The photocatalytic degradation of dinitrophenol (DNP) was studied in an aqueous solution over porous TiO2/SiO2 in a laboratory scale fixed bed circulation type reactor. DNP is consecutively decomposed to NO3- and NH4+ via NO2- and about 90% of aromatic compounds decomposed completely to CO2 in 100 min. Reaction rate equations were found to obey a first order rate law. A pilot scale fixed bed flow reactor packed with the commercially available titania/silica was set outdoors under solar light for treating city water. Concentration of total organic carbon (TOC) and double bonded organic compounds were found to decompose on the photocatalyst under sunbeam. The experiment was done during 3 months and the catalyst was fouled because of the adhesion of red iron agglomerates. We examined a method to remove the adhesion metal by dipping the fouled catalyst into hydrochloric acid, nitric acid and citric acid, and deionized water for I week. Activity recovered for the catalyst washed by acid, but activity does not recover as for the catalyst washed in deionized water. (C) 2003 Elsevier B.V. All rights reserved.