Abstract
UV disinfection technology is of growing interest in the water industry
since it was demonstrated that UV radiation is very effective against
(oo)cysts of Cryptosporidium and Giardia, two pathogenic micro-organisms
of major importance for the safety of drinking water. Quantitative
Microbial Risk Assessment, the new concept for microbial safety of
drinking water and waste water,
... read more
requires quantitative data of the
inactivation or removal of pathogenic micro-organisms by water treatment
processes. The objective of this study was to review the literature on UV
disinfection and extract quantitative information about the relation
between the inactivation of micro-organisms and the applied UV fluence.
The quality of the available studies was evaluated and only high-quality
studies were incorporated in the analysis of the inactivation kinetics.
The results show that UV is effective against all waterborne pathogens. The
inactivation of micro-organisms by UV could be described with first-order
kinetics using fluence-inactivation data from laboratory studies in
collimated beam tests. No inactivation at low fluences (offset) and/or no
further increase of inactivation at higher fluences (tailing) was observed for
some micro-organisms. Where observed, these were included in the
description of the inactivation kinetics, even though the cause of tailing is
still a matter of debate. The parameters that were used to describe
inactivation are the inactivation rate constant k (cm2/mJ), the maximum
inactivation demonstrated and (only for bacterial spores and Acanthamoeba)
the offset value. These parameters were the basis for the calculation of the
Microbial Inactivation Credit (MIC = “log-credits”) that can be assigned to
a certain UV fluence. The most UV resistant organisms are viruses,
specifically Adenoviruses, and bacterial spores. The protozoon
Acanthamoeba is also highly UV resistant. Bacteria and (oo)cysts of
Cryptosporidium and Giardia are more susceptible with a fluence
requirement of <20 mJ/cm2 for a MIC of 3 log.
Several studies have reported an increased UV resistance of environmental
bacteria and bacterial spores, compared to lab-grown strains. This means
that higher UV fluences are required to obtain the same level of
inactivation. Hence, for bacteria and spores, a correction factor of 2 and 4
was included in the MIC calculation, respectively, whereas some
wastewater studies suggest that a correction of a factor of 7 is needed under
these conditions. For phages and viruses this phenomenon appears to be of
little significance and for protozoan (oo)cysts this aspect needs further
investigation. Correction of the required fluence for DNA-repair is considered unnecessary under the conditions of drinking water practice (no
fotorepair, dark repair insignificant, esp. at higher (60 mJ/cm2) fluences)
and probably also wastewater practice (fotorepair limited by light
absorption). To enable accurate assessment of the effective fluence in
continuous flow UV systems in water treatment practice, biodosimetry is
still essential, although the use of Computational Fluid Dynamics (CFD)
improves the description of reactor hydraulics and fluence distribution. For
UV systems that are primarily dedicated to inactivate the more sensitive
pathogens (Cryptosporidium, Giardia and pathogenic bacteria), additional
model organisms are needed to serve as biodosimeter.
show less