Inactivation of natural enteric bacteria in real municipal wastewater by solar photo-Fenton at neutral pH

Highlights

• 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 irrigation reuse.

• In spite to the seasonal variability 99% of disinfection was achieved in 40 min.

Abstract

This study analyses the use of the solar photo-Fenton treatment in compound parabolic collector photo-reactors at neutral pH for the inactivation of wild enteric Escherichia coli and total coliform present in secondary effluents of a municipal wastewater treatment plant (SEWWTP). Control experiments were carried out to find out the individual effects of mechanical stress, pH, reactants concentration, and UVA radiation as well as the combined effects of UVA-Fe and UVA-H₂O₂. The synergistic germicidal effect of solar-UVA with 50 mg L⁻¹ of H₂O₂ led to complete disinfection (up to the detection limit) of total coliforms within 120 min. The disinfection process was accelerated by photo-Fenton, achieving total inactivation in 60 min reducing natural bicarbonate concentration found in the SEWWTP from 250 to 100 mg L⁻¹ did not give rise to a significant enhancement in bacterial inactivation. Additionally, the effect of hydrogen peroxide and iron dosage was evaluated. The best conditions were 50 mg L⁻¹ of H₂O₂ and 20 mg L⁻¹ of Fe²⁺. Due to the variability of the SEWWTP during autumn and winter seasons, the inactivation kinetic constant varied between 0.07 ± 0.04 and 0.17 ± 0.04 min⁻¹. Moreover, the water treated by solar photo-Fenton fulfilled the microbiological quality requirement for wastewater reuse in irrigation as per the WHO guidelines and in particular for Spanish legislation.

Introduction

The World Health Organization (WHO) estimates that half of the world's population will be living in water stressed areas by 2025. This water scarcity will force a better use of wastewater (WW) as an important source of irrigation water all over the world. However, it may pose many health risks to the environment and end users if the treatment is inadequate or incomplete.

In the last few decades, advanced oxidation processes (AOPs) have been growing in importance for tertiary treatments (Bernabeu et al., 2011). The effectiveness of these methods is based on the generation of non-selective and strongly oxidizing radicals, which makes them highly appropriate for the treatment of a wide variety of contaminants in water and WW disinfection (Malato et al., 2009). Amongst these treatments, solar photo-Fenton process has been of special interest due to its efficiency for degradation of a number of organic compounds and the use of solar radiation. In the last few years photo-Fenton reactions have also been studied for inactivation of different bacteria, fungi, nematodes and virus (Spuhler et al., 2010, Polo-López et al., 2012). However, most of publications are focused on inactivation of microorganisms from collection instead of wild enteric species. Argulló Barceló et al., inactivated enteric F-specific RNA bacteriophages and Escherichia coli during photo-Fenton treatment (10 mg L⁻¹ of Fe²⁺, 20 mg L⁻¹ of H₂O₂) at natural pH in real WW from WWTP although complete disinfection was not achieved for somatic coliphages and sulphite reducing chlostridia (Agulló-Barceló et al., 2013). Ndounla et al., 2013 treated water from household wells containing wild enteric bacteria (total coliform/E. coli and salmonella) with photo-Fenton at natural pH with 10 mg L⁻¹ of H₂O₂ and natural iron and solid iron oxides (Ndounla et al., 2013). Wild strain of Fusarium solani isolate from rainfall over a river was also inactivated by photo-Fenton (5 mg L⁻¹ of Fe²⁺, 10 mgL⁻¹ of H₂O₂) at neutral pH (Polo-López et al., 2012).

During the photo-Fenton reaction (Equations (1), (2), (3), (4), (5), (6), (7), (8)), hydrogen peroxide reacts with iron forming hydroxyl radicals HO (which are the main reason for the inactivation of microorganisms), Fe³⁺ and HO⁻. In the presence of UV–vis radiation, the ferric ions (Fe³⁺) produced in Equation (1) are photo-catalytically converted to ferrous ions (Fe²⁺), with the formation of an additional equivalent of hydroxyl radical (Equation (2)). Radicals can react with organic matter, with hydrogen peroxide or with other radicals. These parallel reactions are, in most cases, oxygen-generating, whilst other oxidation steps are oxygen consuming. Therefore, changes in photo-Fenton process are reflected in the dissolved oxygen concentration evolution (Pignatello et al., 2006).

$${\text{Fe}}^{2+}+{\text{ H}}_2{\text{O}}_2\to {\text{Fe}}^{3+}+{\text{HO}}^{-}+{\text{HO}}^{}$$

$${\text{Fe}}^{3+}+{\text{ H}}_2\text{O}+hv\to {\text{Fe}}^{2+}+{\text{H}}^{+}+{\text{HO}}^{}$$

$${\text{R}}^{}+{\text{O}}_2\to \text{R}-{\text{O}}_2^{}$$

$$\text{R}-{\text{O}}_2^{}+{\text{ H}}_2\text{O}\to \text{ROH}+{\text{HO}}_2^{}$$

$${\text{HO}}^{}+{\text{H}}_2{\text{O}}_2\to {\text{H}}_2\text{O}+{\text{HO}}_2^{}$$

$${\text{HO}}_2^{}\to {\text{O}}_2^{·-}+{\text{H}}^{+}$$

$${\text{O}}_2^{-}+{\text{H}}_2{\text{O}}_2\to {\text{O}}_2+{\text{HO}}^{}+{\text{HO}}^{-}$$

$$2{\text{HO}}_2^{}\to {\text{H}}_2{\text{O}}_2+{\text{O}}_2$$

The elevated number of parameters such as reagent dosage (H₂O₂ and iron), irradiance, pH, temperature, different types of enteric microorganisms and inorganic and organic matter present in water make more difficult the understanding of the inactivation processes carried out during the treatment of a real WW by solar photo-Fenton. The presence of different ions like carbonate $\left({{\text{CO}}_3}^{2\mathrm{-}}\right)$, phosphate $\left({{\text{PO}}_3}^{\mathrm{4-}}\right)$, sulfate $\left({{\text{SO}}_4}^{\mathrm{2-}}\right)$ and chlorine (Cl⁻) has an effect on the equilibrium of iron in water. These ions have a negative impact on the photo-Fenton process (Spuhler et al., 2010, Polo-López et al., 2012, Ortega-Gómez et al., 2013, Agulló-Barceló et al., 2013). In particular, carbonate and phosphate have a doubly detrimental effect on the photocatalytic reaction, as they precipitate the iron and act as a scavenger of the hydroxyl radicals. Dissolved iron concentration and pH are closely related as at pH higher than 3 (optimum for photo-Fenton) Fe²⁺ ions are easily transformed into Fe³⁺, forming hydroxyl-complexes and causing iron precipitation. However, some research has demonstrated the possibility of carrying out the photo-Fenton reaction at neutral or near neutral pH for detoxification and disinfection of WW, thus reducing the operational costs associated with acidification and neutralization (Klamerth et al., 2012). Some strategies have been developed to avoid iron precipitation in solution at neutral pH such as immobilized photo-Fenton in woven inorganic silica (Moncayo-Lasso et al., 2008), by immobilizing the ferrous ion on porous activated carbon (Ramírez et al., 2007), or a photo-ferrioxalate disinfection system (Cho et al., 2004). Recently, a new strategy of operating at neutral pH consisting of a sequential iron dosage was reported (Carra et al., 2013). Working in this way, photo-Fenton treatment of a real WW enriched with a mixture of pesticides reached similar reaction rates and degrees of mineralization at neutral pH and at pH 3.

The purpose of this paper was to assess the capability of solar photo-Fenton at neutral pH for disinfecting a real secondary effluent of a municipal wastewater treatment plant (SEWWTP). The photo-Fenton treatment was carried out in a pilot plant with compound parabolic collectors (CPC) under natural sun light. The inactivation of wild enteric bacteria (E. coli and total coliforms (TC)) from the SEWWTP was evaluated and the effects of the main process variables on bacterial inactivation were studied. With this purpose, the effects of bicarbonate concentration and reagent dosage (iron and hydrogen peroxide) were investigated. The concentration of TC was monitored as an indicator of faecal contamination (faecal coliform, E. coli, enterococci) in polluted water. TC represents the potential occurrence of a wide number of pathogenic microorganisms (Sivaraja and Nagarajan, 2014). Finally, the seasonal stability of the disinfection process was studied during the autumn and winter, having these seasons the most unfavorable environmental conditions.