Continuous flow disinfection of WWTP secondary effluents by solar photo Fenton at neutral pH in raceway pond reactors at pilot plant scale

The feasibility of low cost reactors such as raceway pond reactors (RPRs) for micropollutant removal as well as pathogen inactivation has recently been reported. This study presents, for the first time, the inactivation of Total Coliforms, Escherichia colt and Enterococcus sp. in WWTP secondary effluents by the solar photo-Fenton process in continuous flow at neutral pH in open reactors. Firstly, indoor assays were carried out with three hydraulic residence times (HRTs), 15, 30 and 60 min under simulated solar radiation of 30 W m(-2), 5 cm liquid depth, 20 mg Fe2+ L-1 and 50 mg H2O2 L-1. At 60 and 30 min of HRT, wastewater disinfection below 1 CFU mL(-1) was achieved at steady state for the three microbial groups selected. Nevertheless, at 15 min of HRT only 1-log inactivation was obtained. Taking into account the results obtained at laboratory scale, the best HRTs (60 and 30 min) were tested outdoors in 5 cm-deep RPRs in both summer and winter. The start-up began with a batch operation for 120 min with 20 mg Fe2+ L-1 and 50 mg H2O2 L-1, followed by continuous operation over three consecutive days with 20 mg Fe2+ L-1 and 30 mg H2O2 L-1 to avoid large excess of hydrogen peroxide in steady state. Bacterial inactivation results showed 30 min of HRT to be the most efficient operation condition capable of producing 305 m(3) m(-2) year(-1) of disinfected water while complying with the Spanish Law (RD 1620/2007) for water reuse. Additionally, a reduction in iron supply ranging from 10 to 2.5 mg L-1 in continuous mode with 30 mg H2O2 L-1 was evaluated. Efficient microorganism inactivation was reached for 10 and 5 mg Fe2+ L-1, thus reducing the iron consumption at steady state four-fold. This research opens up a new line of research on the optimisation of wastewater disinfection by large-scale solar photo-Fenton.