1. What are germs?
2. Getting germs
3. Germ environment
4. Germ travel
6. Water's role
7. Water suppliers
Diseases & pathogens
Note on E. coli
can water suppliers do?
So, since microbes are everywhere, what are the protective measures we can take? We have already begun to discuss this in general terms, but let us consider a specific series of barriers that water suppliers can put in place to preclude the spread of disease through drinking water.
pollutants out of source waters
In clarification tanks, a simple chemical is often added to encourage small particles, whether soil or vegetation or micoorganisms, to clump together. Then they either settle at the bottom or float to the top, leaving the water much cleaner. MWRA does not have clarification tanks now, though we may in the future. Our water resides so long in Quabbin and Wachusett Reservoirs that natural sedimentation cleans the water effectively.
In filtration systems, water seeps through layers of sand or other materials, imitating nature's filtration of groundwater. If a filter has small enough pores, water will seep through, but solid particles, including microorganisms, will be screened out. At this time, MWRA water from Quabbin and Wachusett Reservoirs is not filtered, though that too may change in years to come.
Disinfection kills bacteriaThe final barrier, also to eliminate pathogens from the water, is called disinfection. Disinfection can be accomplished in two ways: by heat or by chemicals. 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. Water suppliers use chemical disinfection to deactivate (kill) microorganisms. 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.
barriers have costs. Water suppliers must consider the quality of both
their source and treated ("finished") water, as well as cost, to decide
which form of protection is best.