Reuse of Salt-Containing Dyeing Effluents by Means …
5
Consequently, dyeing processes with reactive dyes produce effluents with:
• High colouration
• High salts content
• Alkaline pH
The treatment of these effluents is very difficult due to both the high salinity and
the low biodegradability of dyes. Then, the use of specific treatments is required.
The most usual ones are based on the separation of dyes from the effluent. But
they generate a waste which requires a further treatment to remove the concentrated
pollutants.
On the other hand, there are not feasible treatments able to remove salts. Increasing
salinity of riverbeds (due to saline wastewater discharges) is a major concern in most
of countries, mainly those with scarce riverbeds. For these reasons, a ground-breaking
system, based on an electrochemical-UV combined treatment, able to reuse water
and salts is the most appropriate alternative to encourage the circular economy in
the textile industry. This system could be applied to any type of saline and coloured
effluents. In particular, our studies have been focussed on reactive dye exhausted
dyebaths for three reasons: (1) this type of dyes are one of the most used in the
textile industry, (2) they have one of the lowest fixation rates onto the fibre and (3)
they require the use of a significant amount of salt to be applied, which cannot be
further removed from the wastewater.
3 Methods for Colour Removal of Textile Effluents
As previously mentioned, the wastewater generated by the textile industry is characterised by containing residual dyes. Modern dyes offer great resistance to chemical
and photochemical degradation since the manufacturers are constantly researching
to obtain textile dyes with better features: brighter shades and high fastnesses values
to light, water, soap, etc. As a consequence, they cannot be effectively removed
from the wastewater by traditional methods and wastewater with high colouration
could be discharged into the environment. Although many dyes could be non-toxic
compounds, small amount of them (about 1 mg/L) is sufficient to produce an intense
colouration that can impact or even impede the aquatic life in the riverbeds [21].
The methods currently used for the treatment of textile wastewater can be classified
into three categories: biological, chemical and physical processes (Fig. 3). It should
be noted that a unique method is not applied to treat the textile wastewater, being
required to combine different treatments in order to meet with legislative regulations
[22, 23].
Currently, many textile companies use biological processes to treat their wastewater. This method shows high efficiency in eliminating biodegradable organic matter
with low operational cost. However, due to the structure and high molecular weight of
dyes, biological processes provide low colour removal [24]. In fact, the removal of the
5
Consequently, dyeing processes with reactive dyes produce effluents with:
• High colouration
• High salts content
• Alkaline pH
The treatment of these effluents is very difficult due to both the high salinity and
the low biodegradability of dyes. Then, the use of specific treatments is required.
The most usual ones are based on the separation of dyes from the effluent. But
they generate a waste which requires a further treatment to remove the concentrated
pollutants.
On the other hand, there are not feasible treatments able to remove salts. Increasing
salinity of riverbeds (due to saline wastewater discharges) is a major concern in most
of countries, mainly those with scarce riverbeds. For these reasons, a ground-breaking
system, based on an electrochemical-UV combined treatment, able to reuse water
and salts is the most appropriate alternative to encourage the circular economy in
the textile industry. This system could be applied to any type of saline and coloured
effluents. In particular, our studies have been focussed on reactive dye exhausted
dyebaths for three reasons: (1) this type of dyes are one of the most used in the
textile industry, (2) they have one of the lowest fixation rates onto the fibre and (3)
they require the use of a significant amount of salt to be applied, which cannot be
further removed from the wastewater.
3 Methods for Colour Removal of Textile Effluents
As previously mentioned, the wastewater generated by the textile industry is characterised by containing residual dyes. Modern dyes offer great resistance to chemical
and photochemical degradation since the manufacturers are constantly researching
to obtain textile dyes with better features: brighter shades and high fastnesses values
to light, water, soap, etc. As a consequence, they cannot be effectively removed
from the wastewater by traditional methods and wastewater with high colouration
could be discharged into the environment. Although many dyes could be non-toxic
compounds, small amount of them (about 1 mg/L) is sufficient to produce an intense
colouration that can impact or even impede the aquatic life in the riverbeds [21].
The methods currently used for the treatment of textile wastewater can be classified
into three categories: biological, chemical and physical processes (Fig. 3). It should
be noted that a unique method is not applied to treat the textile wastewater, being
required to combine different treatments in order to meet with legislative regulations
[22, 23].
Currently, many textile companies use biological processes to treat their wastewater. This method shows high efficiency in eliminating biodegradable organic matter
with low operational cost. However, due to the structure and high molecular weight of
dyes, biological processes provide low colour removal [24]. In fact, the removal of the