Treatment of wastewater from textile dyeing by ozonization

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

Pump and ejector design in wastewater treatment pilot equipment

Ozone treatment is an oxidative process used in wastewater treatment plant to demolish complex organic molecule. In the case of textile industry is required to adequately remove residual color, demolishing the chromophoric bonds or groups in the dye molecules. A useful method for adding the ozone gas into water and maximize ozone-water mixing to increase mass transfer, is the use of Venturi ejectors. Forcing water through Venturi body, it creates a differential pressure between the fluid inlet and outlet, which in turn creates a vacuum inside the ejector body. In this part, it is possible introduce ozone. COMSOL Multiphysics® is used to define the design parameter of the ejector

Comparative ozone decolouration in bubble column and in ejector mixer units

Ozone treatments are very effective to reduce the residual coloration in wastewater from wet textile processes. The oxidative treatment is very flexible as a result of the easy and prompt regulation of the ozone generator, together with the high solubility of the reactant. Two equipment configurations were considered to evaluate the 03 mass transfer to wastewater. The two schemes are competitive since the ejector mixer prevails in terms of local mass transfer coefficient, while the bubble column has a more favourable hydrodynamics as far as the liquid phase is concerned, since a partial plug flow between the liquid and the gas phase has been demonstrated. The two units, which appear below, were compared in laboratory and in the field tests by operating at strictly controlled conditions. In the laboratory synthetic exhausted dye-liquors were prepared and fed in parallel to the two units by setting a residence time of approximately 30 min. When the experimentation was carried out in the field, real wastewater from a dye house was treated at the conditions suggested by the laboratory tests. Oxygen was fed to the generator at a rate of 200 NL/h; ozone was produced to have a dosage of 30 to 80 g/m3 of treated water. The industrial experimentation has revealed almost identical performances in terms of colour removal (> 98% in all trials), COD reduction (in the range from 15 to 45%), surfactants reduction (from 70 to 95% of the influent) in the two units. A certain performance difference was evident depending on the class of dyes considered. As a fundamental study, the bubble unit was investigated to draw the ozone concentration profile along the column height and to measure the off-gas ozone concentration. A technological conclusion suggests that bubble systems are favoured due to their simple structure and to the overall energy requiremen

Design criteria of continuously operating biological aerated filters

Biological Aerated Filter (BAF) represent a compact wastewater technology aimed at improving treatment effectiveness by contemporarily minimizing the volume of the sludge-to-water contactor. BAFs can be classified as three-phase systems as far as their hydrodynamics is concerned and make use of granulated activated carbon (GAC) as a solid support; the macro-pores of this solid immobilize the growing biomass and assure a high sludge-to wastewater flow rate ratio. Our study stars from the standard design criteria adopted for a batch unit and extends the application of BAFs to continuous systems to attain simpler operations. The new concept unit is implemented with an air-lift regulated GAC recirculation and a section for separating the sludge from GAC. The treated water is segregated from the sludge suspension thanks to a careful regulation of flow rates in the biological reactor, which has some similarities with the so-called "backwash upflow sand filter". The experimental work, ahead of treating wastewater from textile industries, defines all the operating variables suitable to steadily maintain the contact time between the liquid and solid phases, the hold-up and recirculation time of GAC and the flow rate ratio between the cleared stream and purge liquid. The experimentation has brought to define: the geometrical features of the unit, GAC hold-up and recirculation rate, main and air lift flow rates. In this research program a 0.28 m ID, 2 m high Perspex unit is used for the hydrodynamical study and an identical geometry SS biological contactor for real industrial wastewater treatmen

Washing intensification by ultrasounds

The ultrasound (US) technology was applied to wool fabrics washing processes. The wool fabrics were impregnated with different types of oils. After drying the samples were washed both without and with ultrasounds working at 25 and 46 kHz frequencies. Different temperatures and ultrasound contact times were analysed in order to optimize the process. To compare the washing effectiveness of the processes the percentage differences of the total color differences ΔE of the samples were calculated. It was found that 46 kHz and 1 min ultrasound contact time at 40°C were the best condition

Evaluation of actual dyestuff penetration in ultrasonic-assisted wool dyeing

The application of ultrasounds (US) in dyeing and washing at industrial scale has become an important research topic in the recent years, as this technology appears to have a high potential to reduce the environmental impact of textile productions, as well as to increase the effectiveness of these basic operations. As far as dyeing is concerned, several studies [1, 2] indicate the possibility to operate at a temperature lower than the one typically suggested, yet achieving excellent dye exhaustion, apparently comparable to those obtained in conventional processes. Natural fibres (wool, cotton and silk) seem to be the most suitable materials for USintensified dyeing, since it is not necessary to exceed the glass transition temperature of the constituent polymers, as the dyestuff diffusion within the fibre matrix is a combined effect of hygroscopic pore swelling and US promoted cavitation. Usually, dye-bath exhaustion given by spectrophotometric analysis in the visible range, is one of the promptest parameters to monitor dyeing with time. However in our US-assisted tests, we noticed that this method provided questionable results by varying the operation parameters, namely temperature. In US-assisted dyeing, cavitation phenomena reduce the interphase mass transfer resistance, thus promoting dyestuff adsorption on the fibre surface. In the subsequent rinsing steps, variable amounts of dyestuff leave the textile substrate due to penetration deficiency. For this reason, a new method aimed at determining correctly the dye uptake was based on DMSO (dimethyl sulfoxide) extractions, either after dyeing or rinsing. By comparing the extracted amount, the quantity of adsorbed dyestuff was determined, thus highlighting the mass transfer intensification due to US effec

Mapping of ultrasonic fields for dyeing applications

In the last years textile industries have focused their attention on the development of innovative environmental friendly processes and on the improvement of product quality. The use of ultrasounds (US) waves to improve dyeing processes of natural fibers has been investigated since the begin of fifties; it was shown that US are effective to reduce temperature and operative time of the dyeing. In particular this improvement is very important for natural fibers since reduction of temperature and dyeing time helps in maintaining optimal fiber properties. The tests, carried out in the frame of the Piemonte Regional Project INTEXUSA (INnovation in TEXtile productions by UltraSound Application), were aimed at evaluating the influence of the system geometry on the ultrasound waves propagation in a water-based medium. These runs were performed by monitoring the system with an Ultrasonic Energy Meter (by PPB Megasonics) to evaluate the cavitation energy distribution inside a dyeing prototypal equipment operating with different liquor depths. The main goal of this study was addressed to analyze the influence of the liquid depth on the cavitation phenomena, whose output can be useful to determine an optimal geometric configuration in a novel dyeing equipment provided by US transducers and minimize liquor ratio

Washing off intensification of cotton and wool fabrics by ultrasound

Wet textile washing processes were set up for wool and cotton fabrics to evaluate the potential of ultrasound transducers (US) in improving dirt removal. The samples were contaminated with an emulsion of carbon soot in vegetable oil and aged for three hours in fan oven. Before washing, the fabrics were soaked for 3 min in a standard detergent solution and subsequently washed in a water bath. The dirt removal was evaluated through colorimetric measurements. The total color differences DE of the samples were measured with respect to an uncontaminated fabric, before and after each washing cycle. The percentage of DE variation obtained was calculated and correlated to the dirt removal. The results showed that the US transducers enhanced the dirt removal and temperature was the parameter most influencing the US efficiency on the cleaning process. Better results were obtained at a lower process temperatur