Environmental Engineering Reference
In-Depth Information
13.3.2.3 Spores as indicators
Helminth eggs are not the only microorganisms with the capability to resist conven-
tional water disinfection processes. Other well-known pathogenic species with such
high resistance are
Cryptosporidium parvum
oocyst and
Giardia lamblia
cysts, among
many others. Resistance of
C. parvum
oocysts to chlorine disinfection has been exten-
sively documented. Chlorine concentrations of 4 mg/L, the highest residual allowed
by U.S. regulations, require more than 15 hours of contact time to inactivate 99% of
C. parvum
oocysts at pH 6.0 and 20
◦
C. The chlorine dosage necessary for treatment
increases dramatically with increasing pH and decreasing temperature (Guisar et al.,
2007).
Having in mind all these considerations, it is clear that using
E. coli
as pathogenic
indicator of drinking water disinfection is, by far, not enough. In the search for a more
reliable indicator,
Bacillus
spores have emerged as an interesting alternative. It is known
that spores of Bacillus spp., are highly resistant to inactivation by different physical
stress conditions such as toxic chemicals or biocidal agents, desiccation, extreme pres-
sure and temperature, as well as exposure to high doses of UV or ionizing radiation.
Spores of
Bacillus subtilis
are commonly used test organisms for inactivation studies
due to their high degree of resistance to various sporicidal treatments, reproducible
inactivation response, safety (
B. subtilis
is not pathogenic to humans) and resistance
to UV radiation, and have become a useful conservative index for water disinfection
anytime it is considered that, once
B. subtilis
spores has been inactivated, anything
else with less resistance will surely be inactivated (Bandala et al., 2011b).
SDPAF processes have been demonstrated as highly efficient for
Bacillus subtilis
spore inactivation in water as a surrogate microorganism for
C. parvum
oocysts. Using
relatively low Fenton reagent concentrations and low solar radiation intensity (about
1 sun), Guisar et al., (2007) obtained up to 96% spore viability reduction in only
1.5 hours of exposure to solar radiation. Bandala et al. (2011a) used heminth eggs to
demonstrate that the use of highly resistant microorganisms as a conservative index for
water disinfection is desirable and that an adequate solar radiation dose is required to
ensure the final required water quality. In their work, these authors used
Ascaris suum
eggs, very similar to
A. lumbricoides,
the actual specie infecting humans, to assess the
amount of radiation necessary to inactivate
>
5-log (99.999% removal) of helminth
eggs. They found that approximately 140 kJ L
−
1
was required to achieve this task.
When they tested the same experimental conditions (Fe
2
+
and H
2
O
2
concentration)
for the inactivation of
E. coli
and
P. aeruginosa
, they found that less than 10 kJ L
−
1
were required to reach up to
>
6-log inactivation (99.9999% removal) of both bacteria.
Finally, they found no significant increase in the inactivation dose required when up
to 5mgL
−
1
natural organic matter (NOM) was added to the bacterial suspension.
In relation to
Bacillus subtilis
spore inactivation, several different Fenton reagent
concentrations were tested in combination with solar UV-A radiation (
λ
max
=
365 nm)
for spore inactivation. The best spore inactivation conditions were found to be
[Fe
2
+
]
100 mM and radiation. Under these conditions, over
9-log inactivation was reached after only 20 min of reaction. The effect of ionic strength
and natural organic matter (NOM) on spore inactivation kinetics was also tested.
In both cases, an important decrease of the inactivation rate, fitted to the delayed
Chick-Watson inactivation kinetics, was observed (Bandala et al., 2011b).
=
2.5 mM and [H
2
O
2
]
=
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