Study On The Advanced Treatment Of Secondary Effluents By Ag-TiO₂/Pumices

Secondary effluents from sewage treatment plants contain of physical,chemical and biological constituents.The concentration of the constituents is much lower than other wastewater.They can be reused for agriculture and aquaculture after advanced treatment. Semiconductor photocatalysis has been recognized as the most promising technology for degradation of aqueous organic pollutants and the complete mineralization of large number of compounds.Titanium dioxide(TiO₂) is the most often used photocatalyst due to its considerable photocatalytic activity,high stability,non-environmental impact and low cost.TiO₂ powder is hardly recycled in aqueous solution.Besides,the agglomeration of the TiO₂ powder may reduce the photocatalytic oxidation efficiency.To overcome these problems,various methods were used to immobilise the TiO₂ catalyst on various carriers in the form of thin films or coatings. TiO₂/pumices were prepared by the Sol-Gel method.Ag was loaded on the surface of the TiO₂ film.The purpose of this research was to evaluate the photocatalytic oxidation efficiency of the Ag-TiO₂/pumices for mineralization and disinfection of the secondary effluent from sewage treatment plants.TiO₂/pumices were prepared by the Sol-Gel method according to the literatures.Ag was loaded on the surface of TiO₂ by the photodeposition method.Ag loading rate was 1.0%(Ag/ TiO₂),donated as Ag-TiO₂/pumice.Ag-TiO₂/pumices did well in degrading the Methylene Blue whose original concentration was 10 mg/L.After 60 min reaction,the degradation efficiency of Methylene Blue reached almost 100%using Ag-TiO₂/pumices,which was close to the P25 powder.The TiO₂ films showed good durability and stability of the photocatalytic activity.After 30 times using,the degradation of the Methylene Blue dropped to 75.1%from 91.5%.But the photocatalytic activity resumed after regeneration.The TiO₂ films were intact after 60 times using.Dimethyl phthalate(DMP) and phenol were chosen to investigate the photocatalytic activity of Ag-TiO₂/pumices for the refractory organic matters,which were found in the secondary effluents all around the world.The original concentration of DMP and phenol was 10 mg/L. After 180 min reaction,the degradation efficiency of DMP and Phenol reached 86.4%and 78.5%, respectively. The removal efficiency of total organic compounds(TOC) of the secondary effluent reached 47.5%after 6 h photocatalytic reaction by Ag-TiO₂/pumices.GC-MS analysis showed some regractory organic matters were not determined after treatment.The concentration of phthalate esters was also decreased after 6 h photocatalytic reaction.The biodegradability of the effluent was improved.The BOD/COD increased to 0.68 from 0.28.The sterilization rate was 100% using Ag-TiO₂/Pumices after 1 h reaction.The secondary effluent is a mixture which contains inorganic and organic substances.Some inorganic ions will inhibit the degradation of organic compounds,such as chloride,sulfate, carbonate and nitrate.In order to study further the inhibition of inorganic ions on the degradation of organic compounds,a photocatalytic degradation experiment of dimethyl phthalate(DMP) in a model solution which contained the same kinds and approximately equal concentrations of inorganic ions of the secondary effluent was carried out.The original concentration of the DMP was 5 mg/L.After 120 min reaction,the degradation efficiency of DMP in deionized water reached 93.2%,while the one in model solution was only 51.4%.