Here is the response from the Hofbrau Brewery, in Munich, when asked about water treatment:
The well water runs through the so-called double-deck filter , means it runs through gravel + active carbon filters, ion exchangers (strongly acidic) and remove Na +, Ca2 + and Mg +.
The degree of hardness is further adjusted with lime saturators, softening the water.
Then an activated carbon filter is used again for the perfect brewing water.
So it looks like they mainly taketh away, adding nothing to the water.