Effect of residence time on micropollutant removal in WWTP secondary effluents by continuous solar photo-Fenton process in raceway pond reactors

Abstract

The removal of micropollutants (MPs) in wastewater treatment plants (WWTPs) secondary effluents by solar photo-Fenton is commonly carried out with low concentrations of iron and hydrogen peroxide due to the low pollutant concentration (ngL−1). This led to two main consequences being observed: short treatment times (tens of minutes) and excess of photons when using 5cm-diameter tubular reactors. To make better use of most of the photons reaching the reactor surface, the use of raceway pond reactors (RPRs) with longer light path length was implemented successfully. However, little attention was paid to the treatment time itself and the photo-Fenton process was applied in batch mode despite filling and emptying stages being much longer than reaction time. This paper deals with the performance of the solar photo-Fenton process in RPRs operated in continuous mode to remove MPs in WWTP secondary effluents. In this regard, first the continuous degradation of a model emerging contaminant, the pesticide Acetamiprid (ACTM), was studied in a synthetic secondary effluent at pH 2.8. Next, real secondary effluents were treated at two liquid depths, 5 and 15cm and three hydraulic residence times (HRTs), 80, 40 and 20min. Reactant concentrations were 5.5mgL−1 Fe2+ and 30mgL−1 H2O2 giving rise to a treated wastewater volume of 450Lm−2day−1 for a 5cm-deep RPR operated in continuous mode at 40min HRT for 6h a day in winter, with 84% MP removal and 66% hydrogen peroxide conversion. As far as the authors know, this is the first report on the solar photo-Fenton process operated in continuous mode displaying its advantages with regard to the batch operation.