Application of solar photo-Fenton oxidation coupled with granular activated carbon as a viable option for the removal of contaminants of emerging concern in the urban wastewater of India

It has been estimated that approximately 62 billion litres of sewage are generated every day in India, with only 37% of sewage being treated (Saxena et al., 2021). Wastewater treatment issues such as dysfunctional UWTPs with many of them operating at low service levels compared to the design capacity and a lack of a tertiary treatment facility, lead to the direct discharge of untreated or inadequately treated sewage into receiving environments, daily. Moreover, this type of untreated/partially treated sewage was shown to be popular for agricultural irrigation in water stressed rural regions around the country. This practice carries severe health risks to exposed farmers, due to their continued exposure to pharmaceutical residues and to pathogenic bacteria, which may not be treatable by commonly used antibiotics due to the development of antibiotic resistance (Saxena et al., 2021). Pathogenic bacteria were also shown to persist in secondary-treated effluents of urban wastewater treatment plants (UWTPs) in India, thus being still detected in irrigation canals that supply irrigation water to farmers (Saxena et al., 2021), highlighting the risk for an increase in the prevalence of antibiotic-resistant bacteria (ARB) due to the selective pressure applied by the presence of antibiotic residues in contact with the remaining bacteria. Therefore, the objective of this study was to evaluate the application a solar-driven Advanced Oxidation Process namely the solar photo-Fenton oxidation, coupled with a polishing adsorption step using Granular Activated Carbon (GAC) at a pilot scale, for the treatment of simulated Indian wastewater (SWW). This investigation took place in the framework of the EU-India ‘PANIWATER’ project (H2020-SC5-2018-1/820718), with the goal of applying a real-case scenario where a mixture of chemical and microbial CEC in wastewater of a rural region in India is effectively treated under real solar irradiation, in order to produce a safe-to-use treated effluent. The optimum conditions of the solar photo-Fenton oxidation for the assessment of the removal of the chemical and microbial CEC were: [H2O2]0=100 mg L-1, [Fe2+]0=5 mg L-1 and [pH]=2.8. The pilot-scale GAC adsorption step involved the use of 6.5 kg of a charcoal-based GAC (Norit ROZ3) in a two-column setup. The assessment of the removal of the CEC that were spiked into the SWW at an initial concentration of [C]ini=100 μg L-1 at the end of the solar photo-Fenton oxidation (T30W,n=247.7 min), led to an efficient removal of the chemical CEC. More specifically, acetaminophen and diclofenac were completely removed (>99%), followed by erythromycin (98%) and ofloxacin (90%). The contact time of 15 minutes with GAC after solar photo-Fenton oxidation of the treated effluent, was adequate for further effective CEC removal. All compounds exhibited a final removal >90% after GAC except sulfamethoxazole, which exhibited a removal of 88%. In more detail, the GAC contact contributed to the additional removal of compounds by 1-50%. The solar photo-Fenton oxidation process was effective for the inactivation of total pathogenic Escherichia coli, Enterococcus faecalis and Salmonella enteritidis after T30W,n=42.9 min. This finding showcases the potential of such an advanced oxidation process for the effective disinfection of wastewater effluents, thus rendering them clear from important pathogenic bacterial groups such as the ones examined herein. The spiked ARB into the SWW of an initial concentration of 4x104 CFU mL-1, were inactivated after 5 min of oxidative treatment. Due to the fact that ARB are a sub-category of total bacteria and as a result behave in the same manner as total bacteria once under oxidative stress, the addition of H2O2, the acidic pH and the exposure of the experimental reactor to real solar irradiation achieved their complete inactivation (<LOD) after 5 minutes of solar photo-Fenton oxidation. Finally, the solar photo-Fenton oxidation coupled with GAC adsorption was shown to be a valuable treatment combination for use in developing countries such as India, for the removal of chemical and microbial CEC that may pose serious human health risks when found in wastewater effluents used for disposal or agricultural irrigation purposes.