Chlorine is present in most municipal water supplies. It is added to water to eliminate bacteria or other contaminants. However, excessive chlorine can greatly affect other beer ingredients, resulting in a harsh aroma and flavor. Chlorine can easily be removed by boiling or filtration. Another unwanted aspect of public water is its lack of hardness.
Municipal water is generally soft due to the lack of minerals from filtration.
Beers brewed from soft water tend to exhibit a milder flavor than those made from hard water. Looking at the other end of the spectrum of water being too clean, it can also be contaminated. Bacteria is a major concern in water quality. The presence of bacteria can spoil the wort and cause a souring of the beer. Boiling is again effective in the removal of contaminants.