Assessment of photo-Fenton and biological treatment coupling for Diuron and Linuron removal from water

The coupling of photo-Fenton (chemical) and biological treatments has been used for the removal of Diuron and Linuron herbicides from water. The chemical reaction was employed as a pre-treatment step for the conversion of the toxic and non-biodegradable herbicides into biodegradable intermediates that were subsequently removed by means of a biological sequencing batch reactor (SBR). Multivariate experimental design was used to select four photo-Fenton reagent dose combinations for the coupling experiments. Concentrations of hydrogen peroxide between 10 and 250 mg L−1, and iron (II) concentrations between 2 and 20 mg L−1 have been tested. 15.9 mg L−1 of Fe(II) and 202 mg L−1 of H2O2 were needed to convert initial toxic and non-biodegradable herbicides into suitable intermediates for a subsequent biological treatment. Detrimental effects due to the excess of reactants were detected. Chemical oxygen demand (COD), average oxidation state (AOS), total organic carbon (TOC) and hydrogen peroxide concentration are the parameters used to trace the experiments course. Also, toxicity (EC5015) and biodegradability (BOD5/COD) tests were carried out at the end of each chemical oxidation. Complete disappearance of the herbicides from water was observed after the chemical treatment, while 3,4-dichloroaniline and 3,4-dichlorophenyl isocyanate were identified as the main by-products of the degradation process. Complete TOC removal was achieved after biological treatment in a SBR using a hydraulic retention time (HRT) of 2 days.