Solar photo-Fenton oxidation followed by adsorption on activated carbon for the minimisation of antibiotic resistance determinants and toxicity present in urban wastewater

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Highlights

  • 7 antibiotics were subjected to solar photo-Fenton treatment in urban wastewater.

  • Solar photo-Fenton was effective in ARB and ARGs elimination.

  • Increased toxicity towards D. magna was observed during solar photo-Fenton.

  • The application of GAC to solar photo-Fenton treated flow eliminated toxicity.

  • Combination of solar photo-Fenton and GAC removed antibiotics from wastewater.

Abstract

This work evaluated the removal of a mixture of antibiotics from urban wastewater, by a combined process consisting of solar photo-Fenton (SPF) followed by granular activated carbon (GAC). The effects of the SPF process were investigated at a toxicological, microbiological and genomic level, using species of plants and aquatic organisms, bacteria, antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs). The results demonstrated that SPF could completely degrade antibiotics present in two types of effluent deriving from a conventional activated sludge system (CAS) and a membrane bioreactor (MBR), operated at the optimum oxidant dose ([H2O2]CAS = 100 mg/L; [H2O2]MBR = 50 mg/L) and illumination time (tCAS = 115 min; tMBR = 111 min) at acidic pH (2.8–2.9). Moreover, total disinfection was achieved by the SPF process, as cultivable bacteria, including ARB, were inactivated after 60 min of treatment, disabling also the bacterial regrowth after 24 h of storage of the treated effluent. Furthermore, SPF was shown to be effective in degrading the cellular DNA of the effluent, which was reduced to the detection limit after 60 min of treatment in both effluents. The abundance of 16S rRNA was found to be preferentially decreased by the SPF in the MBR-treated effluent (8 folds) compared to the CAS-treated effluent (7.4 folds). The abundance of blaOXA, blaCTX-M, qnrS, sul1, and tetM genes was decreased to the limit of quantification after 60 min of SPF treatment in both effluents. However, the SPF treated flow resulted in increased toxicity, probably due to the oxidation of the dissolved effluent organic matter of the wastewater leading to the formation of toxic products. Therefore, SPF-treated samples collected at different time intervals (30, 60, 90, 120, and 180 min) were subjected to adsorption onto GAC (500 mg/L), and the removal of both the toxicity and the residual antibiotics remaining after SPF, was explored. The combined processes (30 min SPF; 15 min GAC) provided almost complete removal of toxicity and elimination of antibiotics, ensuring wastewater decontamination.

Introduction

The inclusion of some antibiotics in the revised Watch List drafted by the European Commission [1] and the appearance of new challenges in wastewater reuse applications regarding contaminants of emerging concern (CECs), including antibiotics, antibiotic-resistant bacteria (ARB), and resistance genes (ARGs), have demonstrated the need for the application of new and improved wastewater treatment technologies, able to effectively remove these microcontaminants from urban wastewater [2]. A potential alternative solution could be the utilization of advanced treatment technologies which can increase the removal of CECs and, consequently, enhance the quality of the effluents before their reuse or discharge into the environment.

Advanced chemical oxidation processes (AOPs) have attracted major scientific interest as a promising alternative for the degradation of antibiotics in wastewater [3]. They involve chemical reactions that generate highly reactive radical species, such as the hydroxyl radicals (HOradical dot). Among the existing AOPs, Fenton process, which is based on the production of HOradical dot from the reaction between hydrogen peroxide and ferrous ions in acidic medium, can be accelerated when the solution is irradiated with light (photo-Fenton) and promotes the oxidation and mineralisation of organic contaminants. Some of the advantages of photo-Fenton are its high efficiency in degrading different types of organic substrates and the use of solar irradiation to stimulate the radicals’ formation, resulting thus, in a low-cost application [4]. Solar photo-Fenton (SPF) has been shown to be capable of completely oxidizing various organic microcontaminants, while also providing sufficient disinfection of wastewater. One major constraint of this process, as well as of all the light-driven oxidation processes, is the formation of oxidation products deriving from the dissolved effluent organic matter (dEfOM), which may induce toxic effects [5]. The dEfOM present in biologically treated urban wastewater, consists of a heterogeneous mixture of refractory organic compounds with diverse structures and varying origin, including dissolved natural organic matter, soluble microbial products, endocrine disrupting compounds, pharmaceuticals and personal care products residues, disinfection by-products, metabolites/transformation products and others [6].

Granular activated carbon (GAC) has been widely tested in wastewater remediation for the removal of various CECs due to its ability to adsorb organic contaminants [7]. The efficiency and life-time of GAC depend however on several parameters, such as the quality of the effluent to be treated, the type of GAC used and the contact time. This makes the direct application of GAC after the biological treatment economically non-feasible, as the effluent and the dEfOM contained, could exhaust GAC in a short period of time, imposing thus the application of extensive amounts of GAC and long contact times for the wastewater decontamination. Therefore, the application of GAC as a post-treatment to SPF, when the treated effluent contains reduced organic load and simpler organic molecules, could lead to the cost-efficient further remediation of the wastewater.

Within this context, the overall objectives of this study were two-fold: (a) to investigate the efficiency of SPF process in removing antibiotic resistance determinants and toxicity from urban wastewater (i.e. (i) degradation of selected antibiotics when present as a mixture in secondary-treated effluent, (ii) evaluation of phyto- and eco-toxicity of the treated effluent towards three plant species and Daphnia magna, respectively, (iii) evaluation of the disinfection potential of the process as to the inactivation of frequently encountered bacteria in wastewater, including those harbouring resistance to selected antibiotics [faecal coliforms, Pseudomonas aeruginosa, enterococci, total heterotrophs], as well as their regrowth potential after treatment, (iv) evaluation of the capacity of SPF to remove selected ARGs [e.g. 16S rRNA, blaTEM, tetM, sul1 etc.]), and (b) to investigate the capacity of GAC, applied as post-treatment to SPF, to remove residual ecotoxicity and antibiotics not completely degraded during the early stages of SPF. The results of this study could contribute to the understanding of the fate of both the antibiotics and antibiotic resistance during advanced wastewater treatment and provide guidance for the practical application of the SPF-GAC process. According to the authors’ knowledge, this is the first study investigating the combination of SPF and GAC processes, evaluating its efficiency in removing selected antibiotics, bacteria, ARB, ARGs, and toxicity from urban wastewater. This work suggests the SPF-GAC process could be a comprehensive solution, being not only able to cope with the toxicity problem of SPF, but also to provide in a short time, total removal of microcontaminants enabling the safe disposal of treated urban wastewater to the environment.

Section snippets

Reagents

All antibiotics tested (ampicillin, clarithromycin, erythromycin, ofloxacin, sulfamethoxazole, tetracycline and trimethoprim) were of >90% purity (Sigma-Aldrich) and were spiked in the effluent as a mixture (100 μg/L each) from individually prepared stock solutions (5000 mg/L). The stock solutions were kept in refrigerator and their stability was routinely checked through chromatographic analysis. During one-month period no statistical differences were observed. Iron(II) sulfate heptahydrate

Antibiotics degradation

In order to determine the optimum H2O2 dose, in which sufficient degradation of antibiotics could be accomplished during SPF experiments, several oxidant concentrations (ranging from 50 to 125 and 25–75 mg/L for CAS and MBR effluents, respectively) were tested. The range of H2O2 concentrations examined was based on the findings of previous studies [5,13]. The optimum concentration was found to be 100 and 50 mg/L of H2O2 for CAS and MBR effluents, respectively (results not shown). The need for

Conclusions

The present study focused primarily on the minimisation of antibiotics in urban wastewater, while the fate of antibiotic resistance determinants during the SPF-GAC treatment, was also examined. The combination of SPF with adsorption was proved to be effective not only for the removal of antibiotic related microcontaminants but also for the elimination of toxicity in urban wastewater. Briefly, the main conclusions are:

SPF was found to be efficient in degrading antibiotics but the process

Funding

This work was prepared in the framework of the StARE project (KOINA/ΠKΠ/0113/15), financed by the Cyprus Research Promotion Foundation (DESMI 2009-2010).

Acknowledgements

This Special Issue is dedicated to honour the retirement of Prof. César Pulgarin at the Swiss Federal Institute of Technology (EPFL, Switzerland), a key figure in the area of Catalytic Advanced Oxidation Processes. The authors acknowledge the COST Action ES1403 NEREUS “New and emerging challenges and opportunities in wastewater reuse” supported by European Cooperation in Science and Technology (www.cost.eu) for enabling the collaboration among the authors of the paper.

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