Topics in Current Chemistry (2020) 378:7
1 3
carboxylate components in the cell wall [118, 119], and consequently they are
attracted to positively charged semiconductor photocatalysts. These effects also
influence the rate of removal/inactivation in other ways; for example, bicarbonate
ions are known to adsorb to titania, reducing its activity [120] by preventing electron transfers that generate ROS to take place at the site of adsorption.
4.1.3 Irradiance and Transmittance Through Wastewater
The presence of TSS/turbidity and light-absorbing substances in wastewater results
in a loss of transmittance (via absorption but also scattering and reflection) and, consequently, a loss of energy because of the reduced light intensity that will reach the
photocatalyst’s surface (as we already saw in Fig. 3). In the day-to-day operation of
an HPC tertiary treatment process, the additional variability resulting from this issue
can negatively affect process efficiency. Just like ozone content is dosed relative to
the DOC load in water, some HPC process parameters can be modified depending on the type of reactor utilized. An online measure of UV–Vis transmittance at
appropriate wavelengths has been suggested as a proxy measure for contaminant
load [121]. Thus, this can be used to adjust some HPC parameters, e.g., in semibatch reactors such as raceway reactors [122] the volume of wastewater treated can
be increased and decreased accordingly. A lower volume in a fixed reactor results in
a shorter water column and thus higher transmittance. Batch reactor residence time
can be adjusted as well.
4.1.4 Requirements for Widespread Adoption of Photocatalysis: A Technological
Niche for HPC?
Photocatalytic reactors, including solar reactors, have been reviewed in detail in
other publications (see [123, 124]), and the obstacles to widespread use are not so
much technological, but rather a lack of market demand given the higher costs and
levels of treatment obtained by HPC-treated wastewater.
The most critical requirement for widespread adoption of HPC is the establishment of regulations mandating reduced CEC load in discharged effluents and/or
water intended for reuse. The case for reducing CECs in water intended for reuse,
specifically agricultural irrigation, is especially strong. Paltiel et al. demonstrated
that persons consuming crops irrigated with reused wastewater had significantly
higher blood levels of carbamazepine compared with those who consumed vegetables irrigated with fresh water [125]. While the levels recorded are orders of magnitudes less than therapeutic levels, there is still cause for concern, both because of
chronic exposure to such compounds and due to the cocktail effects from the presence of numerous compounds and their transformation products, many of which
are not known. High-level treatment of wastewater intended for food crop irrigation
would thus be required if one were applying precautionary measures.
Another potential issue that might make HPC a necessity is the environmental
dimension of antibiotic resistance. Conventional tertiary treatment processes and
ozonation are both associated with an increased prevalence of antibiotic-resistant
bacteria (ARB) and antibiotic resistant genes (ARGs) [126]. Such an increase
242
Reprinted from the journal
1 3
carboxylate components in the cell wall [118, 119], and consequently they are
attracted to positively charged semiconductor photocatalysts. These effects also
influence the rate of removal/inactivation in other ways; for example, bicarbonate
ions are known to adsorb to titania, reducing its activity [120] by preventing electron transfers that generate ROS to take place at the site of adsorption.
4.1.3 Irradiance and Transmittance Through Wastewater
The presence of TSS/turbidity and light-absorbing substances in wastewater results
in a loss of transmittance (via absorption but also scattering and reflection) and, consequently, a loss of energy because of the reduced light intensity that will reach the
photocatalyst’s surface (as we already saw in Fig. 3). In the day-to-day operation of
an HPC tertiary treatment process, the additional variability resulting from this issue
can negatively affect process efficiency. Just like ozone content is dosed relative to
the DOC load in water, some HPC process parameters can be modified depending on the type of reactor utilized. An online measure of UV–Vis transmittance at
appropriate wavelengths has been suggested as a proxy measure for contaminant
load [121]. Thus, this can be used to adjust some HPC parameters, e.g., in semibatch reactors such as raceway reactors [122] the volume of wastewater treated can
be increased and decreased accordingly. A lower volume in a fixed reactor results in
a shorter water column and thus higher transmittance. Batch reactor residence time
can be adjusted as well.
4.1.4 Requirements for Widespread Adoption of Photocatalysis: A Technological
Niche for HPC?
Photocatalytic reactors, including solar reactors, have been reviewed in detail in
other publications (see [123, 124]), and the obstacles to widespread use are not so
much technological, but rather a lack of market demand given the higher costs and
levels of treatment obtained by HPC-treated wastewater.
The most critical requirement for widespread adoption of HPC is the establishment of regulations mandating reduced CEC load in discharged effluents and/or
water intended for reuse. The case for reducing CECs in water intended for reuse,
specifically agricultural irrigation, is especially strong. Paltiel et al. demonstrated
that persons consuming crops irrigated with reused wastewater had significantly
higher blood levels of carbamazepine compared with those who consumed vegetables irrigated with fresh water [125]. While the levels recorded are orders of magnitudes less than therapeutic levels, there is still cause for concern, both because of
chronic exposure to such compounds and due to the cocktail effects from the presence of numerous compounds and their transformation products, many of which
are not known. High-level treatment of wastewater intended for food crop irrigation
would thus be required if one were applying precautionary measures.
Another potential issue that might make HPC a necessity is the environmental
dimension of antibiotic resistance. Conventional tertiary treatment processes and
ozonation are both associated with an increased prevalence of antibiotic-resistant
bacteria (ARB) and antibiotic resistant genes (ARGs) [126]. Such an increase
242
Reprinted from the journal