Journal of Hazardous Materials 271 (2014) 73–81

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Removal of E. coli from urban stormwater using antimicrobial-modified filter media Ya L. Li a,b,∗ , Ana Deletic a,b , David T. McCarthy a,b a Environmental and Public Health Microbiology Lab (EPHM LAB), Monash Water for Liveability, Department of Civil Engineering, Monash University, Melbourne, Vic 3800, Australia b CRC for Water Sensitive Cities, Melbourne, Vic 3800, Australia

h i g h l i g h t s • • • • •

15 antibacterial filter media were prepared for enhanced bacterial removal from urban stormwater. Their performances were evaluated over 24 weeks under typical stormwater operational conditions. Filter media modified with copper compounds exhibited robust antibacterial efficiency. Filter media modified with Cu2+ and Cu(OH)2 showed effective bacteria removal during wet events. Filter media modified with Cu(OH)2 showed very good stability in stormwater.

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Article history: Received 5 November 2013 Received in revised form 15 January 2014 Accepted 26 January 2014 Available online 12 February 2014 Keywords: Stormwater Antibacterial Filtration Pathogen Water sensitive urban design

a b s t r a c t Stormwater filters featuring traditional sand filter media cannot reliably treat indicator bacteria for stormwater harvesting. In this work, copper-modified zeolite and granular activated carbon (GAC) were prepared through Cu2+ impregnation and in situ Cu(OH)2 precipitation. Their antibacterial properties and stability in natural stormwater were studied in gravity-fed columns for 24 weeks, under typical stormwater operational conditions. 11 types of other filter media, prepared using zinc, iron, titanium and quaternary ammonium salts as antibacterial agents, were tested in parallel by way of comparison. Cu2+ -immobilised zeolite and Cu(OH)2 -coated GAC yielded an estimated 2-log reduction of E. coli within 40 min with the presence of other native microbial communities in natural stormwater. Even at high flow velocity (effective contact time of 4.5 min), both media demonstrated 0.8 log removal. Both media and Cu2+ -treated GAC showed effective inactivation of the removed E. coli during dry periods. Copper leaching from Cu(OH)2 -coated GAC was found to be below the NHMRC specified drinking water standard, while that from Cu2+ -immobilised zeolite varied with the salinity in stormwater. These findings could provide useful information for further development of passive stormwater harvesting systems. © 2014 Elsevier B.V. All rights reserved.

1. 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 effec-

∗ Corresponding author at: Monash Water for Liveability, Department of Civil Engineering, Monash University, Melbourne, Vic 3800, Australia. Tel.: +61 399056202; fax: +61 399054944. E-mail addresses: [email protected] (Y.L. Li), [email protected] (A. Deletic), [email protected] (D.T. McCarthy). http://dx.doi.org/10.1016/j.jhazmat.2014.01.057 0304-3894/© 2014 Elsevier B.V. All rights reserved.

tively by means of biological uptake, straining and adsorption [1–3]. However, field and laboratory investigations have shown that their effluent seldom meets bacterial indicator targets for outdoor irrigation [4–7]. 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

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Y.L. Li et al. / Journal of Hazardous Materials 271 (2014) 73–81

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/Cu2 O-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:

• Develop simple yet scalable processes to modify GAC and zeolite with Cu(OH)2 , CuO, and Cu2+ producing 4 types of copper modified media; • 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; • Investigate the main bacterial removal mechanisms subjected to stormwater conditions; • 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).

2. Experiments 2.1. 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]. Natural zeolite (particle size 0.3–0.6 mm), Zeolite Australia; and granular activated carbon (GAC) (particle size 0.3–0.6 mm), Activated Carbon Technologies Pty Ltd, constituted base media. The basic physicochemical properties of natural zeolite and GAC were listed in [21,22]. Modification of zeolite by Cu2+ , Fe3+ , Zn2+ : Zeolite was mixed with 2 M NaCl for 72 h to produce Na-zeolite. NaCl solution was replaced every 24 h. After being washed with deionised (DI) water, Na-zeolite was mixed with 0.015 M CuCl2 solution (metal content 5% by weight of zeolite) for 48 h. The CuCl2 solution was replaced every 24 h. This sample was then washed and dried at 105 ◦ C overnight to produce Cu2+ modified zeolite, denoted as Cu-Z. Following a similar procedure, Fe-Z and Zn-Z were prepared. Zn/Cu/Fe-Z was concocted by impregnating Na-zeolite in 0.015 M solutions of ZnSO4 , CuCl2 and FeCl3 for 24 h each sequentially. All mixing was under agitation from a rotary platform at 150 rpm. Modification of zeolite by CuO: 3 g CuCl2 was added into aqueous slurry of zeolite (80 g zeolite in 300 mL DI water). The mixture was rotated gently for 1 h, after which the pH of the slurry was adjusted to 8 with 2 M NaOH. After mixing for 1 h, the particles were separated, washed with DI water and dried at 60 ◦ C overnight. The dry media was then heat-treated at a rate of 5 ◦ C/min to 400 ◦ C and maintained at that temperature for 1 h before cooling by means of a temperature controlled programmer. After cooling, the CuO impregnated particles were washed five times with water. The washed sample was dried at 105 ◦ C to attain the final product CuOZ. Modification of GAC by Cu2+ : 10 g GAC was mixed with 500 mL of 0.015 M CuCl2. The slurry was gently rotated for 24 h. Particles were then separated, washed and dried at 105 ◦ C for use (Cu-G). Modification of GAC by Cu(OH)2 : The preparation procedure was similar to that of CuO-Z, excepting a lower heat treatment temperature of 180 ◦ C for preparing Cu(OH)2 -G. Modification by other antibacterial media: Zn(OH)2 , Fe(OH)3 , TiO2 , QAC, and Si-QAC modified media were prepared following the aforementioned methods [12,23–26]. Prepared media were denoted as Zn(OH)2 -Z, Zn(OH)2 -G, Fe(OH)3 -Z, TiO2 -Z, TiO2 -G, QACZ, SiQAC-Z and SiQAC-G.

Table 1 Combinations of base media with antibacterial agents. Antibacterial agent

Nil Metal ions Metal hydroxide Metal oxide Quatsa a b

Modified filter media Zeolite-based media

GAC-based media

Z0 Cu-Z, Zn-Z, Fe-Z, Zn/Cu/Fe-Zb Fe(OH)3 -Zb , Zn(OH)2 -Zb CuO-Zb , TiO2 -Zb SiQAC-Zb , QAC-Z

G0 Cu-Gb Zn(OH)2 -G, Cu(OH)2 -Gb TiO2 -Gb SiQAC-Gb

Quats – Quaternary ammonium salts. Media have never been tested for water treatment, while all the listed media have never been tested for stormwater applications.

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In total, zeolite and GAC were modified with 10 types of antibacterial materials, while only 15 combinations were examined (Table 1) due to resource limitation. A greater number of zeolite configurations were examined due to low cost and high cation exchange capacities [27]. GAC was investigated to immobilise Cu2+ , Cu(OH)2 etc. due to its vast surface area, while some combinations, for example G-CuO could adduce reported work [18]. Untreated zeolite (Z0) and GAC (G0) were used as controls. The metal content of antibacterial media was measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) in a NATA-accredited laboratory. The morphology and surface elements of antibacterial media were examined using Scanning Electron Microscope (SEM, JEOL JSM-7001F) equipped with Energydispersive X-ray spectroscopy (EDS). 2.2. Test water Natural stormwater (SW) was collected from outlets of two stormwater sedimentation basins, Melbourne (details in Table 2) and utilised on the same day. The water collected during wet weather operated in its raw state as a natural source of E. coli, whereas water collected during dry weather was spiked with labstrain E. coli (ATCC#11775) (ALS Environmental, Melbourne) as a source of mixture comprising natural E. coli and lab-strain E. coli. The test water was stored in a 60 L water tank with continuous stirring during dosing, to avoid sedimentation. DI water was used as microbial contamination-free test water to differentiate bacterial survival and detachment within filter media. The quality of test water – measured using a multi-parameter water quality probe (U50, Horiba Ltd., Japan) – reflected the variability of natural stormwater in terms of E. coli concentration, presence of other native microbial communities (as in natural stormwater), salinity, turbidity and temperature [5] (Table 2). 2.3. Filtration tests Stability and bacterial removal efficiency of the 15 types of modified media and two untreated controls were testedin columns at

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two stages: Stage 1 pure media assessment to understand performance and several processes of each individual medium under stormwater operational conditions and differences between media types. This was to identify promising antibacterial media for stormwater treatment; Stage 2 sand filter application is to apply the promising media in stormwater sand filters which are common in stormwater treatment [1–3]. The experimental arrangement and operational conditions are summarised in Table 2. Experimental set up – Stage 1: The filter media were packed into columns in accordance with the method previously described [17]: three replicates for all media types except for Cu(OH)2 -G, which had one replicate, due to experimental difficulties. Once packed, all columns were flushed using 18 × 80 mL pulses DI water to remove the fine particles produced during packing and to allow the media to settle. Experimental set up – Stage 2: After completion of Stage 1 experiments, 3 best performing filter media and their control media from Stage 1 were emptied from columns and dried at 60 ◦ C for 24 h. The dried media was then mixed with fine sand (particle size 0.15–0.30 mm) in a ratio of 1:1.1 by volume, repacked into initial columns as per Stage 1. Thereby depth of media was increased from 100 mm to 210 mm, yet actual contact time with modified media remained unchanged. The columns were then flushed using 25 × 80 mL pulses DI water. As shown in Table 2, Stage 1 and 2 columns were dosed with test water intermittently over 24 weeks, covering 10 simulated rain events and 9 dry periods between rain events of varied duration 1–4 weeks. During rain events, columns were dosed with test water by maintaining a constant head, whereas during dry periods, they received no inflow. Columns’ outlets were restricted to attain the desired superficial flow velocity (Table 2): 860 mm/h in Event 1, 720 mm/h in Event 7 and 86 mm/h in other Events, translating to effective contact time with modified media of 4 min, 4.5 min and 40 min, respectively. This mimicked a hydraulic loading range typical for stormwater filters and biofilters. 2500 mL stormwater was applied to each column during Event 1 (65 pore volumes of modified media). 350 mL of water (9 pore volumes) was dosed during other Events, which is equivalent to the inflow into a filter sized at 2.5% of its impervious catchment area during a 14 mm rain event

Table 2 Experimental setup and operational conditions during filtration test. (a) Variables Filter media E. coli inflow concentration Column design (b) Experimental set-up

Zeolite based media (10 types + control); GAC based media (5 types + control) Natural stormwater (SW); microbial-contamination free water (DI water) Stage 1 pure media assessment; Stage 2 sand filter application

(c) Experimental conditions Stage 1 pure media assessment Rain event Time (week) Test water E. coli (MPN/100 mL) Flow velocity (mm/h) EC (␮S/cm) T (◦ C) pH Turbidity (NTU)

1 1 SW 2965 860 626 21 7.1 30

2 3 DI