1 3
Topics in Current Chemistry (2020) 378:7
already mentioned for pharmaceutical wastewater, Fenton processes are generally
preferred for industrial wastewater of this type, since the lower water volumes make
pH alteration economically feasible. The authors reached the same conclusion, since
they estimated that the best-performing HPC treatment was 1.5 times as expensive
as solar photo-Fenton per unit volume of wastewater treated. Other research has
been carried out on in situ treatment of wastewater from the agro-industry using
innovative solar photocatalytic reactors [154]. Wastewater generated from the washing of equipment used for agricultural pesticide application was treated using commercial titania. While the COD loads of the treated wastewater were not high, in the
range of 0.1 g/L, the treatment was able to reduce it by more than 75% of the initial
value, with contained costs of operation. Kushniarou et al. also applied a photocatalytic treatment for agro-industry wastewater generated from washing equipment that
had been in direct contact with pesticides. With the use of commercial TiO 2 -P25
and 150 mg/L of persulfate, the grouped concentration of 12 selected pesticides was
reduced by > 99% in 1 h [155].
In all cases, commercial unmodified titania was used as the photocatalyst under
solar or solar-simulated conditions. This catalyst, with a bandgap of about 3.2 eV, is
mostly active under UV radiation, which is a minor fraction of the solar spectrum.
This presents an opportunity to further enhance the photocatalytic process in the
removal of pesticides from agro-industry wastewater, since the use of a doped photocatalyst with a lower bandgap would result in the utilization of a higher fraction of
the solar spectrum and higher expected rates of removals. This improvement might
be enough to reduce the cost while also having a major advantage over solar photoFenton processes in ease of automation, as it does not require the addition of an oxidant such as hydrogen peroxide.
4.3.2 Food Industry Wastewater Treatment and Energy Recovery
Food industry wastewater, in addition to biodegradable organic substances, can also
include organic substances that are not easily biodegradable, such as food dyes.
Synthetic dyes are the largest group of additives used in the food industry, and
their by-products, such as phenolic compounds and aromatic amines, are toxic to
the aquatic environment because of their carcinogenic and mutagenic nature [156].
These dyes generally contain recalcitrant organic and inorganic groups, and their
release into the aquatic environment results in a reduction in light transmittance,
thus reducing the penetration of solar radiation through the receiving body of water.
Dyes have high thermal and photo-stability, and this makes them resistant to biodegradation [157]. For this reason, they are persistent molecules which remain in
the environment for long periods. The main consequence of the presence of dyes in
aquatic environments is on plants, since the light absorption by the dye present in
the water reduces photosynthesis activity and influences the food chain [157]. Furthermore, some dyes can cause carcinogenic and genotoxic effects on humans [67,
158], and AOPs appear to be an attractive option for removing such pollutants from
wastewater.
Although homogeneous Fenton reaction is effective in the oxidation of several recalcitrant pollutants and is used in industrial wastewater treatment as a
247
Reprinted from the journal
Topics in Current Chemistry (2020) 378:7
already mentioned for pharmaceutical wastewater, Fenton processes are generally
preferred for industrial wastewater of this type, since the lower water volumes make
pH alteration economically feasible. The authors reached the same conclusion, since
they estimated that the best-performing HPC treatment was 1.5 times as expensive
as solar photo-Fenton per unit volume of wastewater treated. Other research has
been carried out on in situ treatment of wastewater from the agro-industry using
innovative solar photocatalytic reactors [154]. Wastewater generated from the washing of equipment used for agricultural pesticide application was treated using commercial titania. While the COD loads of the treated wastewater were not high, in the
range of 0.1 g/L, the treatment was able to reduce it by more than 75% of the initial
value, with contained costs of operation. Kushniarou et al. also applied a photocatalytic treatment for agro-industry wastewater generated from washing equipment that
had been in direct contact with pesticides. With the use of commercial TiO 2 -P25
and 150 mg/L of persulfate, the grouped concentration of 12 selected pesticides was
reduced by > 99% in 1 h [155].
In all cases, commercial unmodified titania was used as the photocatalyst under
solar or solar-simulated conditions. This catalyst, with a bandgap of about 3.2 eV, is
mostly active under UV radiation, which is a minor fraction of the solar spectrum.
This presents an opportunity to further enhance the photocatalytic process in the
removal of pesticides from agro-industry wastewater, since the use of a doped photocatalyst with a lower bandgap would result in the utilization of a higher fraction of
the solar spectrum and higher expected rates of removals. This improvement might
be enough to reduce the cost while also having a major advantage over solar photoFenton processes in ease of automation, as it does not require the addition of an oxidant such as hydrogen peroxide.
4.3.2 Food Industry Wastewater Treatment and Energy Recovery
Food industry wastewater, in addition to biodegradable organic substances, can also
include organic substances that are not easily biodegradable, such as food dyes.
Synthetic dyes are the largest group of additives used in the food industry, and
their by-products, such as phenolic compounds and aromatic amines, are toxic to
the aquatic environment because of their carcinogenic and mutagenic nature [156].
These dyes generally contain recalcitrant organic and inorganic groups, and their
release into the aquatic environment results in a reduction in light transmittance,
thus reducing the penetration of solar radiation through the receiving body of water.
Dyes have high thermal and photo-stability, and this makes them resistant to biodegradation [157]. For this reason, they are persistent molecules which remain in
the environment for long periods. The main consequence of the presence of dyes in
aquatic environments is on plants, since the light absorption by the dye present in
the water reduces photosynthesis activity and influences the food chain [157]. Furthermore, some dyes can cause carcinogenic and genotoxic effects on humans [67,
158], and AOPs appear to be an attractive option for removing such pollutants from
wastewater.
Although homogeneous Fenton reaction is effective in the oxidation of several recalcitrant pollutants and is used in industrial wastewater treatment as a
247
Reprinted from the journal