Kinetic assessment of antibiotic resistant bacteria inactivation by solar photo-Fenton in batch and continuous flow mode for wastewater reuse
Graphical abstract
Introduction
The reuse of treated wastewater for crop irrigation has become an increasingly recommended practice to address water scarcity in the EU and worldwide (Michael-Kordatou et al., 2018). However, the presence of the so-called “new contaminants” or “contaminants of emerging concern” in treated wastewater has led to increased concern about the potential direct and indirect effects on the environment and possible implications for human health (Kümmerer et al., 2016). In this regard, special attention has been paid to the contribution of treated wastewater concerning the emergence of antibiotic resistance in pathogenic bacteria, claimed to be “one of the major global threats to society” in the twenty first century by the World Health Organization (WHO, 2014).
Municipal wastewater treatment plants (MWWTPs) are considered to be significant reservoirs of antibiotic resistant bacteria as they combine a high load of bacteria with a residual concentration of antibiotics (Fatta-kassinos, 2016). The biological treatments typically applied in MWWTPs are poorly effective in the complete disinfection of the wastewater, generating secondary effluents with about 109-1012 Colony Forming Units (CFU) per day and per inhabitant equivalent. Among these, at least 107–1010 CFU could have acquired antibiotic resistance (Rizzo et al., 2013a, Rizzo et al., 2013b). Thus, the large amount of antibiotic resistant bacteria (ARB) discharged when treated wastewater is used for irrigation which may proliferate in soils or plants (Christou et al., 2017).
Therefore, conventional MWWTPs should be upgraded with more efficient technologies such as tertiary treatments to provide enough protection for the potential risks related to water reuse, not only by meeting the current quality standards, but also by controlling the spread of ARB into the environment. In fact, currents standards in water reuse for irrigation purposes relies on the control of chemical and microbiological parameters like heavy metals or enteric pathogens (Escherichia coli and Enterococcus sp), but do not evaluate ARB levels (Fatta-kassinos, 2016). In this regard, the measurement of bacteria resistant to Cefotaxime (third-generation cephalosporin, which is on the WHO essential list of medicine) could be used as a reliable indicator of wastewater resistance levels as proposed by the Scientific Committee on Health, Environmental and Emerging Risks (SCHEER) on the “Proposed EU minimum quality requirements for water reuse in agricultural irrigation and aquifer recharge”, a report prepared by the European Commission Joint Research Centre (SCHEER, 2017).
Despite the most common tertiary treatments used as a disinfection step before secondary effluent reuse having proved to be efficient in microorganism inactivation (namely, ozonation, chlorination or UVC irradiation) (Xu et al., 2002; Rodriguez-Chueca et al., 2015; Rizzo et al., 2019), their efficacy for effective reduction of antibiotic resistance is not completely agreed upon (Luigi Rizzo et al., 2013; Yuan et al., 2015; Sousa et al., 2017; Michael- Kordatou et al., 2018). Additionally, other drawbacks related to the generation of undesirable disinfection by-products through ozonation (Stalter et al., 2010) or chlorination (Watson et al., 2012) has forced research to seek out disinfection alternatives that are environmentally safer than conventional tertiary treatments, namely the solar photo-Fenton process. This Advanced Oxidation Process (AOP) has been found to be efficient, not only at degrading antibiotics, but also at inactivating microorganisms and ARB (Giannakis et al., 2018a) without increasing toxicity (Michael et al., 2012). It is based on the generation of highly oxidative and non-selective species, hydroxyl radicals (HO•), that degrade the external membrane of bacteria, increasing its permeability and inducing internal reactions that eventually inactivate the microorganisms (Giannakis et al., 2018b). The generation of HO• in the Fenton process relies on the repeated oxidation and reduction of iron (catalyst) by hydrogen peroxide. Moreover, in the presence of UV–vis radiation, the HO• production rate is increased due to the regeneration of Fe2+ by the photo-conversion of ferric iron to ferrous iron (Pignatello et al., 2006). Despite the most desirable pH for radical production being around 2.8–3.0, owing to the low solubility of Fe3+ at higher pH, many recent studies have confirmed the effectiveness of the photo-Fenton process for bacterial inactivation at neutral pH (Giannakis et al., 2016b). Nevertheless, to our knowledge, the industrial application of this solar process as a tertiary treatment for wastewater disinfection has not yet been developed, even though efforts have been directed towards making its application at large scale feasible via compound parabolic collectors (CPCs) (Rodríguez-Chueca et al., 2014) or low cost reactors such as raceway pond reactors (RPRs) (Esteban García et al., 2018). In particular, the latter has emerged as an attractive option, not only for the remarkable reduction in investment cost (RPR construction costs are 40 times cheaper than those for CPCs), but also for the larger volume of wastewater than can be treated per surface area (Carra et al., 2014). In fact, to bring closer to reality the solar photo-Fenton implementation, the large amount of secondary effluent generated in MWWTPs needs to be dealt with. With this in mind, the feasibility of the continuous flow operation of the solar photo-Fenton process in RPR for secondary effluent disinfection has recently been investigated with promising results (De la Obra et al., 2019).
The objective of this work was to carry out an exhaustive and comparative kinetic study on the inactivation of both total and cefotaxime resistant bacteria by solar photo-Fenton at neutral pH in a wide range of operation conditions, taking into account different bacterial species (Total coliform, Escherichia coli and Enterococcus sp). To this end, the effects of solar irradiance and iron concentration as well as operation mode, batch vs. continuous, were analysed.
Section snippets
Wastewater source
Secondary effluent batches of 1 m3 were taken from the Municipal Wastewater Treatment Plant named “El Bobar” (Almeria, Southeastern Spain). A characterization of wastewater samples was carried out during the first three days of experimentation to assess their variability. The average values and standard deviation of the main physico-chemical parameters during the experimental period (from October 2017 to February 2019) are included as Supplementary Material in Table S1. All experiments were
Influence of UV irradiance and iron concentration in batch mode operation on CFX-R bacteria inactivation
The inactivation kinetics of ARB is studied in batch mode, since a simple first-order model is used to calculate the rate constants for comparison purposes. Fig. 2 shows the variation in the inactivation rate constants, ki, for total and CFX-R bacteria at three irradiance levels related to winter, spring and summer in Almería, Spain. In addition, the inactivation rate constants from Fenton control experiments were also included. In order to obtain accurate results and taking into account that
Conclusions
For the first time, the inactivation kinetics of total and cefotaxime resistant bacteria by solar photo-Fenton have been compared under very different treatment conditions, with no significant differences. The target of wastewater disinfection treatments can therefore be defined as total pathogen bacteria inactivation, as once the detection limit has been achieved, ARB are also inactivated. The value obtained for the E. coli inactivation rate constant in continuous mode operation was around
Acknowledgements
This research has been supported by the Andalusian Regional Government (P12-RNM-1437) and the European Regional Development Fund (ERDF). I. de la Obra would like to acknowledge the Andalusian Regional Government for her grant. Ph.D. Gracia Rivas Ibáñez wishes to thank MICINN for her Juan de la Cierva formation grant (FJCI-2017-34059).
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2021, Science of the Total EnvironmentCitation Excerpt :Equivalent results to those obtained with 20 mg/L of iron were found for 10 and 5 mg/L of iron, while working with 2.5 mg/L of iron did not allow reaching complete bacterial inactivation neither during the batch operation nor during continuous flow operation. These optimum operating conditions (5 mg/L of iron and 30 mg/L of hydrogen peroxide) were used in other work to study the continuous process performance with 22.4, 16.3, 9.0 and 4.7 min of HRT for CFX resistant bacteria (De la Obra Jiménez et al., 2019b). After 2 h of batch operation in which bacterial inactivation was achieved in 70 min, only the 22.4 min HRT allowed to reach complete inactivation.