At this time, MWRA water from Quabbin and Wachusett Reservoirs is not filtered, though that too may change in years to come. The final barrier, also to eliminate pathogens from the water, is called disinfection . If water is boiled, most microorganisms in it will not survive. They aren't removed, but since they are no longer alive, they won't make you sick. Boiling can be an effective disinfection practice on a small scale, such as when camping or in an emergency, but on a large scale, such as municipal water supply, its energy costs would be enormous.
Chlorine is the most common chemical disinfectant, but ozone is also an important option.
The MWRA presently uses chlorine to disinfect, adding a small amount of ammonia to preserve the disinfectant effect as the water travels through aqueducts and pipes to the consumer. All these barriers have costs.