a lot of calcium and magnesium; soft water doesn’t. Water softeners work by chemically replacing the calcium and magnesium in the water with sodium or potassium.
And here is the problem for brewers: recall that I said that good brewing water should be moderately hard. It should have a minimum level of total hardness of about 150 ppm as calcium carbonate (CaCO3). Water softeners remove the hardness but leave the alkalinity behind.
Hardness and alkalinity are opposites. While hardness is calcium and magnesium concentration, alkalinity is carbonate and bicarbonate concentration. Alkaline water is high in bicarbonates. The alkalinity in water acts to raise the pH of the water and the beer, and this can be a problem for the beer flavor, especially for the paler styles.
However, the pH of the water isn’t what really matters. What really matters is the chemistry of the mash and wort. The grain bill can significantly affect the pH of the mash. For example, using dark roasted malts in the mash can neutralize alkaline water to achieve a proper mash pH.
So while knowing the water pH is slightly useful, the mineral composition of the water—and its effect on the wort and beer pH—is most important. Higher beer pH makes the beer taste dull. (Try adding half a spoonful of baking soda to a glass of tomato juice to taste the effect.) Low beer pH attenuates the beer flavors, and the beer will lose complexity.
How much alkalinity is high? Generally, high alkalinity is anything greater than 100 ppm as calcium carbonate. However, alkalinity greater than 50 ppm can be considered high for extract brewing because you are rehydrating a dehydrated wort that already has minerals and alkalinity in it. The alkalinity in your water will add to what’s already there.
Here is the bottom line: If your water is softened or highly alkaline, you should not use it for either extract or all-grain brewing. The alkalinity of water can be reduced by aeration and pre-boiling or by diluting it with distilled or reverse-osmosis water.