Wet processes for textile production are one of the largest water consuming and polluting sources. Quite usually, at the end of the dyeing process, a noticeable amount of dyes remains in wastewater as is not absorbed by fibres, leading to wastewater colouration. Dyes show resistance to degradation in the environment, since their peculiarity is chemical stability. Besides visual problems, the effect of residual dyes is negative on aquatic life because they inhibit sunlight transmission and may enter in the food chain. Generally, conventional biological treatment alone cannot guarantee adequate characteristics to treated water to allow the discharge into the environment or reuse in other processes. Specifically, a high salt content and residual colour are still present in the treated water after secondary treatments. Salt content can be remove using membrane filtration equipment instead the most profitable operation to remove colour appears oxidation. Other techniques, such as coagulation-flocculation, adsorption, membrane filtration, activated sludge, were studied to remove colour but land filling or incineration must be considered as final process. On the contrary, oxidation steps demolish the contaminant at molecular scale, even though not necessary the oxidation is complete. Generally ozone, being an oxidant agent, has a high oxidation potential (even at a low concentration), high efficiency in decomposition of organic matter, adds oxygen to water and has process low sensitivity to changes in temperature. Ozone is able to break up the conjugated bonds of organic matter thanks to a direct reaction between ozone and the organic compound or indirectly through the generation of hydroxyl radicals. The degradation of dyes with O3 is a typical two phase reaction where an effective transfer of ozone from gas to liquid is a critical point. On the other hand, the kinetics of decolouration is usually fast. Therefore, the mass transfer is the rate limiting step. To achieve the best mass transfer condition, several gas diffusers and gas–liquid contactors have been proposed in literature such as turbines, ejectors, gas diffusers (sintered glass diffuser), etc. An innovative operative procedure took into account in this work was cavitation: it was considered as the mean to increase mass transfer of ozone in liquid medium. For this reason, an experimental equipment (Multi-task reactor) was designed and built (Fig. 1). Two types of cavitation were considered: hydrodynamic cavitation by ejector and ultrasound cavitation. The two types of cavitations were used separately or simultaneously in order to clean wastewater from different dyes typology (namely acid, cationic, reactive and disperse dyes). In addition, hydrodynamic and ultrasonic cavitation was used to work alone to decolourise wastewater. Cavitations are able to produce free radicals, such as hydroxyl radicals, which can be used to attack dye cromophores groups of dye molecule. A bubble column reactor was built to compare the decolourisation results obtained in the Multi-task reactor. Bubble column was used as benchmark because represent the most common technology in wastewater decolouration. First of all, decolouration experiments were performed in the multi-task equipment in liquid batch conditions. After that, continuous tests were carried out and the results were compared with bubble column equipment decolouration experiments at the same operational conditions (liquid residence time, gas flow rate, ozone dose, dyestuffs and its concentration). Taking into account the final experiment results, only ultrasound cavitation was able to improve decolouration degree in the case of disperse dye. Comparing the experimental decolouration results obtained with the mentioned technologies, bubble reactor seem to be the best technology for oxidizing treatment. Moreover, fluid dynamic study was performed to bubble column reactor in order to study dye transport mechanisms along the reactor height considering different physical-chemical characteristics. Finally, dyeing test were performed using ozonated wastewater. Wastewater originated from an industrial wool dyeing process was ozonated at different treatment time to obtain different decolouration degree. After that, treated water was reused to dye wool. The benchmark wool dyed with fresh water and wool dyed using ozonated wastewater were compared using a reflection spectrophotometry. In this way, minimum decolouration percentage was discover to obtain a quality parameter to reuse water in dyeing processes, namely color reproducibility.

Treatment of wastewater from textile dyeing by ozonization / ACTIS GRANDE, Giuseppe. - (2015). [10.6092/polito/porto/2591378]

Treatment of wastewater from textile dyeing by ozonization

ACTIS GRANDE, GIUSEPPE
2015

Abstract

Wet processes for textile production are one of the largest water consuming and polluting sources. Quite usually, at the end of the dyeing process, a noticeable amount of dyes remains in wastewater as is not absorbed by fibres, leading to wastewater colouration. Dyes show resistance to degradation in the environment, since their peculiarity is chemical stability. Besides visual problems, the effect of residual dyes is negative on aquatic life because they inhibit sunlight transmission and may enter in the food chain. Generally, conventional biological treatment alone cannot guarantee adequate characteristics to treated water to allow the discharge into the environment or reuse in other processes. Specifically, a high salt content and residual colour are still present in the treated water after secondary treatments. Salt content can be remove using membrane filtration equipment instead the most profitable operation to remove colour appears oxidation. Other techniques, such as coagulation-flocculation, adsorption, membrane filtration, activated sludge, were studied to remove colour but land filling or incineration must be considered as final process. On the contrary, oxidation steps demolish the contaminant at molecular scale, even though not necessary the oxidation is complete. Generally ozone, being an oxidant agent, has a high oxidation potential (even at a low concentration), high efficiency in decomposition of organic matter, adds oxygen to water and has process low sensitivity to changes in temperature. Ozone is able to break up the conjugated bonds of organic matter thanks to a direct reaction between ozone and the organic compound or indirectly through the generation of hydroxyl radicals. The degradation of dyes with O3 is a typical two phase reaction where an effective transfer of ozone from gas to liquid is a critical point. On the other hand, the kinetics of decolouration is usually fast. Therefore, the mass transfer is the rate limiting step. To achieve the best mass transfer condition, several gas diffusers and gas–liquid contactors have been proposed in literature such as turbines, ejectors, gas diffusers (sintered glass diffuser), etc. An innovative operative procedure took into account in this work was cavitation: it was considered as the mean to increase mass transfer of ozone in liquid medium. For this reason, an experimental equipment (Multi-task reactor) was designed and built (Fig. 1). Two types of cavitation were considered: hydrodynamic cavitation by ejector and ultrasound cavitation. The two types of cavitations were used separately or simultaneously in order to clean wastewater from different dyes typology (namely acid, cationic, reactive and disperse dyes). In addition, hydrodynamic and ultrasonic cavitation was used to work alone to decolourise wastewater. Cavitations are able to produce free radicals, such as hydroxyl radicals, which can be used to attack dye cromophores groups of dye molecule. A bubble column reactor was built to compare the decolourisation results obtained in the Multi-task reactor. Bubble column was used as benchmark because represent the most common technology in wastewater decolouration. First of all, decolouration experiments were performed in the multi-task equipment in liquid batch conditions. After that, continuous tests were carried out and the results were compared with bubble column equipment decolouration experiments at the same operational conditions (liquid residence time, gas flow rate, ozone dose, dyestuffs and its concentration). Taking into account the final experiment results, only ultrasound cavitation was able to improve decolouration degree in the case of disperse dye. Comparing the experimental decolouration results obtained with the mentioned technologies, bubble reactor seem to be the best technology for oxidizing treatment. Moreover, fluid dynamic study was performed to bubble column reactor in order to study dye transport mechanisms along the reactor height considering different physical-chemical characteristics. Finally, dyeing test were performed using ozonated wastewater. Wastewater originated from an industrial wool dyeing process was ozonated at different treatment time to obtain different decolouration degree. After that, treated water was reused to dye wool. The benchmark wool dyed with fresh water and wool dyed using ozonated wastewater were compared using a reflection spectrophotometry. In this way, minimum decolouration percentage was discover to obtain a quality parameter to reuse water in dyeing processes, namely color reproducibility.
2015
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11583/2591378
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