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Topics in Current Chemistry (2020) 378:7
4 Application of Heterogeneous Photocatalysis to Wastewater
Treatment
4.1 Tertiary Treatment of Urban Wastewater
The three conventional stages of urban wastewater treatment consist of a primary
stage of physical separation by sedimentation of solids and skimming off of floating contaminants, a secondary stage for reducing biodegradable organic load by
bacterial growth, and a tertiary stage of nutrient removal (typically achieved by
biological processes) and/or disinfection and/or removal of micropollutants such
as CECs (including pharmaceuticals, pesticides, personal care products, and any
other pollutant that is found in micrograms-per-liter or lower quantities in wastewater). The tertiary stage is not considered essential, and only a few countries
have regulation that require the use of tertiary treatment to remove one or more
of the contaminants explained above. When tertiary treatment is used, the main
factors driving the choice of treatment are the efficiency in removing the target
contaminants set by the local/national regulation and the cost. The tertiary treatment applied depends on the target parameter(s) that need to be brought within
regulatory limits. If such a target is the reduction of bacterial load, then chlorination, peracetic acid, or UVC disinfection is commonly employed [98–100]. On
the other hand, if CECs should be removed, disinfection processes are no longer
effective, and advanced treatment methods are necessary [101]. Indeed, in Switzerland, according to the new national water act (Swiss Federal Council Waters
Protection Ordinance) [102], 70% of the urban wastewater treatment plants
(UWTPs) must be upgraded with ozonation or adsorption treatment methods to
remove 14 selected CECs by 80% of inflow levels. In such a context, homogeneous photo-driven AOPs may be competitive with consolidated technologies in the
short term, and HPC may be feasible if some limitations/drawbacks are successfully addressed [103].
4.1.1 HPC for Micropollutant Removal and Bacterial Inactivation
HPC has been intensively studied as a potential technology for the mineralization of organic contaminants and the inactivation of bacteria in the effluents of
secondary treated urban wastewater. While full mineralization is not considered
feasible for urban wastewater, a wide variety of catalysts have been employed for
pollutant degradation and disinfection. These include dyes, [104, 105] endocrine
disruptors [106], pharmaceuticals [107] including specifically antibiotics [108,
109], and agricultural chemicals [110, 111], and for bacterial inactivation [112,
113. The most imposing obstacle hindering widespread use of heterogeneous
photocatalysis as a tertiary treatment in UWTPs is the high cost associated with
photocatalysis, which can be more than an order of magnitude greater than established AOPs [103]. Nevertheless, HPC could find application in the treatment of
wastewater intended for reuse, which has stricter water quality requirements than
239
Reprinted from the journal
Topics in Current Chemistry (2020) 378:7
4 Application of Heterogeneous Photocatalysis to Wastewater
Treatment
4.1 Tertiary Treatment of Urban Wastewater
The three conventional stages of urban wastewater treatment consist of a primary
stage of physical separation by sedimentation of solids and skimming off of floating contaminants, a secondary stage for reducing biodegradable organic load by
bacterial growth, and a tertiary stage of nutrient removal (typically achieved by
biological processes) and/or disinfection and/or removal of micropollutants such
as CECs (including pharmaceuticals, pesticides, personal care products, and any
other pollutant that is found in micrograms-per-liter or lower quantities in wastewater). The tertiary stage is not considered essential, and only a few countries
have regulation that require the use of tertiary treatment to remove one or more
of the contaminants explained above. When tertiary treatment is used, the main
factors driving the choice of treatment are the efficiency in removing the target
contaminants set by the local/national regulation and the cost. The tertiary treatment applied depends on the target parameter(s) that need to be brought within
regulatory limits. If such a target is the reduction of bacterial load, then chlorination, peracetic acid, or UVC disinfection is commonly employed [98–100]. On
the other hand, if CECs should be removed, disinfection processes are no longer
effective, and advanced treatment methods are necessary [101]. Indeed, in Switzerland, according to the new national water act (Swiss Federal Council Waters
Protection Ordinance) [102], 70% of the urban wastewater treatment plants
(UWTPs) must be upgraded with ozonation or adsorption treatment methods to
remove 14 selected CECs by 80% of inflow levels. In such a context, homogeneous photo-driven AOPs may be competitive with consolidated technologies in the
short term, and HPC may be feasible if some limitations/drawbacks are successfully addressed [103].
4.1.1 HPC for Micropollutant Removal and Bacterial Inactivation
HPC has been intensively studied as a potential technology for the mineralization of organic contaminants and the inactivation of bacteria in the effluents of
secondary treated urban wastewater. While full mineralization is not considered
feasible for urban wastewater, a wide variety of catalysts have been employed for
pollutant degradation and disinfection. These include dyes, [104, 105] endocrine
disruptors [106], pharmaceuticals [107] including specifically antibiotics [108,
109], and agricultural chemicals [110, 111], and for bacterial inactivation [112,
113. The most imposing obstacle hindering widespread use of heterogeneous
photocatalysis as a tertiary treatment in UWTPs is the high cost associated with
photocatalysis, which can be more than an order of magnitude greater than established AOPs [103]. Nevertheless, HPC could find application in the treatment of
wastewater intended for reuse, which has stricter water quality requirements than
239
Reprinted from the journal