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Inactivation of natural enteric bacteria in real municipal wastewater by solar photo-Fenton at neutral pH
Accepted Manuscript Inactivation of Natural Enteric Bacteria in Real Municipal Wastewater by Solar PhotoFenton at Neutral pH E. Ortega-Gómez , B. Esteban García , M.M. Ballesteros Martín , P. Fernández Ibáñez , J.A. Sánchez Pérez PII:
S0043-1354(14)00395-9
DOI:
10.1016/j.watres.2014.05.034
Reference:
WR 10688
To appear in:
Water Research
Received Date: 4 February 2014 Revised Date:
13 May 2014
Accepted Date: 19 May 2014
Please cite this article as: Ortega-Gómez, E., Esteban García, B., Ballesteros Martín, M.M., Fernández Ibáñez, P., Sánchez Pérez, J.A., Inactivation of Natural Enteric Bacteria in Real Municipal Wastewater by Solar Photo-Fenton at Neutral pH, Water Research (2014), doi: 10.1016/j.watres.2014.05.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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INACTIVATION OF NATURAL ENTERIC BACTERIA IN REAL MUNICIPAL
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WASTEWATER BY SOLAR PHOTO-FENTON AT NEUTRAL pH
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E. Ortega-Gómeza,b, B. Esteban Garcíaa,b, M.M. Ballesteros Martínb,c, P. Fernández
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Ibáñezb,d, J.A. Sánchez Pérez a,b*
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a
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b
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c
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de Olavide, 41013 Sevilla, Spain
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d
Department of Chemical Engineering, University of Almería, 04120 Almería, Spain
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CIESOL, Joint Centre of the University of Almería-CIEMAT, 04120 Almería, Spain
Department of Molecular Biology and Biochemical Engineering, University of Pablo
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Plataforma Solar de Almería, CIEMAT, 04200 Tabernas, Almería, Spain
*Corresponding author: [email protected]
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Abstract
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This study analyses the use of the solar photo-Fenton treatment at neutral pH for the
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inactivation of wild enteric Escherichia coli and total coliform present in secondary
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effluents of a municipal wastewater treatment plant (SEWWTP). With this purpose in
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mind, the bacterial inactivation was evaluated under several conditions in compound
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parabolic collector photo-reactors. Control experiments were carried out to find out the
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individual effects of mechanical stress, pH, reactants concentration, and UVA radiation
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as well as the combined effects of UVA-Fe and UVA-H2O2. The synergistic germicidal
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effect of solar-UVA with 50 mg L-1 of H2O2 led to complete disinfection (up to the
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detection limit) of total coliform within 120 min. The disinfection process was
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accelerated by photo-Fenton, achieving total inactivation in 60 min. Reducing natural
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bicarbonate concentration found in the SEWWTP from 250 to 100 mg L-1 did not give
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rise to a significant enhancement in bacterial inactivation. Additionally, the effect of
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hydrogen peroxide and iron dosage was evaluated. The best conditions were 50 mg L-1
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of H2O2 and 20 mg L-1 of Fe2+. Due to the variability of the SEWWTP during autumn
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and winter seasons, the inactivation kinetic constant varied between 0.07 ± 0.04 and
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0.17 ± 0.04 min-1. Moreover, the water treated by solar photo-Fenton fulfilled the
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microbiological quality requirement for wastewater reuse in irrigation as per the WHO
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guidelines and in particular for Spanish legislation.
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Keywords: solar disinfection, photo-Fenton, neutral pH, E. coli, total coliform,
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reclaimed water.
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1. Introduction
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The World Health Organization (WHO) estimates that half of the world’s population
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will be living in water stressed areas by 2025. This water scarcity will force a better use
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of wastewater (WW) as an important source of irrigation water all over the world. The
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reuse of WWs from treatment plants is a widespread practice in many countries because
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of the economic benefits that it brings (Maimon et al., 2010). However, it may pose
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many health risks to the environment and end users if the treatment is inadequate or
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incomplete.
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In the last few decades, advanced oxidation processes (AOPs) have been growing in
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importance for tertiary treatments (Bernabeu et al., 2011). The effectiveness of these
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methods is based on the generation of non-selective and strongly oxidizing radicals,
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which makes them highly appropriate for the treatment of a wide variety of
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contaminants in water and WW disinfection (Malato et al., 2009). Amongst these
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treatments, solar photo-Fenton process has been of special interest due to its efficiency
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for degradation of a number of organic compounds (Pignatello et al., 2006) and the use
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of solar radiation. In the last few years photo-Fenton reactions have also been studied
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for inactivation of different bacteria, fungi, nematodes and virus (Spuhler et al., 2010;
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Polo-López et al., 2012). However, most of publications are focused on inactivation of
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microorganisms from collection instead of wild enteric species. Argulló-Barceló et al.,
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inactivated enteric F-specific RNA bacteriophages and E. coli during photo-Fenton
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treatment (10 mg L-1 of Fe2+, 20 mg L-1 of H2O2) at natural pH in real WW from
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WWTP although complete disinfection was not achieved for somatic coliphages and
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sulphite reducing chlostridia (Agulló-Barceló et al., 2013). Ndounla et al., 2013 treated
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water from household wells containing wild enteric bacteria (total coliform/E. coli and
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salmonella) with photo-Fenton at natural pH with 10 mg L-1 of H2O2 and natural iron
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and solid iron oxides (Ndoula et al., 2013). Wild strain of Fusarium solani isolate from
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rainfall over a river was also inactivated by photo-Fenton (5 mg L-1 of Fe2+, 20 mg L-1
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of H2O2) at neutral pH.
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During the Fenton reaction (Equations (1-4)), hydrogen peroxide reacts with iron
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forming hydroxyl radicals HO• (which are the main reason for the inactivation of
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microorganisms), Fe3+ and HO-. In the presence of UV-vis radiation (photo-Fenton
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process), the ferric ions (Fe3+) produced in Equation (1) are photo-catalytically
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converted to ferrous ions (Fe2+), with the formation of an additional equivalent of
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hydroxyl radical (Equation (5))
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Fe H O → Fe HO HO
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Fe HO → Fe HO
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HO RH → H O R
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R Fe → R Fe
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Fe H O → Fe H HO
(1) (2) (3) (4) (5)
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The elevated number of parameters such as reagent dosage (H2O2 and dissolved iron),
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irradiance, pH, temperature, different types of enteric microorganisms and inorganic
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and organic matter present in water make more difficult the understanding of the
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inactivation processes carried out during the treatment of a real WW by solar photo-
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Fenton. The presence of different ions like carbonate (CO
), phosphate (PO ),
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sulphate (SO
) and chlorine (Cl ) has an effect on the equilibrium of iron in water.
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These ions have a negative impact on the photo-Fenton process (Pignatello et al., 2006;
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Spulher et al., 2010; Polo-López et al., 2012; Klamerth et al., 2012; Ortega-Gómez et
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al., 2013; Rodríguez-Chueca et al., 2013; Ndoula et al., 2013; Agulló-Barceló et al.,
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2013). In particular, carbonate and phosphate have a doubly detrimental effect on the
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photocatalytic reaction, as they act as a scavenger of the hydroxyl radicals and
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precipitate the iron. Dissolved iron concentration and pH are closely related as at pH
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higher than 3 (optimum for photo-Fenton) Fe2+ ions are easily transformed into Fe3+,
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forming hydroxyl-complexes and causing iron precipitation. However, some research
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has demonstrated the possibility of carrying out the photo-Fenton reaction at neutral or
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near neutral pH for detoxification and disinfection of WW, thus reducing the
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operational costs associated with acidification and neutralization (Klamerth et al.,
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2012). Some strategies have been developed to avoid iron precipitation in solution at
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neutral pH such as immobilized photo-Fenton in woven inorganic silica (Moncayo-
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Lasso et al., 2008), by immobilizing the ferrous ion on porous activated carbon
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(Ramírez et al., 2007), or a photo-ferrioxalate disinfection system (Cho et al., 2004).
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Recently, a new strategy of operating at neutral pH consisting of a sequential iron
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dosage was reported (Carra et al., 2013). Working in this way, photo-Fenton treatment
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of a real WW enriched with a mixture of pesticides reached similar reaction rates and
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degrees of mineralization at neutral pH and at pH 3.
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The purpose of this paper was to assess the capability of solar photo-Fenton at neutral
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pH for disinfecting a real secondary effluent of a municipal wastewater treatment plant
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(SEWWTP). The photo-Fenton treatment was carried out in a pilot plant with
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compound parabolic collectors (CPC) under natural sun light. The inactivation of wild
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enteric bacteria (Escherichia coli and total coliform (TC)) from the SEWWTP was
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evaluated and the effects of the main process variables on bacterial inactivation were
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studied. With this purpose, the effects of bicarbonate concentration and reagent dosage
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(iron and hydrogen peroxide) were investigated. The concentration of TC was
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monitored as an indicator of faecal contamination (faecal coliform, E. coli, enterococci)
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in polluted water. TC represents the potential occurrence of a wide number of
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pathogenic microorganisms (Ndoula et al., 2013). In addition to this, the photo-Fenton
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process was evaluated as a WW tertiary treatment to reduce the microbiological risks
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associated with WW reuse in order to comply with WHO guidelines and Spanish
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legislation. Finally, the seasonal stability of the disinfection process was studied during
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the autumn and winter, having these seasons the most unfavorable environmental
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conditions.
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2. Materials and methods
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2.1. Reagents
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Ferrous sulfate heptahydrate (FeSO4·7H2O; >99%, Fluka, Spain) was purchased from
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Fluka (Spain) and the hydrogen peroxide (30%, w/v aqueous solution) from Sigma-
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Aldrich (Spain). Sulfuric acid was acquired from Panreac, Spain (95-98%). Hydrogen
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peroxide present in the samples was removed using catalase (Fluka, Spain). Tween 80
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(Panreac, Spain), sodium acetate (Sigma-Aldrich, Spain), acetic acid (Panreac, Spain),
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diethyl ether (Panreac, Spain) and zinc sulphate (Riedel-de Haën, Germany) were used
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for nematode egg counting.
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2.2. Municipal wastewater treatment plant effluent
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All experiments were conducted using secondary effluents from a municipal WWTP
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located in Almeria (Spain), namely “El Bobar”. This plant generates 11,594,704 m3 of
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secondary effluent per year using activated sludge and decantation as a treatment
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process. The collected samples were transported in batches of 20 L and used for in the
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same day. Due to seasonal variation, average values of the WW parameters during the
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experimental period (February 2012- February 2013) are shown in Table 1.
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2.3. Photochemical reactor
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All experiments were carried out in a WW disinfection plant (Ortega-Gómez et al.,
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2013) with compound parabolic collectors (CPC). This plant consists of two twin photo-
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reactors, with 7 L of total volume. Each photo-reactor is made of two Pyrex tubes (1.5-
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meter length, 50 mm in diameter and 2.5 mm thick) fitted onto the focus of two CPC
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mirrors with an illuminated surface of 0.21 m2. Modules are faced south and tilted 37º
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from the horizontal (local latitude). SEWWTP was driven by a centrifugal pump (Pan
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World NH-40PX model) and maintained in recirculation (flow rate = 11 L min-1, Re =
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30000). pH (Crison 53 35), UVA radiation (Delta OHM LP UVA 02 model),
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temperature (Crison 60 50) and dissolved oxygen (Crison 60 50) were monitored
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throughout the experiment. Dissolved oxygen concentration was measured as
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percentage of saturation with air. Data was acquired by a data acquisition card (LabJack
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U12) connected to a computer.
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2.4. Control experiments
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The effect of different pH (2.8, 5 and 7) on enteric E. coli and TC viability in this WW
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was evaluated in the photoreactors reducing the pH of the SEWWTP with sulfuric acid
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2N. These pH assays were performed in dark condition and without any reagent (neither
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H2O2 nor Fe2+), and the temperature was maintained at 25 ± 1.2 ºC. Control experiments
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were carried out in the reactor fitted with a CPC pilot-plant under the following
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conditions: i) in the dark and without any reagent (neither H2O2 nor Fe2+) - to find out
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the mechanical stress effects for the flow conditions set for this reactor; ii) in the
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presence of solar light without any additives (solar photo-inactivation) - to determine
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the irradiation effect on TC cells; iii) and iv) in the presence of 50 and 100 mg L-1 of
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H2O2, respectively, in the dark - to observe the unique effect of this oxidant agent on
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cells; v) the synergistic effect of hydrogen peroxide (50 mg L-1 of H2O2) in tandem with
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solar light; vi) in the presence of 20 mg L-1 of Fe2+ in the dark to determine the effect of
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iron on the bacterial cells; vii) the UVA-Fe2+ process with 20 mg L-1 of iron, and finally
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viii) the Fenton process at neutral pH (50 mg L-1 of H2O2 and 20 mg L-1 of Fe2+).
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Average UV irradiance and temperature were 15 ± 2.6 W m-2 and 33 ± 1.8 ºC,
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respectively. All experiments were repeated twice using the twin photo-reactors of the
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pilot plant. SEWWTP was mixed for 5 min in the dark and pH was adjusted to 7, then a
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control sample was taken. This was kept in the lab in dark conditions at room
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temperature and plated at the start and end of the experiment to check the enteric
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bacteria viability.
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2.5 Photo-Fenton experiments
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In all photo-Fenton tests, pH was adjusted to 7 and reagents were added and mixed for 5
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min in the dark. A control sample was taken and subsequently, solar photo-reactors
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were exposed to sunlight to start the photoreactions. During the experiments, the
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evolution of dissolved oxygen in all tests was similar. The dissolved oxygen increases
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in the first few minutes of reaction due to the initial reaction between iron and the
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hydrogen peroxide added, showing that photo-Fenton process was taking place.
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All assays were repeated three times on different days with the SEWWTP as received.
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In order to work with the same environmental conditions (SEWWTP sample, irradiance
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and temperature), experiments were carried out in the twin photo-reactors evaluating
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two experimental conditions simultaneously. The pairs of experimental conditions were:
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i) photo-Fenton at neutral pH (50 mg L-1 of H2O2 and 20 mg L-1 of Fe2+) versus UVA-50
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mg L-1 of H2O2,
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ii) photo-Fenton (50 mg L-1 of H2O2 and 20 mg L-1 of Fe2+) with a SEWWTP as
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-1 received (250 ± 12 mg L-1 of HCO
) versus photo-Fenton (50 mg L of H2O2 and 20
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-1 mg L-1 of Fe2+ ) with the same SEWWTP but a HCO
concentration of 100 ± 5 mg L
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(reduced from 250 ± 12 mg L-1 with sulfuric acid 2N),
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iii) photo-Fenton with 50 mg L-1 of H2O2 and one addition of iron (20 mg L-1 of Fe2+)
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versus photo-Fenton with 50 mg L-1 of H2O2 and sequential additions of iron (20-10-10
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mg L-1 of Fe2+),
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iv) photo-Fenton (100 mg L-1 of H2O2 and 20 mg L-1 of Fe2+) versus UVA-100 mg L-1
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of H2O2.
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In order to study the variability of the process, disinfection by photo-Fenton (50 mg L-1
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of H2O2 and 20 mg L-1 of Fe2+) of five different effluents (SEWWTP 1-5) from the
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WWTP was carried out in February 2012, October 2012, November 2012, January 2013
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and February 2013. Winter and autumn months were chosen in order to work with a
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similar irradiance and water temperature range. Additionally, these months have the
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most unfavorable environmental conditions in the year. Average UV irradiance and
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temperature were 21.7 ± 2.2 W m-2 and 30.2 ± 3.2 ºC, respectively.
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2.6. Bacterial (E. coli, Legionella spp. and TC) and nematode eggs quantification
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The TC and E. coli colonies were isolated from SEWWTP. The samples were
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enumerated using the standard plate count method with serial dilutions in a selective
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medium namely Chromocult (26.5 g L-1, Biolife). Colonies were counted after 24 h of
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incubation at 37ºC. This procedure was carried out in triplicate for each sample. The
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bacteriological analysis of Legionella spp. was performed with selective media
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(Buffered Charcoal Yeast Extract (BCYE) Agar Base DM258 (MAST) using the same
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counting procedure. In order to test cell recovery post-treatment, the samples were
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maintained in the dark after the treatment for predetermined exposure times. The
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samples were plated for colony counting after a 24 h period. Any regrowth was
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observed. The detection limit was 1 CFU mL-1.
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The nematode eggs search was performed with an optic microscope in SEWWTP
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samples. The volume used was 1 L for SEWWTP as received and 10 L for photo-
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treated SEWWTP. Once the water had been decanted for 2 h, the pellet was washed
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with surfactant reagents and centrifuged at 2500 rpm for 15 min. Supernatants were
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removed using a pump, leaving a volume of 50 mL in the container. Successive
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additions of acetic buffer and diethyl ether, followed by centrifugation steps were
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carried out until clear sediment was obtained. Next, the eggs were floated in a zinc
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sulfate solution. Finally, the samples were observed under a microscope 10x (Leica
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DMIL; Leica Microsystems GmbH, Wetzlar, Germany) and nematode eggs were
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counted.
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2.7. Chemical analyses
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Before chemical analysis all samples were filtered with 0.2 µm syringe-driven filters
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(Millex®, MILLIPORE). Concentration of iron was analyzed by the o-phenantroline
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standardized method according to ISO 6332. Hydrogen peroxide concentration was
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measured with ammonium metavanadate by spectrophotometry (method DIN 38 402
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H15). Dissolved organic carbon (DOC) and carbonate and bicarbonate (as inorganic
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carbon) were measured by direct injection of samples into a Shimadzu-VCPH analyzer
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and turbidity measurements were made using a turbidimeter (HANNA instruments,
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Romania). Ion concentrations were analyzed with ion chromatography (881 Compact IC
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pro, METHOHM, USA).
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2.8. Kinetics analysis
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The decrease of the microbial population was fitted to first order kinetics according the
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Equation (6):
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log C / C0 = −k ⋅ t
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(6)
Where C/C0 represents the reduction in the microorganism concentration, k is the
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disinfection rate constant (min-1) and t is the time of treatment (min). This model was
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used for direct comparison the different treatments evaluated under similar irradiance
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conditions.
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3. Results and discussion
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3.1 Individual effects of the key factors: pH, mechanical stress (flow rate), solar
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UVA, H2O2 and Fe2+
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The efficiency of the photo-Fenton process is higher at a pH of around 2.8 (Pignatello et
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al., 2006), although some authors suggest applying the photo-Fenton treatment at near
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neutral pH in order to reduce overall costs (Klamerth et al., 2012). Acidic pH can be
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lethal for the enteric microorganism; so a precursory study for survival of enteric TC
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and E. coli was carried out in order to determine the effect of low pH values on the
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cells. Previous experiments corroborated that pH 2.8 affected E. coli and TC viability.
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On the other hand, at pH 5 and 7 the bacterial concentration was constant throughout
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the experimental time (data not shown). Therefore, in the subsequent photo-Fenton
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study at pH 7, any observed bacterial inactivation could be attributed to the
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photocatalytic treatment.
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Prior to studying the use of photo-Fenton process at neutral pH for SEWWTP
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disinfection, the effect of the main factors on TC inactivation was experimentally
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determined. Initial concentration of TC varied within the range of 104-105 CFU mL-1
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due to the variability of the collected samples of SEWWTP.
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Mechanical stress due to flow and pumping through the experimental system had no
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effect on enteric bacterial cell viability (Fig. 1). The effect of solar irradiation led to 1-
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log decrease (k = 0.013 ± 0.002 min−1; R2 = 0.9) of TC concentration after 180 min of
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treatment (Fig. 1) which is in line with other studies where solar disinfection was used
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to inactivate E. coli and TC in greywater (Pansonato et al., 2011). TC inactivation was
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enhanced, leading to a higher inactivation rate constant (k = 0.034 ± 0.003 min−1; R2 =
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0.96) when 100 mg L-1 of H2O2 was added in the dark giving a 2.5-log decrease
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although the addition of 50 mg L-1 of H2O2 in the dark (k = 0.0035 ± 0.0005 min−1; R2 =
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0.89) did not produce any significant bacterial damage (Fig. 1). Hydrogen peroxide has
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been used as an alternative to chlorine for WW treatment because it displays advantages
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connected with its decomposition and high efficiency for organic compound
281
mineralization. However, for adequate disinfection, the concentration of hydrogen
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peroxide required needs to be high as the number of TC exponentially decreased with an
283
increasing dose (Ksibi, 2006). A slight enhancement in bacterial inactivation (k = 0.018
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± 0.003 min−1; R2 = 0.89) was observed (1-log after 180 min of experiment) using the
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Fenton process with 20 and 50 mg L-1 of Fe2+ and hydrogen peroxide, respectively (Fig.
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1). This result indicates that the Fenton reagent at neutral pH is more effective than the
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addition of hydrogen peroxide only which is in line with recent research (Ndounla et al.,
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2013) carried out in order to disinfect wild enteric bacteria (TC, E. coli and Salmonella
289
spp.) in water from wells with different pH (4.9 and 6.3). However, the only addition of
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iron in the dark (Fig. 1) had a slight effect over TC as no significant reduction in viable
291
counts was observed as previously demonstrated (Ksibi, 2006). With the same
292
concentration of iron, a reduction in bacterial viability of 1.5-log was observed under
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natural light.
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Many authors have looked into the effect of the UVA-H2O2 combination on the
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inactivation of several microorganisms with promising results (Sichel et al., 2009;
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Sciacca et al., 2010; García-Fernández et al., 2012). Fig. 1 shows that when 50 mg L-1
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of H2O2 was added in the presence of natural solar radiation, the detection limit was
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reached after 180 min (k = 0.036 ± 0.009 min−1; R2 = 0.89). The combination of both
299
effects enhances the individual influence of each parameter due to the UVA action on
300
damaged cells exposed to hydrogen peroxide (Feuerstein et al., 2006; Sichel et al.,
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2009). The combined system has been also used by other authors for the inactivation of
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enteric microorganisms in SEWWTPs with a reduction of more than 4-log of fecal
303
coliform concentration (Xie et al., 2007).
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3.2 Disinfection by solar photo-Fenton process
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Photo-Fenton at neutral pH (50 mg L-1 of H2O2 and 20 mg L-1 of Fe2+) and the most
306
efficient control condition shown in Fig. 1 (UVA-50 mg L-1 of H2O2) were
307
simultaneously tested in order to compare both processes for the inactivation of E. coli
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and TC in SEWWTP. Fig. 2 shows that the addition of iron led to a significant
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difference in the inactivation time, as the detection limit was reached for E. coli and TC
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in 45 and 60 min of solar exposure, respectively for the photo-Fenton treatment;
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whereas the same limit was reached in 60 and 120 min in the UVA-H2O2 process.
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Moreover, the inactivation kinetic constant increased from 0.053 ± 0.011 min−1, R2 =
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0.8 (solar UVA-H2O2) to 0.11 ± 0.01 min−1, R2 = 0.96 (solar photo-Fenton). Therefore,
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although at neutral pH a high amount of the added iron precipitated, the remaining
315
concentration of iron in solution favours the generation of hydroxyl radical.
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Additionally, a contribution of heterogeneous photo-Fenton due to iron hydroxide
317
activity could take place as proposed by Lam et al. (2005). These results are in line with
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the hydrogen peroxide consumption (Fig. 2), which was slower in the UVA-H2O2
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treatment than in the photo-Fenton process because hydrogen peroxide was consumed
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by the formation of hydroxyl radicals. During the first minutes of photo-Fenton and
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UVA-H2O2 treatments, inactivation curves for E. coli and TC were overlapped (Fig. 2).
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The attack of hydroxyl radicals generated during photo-Fenton process was responsible
323
for the cell damage and therefore, the disinfection process was accelerated, achieving
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shorter inactivation times (a reduction of 50%). Photo-Fenton was also more effective
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than Fenton (k = 0.018 ± 0.003 min−1) treatment at neutral pH (Fig. 1) due to the
326
regeneration of Fe2+ caused by the photochemical effect of sun light and the higher
327
generation rate of hydroxyl radicals. This enhancement leads to an increase in the
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inactivation rate of 82%. The same behavior was observed by Moncayo-Lasso et al.
329
(2009) for E. coli inactivation at neutral pH in river water.
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The use of photo-Fenton treatment at neutral pH for disinfection of enteric
331
microorganisms from WWTP effluents has barely been studied. However, the use of
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solar photo-Fenton at neutral pH monitoring the wild enteric bacterial concentration for
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WW recovery and reuse has not been proven or assessed in detail until now. Depending
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on the needs of WW reuse (urban, agricultural, industrial, recreational and
335
environmental uses) the tertiary treatment must ensure the water quality follows the
336
guidelines of each country. As agriculture consumes over 70% of the world’s total fresh
337
water (FAO 2012), the viability of the photo-Fenton treatment for water reuse with
338
agrarian purposes was studied. Reviewing current WW quality guidelines, one of the
339
most important microbiological parameters used as a fecal indicator is E. coli
340
concentration. The WHO guidelines stipulate a reduction in E. coli concentration up to
341
10 CFU mL-1, whereas the Spanish legislation requirements are more restrictive in this
342
sense. The Spanish Royal Decree (RD 1620/2007) establishes a level of 1 CFU mL-1. In
343
both cases disinfection by photo-Fenton process satisfied this legislation. Moreover,
344
Legionella spp. (another microbiological parameter) was not detected either in the
345
SEWWTP as received (before the treatment) or after photo-Fenton disinfection.
346
Additionally, only one nematode egg (in 10 L) was found in the SEWWTP and after the
347
photocatalytic treatment not even one nematode egg was found.
348
3.2.1. Effect of natural bicarbonate concentration on the photo-Fenton treatment
349
Many authors have demonstrated that the presence of carbonate and bicarbonate anions
350
in WW has a negative impact on the photo-Fenton process (Klamerth et al., 2009;
351
Pignatello et al., 2006). Both anions act as radical scavengers generating carbonate
352
radical by Equations (7) and (8) (Kochany et al., 1992).
353
At the working pH (7 - 7.4), CO∙
is formed mainly by Equation (7) as the main species
354
is HCO
(80%). Carbonate radical is an oxidant specie (1.78 V at pH 7), which can
355
degrade organic contaminants (Dell’Arciprete et al., 2012) although its redox potential
356
is lower than that of hydroxyl radical (2.8 V at pH 7).
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∙
HO HCO
→ H O CO
(7)
358
∙
HO CO
→ HO CO
(8)
359
Some strategies have previously been proposed to avoid the scavenging of hydroxyl
360
radicals. One of them is the use of higher amounts of iron since bicarbonate promotes
361
iron precipitation (Carra et al., 2013). Another alternative is reducing the concentration
362
of carbonate and bicarbonate anions in WW before photo-Fenton by adding an adequate
363
quantity of concentrated acid (Klamerth et al., 2009). The photo-Fenton process (50 mg
364
L-1 of H2O2 and 20 mg L-1 of Fe2+) was applied to a SEWWTP as received (HCO
=
365
250 ± 12 mg L-1) and to the same water with a reduced HCO
concentration of 100 ± 5
366
mg L-1 (by sulphuric acid addition) in order to compare the disinfection efficiency under
367
both conditions. The final HCO
concentration was enough to maintain the buffer
368
capacity of the water. It could be observed (Fig. 3) that TC concentration reached the
369
detection limit in 80 min for both WW matrices although the detection limit was
370
reached for E. coli at 80 min (SEWWTP as received) and at 40 min when the HCO
371
concentration was 100 ± 5 mg L-1. The competence for reactive oxidant species (ROS)
372
between microorganisms, organic matter, and bicarbonate anions was not observed
373
because very oxidant conditions were applied during photo-Fenton process and an
374
excess of radicals were generated. This hypothesis is supported by previous results
375
where photo-Fenton treatment was used for the inactivation of Enterococcus faecalis in
376
the presence of resorcinol as hydroxyl radical scavenger (Ortega-Gómez et al., 2014).
377
At low concentrations of H2O2 and Fe2+, when the scavenger was present, a marked
378
delay in the disinfection process was attained. However, at elevated concentration of
379
reagents (H2O2/Fe2+: 50/20 mg L-1) the formation of radicals was so high that the attack
380
to the scavenger molecule does not result in a detrimental effect on the bacterial
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inactivation. Therefore, in order to use photo-Fenton treatment at a stable pH around 7,
382
it is not worth reducing the natural bicarbonate concentration of WW. Complete
383
removal of bicarbonate would produce an increase in inactivation rate as well as a sharp
384
pH drop towards acidic values.
385
3.2.2 Effect of iron addition on the photo-Fenton process
386
Photo-Fenton treatment has traditionally been carried out at pH around 2.8 due to
387
photo-active Fe species predominating at this acidic pH. However, in recent years,
388
many authors (García-Fernández et al., 2012; Klamerth et al., 2012) have shown a
389
growing interest in photo-Fenton treatment (pollutant degradation and disinfection) at
390
natural pH of WW in order to reduce operating costs. At pH between 6 to 8, iron
391
precipitation is inevitable and the photo-Fenton process is less effective (Klamerth et
392
al., 2012). In order to operate at neutral pH, a new strategy consisting of a sequential
393
iron dosage was recently proposed (Carra et al., 2013). Treating real WW enriched with
394
a mixture of pesticides (50 mg L-1 of DOC), similar reaction rates and degrees of
395
mineralization were obtained by photo-Fenton at neutral pH with sequential iron
396
additions (20-20-10-10-10 mg L-1 of Fe2+at 0, 5, 15, 25 and 35 min, respectively)
397
compared to photo-Fenton at pH 2.8 with one dose of 20 mg L-1 of Fe2+. Following the
398
proposed strategy of iron dosage, two parallel assays were carried out: in one loop,
399
sequential additions of 20, 10 and 10 mg L-1 of Fe2+ every 5 min (mixing time) were
400
made; while in the other loop a unique initial 20 mg L-1 of iron addition was added at
401
the beginning of the experiment. In both cases, SEWWTP was used as received and the
402
hydrogen peroxide concentration was 50 mg L-1. As Fig. 4 shows, photo-Fenton with
403
iron additions slowed down the disinfection rate with regard to a unique iron addition as
404
inactivation time increased to 90 min for E. coli and 120 min for TC. Other iron
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addition strategies were tested, but in any case the disinfection rate was faster than that
406
reached with one addition of iron (data not shown). With sequential iron additions,
407
hydrogen peroxide consumption was higher (Fig. 4) than with one addition, since more
408
hydroxyl radicals were generated.
409
These results point out that higher iron concentration values have a negative effect on
410
the bacterial inactivation by photo-Fenton at neutral pH. This detrimental effect could
411
be caused by: (i) iron precipitation promoting by the formation of floccules which
412
favored even more iron precipitation, (ii) an excess of iron concentration increasing the
413
HO• scavenging potential (Equation (2)) (Ndoula et al., 2013; Pansonato et al., 2011)
414
and (iii) the reduction of light transmission due to the increased turbidity of water. Some
415
mechanisms which cause cellular damage is based in absorb light in the UVA and
416
visible spectrum (Spuhler et al., 2010). For this reason, if these pathways involved in
417
photo-inactivation are disfavored the cellular damage is diminished.
418
3.2.3 Effect of hydrogen peroxide concentration on photo-Fenton efficiency
419
With the purpose of minimizing the inactivation time in the photo-Fenton process, a
420
higher concentration of hydrogen peroxide (100 mg L-1) was tested. This experiment
421
and a test of UVA-100 mg L-1 of H2O2 were carried out simultaneously. Fig. 5 shows
422
the same inactivation profiles regardless of whether iron was added or not, reaching the
423
detection limit at 30 min for E. coli and 45 min for TC. When this higher amount of
424
H2O2 is used, photo-Fenton process did not suppose any significant enhancement versus
425
the UVA/H2O2 process for bacterial inactivation, in spite of the major oxidant
426
consumption. In the UVA-H2O2 process, the predominant ROS is H2O2 that is stable,
427
long living compound and at high concentration is toxic for the cells. The mere addition
428
of 100 mg L-1 of H2O2 causes a reduction of 2-log in the bacterial concentration (Fig. 1).
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Additionally, there is a synergistic effect between UVA and H2O2 in microorganism’s
430
inactivation (Feuerstein et al., 2006). This synergy is due to the absorption of UV
431
photons by the bacteria which cause sub-lethal damage followed by the oxidative attach
432
of H2O2 causing a total inactivation. However, although the HO• radical is the unique
433
specie which can directly damage DNA and no cellular defense mechanism against this
434
attack has been identified until today (unlike H2O2) (Satter et al., 2000) the elevated
435
concentration of H2O2 combined with the UVA radiation are capable to produce the
436
same damage.
437
The times for bacterial inactivation were shorter than those obtained when the hydrogen
438
peroxide concentration was 50 mg L-1 (Fig. 2). When the concentration of hydrogen
439
peroxide increased from 50 to 100 mg L-1, inactivation times by photo-Fenton were
440
reduced by 33% and 25% for E. coli and TC, respectively, although the amount of
441
residual hydrogen peroxide was increased two fold (Fig. 2, Fig. 5).
442
Consequently, according to these results, duplicate initial hydrogen peroxide
443
concentrations lead to a slight enhancement in disinfection times and if elevated
444
concentration of oxidant is employed, the addition of iron is not a practicable strategy.
445
3.3 Effect of SEWWTP seasonal variability
446
The performance of photo-Fenton treatment is affected by the WW variability as
447
parameters such as pH, turbidity, wild enteric bacterial concentration, water
448
composition (organics and inorganics), etc. vary with time, day and season. To assess
449
the effects of SEWWTP variability on the photo-Fenton process, several disinfection
450
assays were carried out with the same reagent concentrations (50 mg L-1 of H2O2 and 20
451
mg L-1 of Fe2+) in winter and autumn months for one year (named as SEWWTP1 to
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SEWWTP5 in Table 2). During the first 45 min (Fig. 6), the TC inactivation curves
453
were similar in all cases. At this time, 2-log of bacteria were inactivated (99% with
454
regard to the detection limit) and the remaining cells were more resistant to inactivation.
455
Total coliform disinfection displayed different behavior involving different inactivation
456
times (from 60 to 120 min, Fig. 6). The elevated number of parameters involved in this
457
process obstructs the attribution of some of them to the differences in the inactivation
458
times. Therefore, in order to apply the photo-Fenton process as tertiary treatment in
459
municipal WWTPs it must be taken into account that the inactivation kinetic constant
460
varies between 0.07 and 0.17 min-1 to reach the detection limit (Table 2). As a result, in
461
order to assess what is an effective operating time for the total disinfection,
462
independently of the environmental conditions and WW composition, the minimum
463
inactivation rate must be considered.
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4. Conclusions
466
An analysis of the capability of the photo-Fenton treatment at neutral pH for enteric
467
bacteria disinfection in a secondary effluent from a WWTP was carried out. The
468
synergic effect of the solar-H2O2 (50 mg L-1) was enough to observe a complete
469
disinfection of TC although the addition of iron (solar photo-Fenton: 50 mg L-1 of H2O2
470
and 20 mg L-1 of Fe2+) accelerated the disinfection process, shortening inactivation
471
times by half. Therefore, photo-Fenton is a suitable process to assess the quality of the
472
treated water for reuse in terms of WHO guidelines and in particular Spanish legislation
473
for treated WW reuse.
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With regard to photo-Fenton process operation, reducing the natural concentration of
475
bicarbonate present in WW did not lead to a significant reduction in the inactivation
476
times, since its role as scavenger is overlapped by the excess of HO• radical in solution.
477
Also, sequential iron additions slowed down the disinfection process due to several
478
pathways involved in photo-inactivation are disfavored.
479
Finally, the addition of a high concentration (100 mg L-1) of hydrogen peroxide resulted
480
in a slight enhancement in disinfection times but residual peroxide was twice as high.
481
Moreover, photo-Fenton treatment caused the same inactivation degree than the
482
combination of UVA-H2O2 when elevated H2O2 concentrations were employed.
483
The WW variability led to different inactivation times (between 60 and 120 min)
484
although at 45 min, 99% disinfection was achieved in all the cases. Therefore, attention
485
to this variability should be paid in order to remove the residual surviving bacteria,
486
causing longer treatment times to reach proper disinfection in SEWWTP.
487
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474
Acknowledgments
489
This research was funded by the Ministry for Economy and Competitiveness
490
(CTQ2010-20740-C03-01),
491
AQUAPYME project (P10-RNM-05951) and the European Regional Development
492
Fund (ERDF). Elisabet Ortega Gómez would like to acknowledge the Ministry for
493
Economy and Competitiveness for her F.P.I. scholarship (Ref: BES-2011-043886). The
494
authors would also like to acknowledge the collaboration of "El Bobar" (AQUALIA)
495
WWTP.
AQUASUN
project
(CTM2011-29143-C03-03),
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Sichel, C., Fernández-Ibáñez, P., Tello, J., de Cara, M. Lethal synergy of solar UV-
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Spuhler, D., Andrés Rengifo-Herrera, J., Pulgarin, C. The effect of Fe2+, Fe3+, H2O2
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and the photo-Fenton reagent at near neutral pH on the solar disinfection (SODIS) at
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Environmentaol 96 (2010), 126-141.
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Xie, R.J., Gomez, M.J., Xing, Y.J. Field investigation of advanced oxidation of
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598
Environmental Science and Health-Part A Toxic/Hazardous Substances Environmental
599
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Journal
of
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601
Table and Figure captions
602 Table 1. Chemical and microbiological characterization of the SEWWTP (El Bobar, Almería,
604
Spain).
RI PT
603
605
Table 2: Inactivation kinetic constants for TC disinfection by photo-Fenton on different
607
days during autumn and winter seasons.
SC
606
M AN U
608
Fig. 1. TC inactivation in the CPC reactor during control assays. Dark experiments:
610
Mechanical stress (-○), 50 mg H2O2 L-1 (-□-),100 mg H2O2 L-1 (--), 20 mg Fe2+ L-1 (-
611
∇-), Fenton (--). With solar radiation experiments: UVA (-●-), UVA-20 mg Fe2+ L-1
612
(-■-), and UVA-50 mg H2O2 L-1 (-♦-).
TE D
609
613
Fig. 2. TC and E. coli inactivation by UVA-50 mg H2O2 L-1 treatment (open symbols)
615
and photo-Fenton (50 mg H2O2 L-1-20 mg Fe2+ L-1) process (closed symbols). TC (-◌-),
616
E. coli (--) and H2O2 concentration (-□-).
AC C
617
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614
618
Fig. 3. Effect of bicarbonate concentration on water disinfection by photo-Fenton (50
619
-1 mg H2O2 L-1-20 mg Fe2+ L-1). Open symbols, HCO
= 100 ± 5 mg L . Closed symbols,
620
-1 HCO
= 250 ± 12 mg L . TC (-◌-), E. coli (--) and H2O2 (-□-).
27
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Fig. 4. Effect of iron dosage on water disinfection by photo-Fenton. Open symbols, iron
622
additions 20 mg Fe2+ L-1, at t=0; 10 mg Fe2+ L-1, at t=5 min; and 10 mg Fe2+ L-1, at t=10
623
min. Closed symbols, one addition of 20 mg Fe2+ L-1. TC (-◌-), E. coli (--) and H2O2
624
(-□-).
RI PT
621
625
Fig. 5. Water disinfection with high hydrogen peroxide concentration. Open symbols,
627
UVA-100 mg H2O2 L-1. Closed symbols, photo-Fenton (100 mg H2O2 L-1-20 mg Fe2+ L-
628
1
M AN U
). TC (-◌-), E. coli (--) and H2O2 (-□-).
SC
626
629
Fig. 6. TC inactivation by photo-Fenton (50 mg H2O2 L-1-20 mg Fe2+ L-1) replicas with
631
SEWWTPs obtained on five different days over one year: SEWWTP1 (-■-),
632
SEWWTP2 (-●-), SEWWTP3 (-♦-), SEWWTP4 (-□-) and SEWWTP5 (--).
TE D
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pH -1
Conductivity (µS cm ) Turbidity (NTU) -1
Dissolved organic carbon, DOC (mg L ) -1
Maximum
Minimum
7.48 ± 0.33
8.34
7.08
1248 ± 166
1659
858
8.98 ± 4.2
15.23
2.91
23.78 ± 6.21
35.53
9.85
399.45 ± 2.15
605.15
-1
2.50 ± 0.43
2.76
-1
248.43 ± 40.92
Bromide (mg L ) Chloride (mg L ) -1
-2
Fluoride (mg L )
1.25·10 ± 2.710
-1
4.6 ± 7.6
-1
5.43 ± 5.43
Nitrite (mg L )
Nitrate (mg L ) -1
Sulfide (mg L )
268.66 -3
4
0.05
3
1.16·10 ± 1.8·10
TC (CFU mL-1)
3.24·104 ± 2.3·102
TE D EP AC C
0
21.16
0
110.41
79.30
3.4·10
4
6.12·105
M AN U
E. coli (CFU mL )
216.87
0
14.69
89.71 ± 13.94 -1
122.15 0.67
SC
Bicarbonate (mg L )
Mean
RI PT
Parameter
2.13·102
1.63·103
ACCEPTED MANUSCRIPT
k (min-1)
Sample
(2-log decrease)
R2
k (min-1) (from C0 to DL)
R2
0.12
0.94
0.15
0.96
SEWWTP 2
0.14
0.99
0.17
0.97
SEWWTP 3
0.08
0.93
0.14
0.93
SEWWTP 4
0.10
0.91
0.09
SEWWTP 5
0.11
0.97
0.07
Average
0.11
-
0.13
Standard deviation
0.02
-
0.04
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0.96
0.92
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Disinfection by solar photo-Fenton at neutral pH as tertiary treatment was studied Photo-Fenton reduces 50 % the inactivation time respect to UVA-H2O2 Total inactivation by photo-Fenton was achieved in 60 min (H2O2/Fe2+: 50/20 mg L-1) Wastewater treated by photo-Fenton was adequate for its reuse for irrigation In spite to the seasonal variability 99% of disinfection was achieved in 40 min
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S0043-1354(14)00395-9
DOI:
10.1016/j.watres.2014.05.034
Reference:
WR 10688
To appear in:
Water Research
Received Date: 4 February 2014 Revised Date:
13 May 2014
Accepted Date: 19 May 2014
Please cite this article as: Ortega-Gómez, E., Esteban García, B., Ballesteros Martín, M.M., Fernández Ibáñez, P., Sánchez Pérez, J.A., Inactivation of Natural Enteric Bacteria in Real Municipal Wastewater by Solar Photo-Fenton at Neutral pH, Water Research (2014), doi: 10.1016/j.watres.2014.05.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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INACTIVATION OF NATURAL ENTERIC BACTERIA IN REAL MUNICIPAL
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WASTEWATER BY SOLAR PHOTO-FENTON AT NEUTRAL pH
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E. Ortega-Gómeza,b, B. Esteban Garcíaa,b, M.M. Ballesteros Martínb,c, P. Fernández
4
Ibáñezb,d, J.A. Sánchez Pérez a,b*
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a
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b
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c
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de Olavide, 41013 Sevilla, Spain
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d
Department of Chemical Engineering, University of Almería, 04120 Almería, Spain
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CIESOL, Joint Centre of the University of Almería-CIEMAT, 04120 Almería, Spain
Department of Molecular Biology and Biochemical Engineering, University of Pablo
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Plataforma Solar de Almería, CIEMAT, 04200 Tabernas, Almería, Spain
*Corresponding author: [email protected]
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Abstract
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This study analyses the use of the solar photo-Fenton treatment at neutral pH for the
14
inactivation of wild enteric Escherichia coli and total coliform present in secondary
15
effluents of a municipal wastewater treatment plant (SEWWTP). With this purpose in
16
mind, the bacterial inactivation was evaluated under several conditions in compound
17
parabolic collector photo-reactors. Control experiments were carried out to find out the
18
individual effects of mechanical stress, pH, reactants concentration, and UVA radiation
19
as well as the combined effects of UVA-Fe and UVA-H2O2. The synergistic germicidal
20
effect of solar-UVA with 50 mg L-1 of H2O2 led to complete disinfection (up to the
21
detection limit) of total coliform within 120 min. The disinfection process was
22
accelerated by photo-Fenton, achieving total inactivation in 60 min. Reducing natural
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bicarbonate concentration found in the SEWWTP from 250 to 100 mg L-1 did not give
24
rise to a significant enhancement in bacterial inactivation. Additionally, the effect of
25
hydrogen peroxide and iron dosage was evaluated. The best conditions were 50 mg L-1
26
of H2O2 and 20 mg L-1 of Fe2+. Due to the variability of the SEWWTP during autumn
27
and winter seasons, the inactivation kinetic constant varied between 0.07 ± 0.04 and
28
0.17 ± 0.04 min-1. Moreover, the water treated by solar photo-Fenton fulfilled the
29
microbiological quality requirement for wastewater reuse in irrigation as per the WHO
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guidelines and in particular for Spanish legislation.
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Keywords: solar disinfection, photo-Fenton, neutral pH, E. coli, total coliform,
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reclaimed water.
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1. Introduction
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The World Health Organization (WHO) estimates that half of the world’s population
50
will be living in water stressed areas by 2025. This water scarcity will force a better use
51
of wastewater (WW) as an important source of irrigation water all over the world. The
52
reuse of WWs from treatment plants is a widespread practice in many countries because
53
of the economic benefits that it brings (Maimon et al., 2010). However, it may pose
54
many health risks to the environment and end users if the treatment is inadequate or
55
incomplete.
56
In the last few decades, advanced oxidation processes (AOPs) have been growing in
57
importance for tertiary treatments (Bernabeu et al., 2011). The effectiveness of these
58
methods is based on the generation of non-selective and strongly oxidizing radicals,
59
which makes them highly appropriate for the treatment of a wide variety of
60
contaminants in water and WW disinfection (Malato et al., 2009). Amongst these
61
treatments, solar photo-Fenton process has been of special interest due to its efficiency
62
for degradation of a number of organic compounds (Pignatello et al., 2006) and the use
63
of solar radiation. In the last few years photo-Fenton reactions have also been studied
64
for inactivation of different bacteria, fungi, nematodes and virus (Spuhler et al., 2010;
65
Polo-López et al., 2012). However, most of publications are focused on inactivation of
66
microorganisms from collection instead of wild enteric species. Argulló-Barceló et al.,
67
inactivated enteric F-specific RNA bacteriophages and E. coli during photo-Fenton
68
treatment (10 mg L-1 of Fe2+, 20 mg L-1 of H2O2) at natural pH in real WW from
69
WWTP although complete disinfection was not achieved for somatic coliphages and
70
sulphite reducing chlostridia (Agulló-Barceló et al., 2013). Ndounla et al., 2013 treated
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water from household wells containing wild enteric bacteria (total coliform/E. coli and
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salmonella) with photo-Fenton at natural pH with 10 mg L-1 of H2O2 and natural iron
73
and solid iron oxides (Ndoula et al., 2013). Wild strain of Fusarium solani isolate from
74
rainfall over a river was also inactivated by photo-Fenton (5 mg L-1 of Fe2+, 20 mg L-1
75
of H2O2) at neutral pH.
76
During the Fenton reaction (Equations (1-4)), hydrogen peroxide reacts with iron
77
forming hydroxyl radicals HO• (which are the main reason for the inactivation of
78
microorganisms), Fe3+ and HO-. In the presence of UV-vis radiation (photo-Fenton
79
process), the ferric ions (Fe3+) produced in Equation (1) are photo-catalytically
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converted to ferrous ions (Fe2+), with the formation of an additional equivalent of
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hydroxyl radical (Equation (5))
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Fe H O → Fe HO HO
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Fe HO → Fe HO
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HO RH → H O R
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R Fe → R Fe
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Fe H O → Fe H HO
(1) (2) (3) (4) (5)
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The elevated number of parameters such as reagent dosage (H2O2 and dissolved iron),
90
irradiance, pH, temperature, different types of enteric microorganisms and inorganic
91
and organic matter present in water make more difficult the understanding of the
92
inactivation processes carried out during the treatment of a real WW by solar photo-
93
Fenton. The presence of different ions like carbonate (CO
), phosphate (PO ),
94
sulphate (SO
) and chlorine (Cl ) has an effect on the equilibrium of iron in water.
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These ions have a negative impact on the photo-Fenton process (Pignatello et al., 2006;
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Spulher et al., 2010; Polo-López et al., 2012; Klamerth et al., 2012; Ortega-Gómez et
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al., 2013; Rodríguez-Chueca et al., 2013; Ndoula et al., 2013; Agulló-Barceló et al.,
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2013). In particular, carbonate and phosphate have a doubly detrimental effect on the
99
photocatalytic reaction, as they act as a scavenger of the hydroxyl radicals and
100
precipitate the iron. Dissolved iron concentration and pH are closely related as at pH
101
higher than 3 (optimum for photo-Fenton) Fe2+ ions are easily transformed into Fe3+,
102
forming hydroxyl-complexes and causing iron precipitation. However, some research
103
has demonstrated the possibility of carrying out the photo-Fenton reaction at neutral or
104
near neutral pH for detoxification and disinfection of WW, thus reducing the
105
operational costs associated with acidification and neutralization (Klamerth et al.,
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2012). Some strategies have been developed to avoid iron precipitation in solution at
107
neutral pH such as immobilized photo-Fenton in woven inorganic silica (Moncayo-
108
Lasso et al., 2008), by immobilizing the ferrous ion on porous activated carbon
109
(Ramírez et al., 2007), or a photo-ferrioxalate disinfection system (Cho et al., 2004).
110
Recently, a new strategy of operating at neutral pH consisting of a sequential iron
111
dosage was reported (Carra et al., 2013). Working in this way, photo-Fenton treatment
112
of a real WW enriched with a mixture of pesticides reached similar reaction rates and
113
degrees of mineralization at neutral pH and at pH 3.
114
The purpose of this paper was to assess the capability of solar photo-Fenton at neutral
115
pH for disinfecting a real secondary effluent of a municipal wastewater treatment plant
116
(SEWWTP). The photo-Fenton treatment was carried out in a pilot plant with
117
compound parabolic collectors (CPC) under natural sun light. The inactivation of wild
118
enteric bacteria (Escherichia coli and total coliform (TC)) from the SEWWTP was
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evaluated and the effects of the main process variables on bacterial inactivation were
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studied. With this purpose, the effects of bicarbonate concentration and reagent dosage
121
(iron and hydrogen peroxide) were investigated. The concentration of TC was
122
monitored as an indicator of faecal contamination (faecal coliform, E. coli, enterococci)
123
in polluted water. TC represents the potential occurrence of a wide number of
124
pathogenic microorganisms (Ndoula et al., 2013). In addition to this, the photo-Fenton
125
process was evaluated as a WW tertiary treatment to reduce the microbiological risks
126
associated with WW reuse in order to comply with WHO guidelines and Spanish
127
legislation. Finally, the seasonal stability of the disinfection process was studied during
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the autumn and winter, having these seasons the most unfavorable environmental
129
conditions.
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2. Materials and methods
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2.1. Reagents
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Ferrous sulfate heptahydrate (FeSO4·7H2O; >99%, Fluka, Spain) was purchased from
134
Fluka (Spain) and the hydrogen peroxide (30%, w/v aqueous solution) from Sigma-
135
Aldrich (Spain). Sulfuric acid was acquired from Panreac, Spain (95-98%). Hydrogen
136
peroxide present in the samples was removed using catalase (Fluka, Spain). Tween 80
137
(Panreac, Spain), sodium acetate (Sigma-Aldrich, Spain), acetic acid (Panreac, Spain),
138
diethyl ether (Panreac, Spain) and zinc sulphate (Riedel-de Haën, Germany) were used
139
for nematode egg counting.
140
2.2. Municipal wastewater treatment plant effluent
141
All experiments were conducted using secondary effluents from a municipal WWTP
142
located in Almeria (Spain), namely “El Bobar”. This plant generates 11,594,704 m3 of
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secondary effluent per year using activated sludge and decantation as a treatment
144
process. The collected samples were transported in batches of 20 L and used for in the
145
same day. Due to seasonal variation, average values of the WW parameters during the
146
experimental period (February 2012- February 2013) are shown in Table 1.
147
2.3. Photochemical reactor
148
All experiments were carried out in a WW disinfection plant (Ortega-Gómez et al.,
149
2013) with compound parabolic collectors (CPC). This plant consists of two twin photo-
150
reactors, with 7 L of total volume. Each photo-reactor is made of two Pyrex tubes (1.5-
151
meter length, 50 mm in diameter and 2.5 mm thick) fitted onto the focus of two CPC
152
mirrors with an illuminated surface of 0.21 m2. Modules are faced south and tilted 37º
153
from the horizontal (local latitude). SEWWTP was driven by a centrifugal pump (Pan
154
World NH-40PX model) and maintained in recirculation (flow rate = 11 L min-1, Re =
155
30000). pH (Crison 53 35), UVA radiation (Delta OHM LP UVA 02 model),
156
temperature (Crison 60 50) and dissolved oxygen (Crison 60 50) were monitored
157
throughout the experiment. Dissolved oxygen concentration was measured as
158
percentage of saturation with air. Data was acquired by a data acquisition card (LabJack
159
U12) connected to a computer.
160
2.4. Control experiments
161
The effect of different pH (2.8, 5 and 7) on enteric E. coli and TC viability in this WW
162
was evaluated in the photoreactors reducing the pH of the SEWWTP with sulfuric acid
163
2N. These pH assays were performed in dark condition and without any reagent (neither
164
H2O2 nor Fe2+), and the temperature was maintained at 25 ± 1.2 ºC. Control experiments
165
were carried out in the reactor fitted with a CPC pilot-plant under the following
166
conditions: i) in the dark and without any reagent (neither H2O2 nor Fe2+) - to find out
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the mechanical stress effects for the flow conditions set for this reactor; ii) in the
168
presence of solar light without any additives (solar photo-inactivation) - to determine
169
the irradiation effect on TC cells; iii) and iv) in the presence of 50 and 100 mg L-1 of
170
H2O2, respectively, in the dark - to observe the unique effect of this oxidant agent on
171
cells; v) the synergistic effect of hydrogen peroxide (50 mg L-1 of H2O2) in tandem with
172
solar light; vi) in the presence of 20 mg L-1 of Fe2+ in the dark to determine the effect of
173
iron on the bacterial cells; vii) the UVA-Fe2+ process with 20 mg L-1 of iron, and finally
174
viii) the Fenton process at neutral pH (50 mg L-1 of H2O2 and 20 mg L-1 of Fe2+).
175
Average UV irradiance and temperature were 15 ± 2.6 W m-2 and 33 ± 1.8 ºC,
176
respectively. All experiments were repeated twice using the twin photo-reactors of the
177
pilot plant. SEWWTP was mixed for 5 min in the dark and pH was adjusted to 7, then a
178
control sample was taken. This was kept in the lab in dark conditions at room
179
temperature and plated at the start and end of the experiment to check the enteric
180
bacteria viability.
181
2.5 Photo-Fenton experiments
182
In all photo-Fenton tests, pH was adjusted to 7 and reagents were added and mixed for 5
183
min in the dark. A control sample was taken and subsequently, solar photo-reactors
184
were exposed to sunlight to start the photoreactions. During the experiments, the
185
evolution of dissolved oxygen in all tests was similar. The dissolved oxygen increases
186
in the first few minutes of reaction due to the initial reaction between iron and the
187
hydrogen peroxide added, showing that photo-Fenton process was taking place.
188
All assays were repeated three times on different days with the SEWWTP as received.
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In order to work with the same environmental conditions (SEWWTP sample, irradiance
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and temperature), experiments were carried out in the twin photo-reactors evaluating
191
two experimental conditions simultaneously. The pairs of experimental conditions were:
192
i) photo-Fenton at neutral pH (50 mg L-1 of H2O2 and 20 mg L-1 of Fe2+) versus UVA-50
193
mg L-1 of H2O2,
194
ii) photo-Fenton (50 mg L-1 of H2O2 and 20 mg L-1 of Fe2+) with a SEWWTP as
195
-1 received (250 ± 12 mg L-1 of HCO
) versus photo-Fenton (50 mg L of H2O2 and 20
196
-1 mg L-1 of Fe2+ ) with the same SEWWTP but a HCO
concentration of 100 ± 5 mg L
197
(reduced from 250 ± 12 mg L-1 with sulfuric acid 2N),
198
iii) photo-Fenton with 50 mg L-1 of H2O2 and one addition of iron (20 mg L-1 of Fe2+)
199
versus photo-Fenton with 50 mg L-1 of H2O2 and sequential additions of iron (20-10-10
200
mg L-1 of Fe2+),
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iv) photo-Fenton (100 mg L-1 of H2O2 and 20 mg L-1 of Fe2+) versus UVA-100 mg L-1
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of H2O2.
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In order to study the variability of the process, disinfection by photo-Fenton (50 mg L-1
205
of H2O2 and 20 mg L-1 of Fe2+) of five different effluents (SEWWTP 1-5) from the
206
WWTP was carried out in February 2012, October 2012, November 2012, January 2013
207
and February 2013. Winter and autumn months were chosen in order to work with a
208
similar irradiance and water temperature range. Additionally, these months have the
209
most unfavorable environmental conditions in the year. Average UV irradiance and
210
temperature were 21.7 ± 2.2 W m-2 and 30.2 ± 3.2 ºC, respectively.
211
2.6. Bacterial (E. coli, Legionella spp. and TC) and nematode eggs quantification
212
The TC and E. coli colonies were isolated from SEWWTP. The samples were
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enumerated using the standard plate count method with serial dilutions in a selective
214
medium namely Chromocult (26.5 g L-1, Biolife). Colonies were counted after 24 h of
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incubation at 37ºC. This procedure was carried out in triplicate for each sample. The
216
bacteriological analysis of Legionella spp. was performed with selective media
217
(Buffered Charcoal Yeast Extract (BCYE) Agar Base DM258 (MAST) using the same
218
counting procedure. In order to test cell recovery post-treatment, the samples were
219
maintained in the dark after the treatment for predetermined exposure times. The
220
samples were plated for colony counting after a 24 h period. Any regrowth was
221
observed. The detection limit was 1 CFU mL-1.
222
The nematode eggs search was performed with an optic microscope in SEWWTP
223
samples. The volume used was 1 L for SEWWTP as received and 10 L for photo-
224
treated SEWWTP. Once the water had been decanted for 2 h, the pellet was washed
225
with surfactant reagents and centrifuged at 2500 rpm for 15 min. Supernatants were
226
removed using a pump, leaving a volume of 50 mL in the container. Successive
227
additions of acetic buffer and diethyl ether, followed by centrifugation steps were
228
carried out until clear sediment was obtained. Next, the eggs were floated in a zinc
229
sulfate solution. Finally, the samples were observed under a microscope 10x (Leica
230
DMIL; Leica Microsystems GmbH, Wetzlar, Germany) and nematode eggs were
231
counted.
232
2.7. Chemical analyses
233
Before chemical analysis all samples were filtered with 0.2 µm syringe-driven filters
234
(Millex®, MILLIPORE). Concentration of iron was analyzed by the o-phenantroline
235
standardized method according to ISO 6332. Hydrogen peroxide concentration was
236
measured with ammonium metavanadate by spectrophotometry (method DIN 38 402
237
H15). Dissolved organic carbon (DOC) and carbonate and bicarbonate (as inorganic
238
carbon) were measured by direct injection of samples into a Shimadzu-VCPH analyzer
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and turbidity measurements were made using a turbidimeter (HANNA instruments,
240
Romania). Ion concentrations were analyzed with ion chromatography (881 Compact IC
241
pro, METHOHM, USA).
242
2.8. Kinetics analysis
243
The decrease of the microbial population was fitted to first order kinetics according the
244
Equation (6):
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249
disinfection rate constant (min-1) and t is the time of treatment (min). This model was
250
used for direct comparison the different treatments evaluated under similar irradiance
251
conditions.
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3. Results and discussion
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3.1 Individual effects of the key factors: pH, mechanical stress (flow rate), solar
255
UVA, H2O2 and Fe2+
256
The efficiency of the photo-Fenton process is higher at a pH of around 2.8 (Pignatello et
257
al., 2006), although some authors suggest applying the photo-Fenton treatment at near
258
neutral pH in order to reduce overall costs (Klamerth et al., 2012). Acidic pH can be
259
lethal for the enteric microorganism; so a precursory study for survival of enteric TC
260
and E. coli was carried out in order to determine the effect of low pH values on the
261
cells. Previous experiments corroborated that pH 2.8 affected E. coli and TC viability.
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On the other hand, at pH 5 and 7 the bacterial concentration was constant throughout
263
the experimental time (data not shown). Therefore, in the subsequent photo-Fenton
264
study at pH 7, any observed bacterial inactivation could be attributed to the
265
photocatalytic treatment.
266
Prior to studying the use of photo-Fenton process at neutral pH for SEWWTP
267
disinfection, the effect of the main factors on TC inactivation was experimentally
268
determined. Initial concentration of TC varied within the range of 104-105 CFU mL-1
269
due to the variability of the collected samples of SEWWTP.
270
Mechanical stress due to flow and pumping through the experimental system had no
271
effect on enteric bacterial cell viability (Fig. 1). The effect of solar irradiation led to 1-
272
log decrease (k = 0.013 ± 0.002 min−1; R2 = 0.9) of TC concentration after 180 min of
273
treatment (Fig. 1) which is in line with other studies where solar disinfection was used
274
to inactivate E. coli and TC in greywater (Pansonato et al., 2011). TC inactivation was
275
enhanced, leading to a higher inactivation rate constant (k = 0.034 ± 0.003 min−1; R2 =
276
0.96) when 100 mg L-1 of H2O2 was added in the dark giving a 2.5-log decrease
277
although the addition of 50 mg L-1 of H2O2 in the dark (k = 0.0035 ± 0.0005 min−1; R2 =
278
0.89) did not produce any significant bacterial damage (Fig. 1). Hydrogen peroxide has
279
been used as an alternative to chlorine for WW treatment because it displays advantages
280
connected with its decomposition and high efficiency for organic compound
281
mineralization. However, for adequate disinfection, the concentration of hydrogen
282
peroxide required needs to be high as the number of TC exponentially decreased with an
283
increasing dose (Ksibi, 2006). A slight enhancement in bacterial inactivation (k = 0.018
284
± 0.003 min−1; R2 = 0.89) was observed (1-log after 180 min of experiment) using the
285
Fenton process with 20 and 50 mg L-1 of Fe2+ and hydrogen peroxide, respectively (Fig.
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1). This result indicates that the Fenton reagent at neutral pH is more effective than the
287
addition of hydrogen peroxide only which is in line with recent research (Ndounla et al.,
288
2013) carried out in order to disinfect wild enteric bacteria (TC, E. coli and Salmonella
289
spp.) in water from wells with different pH (4.9 and 6.3). However, the only addition of
290
iron in the dark (Fig. 1) had a slight effect over TC as no significant reduction in viable
291
counts was observed as previously demonstrated (Ksibi, 2006). With the same
292
concentration of iron, a reduction in bacterial viability of 1.5-log was observed under
293
natural light.
294
Many authors have looked into the effect of the UVA-H2O2 combination on the
295
inactivation of several microorganisms with promising results (Sichel et al., 2009;
296
Sciacca et al., 2010; García-Fernández et al., 2012). Fig. 1 shows that when 50 mg L-1
297
of H2O2 was added in the presence of natural solar radiation, the detection limit was
298
reached after 180 min (k = 0.036 ± 0.009 min−1; R2 = 0.89). The combination of both
299
effects enhances the individual influence of each parameter due to the UVA action on
300
damaged cells exposed to hydrogen peroxide (Feuerstein et al., 2006; Sichel et al.,
301
2009). The combined system has been also used by other authors for the inactivation of
302
enteric microorganisms in SEWWTPs with a reduction of more than 4-log of fecal
303
coliform concentration (Xie et al., 2007).
304
3.2 Disinfection by solar photo-Fenton process
305
Photo-Fenton at neutral pH (50 mg L-1 of H2O2 and 20 mg L-1 of Fe2+) and the most
306
efficient control condition shown in Fig. 1 (UVA-50 mg L-1 of H2O2) were
307
simultaneously tested in order to compare both processes for the inactivation of E. coli
308
and TC in SEWWTP. Fig. 2 shows that the addition of iron led to a significant
309
difference in the inactivation time, as the detection limit was reached for E. coli and TC
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in 45 and 60 min of solar exposure, respectively for the photo-Fenton treatment;
311
whereas the same limit was reached in 60 and 120 min in the UVA-H2O2 process.
312
Moreover, the inactivation kinetic constant increased from 0.053 ± 0.011 min−1, R2 =
313
0.8 (solar UVA-H2O2) to 0.11 ± 0.01 min−1, R2 = 0.96 (solar photo-Fenton). Therefore,
314
although at neutral pH a high amount of the added iron precipitated, the remaining
315
concentration of iron in solution favours the generation of hydroxyl radical.
316
Additionally, a contribution of heterogeneous photo-Fenton due to iron hydroxide
317
activity could take place as proposed by Lam et al. (2005). These results are in line with
318
the hydrogen peroxide consumption (Fig. 2), which was slower in the UVA-H2O2
319
treatment than in the photo-Fenton process because hydrogen peroxide was consumed
320
by the formation of hydroxyl radicals. During the first minutes of photo-Fenton and
321
UVA-H2O2 treatments, inactivation curves for E. coli and TC were overlapped (Fig. 2).
322
The attack of hydroxyl radicals generated during photo-Fenton process was responsible
323
for the cell damage and therefore, the disinfection process was accelerated, achieving
324
shorter inactivation times (a reduction of 50%). Photo-Fenton was also more effective
325
than Fenton (k = 0.018 ± 0.003 min−1) treatment at neutral pH (Fig. 1) due to the
326
regeneration of Fe2+ caused by the photochemical effect of sun light and the higher
327
generation rate of hydroxyl radicals. This enhancement leads to an increase in the
328
inactivation rate of 82%. The same behavior was observed by Moncayo-Lasso et al.
329
(2009) for E. coli inactivation at neutral pH in river water.
330
The use of photo-Fenton treatment at neutral pH for disinfection of enteric
331
microorganisms from WWTP effluents has barely been studied. However, the use of
332
solar photo-Fenton at neutral pH monitoring the wild enteric bacterial concentration for
333
WW recovery and reuse has not been proven or assessed in detail until now. Depending
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on the needs of WW reuse (urban, agricultural, industrial, recreational and
335
environmental uses) the tertiary treatment must ensure the water quality follows the
336
guidelines of each country. As agriculture consumes over 70% of the world’s total fresh
337
water (FAO 2012), the viability of the photo-Fenton treatment for water reuse with
338
agrarian purposes was studied. Reviewing current WW quality guidelines, one of the
339
most important microbiological parameters used as a fecal indicator is E. coli
340
concentration. The WHO guidelines stipulate a reduction in E. coli concentration up to
341
10 CFU mL-1, whereas the Spanish legislation requirements are more restrictive in this
342
sense. The Spanish Royal Decree (RD 1620/2007) establishes a level of 1 CFU mL-1. In
343
both cases disinfection by photo-Fenton process satisfied this legislation. Moreover,
344
Legionella spp. (another microbiological parameter) was not detected either in the
345
SEWWTP as received (before the treatment) or after photo-Fenton disinfection.
346
Additionally, only one nematode egg (in 10 L) was found in the SEWWTP and after the
347
photocatalytic treatment not even one nematode egg was found.
348
3.2.1. Effect of natural bicarbonate concentration on the photo-Fenton treatment
349
Many authors have demonstrated that the presence of carbonate and bicarbonate anions
350
in WW has a negative impact on the photo-Fenton process (Klamerth et al., 2009;
351
Pignatello et al., 2006). Both anions act as radical scavengers generating carbonate
352
radical by Equations (7) and (8) (Kochany et al., 1992).
353
At the working pH (7 - 7.4), CO∙
is formed mainly by Equation (7) as the main species
354
is HCO
(80%). Carbonate radical is an oxidant specie (1.78 V at pH 7), which can
355
degrade organic contaminants (Dell’Arciprete et al., 2012) although its redox potential
356
is lower than that of hydroxyl radical (2.8 V at pH 7).
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∙
HO HCO
→ H O CO
(7)
358
∙
HO CO
→ HO CO
(8)
359
Some strategies have previously been proposed to avoid the scavenging of hydroxyl
360
radicals. One of them is the use of higher amounts of iron since bicarbonate promotes
361
iron precipitation (Carra et al., 2013). Another alternative is reducing the concentration
362
of carbonate and bicarbonate anions in WW before photo-Fenton by adding an adequate
363
quantity of concentrated acid (Klamerth et al., 2009). The photo-Fenton process (50 mg
364
L-1 of H2O2 and 20 mg L-1 of Fe2+) was applied to a SEWWTP as received (HCO
=
365
250 ± 12 mg L-1) and to the same water with a reduced HCO
concentration of 100 ± 5
366
mg L-1 (by sulphuric acid addition) in order to compare the disinfection efficiency under
367
both conditions. The final HCO
concentration was enough to maintain the buffer
368
capacity of the water. It could be observed (Fig. 3) that TC concentration reached the
369
detection limit in 80 min for both WW matrices although the detection limit was
370
reached for E. coli at 80 min (SEWWTP as received) and at 40 min when the HCO
371
concentration was 100 ± 5 mg L-1. The competence for reactive oxidant species (ROS)
372
between microorganisms, organic matter, and bicarbonate anions was not observed
373
because very oxidant conditions were applied during photo-Fenton process and an
374
excess of radicals were generated. This hypothesis is supported by previous results
375
where photo-Fenton treatment was used for the inactivation of Enterococcus faecalis in
376
the presence of resorcinol as hydroxyl radical scavenger (Ortega-Gómez et al., 2014).
377
At low concentrations of H2O2 and Fe2+, when the scavenger was present, a marked
378
delay in the disinfection process was attained. However, at elevated concentration of
379
reagents (H2O2/Fe2+: 50/20 mg L-1) the formation of radicals was so high that the attack
380
to the scavenger molecule does not result in a detrimental effect on the bacterial
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inactivation. Therefore, in order to use photo-Fenton treatment at a stable pH around 7,
382
it is not worth reducing the natural bicarbonate concentration of WW. Complete
383
removal of bicarbonate would produce an increase in inactivation rate as well as a sharp
384
pH drop towards acidic values.
385
3.2.2 Effect of iron addition on the photo-Fenton process
386
Photo-Fenton treatment has traditionally been carried out at pH around 2.8 due to
387
photo-active Fe species predominating at this acidic pH. However, in recent years,
388
many authors (García-Fernández et al., 2012; Klamerth et al., 2012) have shown a
389
growing interest in photo-Fenton treatment (pollutant degradation and disinfection) at
390
natural pH of WW in order to reduce operating costs. At pH between 6 to 8, iron
391
precipitation is inevitable and the photo-Fenton process is less effective (Klamerth et
392
al., 2012). In order to operate at neutral pH, a new strategy consisting of a sequential
393
iron dosage was recently proposed (Carra et al., 2013). Treating real WW enriched with
394
a mixture of pesticides (50 mg L-1 of DOC), similar reaction rates and degrees of
395
mineralization were obtained by photo-Fenton at neutral pH with sequential iron
396
additions (20-20-10-10-10 mg L-1 of Fe2+at 0, 5, 15, 25 and 35 min, respectively)
397
compared to photo-Fenton at pH 2.8 with one dose of 20 mg L-1 of Fe2+. Following the
398
proposed strategy of iron dosage, two parallel assays were carried out: in one loop,
399
sequential additions of 20, 10 and 10 mg L-1 of Fe2+ every 5 min (mixing time) were
400
made; while in the other loop a unique initial 20 mg L-1 of iron addition was added at
401
the beginning of the experiment. In both cases, SEWWTP was used as received and the
402
hydrogen peroxide concentration was 50 mg L-1. As Fig. 4 shows, photo-Fenton with
403
iron additions slowed down the disinfection rate with regard to a unique iron addition as
404
inactivation time increased to 90 min for E. coli and 120 min for TC. Other iron
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addition strategies were tested, but in any case the disinfection rate was faster than that
406
reached with one addition of iron (data not shown). With sequential iron additions,
407
hydrogen peroxide consumption was higher (Fig. 4) than with one addition, since more
408
hydroxyl radicals were generated.
409
These results point out that higher iron concentration values have a negative effect on
410
the bacterial inactivation by photo-Fenton at neutral pH. This detrimental effect could
411
be caused by: (i) iron precipitation promoting by the formation of floccules which
412
favored even more iron precipitation, (ii) an excess of iron concentration increasing the
413
HO• scavenging potential (Equation (2)) (Ndoula et al., 2013; Pansonato et al., 2011)
414
and (iii) the reduction of light transmission due to the increased turbidity of water. Some
415
mechanisms which cause cellular damage is based in absorb light in the UVA and
416
visible spectrum (Spuhler et al., 2010). For this reason, if these pathways involved in
417
photo-inactivation are disfavored the cellular damage is diminished.
418
3.2.3 Effect of hydrogen peroxide concentration on photo-Fenton efficiency
419
With the purpose of minimizing the inactivation time in the photo-Fenton process, a
420
higher concentration of hydrogen peroxide (100 mg L-1) was tested. This experiment
421
and a test of UVA-100 mg L-1 of H2O2 were carried out simultaneously. Fig. 5 shows
422
the same inactivation profiles regardless of whether iron was added or not, reaching the
423
detection limit at 30 min for E. coli and 45 min for TC. When this higher amount of
424
H2O2 is used, photo-Fenton process did not suppose any significant enhancement versus
425
the UVA/H2O2 process for bacterial inactivation, in spite of the major oxidant
426
consumption. In the UVA-H2O2 process, the predominant ROS is H2O2 that is stable,
427
long living compound and at high concentration is toxic for the cells. The mere addition
428
of 100 mg L-1 of H2O2 causes a reduction of 2-log in the bacterial concentration (Fig. 1).
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Additionally, there is a synergistic effect between UVA and H2O2 in microorganism’s
430
inactivation (Feuerstein et al., 2006). This synergy is due to the absorption of UV
431
photons by the bacteria which cause sub-lethal damage followed by the oxidative attach
432
of H2O2 causing a total inactivation. However, although the HO• radical is the unique
433
specie which can directly damage DNA and no cellular defense mechanism against this
434
attack has been identified until today (unlike H2O2) (Satter et al., 2000) the elevated
435
concentration of H2O2 combined with the UVA radiation are capable to produce the
436
same damage.
437
The times for bacterial inactivation were shorter than those obtained when the hydrogen
438
peroxide concentration was 50 mg L-1 (Fig. 2). When the concentration of hydrogen
439
peroxide increased from 50 to 100 mg L-1, inactivation times by photo-Fenton were
440
reduced by 33% and 25% for E. coli and TC, respectively, although the amount of
441
residual hydrogen peroxide was increased two fold (Fig. 2, Fig. 5).
442
Consequently, according to these results, duplicate initial hydrogen peroxide
443
concentrations lead to a slight enhancement in disinfection times and if elevated
444
concentration of oxidant is employed, the addition of iron is not a practicable strategy.
445
3.3 Effect of SEWWTP seasonal variability
446
The performance of photo-Fenton treatment is affected by the WW variability as
447
parameters such as pH, turbidity, wild enteric bacterial concentration, water
448
composition (organics and inorganics), etc. vary with time, day and season. To assess
449
the effects of SEWWTP variability on the photo-Fenton process, several disinfection
450
assays were carried out with the same reagent concentrations (50 mg L-1 of H2O2 and 20
451
mg L-1 of Fe2+) in winter and autumn months for one year (named as SEWWTP1 to
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SEWWTP5 in Table 2). During the first 45 min (Fig. 6), the TC inactivation curves
453
were similar in all cases. At this time, 2-log of bacteria were inactivated (99% with
454
regard to the detection limit) and the remaining cells were more resistant to inactivation.
455
Total coliform disinfection displayed different behavior involving different inactivation
456
times (from 60 to 120 min, Fig. 6). The elevated number of parameters involved in this
457
process obstructs the attribution of some of them to the differences in the inactivation
458
times. Therefore, in order to apply the photo-Fenton process as tertiary treatment in
459
municipal WWTPs it must be taken into account that the inactivation kinetic constant
460
varies between 0.07 and 0.17 min-1 to reach the detection limit (Table 2). As a result, in
461
order to assess what is an effective operating time for the total disinfection,
462
independently of the environmental conditions and WW composition, the minimum
463
inactivation rate must be considered.
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4. Conclusions
466
An analysis of the capability of the photo-Fenton treatment at neutral pH for enteric
467
bacteria disinfection in a secondary effluent from a WWTP was carried out. The
468
synergic effect of the solar-H2O2 (50 mg L-1) was enough to observe a complete
469
disinfection of TC although the addition of iron (solar photo-Fenton: 50 mg L-1 of H2O2
470
and 20 mg L-1 of Fe2+) accelerated the disinfection process, shortening inactivation
471
times by half. Therefore, photo-Fenton is a suitable process to assess the quality of the
472
treated water for reuse in terms of WHO guidelines and in particular Spanish legislation
473
for treated WW reuse.
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With regard to photo-Fenton process operation, reducing the natural concentration of
475
bicarbonate present in WW did not lead to a significant reduction in the inactivation
476
times, since its role as scavenger is overlapped by the excess of HO• radical in solution.
477
Also, sequential iron additions slowed down the disinfection process due to several
478
pathways involved in photo-inactivation are disfavored.
479
Finally, the addition of a high concentration (100 mg L-1) of hydrogen peroxide resulted
480
in a slight enhancement in disinfection times but residual peroxide was twice as high.
481
Moreover, photo-Fenton treatment caused the same inactivation degree than the
482
combination of UVA-H2O2 when elevated H2O2 concentrations were employed.
483
The WW variability led to different inactivation times (between 60 and 120 min)
484
although at 45 min, 99% disinfection was achieved in all the cases. Therefore, attention
485
to this variability should be paid in order to remove the residual surviving bacteria,
486
causing longer treatment times to reach proper disinfection in SEWWTP.
487
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Acknowledgments
489
This research was funded by the Ministry for Economy and Competitiveness
490
(CTQ2010-20740-C03-01),
491
AQUAPYME project (P10-RNM-05951) and the European Regional Development
492
Fund (ERDF). Elisabet Ortega Gómez would like to acknowledge the Ministry for
493
Economy and Competitiveness for her F.P.I. scholarship (Ref: BES-2011-043886). The
494
authors would also like to acknowledge the collaboration of "El Bobar" (AQUALIA)
495
WWTP.
AQUASUN
project
(CTM2011-29143-C03-03),
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Xie, R.J., Gomez, M.J., Xing, Y.J. Field investigation of advanced oxidation of
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of
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601
Table and Figure captions
602 Table 1. Chemical and microbiological characterization of the SEWWTP (El Bobar, Almería,
604
Spain).
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603
605
Table 2: Inactivation kinetic constants for TC disinfection by photo-Fenton on different
607
days during autumn and winter seasons.
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608
Fig. 1. TC inactivation in the CPC reactor during control assays. Dark experiments:
610
Mechanical stress (-○), 50 mg H2O2 L-1 (-□-),100 mg H2O2 L-1 (--), 20 mg Fe2+ L-1 (-
611
∇-), Fenton (--). With solar radiation experiments: UVA (-●-), UVA-20 mg Fe2+ L-1
612
(-■-), and UVA-50 mg H2O2 L-1 (-♦-).
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Fig. 2. TC and E. coli inactivation by UVA-50 mg H2O2 L-1 treatment (open symbols)
615
and photo-Fenton (50 mg H2O2 L-1-20 mg Fe2+ L-1) process (closed symbols). TC (-◌-),
616
E. coli (--) and H2O2 concentration (-□-).
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618
Fig. 3. Effect of bicarbonate concentration on water disinfection by photo-Fenton (50
619
-1 mg H2O2 L-1-20 mg Fe2+ L-1). Open symbols, HCO
= 100 ± 5 mg L . Closed symbols,
620
-1 HCO
= 250 ± 12 mg L . TC (-◌-), E. coli (--) and H2O2 (-□-).
27
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Fig. 4. Effect of iron dosage on water disinfection by photo-Fenton. Open symbols, iron
622
additions 20 mg Fe2+ L-1, at t=0; 10 mg Fe2+ L-1, at t=5 min; and 10 mg Fe2+ L-1, at t=10
623
min. Closed symbols, one addition of 20 mg Fe2+ L-1. TC (-◌-), E. coli (--) and H2O2
624
(-□-).
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625
Fig. 5. Water disinfection with high hydrogen peroxide concentration. Open symbols,
627
UVA-100 mg H2O2 L-1. Closed symbols, photo-Fenton (100 mg H2O2 L-1-20 mg Fe2+ L-
628
1
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). TC (-◌-), E. coli (--) and H2O2 (-□-).
SC
626
629
Fig. 6. TC inactivation by photo-Fenton (50 mg H2O2 L-1-20 mg Fe2+ L-1) replicas with
631
SEWWTPs obtained on five different days over one year: SEWWTP1 (-■-),
632
SEWWTP2 (-●-), SEWWTP3 (-♦-), SEWWTP4 (-□-) and SEWWTP5 (--).
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pH -1
Conductivity (µS cm ) Turbidity (NTU) -1
Dissolved organic carbon, DOC (mg L ) -1
Maximum
Minimum
7.48 ± 0.33
8.34
7.08
1248 ± 166
1659
858
8.98 ± 4.2
15.23
2.91
23.78 ± 6.21
35.53
9.85
399.45 ± 2.15
605.15
-1
2.50 ± 0.43
2.76
-1
248.43 ± 40.92
Bromide (mg L ) Chloride (mg L ) -1
-2
Fluoride (mg L )
1.25·10 ± 2.710
-1
4.6 ± 7.6
-1
5.43 ± 5.43
Nitrite (mg L )
Nitrate (mg L ) -1
Sulfide (mg L )
268.66 -3
4
0.05
3
1.16·10 ± 1.8·10
TC (CFU mL-1)
3.24·104 ± 2.3·102
TE D EP AC C
0
21.16
0
110.41
79.30
3.4·10
4
6.12·105
M AN U
E. coli (CFU mL )
216.87
0
14.69
89.71 ± 13.94 -1
122.15 0.67
SC
Bicarbonate (mg L )
Mean
RI PT
Parameter
2.13·102
1.63·103
ACCEPTED MANUSCRIPT
k (min-1)
Sample
(2-log decrease)
R2
k (min-1) (from C0 to DL)
R2
0.12
0.94
0.15
0.96
SEWWTP 2
0.14
0.99
0.17
0.97
SEWWTP 3
0.08
0.93
0.14
0.93
SEWWTP 4
0.10
0.91
0.09
SEWWTP 5
0.11
0.97
0.07
Average
0.11
-
0.13
Standard deviation
0.02
-
0.04
RI PT
SEWWTP 1
0.96
0.92
-
AC C
EP
TE D
M AN U
SC
-
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
• • •
AC C
EP
TE D
M AN U
SC
•
Disinfection by solar photo-Fenton at neutral pH as tertiary treatment was studied Photo-Fenton reduces 50 % the inactivation time respect to UVA-H2O2 Total inactivation by photo-Fenton was achieved in 60 min (H2O2/Fe2+: 50/20 mg L-1) Wastewater treated by photo-Fenton was adequate for its reuse for irrigation In spite to the seasonal variability 99% of disinfection was achieved in 40 min
RI PT
•