Technical NoteRemoval of NOM from drinking water: Fenton’s and photo-Fenton’s processes
Introduction
Water throughout the world contains natural organic matter (NOM) as a result of the interactions between the hydrological cycle and the biosphere and geosphere. These interactions are responsible for the diverse nature of NOM as the organic content of a particular water body is dependent on the surrounding environments biogeochemical cycles. NOM is a complex mixture of organic material and has shown to consist of organics as diverse as humic acids, hydrophilic acids, proteins, lipids, hydrocarbons and amino acids. The range of organic components in NOM varies from water to water and seasonally; this consequently leads to variations in the reactivity of NOM with chemical disinfectants such as chlorine (Goslan et al., 2003). NOM affects potable water quality as it:
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enhances transportation and distribution of organic micropollutants,
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promotes disinfection by-product (DBP) formation (Singer, 1999; Goslan et al., 2003),
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provides an undesirable biological growth substrate (Volk and Lechevallier, 2002),
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lowers the efficiency of treatment processes (Goel et al., 1995),
The conventional treatment process for waters containing NOM is coagulation/flocculation using inorganic coagulants such as ferric chloride or aluminium sulphate. These processes remove NOM by adsorption onto flocs and can typically achieve 50–80% removal of dissolved organic carbon (DOC) (Parsons et al., 2002). Increasing coagulant dose will increase removal but the process is limited when trying to reach total organic carbon (TOC) levels less than 1 mg l−1 (Croué et al., 1993; Nowack et al., 1999). The NOM problem is an intractable one as even at these residual levels of TOC the proposed disinfection by-product standards of 100 μg l−1 maximum admissible concentration (MAC) are difficult to achieve, as NOM has been reported to have the potential to form varying concentration of THMs with raw waters as high as 90 μg THM mg−1 DOC (Parsons et al., 2002) with generally higher hydrophobic waters forming more (51 μg THM mg−1 DOC) when compared to hydrophilic waters (21 μg THM mg−1 DOC) (Krasner et al., 1989). Increasing TOC removal by increasing coagulant dose will inevitably lead to an accompanying increase in the amount of plant residuals (sludge) generated and associated increases in costs and operators’ time. The sludge produced during the coagulation of NOM is also difficult to dewater because of the increased metal ion and organic content (Harbour et al., 1999).
There has been considerable interest in the use of advanced oxidation processes (AOPs) for the treatment of water and wastewater contaminants. These processes are characterised by their ability to form the strong oxidising species, the hydroxyl radical in water. The hydroxyl radical has been reported to oxidise a range of organic compounds significantly faster than ozone (Fukushima et al., 2001). A range of AOPs has been tested for treating water containing commercial humic acid or natural organic matter. A recent study on UV photo-oxidation of a reservoir water rich in NOM showed how as UV dose increased from 1000 to 26 000 mJ cm−2 there was a substantial increase in biodegradability of the residual organic carbon (Thompson et al., 2002). Kleiser and Frimmel (2000) showed that water containing 2 mg l−1 of NOM was resistant to oxidation by ozone but treatment with hydroxyl radicals, generated by the combination of H2O2 (8 mg l−1) and UV radiation reduced the same DOC by 25% after 1050 min of treatment. In comparison Wang et al. (2000) reported an 80% DOC removal from a 5 mg l−1 humic acid solution in 120 min although this was at significantly higher concentration of H2O2.
In this paper we report the treatment of commercial humic acid and NOM rich waters with Fenton’s reagent (FR). In this process hydroxyl radicals are produced during the decomposition of hydrogen peroxide in the presence of ferrous salts.The process has an optimum between pH 3 and 6. Waite (2002) reported that Fenton’s reagent is most effective at a pH between 2 and 4 with an optimum at pH 3 and IJeplaar et al. (2002) showed how for a ground water polluted with pesticides removal fell from 100% at pH 5 to 20% at pH 7.5. Process performance is also dependent on reaction time, ferrous and peroxide dose and organic strength. The process can also be enhanced through irradiation with near-UV and visible light, a process termed the photo-Fenton’s (PFR) process.
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Materials and methods
Initial experiments were performed using humic acid solution prepared from Aldrich humic acid sodium salt. Once optimum reaction conditions were identified the experiments were repeated, under these conditions, using raw water obtained from Albert water treatment works (WTW) reservoir, Yorkshire. Albert WTW is located on the western side of Halifax in the UK (north west England) and treats between 33 000 and 55 000 m3 d−1. Raw water quality during these tests is shown in Table 1.
Ferrous sulphate
Fenton’s reagent
Commercial humic acid and NOM laden raw water have been treated with Fenton’s reagent. The process is controlled by the key parameters pH, Fe2+:H2O2 ratio and mixing speed. Here the optimum pH was found to be ∼pH 4 (Fig. 2) where in excess of 90% removal of UV254 and DOC was achieved for both the commercial humic acid and the raw water. The pH is in the expected range (Arslan and Balcioglu, 1999; Waite et al., 2000; IJeplaar et al., 2002) and moving the pH to the more economic pH 5 had little
Conclusions
Whilst Fenton’s and photo-Fenton’s are not new processes there are few examples of their application in bulk organics removal during potable water treatment. The results presented here show that once optimised both processes have the potential to treat waters rich in NOM. The performance of the two processes is comparable for both DOC and UV254 removal and significantly better than coagulation with iron salts for the same water. The key parameter’s determining the removal efficiency of both
Acknowledgements
CM was funded by a grant from EPSRC with additional support from Yorkshire Water and United Utilities. The opinions expressed are those of the authors and do not necessarily reflect the views of Yorkshire Water or United Utilities.
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