hearings were prompted in part by several major disease outbreaks in recent
years:
Included were a large outbreak of gastroenteritis (thought to be viral in
origin) in 1968 in Angola, New York, and an outbreak of infectious hepatitis
?
aecting the Holy Cross football team in 1969.
One of the problems raised at the hearings was the general lack of knowl—
edge of the spread of viral infection by water. It was pointed out that while
there is the prospect of increased reuse of water, there have been few studies
‘
of viruses in reuse systems over long periods of time and we know little about
the movement of viruses in groundwater or the ability of water treatment
plants to remove viruses [49]. The 1962 PHS drinking water standards do not
include limits for viruses. Although the Advisory Committee on Revision of
U. S. Public Health Service 1946 Drinking Water Standards discussed setting
standards for viruses, it was concluded that not enough was known about
them to limit them or to recommend a routine procedure for their exam—
ination in water supplies. However, it was recommended that those labora—
tories equipped to isolate and identify viruses should evaluate enteroviruses in
;
treated water [50].
A scientic paper submitted to the “Safe Drinking Water” hearings summarized the results of a variety of reports on the efciency of various aspects
of water treatment in deactivating viral contaminants [51]. There are few
eld data, but laboratory experiments revealed a number of deciencies. The
storage of water—one step in some water treatment systems—does not always
help to reduce virus count. In warm months, it may take several weeks to obtain a 99.9 percent reduction of viruses; during prolonged cold weather, vi—
ruses may remain active for months. There is also evidence that infectious
hepatitis agents survived ten weeks in well water stored in a laboratory. Fil—
tering—another step in water treatment—may be ineffective in removing vi—
ruses if the ow of water is too rapid [52]. For those plants With a multistep
system, the last step in the treatment process
is usually chlorination. Some
'
laboratory data show that many viruses are more resistant than bacteria to
chlorine treatment. For example, a 99.9 percent removal of E. coli was obtained in ve minutes with a chlorine concentration of 0.04 ppm,
while it
tOok 30 minutes to remove 99 percent of the infectivity of one type of poli0
virus with a chlorine concentration of0.11—0.20 ppm [53].
When chlorine is added to water, it reacts initially with the Chemicals in
the water. The chlorine remaining after these initial reactions—called the free
residual chlorine—is what is actually available» for killing bacteria. The PHS
recommends that a free chlorine residual of 0.2 ppm be obtained at the treat—
ment plant, and a residual of 0.05—0.1 ppm be maintained in the distribution
system [54]. Most states do not set standards for chlorine residuals, but labora—
tory studies have indicated that a chlorine residual higher than these recom—
mended concentrations would be necessary to kill viruses. In a multistep
treatment process, virus removal could be increased by chlorinating twice—at
the begirming of the process and again at the end [55].
Flocculatz‘on or coagulation—the clumping of particles so that they will
settle which is one part of the clarication step in water treatment—can be
166
Drinking Water
years:
Included were a large outbreak of gastroenteritis (thought to be viral in
origin) in 1968 in Angola, New York, and an outbreak of infectious hepatitis
?
aecting the Holy Cross football team in 1969.
One of the problems raised at the hearings was the general lack of knowl—
edge of the spread of viral infection by water. It was pointed out that while
there is the prospect of increased reuse of water, there have been few studies
‘
of viruses in reuse systems over long periods of time and we know little about
the movement of viruses in groundwater or the ability of water treatment
plants to remove viruses [49]. The 1962 PHS drinking water standards do not
include limits for viruses. Although the Advisory Committee on Revision of
U. S. Public Health Service 1946 Drinking Water Standards discussed setting
standards for viruses, it was concluded that not enough was known about
them to limit them or to recommend a routine procedure for their exam—
ination in water supplies. However, it was recommended that those labora—
tories equipped to isolate and identify viruses should evaluate enteroviruses in
;
treated water [50].
A scientic paper submitted to the “Safe Drinking Water” hearings summarized the results of a variety of reports on the efciency of various aspects
of water treatment in deactivating viral contaminants [51]. There are few
eld data, but laboratory experiments revealed a number of deciencies. The
storage of water—one step in some water treatment systems—does not always
help to reduce virus count. In warm months, it may take several weeks to obtain a 99.9 percent reduction of viruses; during prolonged cold weather, vi—
ruses may remain active for months. There is also evidence that infectious
hepatitis agents survived ten weeks in well water stored in a laboratory. Fil—
tering—another step in water treatment—may be ineffective in removing vi—
ruses if the ow of water is too rapid [52]. For those plants With a multistep
system, the last step in the treatment process
is usually chlorination. Some
'
laboratory data show that many viruses are more resistant than bacteria to
chlorine treatment. For example, a 99.9 percent removal of E. coli was obtained in ve minutes with a chlorine concentration of 0.04 ppm,
while it
tOok 30 minutes to remove 99 percent of the infectivity of one type of poli0
virus with a chlorine concentration of0.11—0.20 ppm [53].
When chlorine is added to water, it reacts initially with the Chemicals in
the water. The chlorine remaining after these initial reactions—called the free
residual chlorine—is what is actually available» for killing bacteria. The PHS
recommends that a free chlorine residual of 0.2 ppm be obtained at the treat—
ment plant, and a residual of 0.05—0.1 ppm be maintained in the distribution
system [54]. Most states do not set standards for chlorine residuals, but labora—
tory studies have indicated that a chlorine residual higher than these recom—
mended concentrations would be necessary to kill viruses. In a multistep
treatment process, virus removal could be increased by chlorinating twice—at
the begirming of the process and again at the end [55].
Flocculatz‘on or coagulation—the clumping of particles so that they will
settle which is one part of the clarication step in water treatment—can be
166
Drinking Water