Environmental Engineering Reference
In-Depth Information
treatment plants are overloaded or break down. Only laboratories with spe-
cialized capabilities can detect the presence of
Cryptosporidium
oocysts in
water. Unfortunately, current sampling and detection methods are unreli-
able. Recovering oocysts trapped on the material used to filter water samples
is difficult. Once a sample is obtained, however, determining whether the
oocyst is alive or whether it is the species
C. parvum
that can infect humans is
easily accomplished by looking at a sample under a microscope. The number
of oocysts detected in raw (untreated) water varies with location, sampling
time, and laboratory methods. Water treatment plants remove most—but not
always all—oocysts. Low numbers of oocysts are sufficient to cause crypto-
sporidiosis, but the low numbers of oocysts sometimes present in drinking
water are not considered cause for alarm in the general public.
Protecting water supplies from
Cryptosporidium
demands multiple barri-
ers, because
Cryptosporidium
oocysts have tough walls that can withstand
many environmental stresses and are resistant to the chemical disinfectants
such as chlorine that are traditionally used in municipal drinking water sys-
tems and swimming pools. Physical removal of particles, including oocysts,
from water by filtration is an important step in the municipal water treat-
ment process. Typically, water pumped from rivers or lakes into a treatment
plant is mixed with coagulants (see
Chapter 11
) which help settle out parti-
cles suspended in the water. If sand filtration is used, even more particles are
removed. Finally, the clarified water is disinfected and piped to customers.
Filtration is the only conventional method now in use in the United States for
controlling
Cryptosporidium
.
Ozone is a strong disinfectant (see
Chapter 11
) that kills protozoa if suf-
ficient doses and contact times are used, but ozone leaves no residual for
killing microorganisms in the distribution system as does chlorine. The high
costs of new filtration or ozone treatment plants must be weighed against the
benefits of additional treatment. Even well-operated water treatment plants
cannot ensure that drinking water will be completely free of
Cryptosporidium
oocysts. Water treatment methods alone cannot solve the problem; water-
shed protection and monitoring of water quality are critical. Land use con-
trols such as septic system regulations and best management practices to
control runoff can help keep human and animals wastes out of water.
Under the Surface Water Treatment Rule of 1989, public water systems must
filter surface water sources unless water quality and disinfection require-
ments are met and a watershed control program is maintained. This rule,
however, did not address
Cryptosporidium
. The USEPA has now set standards
for turbidity (cloudiness) and coliform bacteria (which, as you remember,
indicate that pathogens are probably present) in drinking water. Frequent
monitoring must occur to provide officials with early warning of potential
problems to enable them to take steps to protect public health. Unfortunately,
no water quality indicators can reliably predict the occurrence of crypto-
sporidiosis. More accurate and rapid assays of oocysts will make it possi-
ble to notify residents promptly if their water supply is contaminated with
