Journal of Hazardous Materials 273 (2014) 222–230

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Stable copper-zeolite filter media for bacteria removal in stormwater Ya L. Li a,b,∗ , David T. McCarthy a,b , Ana Deletic 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 • • • • •

Six new filter media were developed by calcination or Cu(OH)2 coating on Cu2+ -treated zeolite (ZCu). All new media showed more than 97% reduction in copper leaching compared to ZCu. Cu(OH)2 -coated ZCu showed better and more stable bacteria removal than ZCu. New media maintained bacterial inactivation efficiency during drying event. Applying new media in sand filter was successful in terms of bacterial removal and inactivation.

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Article history: Received 11 December 2013 Received in revised form 28 February 2014 Accepted 16 March 2014 Available online 28 March 2014 Keywords: Stormwater biofilter Pathogen treatment Antimicrobial filter media Copper E. coli

a b s t r a c t Cu2+ -exchanged zeolite (ZCu) as antibacterial media shows great potential for bacteria removal from stormwater, but its stability in high salinity water needs attention. In this study, stable antibacterial media were developed by modifying ZCu through calcination and in situ Cu(OH)2 coating. Their stability and Escherichia coli removal efficiency along with impact of salinity were examined in gravity-fed columns. While copper leaching from ZCu was 20 mg/L in test water of salinity 250 ␮S/cm, it was reduced by over 97% through Cu(OH)2 coating and/or calcination. ZCu coated with Cu(OH)2 followed by heat treatment at 180 ◦ C (ZCuCuO180) exhibited more consistent E. coli removal (1.7–2.7 log) than ZCu (1.2–3.3 log) in test water of varied salinity but constant contact time 22 min. ZCu calcined at 400 ◦ C (ZCu400) effectively inactivated removed bacteria during 24 h drying period. In the presence of native microbial communities, new sand filters, particularly those having ZCu400 at the top to inactivate bacteria during drying periods and ZCuCuO180 midway to capture and inactivate microbes during wet events, provided the best bacterial removal (1.7 log, contact time 9 min). Copper leaching from this design was 9 ␮g/L, well below long-term irrigation standard of 200 ␮g/L. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Stormwater, via harvesting, is gaining prominence as an alternative water source to ensure reliable water supplies for cities and towns [1]. Stormwater filters and biofilters are becoming popular for stormwater harvesting [2]. They are gravity-fed filter beds, vegetated or non-vegetated, removing microbes mainly by means of sedimentation, straining, adsorption and die-off. However, distinct

∗ Corresponding author at: Environmental and Public Health Microbiology Lab (EPHM LAB), 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] (D.T. McCarthy), [email protected] (A. Deletic). http://dx.doi.org/10.1016/j.jhazmat.2014.03.036 0304-3894/© 2014 Elsevier B.V. All rights reserved.

characteristics of stormwater pose challenges for filter performance, since stormwater biofilters are often located within urban environments, thus exposed to highly variable hydraulic and pollutant loadings, intermittent wetting and drying conditions, and high seasonal variations [3,4]. Limited field and laboratory investigations have shown that their performances are highly variable (ranging from good removal to net leaching), and effluent water quality can hardly meet requirements even for the lowest level of stormwater reuse, i.e. non-restricted irrigation [5–8]. Inadequate microbial removal capacity of sand media used in stormwater filters and biofilters plays a key role in these observations, while survival/growth and remobilisation of microbes from the media contribute to net leaching. Filter performance is further worsened by the highly variable and intermittent nature of stormwater runoff, with the latter exposing filters to varying duration of dry periods [8,9]. Antibacterial media exert bactericidal

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effects through contact with bacterial solution or slow release of antibacterial agents [10–12]. A few studies examined the bactericidal effects of immobilising heavy metals (e.g. Ag, Cu, and Zn) on a range of media (such as activated carbon, zeolite etc.) for wastewater treatment [10–13] where operational conditions of filter media including inflow concentrations, temperature, salinity, and hydraulic loading are relatively stable. Effectiveness of these antibacterial media was further justified for stormwater treatment. 15 types of Cu, Zn, Fe, Ti, and quaternary ammonium salts modified media were developed and examined for 5 months subjected to inflow concentrations (about 1–48,000 MPN Escherichia coli/100 mL), temperature (11 to 21 ◦ C), hydraulic loading (14–100 mm rain events), and drying periods of varying length (1–4 weeks) [14]. It was found that Cu2+ -exchanged zeolite (ZCu) was effective to reduce bacterial level by 2 log consistently. However, high copper concentrations in the outflow (30–40 mg/L) were observed. Excessive leaching of Cu2+ is mainly due to ion-exchange with other cations in solution. This level of copper concentration is much higher than the stormwater harvesting guidelines (i.e. 2 mg/L in drinking water, and 0.2 mg/L in water for irrigation) [6,15], posing significant health issues in humans and toxicity to plants. The aim of this study is to develop stable Cu-zeolite media for effective bacterial removal from stormwater by sand filters, specifically with three main objectives: • to prepare stable yet effective Cu-zeolite media by calcination of ZCu or in situ Cu(OH)2 coating on ZCu; • to investigate the impact of salinity on the stability and E. coli removal performance of the antibacterial media; and • to design and investigate new sand filters with the stable Cuzeolite media for improved bacterial retention and inactivation. 2. Materials and methods 2.1. Materials Natural zeolite (Escott Zeolite from Zeolite Australia, basic physicochemical properties listed in [16]), was used as base media comprising three size fractions: non-graded 0.1–0.6 mm (Z0), graded 0.3–0.6 mm (Z00.3 ) and 0.1–0.3 mm (Z00.1 ). They were washed thrice with 10 volumes of tap water, dried at 105 ◦ C overnight then stored in a dry container for use. Washed sand, washed coarse sand, gravel of size 0.075–0.6 mm, 1.0–2.0 mm and 2.0–3.4 mm respectively were used, Daisy Garden Supplies, Melbourne. The former two were used directly, while gravel was washed 5 times with 10 volumes of tap water and dried in air. The chemicals included NaCl, CuCl2 , NaOH and ethylenediaminetetraacetic acid disodium salt (EDTA) (all from Merck Chemicals). 2.2. Preparation and characterisation of stable antibacterial media 2.2.1. Modification of zeolite by copper chloride Zeolite of three size fractions (Z0, Z00.3 , Z00.1 ) was treated by CuCl2 following the method described in [14] with a slight change in NaCl treatment time: 48 h was used in this study. The so prepared Cu2+ -exchanged zeolite was denoted as ZCu, ZCu0.3 , and ZCu0.1 respectively. 2.2.2. Heat treatment of copper-treated zeolite Heat treatment has been reported to reduce the elution of metal ions into contacting liquid [17]. To test possible improvements to Cu2+ -exchanged zeolite, as well as the impact of different temperatures on the phenomena, ZCu0.3 was heated using LAB Muffle

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Furnace CEMLL at a rate of 5 ◦ C/min to a set temperature, which was then maintained for 2 h before cooling naturally to room temperature. The following set temperatures were trialled: 400 ◦ C, 600 ◦ C, and 800 ◦ C, and the produced media were denoted as ZCu4000.3 , ZCu6000.3 and ZCu8000.3 , respectively. ZCu and ZCu0.1 were treated in a similar way at temperatures 400 ◦ C and 800 ◦ C respectively producing ZCu400 and ZCu8000.1 . 2.2.3. Modification of copper-treated zeolite by CuO An attempt was made to further reduce metal elution using CuO coating. ZCu0.3 was gently stirred in 1 wt% CuCl2 for 10 min; then the pH of the slurry was slowly adjusted to 7 using 2 M NaOH. The mixture was stirred for another 2 h and left still overnight. The media was separated from the mixture and washed once, before being dried at 65 ◦ C overnight. The dry media was then heat-treated at 400 ◦ C following a procedure similar to preparing ZCu4000.3 . After cooling, the media was washed five times with DI water then dried at 105 ◦ C overnight. The produced media was denoted as ZCuCuO4000.3 . ZCu and ZCu0.3 were treated similarly but at 180 ◦ C producing ZCuCuO180 and ZCuCuO1800.3 respectively. In total, nine types of antibacterial media were prepared including: • Graded 0.1–0.3 mm: ZCu8000.1 ; • Graded 0.3–0.6 mm: ZCu0.3 , ZCu8000.3 , ZCu6000.3 , ZCu4000.3 , ZCuCuO4000.3 , ZCuCuO1800.3 ; • Non-graded 0.1–0.6 mm: ZCuCuO180, ZCu400. The seven types of graded media consisting of two size fractions were tested in columns to investigate their stability and bacterial removal efficiency in test water of varied salinity (Section 2.3 and Table 1), and untreated zeolite (Z00.1 , Z00.3 ) were used as controls. The two non-graded antibacterial media were combined with washed sand in a variety of arrangements to investigate the promising layout for bacterial removal (Section 2.4 and Table 1). The copper content of antibacterial media was measured using inductively coupled plasma mass spectrometry (ICP-MS) in a NATA-accredited laboratory. 2.3. Performance evaluation of antibacterial media at various salinity-pure media assessment The stability and bacterial removal efficiency of seven types of antibacterial media and two untreated controls (Z00.3 and Z00.1 ) were tested in columns (three replicates for each media). Table 1 summarises the experimental setting and operational conditions. The filter media were packed into columns (18 mm in diameter, 340 mm in length, with a fine screen mesh placed at the bottom, and sand-blasted interior walls to prevent edge effects) in accordance with the aforementioned method [14]. In brief, moist filter media was added incrementally through the top of each column, which was partially filled with DI water. After addition of filter media, the latter was thoroughly packed by dropping a stainless steel rod (ID 7 mm, weight 110 g) from 10 mm height above the top media surface to remove any trapped air bubbles. The columns were flushed using nine pulses of 70 mL DI water to allow media to settle and remove fine granules produced during the packing. Each pulse was applied after the columns were completely drained. Thereafter, outlets of columns were restricted to maintain superficial velocity of 236 mm/h, translating to contact time between water and media of 22 min. To condition the media, the filter columns were dosed with 29 pulses of 70 mL DI water spiked by NaCl to achieve salinity 250 ␮S/cm. The water was applied in pulses to mimic the intermittent nature of stormwater

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Table 1 Experimental setup and operational conditions during filtration test.

a

DI water spiked with lab-strain E. coli (ATCC#11775) and NaCl. Refer to Section 2.4 for details of each design. c Raw sewage-spiked pond water. d Pond water. e Runs 3 and 4 are discrete samplings during a challenging dosing event while all other runs are composite samplings during regular event. b

runoff. Each pulse of 70 mL herein was equivalent to the inflow into a filter sized at 2% of its impervious catchment area during one average rain event in Melbourne (typical design of stormwater biofilters [18]). The salinity 250 ␮S/cm was chosen, as it represents 75th percentile salinity levels found in urban runoff in six Melbourne catchments (unpublished data). This level of salinity is also consistent with the total dissolved solid level in urban runoff [6,19]. Multiple samples of filtered water were collected and analysed for total copper concentration using ICP-MS. The columns were then exposed to test water over six sampling runs, where a pulse of 70 mL of test water was applied during each run to mimic six rain events, as explained in Table 1. The test water during the first five Runs was bacterial contaminated prepared by dissolving certain amount of NaCl in DI water to achieve the targeted salinity, which was then spiked with lab-strain E. coli ATCC#11775 collected from ALS Environmental, Melbourne. DI water only was applied during the sixth Run, which was to investigate bacterial survival and detachment within various media. During each sampling run, duplicate composite inflow samples were taken, while the entire outflow from each column was collected. The sterilised bottles, used for sample collection, were also pre-treated with 0.01 M EDTA in a ratio of 0.3:100 sample volume, in order to inactivate metal ions. In this way, the impact of post filtration inactivation was eliminated. E. coli concentrations in all inflow and outflow samples were analysed using ColilertTM method (IDEXX-Laboratories, 2007). Total metal concentrations in these samples were measured by ICP-MS in a NATA-accredited laboratory.

2.4. Evaluation of new sand filter designs Sand filter columns (ID 30 mm) were constructed with a fine screen mesh placed at the bottom, filled with 40 cm of filter media and 5 cm of coarse gravel, leaving 20 cm freeboard for extended detention of water (Table 1). In addition to washed sand only columns (denoted as S) as controls (two replicates), four new filter designs (T, TT, MM, TM) were constructed (three replicates) by layering ZCu400 and ZCuCuO180 into washed sand. Layout of filter media (bottom to top) in each design was as follows, as per the diagram in Table 1: • S – 400 mm sand; • T – 250 mm sand, 100 mm sand/ZCu400 mix (1:1), 50 mm ZCu400; • TT – 200 mm sand, 50 mm Z0, 100 mm sand/ZCu400/ZCuCuO180 mix (2:1:1), 50 mm ZCu400/ZCuCuO180 mix (1:1); • MM – 100 mm sand, 50 mm Z0, 100 mm ZCu400/ZCuCuO180 mix (1:1), 150 mm sand; • TM – 150 mm sand, 50 mm Z0, 50 mm ZCuCuO180, 100 mm sand, 50 mm ZCu400. The filter columns were constructed in segments following the procedure reported in [20]: 50 mm of moist media was uniformly spread across cross-section and evenly compacted; another 50 mm layer was added, and so on. Columns with new media, i.e. T, TT, MM, TM, had unrestricted filtration velocities of 850 mm/h, while the S columns manifest velocity of 640 mm/h.

Table 2 Characteristics and performance of seven types of antibacterial media regarding their stability and E. coli removal in test water. Pollutants

Copper

E. coli

DI water with various level of salinity (␮S/cm) 5 250 500

Bacterial contaminated waterb 10,800 (8200, 13,200)

DI waterc