Drinking Water Quality for the 21't Century
291
the regulated dose requirements, chlorine dioxide will be used to treat surface raw
waters for algal control only when the demand is less than about 1.4 mg rl.
Otherwise it is typically added following removal of demand materials after
sedimentation. This disinfectant has certainly been demonstrated to have equal, if
not superior, performance to chlorine as, for example, in the inactivation of
Giardia lamblia (White 1999) and in some cases might be an economically viable
alternative where there would normally be a high chlorine demand by the water.
When considered for disinfection purposes at sites that have traditionally used use
chlorine gas, it is possible to modify the chlorination process with introduction of
sodium chlorite to generate chlorine dioxide (Aieta and Roberts 1985). As a
selective oxidant, chlorine dioxide does not produce the halogenated species
associated with chlorine. Although there is not the wealth of literature on organic
byproducts as compared to chlorine (which in and of itself is a clear statement!), it
appears that what have been identified in finished water are mostly mono- and
dicarboxylic acids (Richardson 1998). Terminal disinfection with chlorine dioxide
is not usually practiced due to the odor and chlorite limitations described above.
Furthermore, it is suspected that using terminal chlorination on a water containing
residual chlorite from upstream chlorine dioxide application may generate
chlorine dioxide in the distribution system. If this is a concern, chloramination
should be practiced.
4.11 Mixed Oxidants
Contamination of drinking water sources by fecal matter is an international
problem, especially where watershed management is not maintained. In addition
to the well-documented viruses and bacteria, parasites such as Cryptosporidium
parvum continue to make their presence felt, and demand more safeguards built
into treatment technology to prevent major epidemics. While traditional
disinfectants such as chlorine have been effective against bacteria and viruses,
these parasites require more powerful treatment to remove them. While mixed
oxidant technology is not new, its application in drinking water disinfection adds a
new tool in the arsenal of water protection. Recent work has demonstrated that
such a product generated from the electrolysis of brine is very effective against
this parasite (Venczel et al. 1997). Although the precise characterization of the
chemical nature of this product remains elusive, the technology is available both at
full scale and in miniature version for use by travelers or in remote locations, and
offers protection against sporadic microbial infiltration into the water supply.
4.12 Membrane Technology
Low-pressure membrane filtration is a term which encompasses ultrafiltration
(UF) and micro filtration (MF) and includes operation in the range 10 to 30 psi.
High-pressure membrane operation includes nanofiltration (NF) and reverse
osmosis (RO) which operate in the range 75 to 250 psi. Membrane filtration is an
291
the regulated dose requirements, chlorine dioxide will be used to treat surface raw
waters for algal control only when the demand is less than about 1.4 mg rl.
Otherwise it is typically added following removal of demand materials after
sedimentation. This disinfectant has certainly been demonstrated to have equal, if
not superior, performance to chlorine as, for example, in the inactivation of
Giardia lamblia (White 1999) and in some cases might be an economically viable
alternative where there would normally be a high chlorine demand by the water.
When considered for disinfection purposes at sites that have traditionally used use
chlorine gas, it is possible to modify the chlorination process with introduction of
sodium chlorite to generate chlorine dioxide (Aieta and Roberts 1985). As a
selective oxidant, chlorine dioxide does not produce the halogenated species
associated with chlorine. Although there is not the wealth of literature on organic
byproducts as compared to chlorine (which in and of itself is a clear statement!), it
appears that what have been identified in finished water are mostly mono- and
dicarboxylic acids (Richardson 1998). Terminal disinfection with chlorine dioxide
is not usually practiced due to the odor and chlorite limitations described above.
Furthermore, it is suspected that using terminal chlorination on a water containing
residual chlorite from upstream chlorine dioxide application may generate
chlorine dioxide in the distribution system. If this is a concern, chloramination
should be practiced.
4.11 Mixed Oxidants
Contamination of drinking water sources by fecal matter is an international
problem, especially where watershed management is not maintained. In addition
to the well-documented viruses and bacteria, parasites such as Cryptosporidium
parvum continue to make their presence felt, and demand more safeguards built
into treatment technology to prevent major epidemics. While traditional
disinfectants such as chlorine have been effective against bacteria and viruses,
these parasites require more powerful treatment to remove them. While mixed
oxidant technology is not new, its application in drinking water disinfection adds a
new tool in the arsenal of water protection. Recent work has demonstrated that
such a product generated from the electrolysis of brine is very effective against
this parasite (Venczel et al. 1997). Although the precise characterization of the
chemical nature of this product remains elusive, the technology is available both at
full scale and in miniature version for use by travelers or in remote locations, and
offers protection against sporadic microbial infiltration into the water supply.
4.12 Membrane Technology
Low-pressure membrane filtration is a term which encompasses ultrafiltration
(UF) and micro filtration (MF) and includes operation in the range 10 to 30 psi.
High-pressure membrane operation includes nanofiltration (NF) and reverse
osmosis (RO) which operate in the range 75 to 250 psi. Membrane filtration is an