Heterogeneous fluidized-bed Fenton process: Factors affecting iron removal and tertiary treatment application

In this research, the factors affecting iron removal were determined for a fluidized-bed Fenton reactor using construction sand as the medium. Most of the particulate iron could be removed by the fluidized-bed Fenton process under optimum conditions. The surface area of the fluidized media available for iron crystallization via heterogeneous nucleation and hydraulic retention time were found to be the key factors controlling the iron removal efficiency. Bed expansion of 50% was sufficient for satisfactory iron removal. Further bed expansion did not provide any significant improvement, and could deteriorate the iron removal performance through scouring effect and increase energy consumption. Formic acid, one of the carboxylic intermediates from organic pollutant oxidation with hydroxyl radicals, remarkably deteriorated the iron removal performance. This is because it not only increased iron solubility through complexation but also hindered heterogeneous nucleation of ferric hydroxide onto the fluidized material. Therefore, in terms of iron removal, the fluidized-bed Fenton process should be applied to treat low-strength wastewater or to serve as a polishing unit to limit the formation of Fe3+-chelating intermediates. As a tertiary treatment unit, the fluidized-bed Fenton reactor could successfully and consistently reduce the COD and color of secondary effluents from pulp and paper mills to below the new and more stringent standards of 120 mg/L and 300 ADMI, respectively. Depending on the water characteristics, total iron removal of 53-81% could be achieved at the optimum retention time.