1 3
Topics in Current Chemistry (2020) 378:7
in prevalence results a process of natural selection in bacterial mortality, that
is, when non-resistant bacteria are more susceptible to treatment than resistant
bacteria. Therefore, while the overall bacterial loads decrease, the percentage of
resistant bacteria within the total bacterial load increases [127–129]. This shift in
population dynamics introduces risks in the downstream environment such that
the more prevalent resistance is now more likely to thrive in the environment.
Chlorination and UVC treatments are known to cause resistance selection [127,
129]. Ozonation, which is also an AOP with high operating costs, is associated
with increased ARB and ARGs as well [126]. Since HPC application for AR
resistance mitigation is largely in its early stages, most studies on the matter have
dealt with its ability to reduce the absolute number of ARB and ARGs, not specifically on reducing relative abundance [94, 112, 130]. Other studies have targeted
abundance [5, 131], but because of the lack of full-scale application in WWTPs,
it has not been fully established whether HPC also increases prevalence. If antibiotic resistance associated with wastewater reuse is deemed a high risk in need of
regulation by the authorities, and HPC is found to be an effective technology for
AR mitigation, the high cost of HPC could be justified by its efficacy and by the
lack of suitable alternative treatment methods.
Without fulfilling such a need, photocatalysis needs to become at least one
order of magnitude more efficient [103], an obstacle that as yet has not been overcome despite the intense level of research on the matter. The preferential use of
modified catalysts over commercially available ones can play a pivotal role. These
modifications must result in both higher efficiency in the generation of radicals
(i.e., quantum yields for hydroxyl radical generation) and the ability to harness
photonic energy beyond the UV range and into the visible range, thus enabling
solar treatment. Energy production in a power plant and the transport and powering of UVA lamps involve substantial energy losses at each stage, which can be
avoided by using solar reactors as a cost-effective strategy. However, solar-driven
treatments have problems as well. Even if future engineering modifications result
in a sufficiently efficient visible-light-active photocatalyst that makes HPC competitive with other AOPs, solar treatments are still limited by the actual duration
of daylight and by the fact that solar reactors need a large footprint in terms of
land area. Thus, they are more feasible in smaller, less densely populated cities,
where the value of land is not as high as it is close to major cities. Additionally,
solar treatments are limited seasonally, with lower treatment potential in winter,
as well as geographically, with latitudes farther from the equator having lower
insolation and hence treatment potential. While this limits solar-based technologies for the treatment of all types of wastewater, it is much less an issue for solarbased treatment of water specifically for agricultural reuse. Firstly, areas with
intense agriculture activity are located far from major cities, and thus the value
of land is low allowing for larger footprints. Additionally, the need for irrigation
is stronger in latitudes and seasons where insolation is higher. A wide variety of
solar reactors have been proposed, including parabolic-based reactors and raceway pond reactors [124, 132]. Parabolic reactors are more suitable in low-insolation conditions but are more expensive than raceway pond reactors. The latter
have been used mostly for homogenous AOP processes [133], since the low flow
243
Reprinted from the journal
Topics in Current Chemistry (2020) 378:7
in prevalence results a process of natural selection in bacterial mortality, that
is, when non-resistant bacteria are more susceptible to treatment than resistant
bacteria. Therefore, while the overall bacterial loads decrease, the percentage of
resistant bacteria within the total bacterial load increases [127–129]. This shift in
population dynamics introduces risks in the downstream environment such that
the more prevalent resistance is now more likely to thrive in the environment.
Chlorination and UVC treatments are known to cause resistance selection [127,
129]. Ozonation, which is also an AOP with high operating costs, is associated
with increased ARB and ARGs as well [126]. Since HPC application for AR
resistance mitigation is largely in its early stages, most studies on the matter have
dealt with its ability to reduce the absolute number of ARB and ARGs, not specifically on reducing relative abundance [94, 112, 130]. Other studies have targeted
abundance [5, 131], but because of the lack of full-scale application in WWTPs,
it has not been fully established whether HPC also increases prevalence. If antibiotic resistance associated with wastewater reuse is deemed a high risk in need of
regulation by the authorities, and HPC is found to be an effective technology for
AR mitigation, the high cost of HPC could be justified by its efficacy and by the
lack of suitable alternative treatment methods.
Without fulfilling such a need, photocatalysis needs to become at least one
order of magnitude more efficient [103], an obstacle that as yet has not been overcome despite the intense level of research on the matter. The preferential use of
modified catalysts over commercially available ones can play a pivotal role. These
modifications must result in both higher efficiency in the generation of radicals
(i.e., quantum yields for hydroxyl radical generation) and the ability to harness
photonic energy beyond the UV range and into the visible range, thus enabling
solar treatment. Energy production in a power plant and the transport and powering of UVA lamps involve substantial energy losses at each stage, which can be
avoided by using solar reactors as a cost-effective strategy. However, solar-driven
treatments have problems as well. Even if future engineering modifications result
in a sufficiently efficient visible-light-active photocatalyst that makes HPC competitive with other AOPs, solar treatments are still limited by the actual duration
of daylight and by the fact that solar reactors need a large footprint in terms of
land area. Thus, they are more feasible in smaller, less densely populated cities,
where the value of land is not as high as it is close to major cities. Additionally,
solar treatments are limited seasonally, with lower treatment potential in winter,
as well as geographically, with latitudes farther from the equator having lower
insolation and hence treatment potential. While this limits solar-based technologies for the treatment of all types of wastewater, it is much less an issue for solarbased treatment of water specifically for agricultural reuse. Firstly, areas with
intense agriculture activity are located far from major cities, and thus the value
of land is low allowing for larger footprints. Additionally, the need for irrigation
is stronger in latitudes and seasons where insolation is higher. A wide variety of
solar reactors have been proposed, including parabolic-based reactors and raceway pond reactors [124, 132]. Parabolic reactors are more suitable in low-insolation conditions but are more expensive than raceway pond reactors. The latter
have been used mostly for homogenous AOP processes [133], since the low flow
243
Reprinted from the journal