Upgrade constructed wetlands wastewater quality by solar-driven photochemical quaternary treatments in raceway pond reactor at pilot plant scale.

This study explores the potential application of solar photochemical processes (SPPs) for simultaneous disinfection and decontamination of urban wastewater (UWW) when combined with constructed wetlands (CWs). Two SPPs based on the addition of low concentrations of hydrogen peroxide and peroxymonosulfate (PMS) were evaluated. SPPs were carried out at pilot plant scale using low-cost solar open photoreactors (Raceway Pond Reactor (RPR)) under natural sunlight. The performance of the SPPs was analyzed by monitoring naturally occurring bacteria (Escherichia coli, Enterococcus spp. and Salmonella spp.) and Contaminants of Emerging Concern (CECs), such as pharmaceutical products, pesticides, antibiotics and their metabolites, simultaneously. SPP best operating conditions were determined by testing a wide range of oxidant concentrations (0-5.9 mM, 0-3.0 mM) and liquid depths (5, 10 and 15 cm) in the RPR. SPP treatment efficacy was tested on the actual inlet and outlet of the CW system, showing the high influence of their different physicochemical characteristics on the oxidants' performance. H ₂ O ₂ /solar SPP in the actual inlet of the CWs showed slightly higher inactivation kinetics when increasing H ₂ O ₂ concentration at both water depths (5 and 10 cm), while no significant CECs degradation rates were obtained. However, much higher efficacy was obtained with PMS/solar process attaining high bacteria inactivation under dark conditions and 79 % CEC degradation at 3.0 mM after only 10 min of reaction time. SPPs assessment on the outlet of the CWs also showed better efficacy of the PMS as oxidant compared to the H ₂ O ₂ for the simultaneous CECs removal (95 % at 1.0 mM of PMS, after only 5 min of treatment) and bacteria inactivation (confirming the no-regrown after 24 h). SPPs have demonstrated to be a low-cost and eco-sustainable polishing alternative to regenerate CW effluents complying with the actual European regulation on the minimum water quality requirements for reusing treated UWW in agriculture.
Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Isabel Oller reports financial support was provided by ENI CBC Mediterranean Sea Basin Program. Isabel Oller reports financial support was provided by Spanish ministry of science and innovation. Isabel Oller reports a relationship with Research Centre for Energy Environment and Technology Energy Department that includes: employment. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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