Photocatalytic methods for the treatment of municipal wastewater

Wastewater has been recognized as one of the most important environmental pressures in Mediterranean region. The last decades centralized conventional wastewater treatment systems were typically provided to large cities and secondary towns. However, the establishment and operation of theses systems in the Mediterranean region have been costly and problematic. As a result, the last decades several efforts have been made towards the development of alternative methods of wastewater treatment systems well established in the environmental and socioeconomic status of Mediterranean region. The increased concern for the use of the “Advanced Oxidation Technologies, AOT” may be explained by the need for seeking of new, alternative to the conventional ones and environmentally friendly technologies. Under this term the scientific community refers to the technologies whose effectiveness is based on the production of the ΟΗ. radicals, one of the most powerful oxidant reagents. They can easily attack the organic molecules leading to the production of organic peroxideradicals and their finally conversion to CO2, H2O and inorganic salts. Among these, heterogeneous and homogeneous solar photocatalytic detoxification methods (TiO2/ H2O2, Fe+3/H2O2) have shown recently great promise for the treatment of industrial wastewater, groundwater and contaminated air, allowing the contribution of the renewable sources of energy (solar energy) to the process of cleaning and restoring the environment. In this study, the photocatalytic oxidation of synthetic, municipal and cesspool wastewater has been studied using homogeneous, heterogeneous and TiO2/Fe3+/H2O2 photocatalysis. In heterogeneous photocatalysis, suspension of TiO2 P-25 and TiO2 A were used as the basic photocatalytic systems, while in homogeneous, the photocatalytic efficiency of the Fenton, photo-Fenton and Ferrioxallate reagent under UV-A and Visible light were examined. The objectives were: a) to examine the extent of the oxidation of the wastewater b) to evaluate the kinetics and the relative photonic efficiency of the photocatalytic oxidation c) to compare the photocatalytic efficiency of the used semiconductors d) to examine the effect of various parameters like the mass of the catalyst, the addition of an oxidant, pH, etc e) to examine the efficiency of the photocatalytic disinfection. As far as heterogeneous photocatalysis is concerned, TiO2 P-25 proved to be more efficient photocatalyst. Parameters such as pH, concentration of the catalyst and the H2O2 play an important role affecting the initial reaction rate and the relative photonic efficiency. By the addition of H2O2 into TiO2 illuminated suspensions a synergistic effect is observed leading to the enhancement of the process. As far as mineralization is concerned, illuminated TiO2 P-25 /H2O2 systems are able to achieve higher percentage of oxidation (maximum about 50%), dependant on the amount of the catalyst, the concentration of the oxidant and the pH of the wastewater. As far as homogeneous photocatalysis is concerned, photo-Fenton reaction is able to achieve higher initial reaction rates and extent of mineralization. It is proved to be much faster. In general, the photo-Fenton and Ferrioxallate reagent give similar efficiency, while Fenton reaction, in the absence of light, is not able to achieve satisfactory results, thus requiring larger quantities or iron and oxidant. The irradiation of the above solutions by UV-A or Visible light leads to the regeneration of the Fe2+, the extra production of hydroxyl radicals providing higher extent of the mineralization. The concentration of iron or peroxide and temperature are parameters that affect the efficiency of the system. Furthermore, the efficiency of the TiO2 /Fe3+/H2O2 reagent was examined. The presence of the ferric ions multiplies the initial reaction rate and the extent of mineralization of the wastewater. Parameters such as the TiO2, Fe3+ and the oxidant concentration and the pH affect severely the efficiency of the above system. The stepwise addition of the oxidant gives the maximum efficiency. Comparison of the three above photocatalytic methods showed that homogeneous photocatalysis and the TiO2 /Fe3+/H2O2 reagent are able to achieve satisfactory extent of the mineralization of the wastewater. However, the TiO2 /Fe3+/H2O2 reagent seems to be the most appropriate, since it can provide high efficiency, while at the same time, a severe reduction of the needed amounts of the reagents can be fulfilled. In order to develop a simple and effective solar photocatalytic method for the treatment of municipal wastewater of small communities and villages in small islands in the area of the Aegean Sea, where there is a wide variation of the population between the winter and the tourist season and where the biological treatment of the wastewater has a lot of disadvantages. More specifically, the system combines the synergetic action of the heterogeneous photocatalytic oxidation with the surface flow constructed wetlands in order to utilize the high solar irradiation and the ability of the constructed wetlands to improve water quality through natural processes. According to the results, the two methods are compatible, under terms, and the combined system may effectively treat synthetic, municipal and cesspool wastewater by reducing the concentration of pollutants at levels required by the Greek and European legislation. Comparing to conventional systems of wastewater treatment, the main advantages of the combined system are: Low cost of establishment and operation Ability to treat wastewater with great variability of hydraulic and pollutants load. Low requirements in chemicals and energy and utilization of solar energy and natural processes No need for additional disinfection method. This combination gives an integrated system for treating municipal wastewater, that has a lower investment cost and has the possibility of treating different volumes, while it makes possible the reuse of the treated wastewater