Photocatalytic-Biochemical Wastewater Treatment Based On The Characterization Of Refractory Organic Pollutants

Developing a suitable treatment for refractory organic wastewater is a difficulty in the field of sewage treatment. In this paper, the combination of photocatalytic and biological treatment is considered to avoid the high running cost and unstable treatment effect based on a full analysis of treatment technology of high concentration organic wastewater. Quantum efficiency of existing photocatalysts is low, so it is difficult to use in wastewater treatment. By a review characteristics of refractory organic pollutants, we find that most refractory organic pollutants have hydrophobic group. Thus, photocatalysts with hydrophobic properties by innovated the surface of nano-TiO₂ photocatalysts will increase the adsorption capacity of refractory organic pollutants. Through the selectively adsorption and degradation by photocatalysis, lower quantum requirement is demanded of high concentration refractory organic wastewater. According to this innovative idea, SDS-CuO/TiO₂ photocatalyst is prepared by modification of nano-TiO₂ using CuO and sodium dodecyl sulfate (SDS). A spiral up-flow type photocatalytic reactor is developed to improve the reaction efficiency and catalyst recovery rate. A photocatalytic-biological system is proposed to treat the refractory organic wastewater, and the key controlling factors and dynamics model are discussed.1) Nano-TiO₂ is modified by CuO to have visible responsibility. By characterization of XRD, it is found that there are two kinds of crystals on CuO/TiO₂, which are CuO and anatase TiO₂. Copper exists mainly in the form of CuO. UV-Vis characterization found CuO/TiO₂ catalyst has good response performance of visible light, and the maximum excitation wavelength reaches 821nm; Three-dimensional fluorescence characterization found the compound probability of electron and hole is decreased in photocatalyst modified by CuO. BET characterization found that modification of CuO does not influence the surface area and pore structure. CuO/TiO₂-visible light system exhibits excellent photocatalytic activity in degradation of methylene blue. Decolorization rate of methylene blue up to 90% within 2h when the initial pH = 11, the initial concentration of methylene blue is 500 times, the catalyst dosage is 0.1g/L wastewater, H2O₂ dosage is 10mL/L wastewater. CuO/TiO₂ has the highest catalytic efficiency with such conditions.2) Hydrophobic SDS-CuO/TiO₂ photocatalyst is prepared by using SDS as the modifier on CuO/TiO₂ surface. This modification does not effect the catalyst crystal type and the properties DRS of CuO/TiO₂. But it really reduces the compound probability of electron and hole. SDS modified catalyst leads to increased crystals size and pore size, and decreased BET surface area. Organic hydrophobic groups obtain on the modified surface of the catalyst, which are mainly C-C and-CH-group. The concentration of SDS is the most important factor on photocatalytic activity when SDS-CuO/TiO₂ is prepared. The photocatalyst has the best activity when SDS concentration is the critical micelle concentration (CMC), because there are maximum amount of hydrophobic groups on photocatalyst surface and the lowest compound probability of electron and hole. BET surface area of photocatalyst prepared with this concentration is 52.1m2/g, and the maximum excitation wavelength is 763nm.3) SDS-CuO/TiO₂-visible light system is used to degrade nitrobenzene. 2h degradation of nitrobenzene was 77% catalyzed by SDS-CuO/TiO₂ when the initial pH = 9, the initial concentration of nitrobenzene was 400mg/L⁵00mg/L, the catalyst dosage is 0.2g/L wastewater, H2O₂ dosage is 6mL/L wastewater.. Results show that SDS-CuO/TiO₂ photocatalyst has visible light response and hydrophobic adsorption capacity by by two steps modification of CuO and SDS. It can be used to preferential adsorb and degrade hydrophobic refractory organic pollutants under visible light irradiation.4) By cyclone separation theory, in order to increase the reactor A/V value and photocatalyst recovery rate, a solar photocatalytic reactor is designed. Its radius and high meet R 3 + 3 R2h2=Q, and light area and radius meet S = 2Ï€(0.02 ? R 3) / 3R. A/V value reaches 13, and loss rate of photocatalysts is 15.5% after 5d reaction When R=0.075m. Loss rate is 26.5% after 20d. SDS-CuO/TiO₂ catalyst has high photocatalytic activity even after continuous use of 20d. Printing and dyeing wastewater is treated under the sunlight. Results show that there is a maximum of promoting of light intensity in photocatalytic process. Aeration can be used to replace H2O₂ as electron capture agent in photocatalytic reaction. 3h is a better hydraulic retention time. Using auxiliary light source can significantly enhance the wastewater treatment. It can be used when the natural light is absence.5) SDS-CuO/TiO₂ photocatalytic-SBR biochemical system can be used for the treatment of refractory organic wastewater. For the treatment of printing and dyeing wastewater, Degradation process of refractory organic pollutants will take about 40min in photocatalysis unit. The wastewater has the best biodegradability, because BOD5 and DHA of wastewater reach the maximum value after 40min. The best operating parameters for photocatalytic reactor is 4g SDS-CuO/TiO₂ photocatalyst, 0.25m3/h aeration, 40min residence time; for SBR biological treatment unit is 0.179kgCOD/ (kgMLSS ? d) sludge load, 0.4m3/h aeration, 10h aeration time. Under these conditions, effluent quality of this photoatalytic-biological systems can achieve I-level standard of "discharge Standard of water pollutants for dyeing and finishing of textile industry " (GB4287-92).6) COD degradation of printing and dyeing wastewater consistent with Langmuir- Hinshelwood kinetic equation photocatalyzed by SDS-CuO/TiO₂. Considering the factors of initial COD concentration of wastewater, the amount of catalyst and light intensity, the dynamic model is:Formation rate of BOD5 in photocatalysis unit is C = 23.729t ? 49.33. The t in this equation relates to the amount of refractory organic pollutants in wastewater, and it between 30 and 40 in this system. By calculating the COD, BOD5 and B/C values in photocatalysis unit, main pollution indicators and biodegradability of influent of biological treatment unit can be obtain. So this dynamic model can be used as basis of operational controls of biological treatment unit.