Removal of E. coli from urban stormwater using antimicrobial-modified filter media
Introduction
Stormwater filters and biofilters are gravity-fed filter beds, vegetated or non-vegetated, placed within urban landscapes that are never back-washed [1]. They remove nutrients and metals effectively by means of biological uptake, straining and adsorption [1], [2], [3]. However, field and laboratory investigations have shown that their effluent seldom meets bacterial indicator targets for outdoor irrigation [4], [5], [6], [7]. This is partially due to the inadequate microbial removal capacity of sand media used in these filters, as well as survival and detachment of microbes under intermittent stormwater inflows [8]. Therefore, stormwater filters and biofilters require novel media in order to ensure effective pathogen removal [9].
As inorganic antimicrobials, zeolites and activated carbon containing Ag, Cu or Zn have proliferated for food preservation, self-disinfection fabrics, etc. [10], [11]. Ag-activated carbon has been tested for water treatment and demonstrated very good bacterial removal efficiency [12], [13]. An investigation of Cu2+-treated zeolite for water treatment, subjected to controlled conditions and 6 h hydraulic residence time, showed around 1–3 log removal for a wide range of microbes [14], [15]. However, longevity in efficiency and stability – yet to be addressed – is essential for field application.
Compared with the metal ion treatment of filter media, metal hydr(oxide) coating exhibits better stability in water due to its low solubility constant. In addition, metal hydr(oxide) coating shows effective bacteria removal through electrostatic attraction [12], [16], [17]. More importantly, the slow release of metal ions from the coating layer in solution at an effective level may exert an antimicrobial effect from the media. Kennedy et al. [18], for example, examined the bactericidal effects of CuO/Cu2O-coated carbon and showed 4 log removal of E. coli. However, the coating processes required expensive organometallic precursor and toxic organic solvent, prohibiting large scale field applications. In addition, Cu(OH)2, an effective component in Bordeaux mixture as a pesticide and fungicide, has not been investigated as antimicrobial coating for any water treatment media. Consequently, CuO coating through Cu(OH)2 precipitation has not been investigated for antimicrobial use.
The above findings cannot be used directly for advancements of stormwater filters and biofilters, as urban runoff is a unique water source. For example, stormwater biofilters are located within an urban environment (often being an amenity feature), thus are exposed to highly variable hydraulic and pollutant loadings, intermittent wetting and drying conditions, and highly seasonal variations [19]. The distinct characteristics of stormwater will pose questions for the aforementioned modified materials: for example, will variability, and sometimes high salinity, of stormwater lead to excessive heavy metal leaching from antimicrobial media?
This study therefore aims to develop and evaluate antibacterial media for urban stormwater treatment. Specifically, this work’s objectives are to:
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Develop simple yet scalable processes to modify GAC and zeolite with Cu(OH)2, CuO, and Cu2+ producing 4 types of copper modified media;
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Evaluate their stability and bacteria removal performance, inactivation efficiency over semi-long term experimental duration, under typical stormwater operational conditions including relatively high filtration rates, intermittent operation patterns, and variable salinity in stormwater;
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Investigate the main bacterial removal mechanisms subjected to stormwater conditions;
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Prepare 11 other antibacterial media using zinc, iron, titanium, and quaternary ammonium salts (consulted in literature for other types of water treatment), evaluating their stability and bacteria removal efficiency for stormwater operational conditions (in comparison with copper treated media).
Section snippets
Modification of zeolite and granular activated carbon with antibacterial agents
The chemicals (CAS number in parentheses) and their sources, featured in this study, comprise zinc sulfate heptahydrate (7446-20-0), copper(II) chloride (7447-39-4), iron(III) chloride (7705-08-0), hexadecyltrimethylammonium chloride (QAC) (11202-7), sodium hydroxide (1310-73-2) and ethylenediaminetetraacetic acid disodium salt (EDTA) (6381-92-6), Merck Chemicals, Australia; dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (Si-QAC) (27668-52-6), Sigma–Aldrich; TiO2 sol–gel [20].
Characterisation of modified media
Concentration of antibacterial agents and their accessibility to bacteria solutions are two of the governing factors for effective bacterial inactivation by antibacterial media. Both factors were controlled by the physical/chemical characteristics of base material, antibacterial chemicals and modification conditions.
Table 3 shows concentration of antimicrobial agents on newly prepared filter media. The net increase in content of Fe3+, Zn2+, Cu2+ in treated zeolite (as opposed to untreated
Conclusions
Removal of pathogens from stormwater discharges is of high importance for effective stormwater harvesting. Filter media, after modification with antibacterial agents, may potentially improve pathogen removal efficiency of current biofilters. Benefits of using antibacterial media include instantaneous improved bacterial removal during rain events and inhibition of bacterial survival during dry events. The findings in this study can be summarised thus:
Among the 15 types of antibacterial media
Acknowledgements
CRC for Water Sensitive Cities are acknowledged for supporting this study. Dr. Walid Daoud and Lorena Lopez-Vanegas are greatly acknowledged for their time and effort in preparing TiO2 modified media. Louisa John-Krol is greatly acknowledged for editing grammatical errors in English. The staff from Civil Engineering, Monash University, especially Richard Williamson, Frank Winston, Peter Kolotelo, Christelle Schang, Javier Neira and Peter Poelsma, are gratefully acknowledged for their active
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