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Assessment of titanium dioxide films for photocatalytic treatment of greywater
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posted on 15.02.2017by Lopez Vanegas, Lorena
As the need for safe and clean water has increased globally over the years, recycling wastewater has gained great importance not only for economic reasons but to help the environment through the reduction of the contamination load by using recycled water for those final purposes that do not require drinking water quality. In this regard, greywater is seen as one of the most appropriate and promising sources of wastewater with great potential for treatment and reuse as it represents between 60% to 75% of the domestic wastewater daily volume and contains a lower concentration of organic compounds and fewer pathogens when compared to other wastewater sources (Nolde, 2000b, Friedler, 2004, Abu Ghunmi et al., 2011, Eriksson et al., 2002).
This project investigated the potential of heterogeneous TiO2 photocatalytic processing for domestic greywater treatment through evaluating the TiO2 photocatalytic activity against methyl orange in the presence of a wide range of individual greywater components. The research was developed in three stages; in stage one the design of highly efficient large-scale TiO2 photocatalytic films was achieved, in stage two the full characterisation of the TiO2 films was done including the evaluation of the photocatalytic activity of the TiO2 films towards UV photodegradation of methyl orange (probe species) and in stage three the assessment of large-scale TiO2 films for the photocatalytic treatment of individual greywater components was carried out.
A TiO2 sol-gel formulation used for the synthesis of the TiO2 colloidal solution was found to be appropriate for the preparation of large-scale TiO2 films since these showed high photocatalytic response. This was the first time that this formulation was successfully used for the preparation of large-scale TiO2 films even though it has been previously used for coatings on different types of fabrics (Tung and Daoud, 2009, Tung and Daoud, 2011). For the immobilisation of this formulation, a doctor blade coating technique was found to allow for preparation of more efficient TiO2 films than a spray coating technique. Acid frosted soda-lime glass substrate allowed for deposition of continuous and uniform nanostructured anatase films with high photocatalytic activity.
The photodegradation of aqueous solution of methyl orange over anatase TiO2 films exhibited pseudo-first-order kinetics only during the first catalytic oxidation cycle. Deviations from first order kinetics in subsequent catalytic runs were ascribed to film activation as a result of modifications of the catalyst surface during the initial cycle. These results were the baseline for the assessment of the TiO2 photocatalytic processing for greywater treatment.
A detailed study of the photocatalytic activity of the TiO2 films towards the UV photodegradation of methyl orange in the presence of twenty four different individual greywater components was carried out. Reactions trends were identified for the effect of the tested individual greywater components during the initial cycle of treatment and the effect of their degradation products during a second cycle. It was found that some key components present in greywater have deleterious effects on both catalyst performance and activation while others were found to assist the catalyst and enhanced its activation. However, it was only possible to develop a partial predictive model of the performance of the TiO2 films for real greywater situations because of the complexity of the competing processes involved.
Overall, in this study the preparation of highly efficient anatase TiO2 photocatalytic films suitable for wastewater treatment was achieved. Reaction trends for the effects of a wide range of domestic greywater components on the TiO2 catalyst performance and re-use were identified. These findings show that there is more to investigate about the behaviour of the TiO2 catalyst towards individual greywater components before TiO2 photocatalytic processing can be fully understood and assessed for the treatment of complex wastewater streams such as greywater.