One of the first of the several steps that municipal water suppliers use to prepare water for distribution is getting it as clear and as particulate-free as possible.  To accomplish this, the water is treated with aluminum sulfate, commonly called alum, which serves as a flocculant.   Raw water often holds tiny suspended particles that are very difficult for a filter to catch.  Alum causes them to clump together so that they can settle out of the water or be easily trapped by a filter.  

Usually  a mixture of water with 48 percent filter alum  is injected into the raw incoming water at a rate of 18 to 24 parts per million. The alum promotes coagulation of fine particles which helps resolve problems of color as well a turbidity. If the process is given enough time to work and is applied properly, it not only corrects problems in the water but actually results in removing most of the aluminum used in the process.  

Although concern over the safety of treating water with aluminum has often been voiced, there is no evidence that aluminum in water, whether it comes from the aluminum sulfate used in treatment or from other sources, is a health issue. Actually, most aluminum that we take in does come from other sources.  One study showed that only between 0.4% and 1.0% of our lifetime intake of aluminum comes from alum used to prepare municipal water. Most aluminum intake is from aluminum that occurs naturally in foods, aluminum used in food packaging, and from products like deodorants and vaccines.

Water treatment for aluminum is normally not needed, but aluminum is easy to remove with reverse osmosis or distillation.

See also “Simple Facts about Aluminum.”

More about alum from other sources:

There are a variety of primary coagulants which can be used in a water treatment plant.  One of the earliest, and still the most extensively used, is aluminum sulfate, also known as alum.  Alum can be bought in liquid form with a concentration of 8.3%, or in dry form with a concentration of 17%. When alum is added to water, it reacts with the water and results in positively charged ions.

Coagulation/flocculation is a process used to remove turbidity, color, and some bacteria from water.  In the flash mix chamber, chemicals are added to the water and mixed violently for less than a minute.  These coagulants consist of primary coagulants and/or coagulant aids.  Then, in the flocculation basin, the water is gently stirred for 30 to 45 minutes to give the chemicals time to act and to promote floc formation.  The floc then settles out in the sedimentation basin.

Coagulation removes colloids and suspended solids from the water.  These particles have a negative charge, so the positively charged coagulant chemicals neutralize them during coagulation.  Then, during flocculation, the particles are drawn together by van der Waal's forces, forming floc.  The coagulation/flocculation process is affected by pH, salts, alkalinity, turbidity, temperature, mixing, and coagulant chemicals.

 Source: http://water.me.vccs.edu/courses/env110/lesson4_3.htm

Aluminum sulfate is widely used as a flocculant in water treatment plants in the United States. It is also widely available in developing countries, sold in blocks of soft white stone, and generally called alum. There are numerous ways to use alum as a flocculant, including to crush it into a powder before adding it to water, stirring and decanting or stirring the whole stone in the water for a few seconds and waiting for the solids to settle. The benefits of alum are that it is widely available, is proven to reduce turbidity, and is inexpensive. The drawback of alum is that the necessary dosage varies unpredictably. Laboratory studies have shown that alum is effective at reducing turbidity and chlorine demand 3.

Source: http://www.cdc.gov/safewater/chlorination-pretreatment.html#3