Water and Germs (Lesson 7)

Introduction

1. What are germs?
2. Getting germs
3. Germ environment
4. Germ travel
5. Symptoms
6. Water's role
7. Water suppliers
8. Rules
9. Conclusion

Diseases & pathogens
Note on E. coli

What can water suppliers do?

ON THIS PAGE:

Keeping pollutants out of source waters
Water treatment removes pollutants
Disinfection kills bacteria

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.

Keeping pollutants out of source waters
The first barrier is preventive. Minimizing contamination of source waters is called source protection. At Quabbin and Wachusett Reservoirs, human contact with the water is carefully limited. Because people aren't allowed to swim in the reservoir, or camp on its shores, exposure of the water to human pathogens is reduced. If the nearest homes or neighborhoods are some distance away, the chance of material from a leaking sewer line or a malfunctioning septic system reaching the reservoir while still viable (able to cause infection) is greatly reduced. Animal populations are monitored, and bird populations are encouraged not to congregate near any intake structure. In surface water systems, another term for source protection is watershed management, and it is a high priority of MWRA and the Metropolitan District Commission, who manage the reservoirs.

Water treatment removes pollutants
The next barriers against microbes in surface water are sedimentation (often called clarification) and filtration. All river sources and many reservoirs need clarification and filtration. Only the cleanest, best protected sources are able to skip these processes.

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 bacteria

The 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.

All these 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.

See the kinds of
germs we're talking
about

Lesson 8>