Environmental Engineering Reference
In-Depth Information
cant drinking water outbreak of cryptosporidiosis was in Mil-
waukee Wisconsin from March to April of 1993, the worst waterborne disease out-
break in the US history. Two water treatment plants supplying water to Milwaukee
used water from Lake Michigan. Both plants used conventional treatment of coag-
ulation,
The most signi
filtration , and chlorination treatment
(Solo-Gabriele and Neumeister 1996 , p.81). Again the failure to remove a protozoon
indicates that these plants functioned as no more than Class 2 treatment systems.
Based on the evidence and the above classi
flocculation, sedimentation, rapid sand
cation system, we are led to the
conclusion that the conventional treatment plants in North America are at best Class
3, and no more than Class 2 when they fail to remove protozoa. Note that this
conclusion is based on treatment technologies and not on the quality of
nal drinking
water, which may be quite good in some areas, depending on the characteristics of
the source water; our focus here is on treatment.
It should also be noted that after a large fall in unit costs of ozonation, many
water utilities are choosing ozonation 1 as the primary treatment option (Class 4). In
Europe the treatment of choice is granular activated carbon, which we classify as
Class 5a. Granular activated carbon (GAC) has been used extensively for the
removal of dissolved organics from drinking water. In the early 1970s, it was
reported that bacteria, which proliferate in GAC
filters may be responsible for a
fraction of the net removal of organics in the
filter. Following this discovery, pre-
ozonation was found to enhance significantly the biological activity on GAC. The
combination of ozonation and GAC is commonly referred to as the biological
activated carbon (BAC) process, or biologically enhanced activated carbon process.
This was implemented in many large water treatment plants in Europe in the 1980s
(Dussert and Stone 2000 ). The ef
cacy of activated carbon in removing all sorts of
contaminants has been further con
rmed by Rodriguez-Mozaz ( 2004 ).
Advanced oxidation processes (with ozonation or UV-based) are essentially the
same as Class 5a, but experiments show a greater ef
cacy of removal of the same
contaminants as those in Class 5a; we, therefore, classify Advanced Oxidation
processes as Class 5b.
We should also note that for 90 percent of the residents of Ontario, the source water
is the Great Lakes, which also receive wastewater that is not always treated to remove
chemicals, particularly pesticides, pharmaceuticals and personal care products; this
topic is deferred to the chapter dealing with wastewater and its impacts on drinking
water.
1 Ozone (O 3 ) and its primary reactive products, the hydroxyl free radical (OH*), are strong
oxidizing agents. However, ozonation can also lead to the formation of potentially harmful
byproducts that include bromate ions (BrO 3 ), aldehydes and peroxides. The use of O 3 as an
alternative disinfectant to chlorine will not produce chlorinated trihalomethanes (THMs), halo-
acetic acids (HAAs) or other chlorinated byproducts; but it can react with natural organic matter
(NOM) to produce a variety of oxidation byproducts that typically include aldehydes, aldo- and
keto-acids, carboxylic acids and peroxides. However, there are technologies that can be used to
minimize these byproducts. For more information on the chemistry of ozonation and how to
minimize these byproducts, see: http://www.wwdmag.com/micro ltration/strategies-minimizing-
ozonation-products-drinking-water .
 
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