Water Pollution - The Drinking Water - page 167

process is feeding a growing enterprise. One alternative that is currently
being given widespread consideration in the United States is ozonation,
which uses ozone gas to kill microorganisms. Ozonation is Europe's pre-
ferred method, and it does not produce trihalomethanes, but the USEPA
does not yet recommend a wholesale switchover to ozone to replace chlorine
or chlorination systems utilizing sodium hypochlorite or calcium hypochlo-
rite. The USEPA points out that ozone also has problems, as it does not pro-
duce a residual disinfectant in the water distribution system, it is much more
expensive, and in salty water it can produce another carcinogen, bromate.
We discuss disinfection alternatives in greater detail in Chapter 11 .
At the present, what drinking water practitioners are doing (in the real
world) is attempting to fine-tune water treatment. What it all boils down to is
a delicate balancing act. Drinking water professionals do not want to cut back
on disinfection; if anything, they would prefer to strengthen it. So, we have
to ask how we can bring into parity the microbial risks versus the chemical
risks. How can both risks be reduced to an acceptable level? Unfortunately,
no one is quite sure how to do this. The problem really revolves around the
enigma associated with a “we don't know what we don't know” scenario.
The disinfection byproducts problem stems from the fact that most U.S.
water systems produce the unwanted byproducts when the chlorine reacts
to decayed organics: vegetation and other carbon-containing materials in
water. Communities that take drinking water from lakes and rivers have a
tougher time keeping the chlorine byproducts out of the tap than those that
use clean groundwater. When a lot of debris is in the reservoir, a water utility
may switch to alternative sources, such as wells. In other facilities, chlorine is
combined with ammonia in a disinfection method called chloramination . This
method is not as potent as pure chlorination, but it does prevent the produc-
tion of unwanted trihalomethanes.
In communities where rains wash leaves, grasses, and trees into the local
water source (such as a lake or river), hot summer days can trigger algae
blooms, upping the organic matter that can produce trihalomethanes. Spring
runoff in many communities exacerbates the problem. With increased runoff
comes agricultural waste, pesticides, and quantities of growth falling into
the water that must be dealt with. Nature's conditions in summer diminish
some precursors for trihalomethanes—the bromides in salty water. Under
such conditions, usually nothing unusual is visible in the drinking water;
however, water that is cloudy due to silt (dissolved organics from decayed
plants) could harbor trihalomethanes.
Most cities today strain out the organics from their water supplies before
chlorinating to prevent the formation of trihalomethanes and haloacetic
acids. In other communities, the move is already on to switch from chlorine
to ozone and other disinfectant methods. The National Resources Defense
Council (NRDC, 2003) believes that most U.S. systems will catch up with
Europe in the next decade or so and use ozone to kill resistant microbes such
as Cryptosporidium . When this method is employed, the finishing touch is