Civil Engineering Reference
In-Depth Information
a standby generator are typical ways to achieve the desired reliability. Each lowpressure
UV lamp requires approximately 100 watts of standby power. A second precaution
that should be considered is not powering the UV system from the same motor control
center (MCC) that powers variable- frequency drives (VFDs). The electronic ballasts
produce harmonics that may require mitigation (active harmonic filters) for the VFDs.
Interactive Disinfectants
In 1988, several reports appeared on the combined efficiency of some disinfectants on
pathogen inactivation. Higher inactivation of pathogens was found by Worley and
Williams
38
using a mixture of free chlorine and organic
N
-halamine, and by Alleman
et al.
39
using free chlorine and sodium bromide. Others found similar results.
Interactive disinfectants can be very significant in water treatment because some of
these combinations are effective for inactivating
Cryptosporidium.
40
Interactive disin-
fectants for primary pathogen inactivation can include many different combinations of
disinfectants:
Chlorine followed by monochloramine
•
Chlorine dioxide followed by chlorine
•
Chlorine dioxide followed by chlorine dioxide
•
Chlorine dioxide followed by monochloramine
•
Ozone followed by chlorine
•
Ozone followed by chlorine dioxide
•
Ozone followed by monochloramine
•
Inactivation Mechanism
The mechanism by which interactive disinfectants pro-
vide a synergistic effect is not clearly understood. Several hypotheses have been put
forth.
41-44
The prevailing thought is that the two disinfectants perform separate functions. For
example, one disinfectant could react with the outer cell membrane, compromising the
membrane integrity. The second disinfectant can now penetrate the cell and attack a
functional group to inactivate the organism.
Finch presented the results of laboratory scale investigations of an AWWARF study
into the efficiency of interactive disinfectants.
45
The preliminary results of an
AWWARF study that investigated the application of multiple disinfectants was pre-
sented at a American Water Works Association Technology Transfer Conference in
Portland, Oregon, in August 1997. The objectives of this study were to screen se-
quential chemical disinfectants (ozone, chlorine, chlorine dioxide, and monochlora-
mine) for inactivation of
Cryptosporidium parvum
,
Giardia muris
, and
Bacillus cereus
and develop design criteria for
Cryptosporidium parvum
inactivation using the best
combinations. The results of the study (see Table 19-16) show the improvement of
the disinfection efficiency as a result of the interaction of the disinfectants.
The improved disinfection efficiency due to synergism is highly variable, ranging
from negative (antagonistic) effects to positive enhancement of disinfection efficiency.
The data at this time show that:
Coliform bacteria inactivation appears to increase with interactive disinfectants.
•
Giardia
cyst inactivation appears to increase with interactive disinfectants.
•
