Stable copper-zeolite filter media for bacteria removal in stormwater

Journal of Hazardous Materials 273 (2014) 222–230

Contents lists available at ScienceDirect

Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

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.

a r t i c l e

i n f o

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

Y.L. Li et al. / Journal of Hazardous Materials 273 (2014) 222–230

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

223

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

224

Y.L. Li et al. / Journal of Hazardous Materials 273 (2014) 222–230

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 <1

Outflow

Concentration (␮g/L)

Log removal Median (10th, 90th)

Number of measurements/samples Filter media type

3

7

3

Concentration (MPN/100 mL) Median (10th, 90th) 15

15

Concentration (MPN/100 mL) Median (min, max) 3

Copper (mg/g media) <0.005

NA

NA

1

6880 (2540, 11,700)

0.20 (−0.03, 0.63)

522 (135, 1724)

9.0

210 (130, 350)

20 (12, 26) mg/L

36 (36, 37) mg/L

81 (5, 940)

2.13 (1.10, 3.34)

7 (3, 10)

0.58

4 (2, 7)

2 (0, 7)

3 (2, 4)

7270 (4350, 11,500)

0.17 (−0.02, 0.40)

191

5.5

42 (16, 68)

103 (20, 150)

447 (320, 560)

4610 (1210, 7440)

0.37 (0.16, 0.95)

10 (1, 14)

6.9

21 (9, 29)

178 (41, 270)

1080 (940, 1200)

4610 (951, 7700)

0.37 (0.15, 1.08)

7 (1, 10)

NA

590 (210, 1200)

54 (15, 150)

50 (50, 50)

6870 (4750, 10,800)

0.20 (0.00, 0.40)

190

6.3

43 (27, 58)

693 (400, 920)

2633 (2200, 3000)

98 (29, 242)

2.04 (1.66, 2.67)

3 (1, 7)

<0.005

NA

NA

NA

2480 (659, 6430)

0.64 (0.23, 1.23)

3070 (1370, 3490)

6.4

62 (17, 120)

109 (21, 210)

390 (280, 570)

1900 (215, 4530)

0.76 (0.38, 1.71)

35 (24, 36)

<10 (median)

>1.5 (median)

<10 (median)

ZCu0.3 ZCu8000.3 ZCu6000.3 ZCu4000.3 ZCuCuO4000.3 ZCuCuO1800.3 Z00.1 ZCu8000.1 Drinking water standard [15] Long-term irrigation (100 years) standard [24] Short-term irrigation (20 years) standard [24] Non-restricted irrigation standard [6] Primary contact recreational water use [25] a b c

Description of the filter media Untreated zeolite (0.3–0.6 mm) Cu2+ exchanged zeolite ZCu0.3 calcined at 800 ◦ C ZCu0.3 calcined at 600 ◦ C ZCu0.3 calcined at 400 ◦ C ZCu0.3 coated with Cu(OH)2 at 400 ◦ C ZCu0.3 coated with Cu(OH)2 at 180 ◦ C Untreated zeolite (0.1–0.3 mm) ZCu0.1 calcined at 800 ◦ C

Mean (min, max)

2000 200 (95th percentile) 5000 (95th percentile)

≤150 (median)

Y.L. Li et al. / Journal of Hazardous Materials 273 (2014) 222–230

Inflow

Z00.3

a

≤150 (median)

Test water was prepared by adjusting DI water with NaCl to achieve the required salinity of 5, 250, 500 ␮S/cm; NA – no data available. Test water – DI water spiked with lab-strain E. coli (ATCC#11775) and mixed with NaCl to achieve the required salinity. DI water only.

225

226

Y.L. Li et al. / Journal of Hazardous Materials 273 (2014) 222–230

After DI water flushing, consisting of 8× 160 mL water pulses, the columns were exposed to pulses of test water over eight sampling runs, as per Table 1. Two types of test water included (i) sewage-spiked pond water – pond water collected from a large stormwater pond in Clayton, Melbourne spiked by NaCl and raw sewage (from Pakehnam Treatment Plant, Melbourne) to achieve the required salinity and bacterial level; (ii) pond water – the aforementioned pond water spiked only with NaCl but not raw sewage. During each sampling run, 350 mL test water was applied to each column, equivalent to 2 pore volumes of total media volume or 15 pore volumes of the new media ZCuCuO180. Exceptions were Runs 3 and 4, which were discrete samplings during a challenging dosing event: 700 mL test water was applied to each column and the first 350 mL of effluent was collected and designated as Run 3 outflow sample; the second 350 mL as Run 4 sample. The column outlets were unrestricted during the first five Runs, whereas they were restricted during Runs 6–8, achieving superficial velocity of 160 mm/h. The former translated to contact time between ZCuCuO180 media (depth 5 cm) and test water of 2 min; the latter 9 min. Composite inflow and the entire outflow from each column were collected and analysed for E. coli and Cu concentrations, as in the pure media tests.

3. Results and discussion 3.1. Characterisation of antibacterial media Table 2 summarises the initial copper content of freshly prepared media and their stability in water of various salinity. Through ion-exchange [22], around 9 mg of copper was loaded into one gram of natural zeolite (ZCu0.3 ). After calcination, however, slightly less copper was detected especially in media treated at high temperatures, e.g. 5.5 mg copper per gram ZCu6000.3 and 6.9 mg copper per gram ZCu4000.3 . During calcination at high temperatures, copper could be lost due to evaporation. However, dehydration of zeolite particles allowing Cu2+ migrating into small cages (rendering them unavailable for re-exchange) is a stronger reason for the observed low detection of copper content [17,23]. The much higher copper content detected in fine zeolite ZCu8000.1 than in ZCu8000.3 also demonstrated the in-depth distribution of Cu2+ in the relatively coarse zeolite particles. Slightly lower copper content on ZCuCuO1800.3 than that on ZCu0.3 could be due to copper desorption during coating and washing. Although the media was labelled with CuO, the actual chemical composition proved to be mainly Cu(OH)2 in a previous study [14], which is reasonable since the thermal decomposition of Cu(OH)2 to CuO occurs at temperatures above 185 ◦ C.

2.5. Inactivation of E. coli by Cu2+ in aqueous phase

3.2. Stability of antibacterial media

Inactivation of E. coli by Cu2+ in an aqueous phase was investigated in a batch system. A conical tube containing 50 mL of test water (DI water mixed with NaCl and lab-strain E. coli ATCC#11775 achieving 260 ␮S/cm and E. coli concentration around 104ˆ MPN/100 mL) was spiked with CuCl2 (final Cu2+ concentration 0.5 mg/L). The tube was agitated in a shaking incubator at 20 ◦ C at a speed of 250 rpm. At a specified time, duplicate samples of 2 mL were collected from the tube into microcentrifuge tubes containing 6.7 ␮l 0.01 M EDTA and mixed thoroughly. E. coli concentration was then analysed using ColilertTM method. Moreover, a control experiment in the absence of Cu2+ was performed under the same conditions.

As shown in Table 2, all filter media including ZCu0.3 demonstrated pleasing stability in contact with DI water (salinity 5 ␮S/cm), with total copper in outflows meeting long-term irrigation standard [24]. One exception was ZCuCuO4000.3 showing slightly higher copper leaching, which might be mainly attributed to fine CuO particles produced during packing. When the antibacterial media were exposed to water with salinity of 250 ␮S/cm, significantly higher copper concentration was detected. ZCu0.3 , particularly, showed 2 orders higher Cu leaching (20 mg/L), which was mainly due to ion-exchange of impregnated Cu2+ with Na+ in water. However, coating a layer of Cu(OH)2 at 180 ◦ C on ZCu0.3 (ZCuCuO1800.3 ) caused 97% reduction in copper leaching: 690 ␮g/L in outflow, which is well below drinking water guidelines [15]. The layer of Cu(OH)2 , having negligible solubility in water, i.e. Ksp 2.20 × 10−20 equivalent to 11 ␮g Cu2+ /L), may act as a barrier against elution of Cu2+ . In addition, migration of OH− into zeolite during Cu(OH)2 coating might cause hydrolysis of previously loaded Cu2+ , resulting in irreversible exchange. Residual NaOH in precipitates might also play a role in stopping Cu2+ leaching. It could also be due to thermal treatment causing phase transition. A control experiment proved that this last hypothesis was not significant: ZCu0.3 calcined at 180 ◦ C showed a similar level of copper leaching to ZCu0.3 (data not shown). However, calcination of ZCu0.3 at temperatures above 400 ◦ C reduced copper leaching by 99%, although the maximum copper concentration in outflow (270 ␮g/L) was still beyond long-term irrigation trigger value [24]. When the salinity in solution rose from 250 ␮S/cm to 500 ␮S/cm, the copper leaching from ZCu0.3 further increased to 36 mg/L, while that from ZCu4000.3 and ZCuCuo1800.3 increased by three to five times. A linear correlation between copper leaching and salinity in test water was identified for ZCu0.3 , ZCu4000.3 , and ZCuCuo1800.3 (p ≤ 0.001) (Fig. 1), indicating that the immobilised Cu2+ in these media was still exchangeable.

2.6. Data analysis Where the E. coli outflow concentration was lower than detection limit, half of the latter was taken as the concentration for statistical analysis. To assess the E. coli removal by pure antibacterial media, the median inflow and outflow concentrations across runs grouped by inflow types were summarised. Outflow concentrations during bacterial contaminated water dosing events were statistically analysed to understand differences between media types. Data was tested for normality using Kolmogorov–Smirnov test. One-way ANOVA analysis along with post hoc tests – Tukey was performed to test the significance of filter media types on E. coli removal. Although normality could not be confirmed for ZCu0.3 probably because 20% of measurements were below detection, it was considered that the high significance of results was unaffected since p value was very low (<0.001) [21]. Log removal values were calculated based on log concentration differences between inflow and outflow samples during each run, presented as boxplots. For selected filter media, linear regression tests were utilised for correlation analysis. For example, log removal rates versus copper leaching, and copper leaching versus salinity in test water, were analysed for ZCu0.3 , ZCu4000.3 and ZCuCuO1800.3 . The E. coli concentrations in the inflow and outflow samples collected from new filter design study were plotted over sampling runs.

3.3. Overall E. coli removal by antibacterial media The antibacterial media were assessed in columns based on conditions specified in Table 1. Median E. coli inflow and outflow concentrations for each media type are summarised in Table 2,

Y.L. Li et al. / Journal of Hazardous Materials 273 (2014) 222–230

227

Fig. 1. Correlation between Cu leaching and salinity level in water (circle solid line), E. coli removal rate and Cu leaching (triangle dashed line) (Note: filter media ZCu, ZCu400 and ZCuCuO180 have grain size 0.3–0.6 mm).

where the data are classified based on two inflow types. For bacterial contaminated test water, the median E. coli inflow concentration of 10,800 MPN/100 mL was within the range of untreated urban stormwater quality [6]. It was found that filter media type had a significant influence on E. coli removal efficiency (p < 0.001, ANOVA). Untreated zeolite Z00.3 showed E. coli outflow concentration 6880 MPN/100 mL (0.2 log removal). Among the antibacterial media, ZCu0.3 and ZCuCuO1800.3 significantly outperformed the other media with similar grain size (p < 0.001; Post Hoc tests). Their outflow E. coli concentrations, 81 and 98 MPN/100 mL respectively, met the primary contact recreational water quality (≤150 E. coli/100 mL) [25]. In addition, performance of both media met the non-restricted irrigation standard regarding log removal (>1.5

log), but not the same standard regarding E. coli concentration [6]. Reducing particle size improved performance for all media types; for example, Z00.1 showed higher removal (0.64 log) than Z00.3 ; ZCu8000.1 showed significantly higher removal (0.9 log) than ZCu8000.3 (p < 0.001; post hoc tests). Reduced particle size suggests enlarged surface area, reduced pore size, thus prominent removal through enhanced adsorption and straining [26,27]. Thereby, ZCuCuO180 prepared using fine zeolite (0.1–0.3 mm) seems a promising candidate to further improve water quality. In addition to enhanced E. coli removal by antibacterial media during rain events, adequate bacterial inactivation during antecedent drying periods is desirable since removed bacteria could survive/reproduce in biofilters and cause leaching during

Fig. 2. Range of E. coli logarithmic reduction (top) and copper leaching (bottom) during pure media assessment grouped by filter media types for sampling runs 1–5 (Note: All filter media have grain size 0.3–0.6 mm except for Z0F and ZCu800F which have grain size 0.1–0.3 mm; three replicates for each media type).

228

Y.L. Li et al. / Journal of Hazardous Materials 273 (2014) 222–230

Fig. 3. Range of E. coli concentrations in inflow and outflow samples grouped by sand filter designs for all eight sampling runs (Note: shaded runs using sewage-spiked pond water and open runs using pond water without sewage; two replicates for inflow samples and S columns while three replicates for other designs).

following rain events, as found by Chandrasena et al. [5,8]. During Run 6, the columns were dosed with DI-only water (E. coli concentration <1 MPN/100 mL). Effluent E. coli concentrations are summarised in Table 2, representing the leached E. coli leaving the columns after a short drying period of 24 h. A high number of E. coli manifest in the outflow from control columns Z00.3 , while only 3 MPN/100 mL E. coli emerged from ZCuCuO1800.3 . Less than 10 MPN/100 mL E. coli came out of ZCu6000.3 and ZCu4000.3 , illustrating their superior antibacterial efficiency despite inadequate instantaneous removal efficiency during filtration. The elution of E. coli from Z00.1 was 3070 MPN/100 mL, indicating that although filter media of finer grain size effectively removed bacteria, their lack of inactivation capacity allowed bacteria to survive and remobilise under intermittent flow patterns. ZCu8000.1 , on the contrary, effectively inactivated the removed bacteria: only 35 MPN/100 mL E. coli leached out.

3.4. Effect of salinity on antibacterial media performance The performance changes of antibacterial media over sampling runs and levels of salinity in test water are graphed in Fig. 2. Z00.3 showed minimal removal (or even net leaching) of bacteria. Furthermore, the bacterial removal by Z00.3 varied with salinity in test water, and highest removal was recorded during Run 3 when test water had the highest salinity (500 ␮S/cm). The removal by Z00.3 was mainly attributed to adsorption through electrostatic force. Both zeolite and E. coli cell have negatively charged surfaces at neutral pH, thereby the interactions between them are controlled by double-layer repulsive forces [10]. Increasing salinity leads to a reduction in electrostatic repulsive force, causing more bacteria to be adsorbed [26,28]. E. coli die-off during filtration in Z00.3 columns proved to be negligible through an individual batch experiment (data not shown). According to a study by Bradford et al. [27], straining should also play a minor role, since coarse grained zeolite (0.3–0.6 mm) with relatively high filtration velocity (236 mm/h) was used in this study. ZCu0.3 showed considerably better removal rates over all runs, demonstrating the effectiveness of Cu2+ -integrated zeolite as antibacterial media. Effectiveness of this media varied with salinity in test water: 3.3 log removal in test water of salinity 500 ␮S/cm (Run 3), in comparison with only 1.2 log in test water of salinity 120 ␮S/cm (Run 1). A linear correlation between bacterial removal and copper leaching from ZCu was identified (p < 0.001, Fig. 1). In addition, the observed removal by ZCu0.3 in this study was less than what was reported earlier (>1.7 log) [14] although in both cases, the test water had a similar level of salinity (around 100 ␮S/cm). This

observation could mainly be attributed to less contact time (22 min) in this study than that in literature (38 min) [14], demonstrating its limited removal efficiency in high flow velocity filters. Consistently good bacterial removal by ZCuCuO1800.3 was observed over all runs and all salinity levels, despite significantly lower copper leaching from this media than from ZCu. It is observed that the bacterial removal by ZCuCuO1800.3 was not significantly correlated with the leached copper in effluent (p = 0.149, Fig. 1). This observation was consistent with the outcome of a separate batch study showing that the inactivation in aqueous phase was negligible (data not shown). The good removal by this media was partially attributed to instantaneous bacterial inactivation by Cu2+ at solid–liquid interface. Observed removal might be due more to enhanced adsorption by the Cu(OH)2 deposition layer. Similar to other metal oxide/hydroxide coating, Cu(OH)2 coating introduced positive charges onto zeolite surface, facilitating attachment of negatively charged bacterial cells. This effect was more obvious when comparing the performance of ZCuCuO1800.3 and ZCu0.3 in water of low salinity (Run 1); ZCuCuO1800.3 achieved 2.1 log reduction with leached copper 0.7 mg/L, while ZCu0.3 achieved only 1.2 log reduction with leached copper 6.2 mg/L. The observed removal by ZCuCuO1800.3 in 22 min was comparable to what was achieved by ZCu0.3 in 38 min, as reported earlier [14], demonstrating its potential benefits in reducing filter size.

3.5. New sand filter designs with media ZCu400 and ZCuCuO180 New sand filters integrated with media ZCu400 and ZCuCuO180, as well as control sand filters, were assessed based on conditions specified in Table 1. Although natural microbial communities were present in the raw sewage-spiked pond water, the concentration of E. coli was monitored to indicate the effectiveness of various filter designs for bacterial treatment. E. coli concentrations in inflow and outflow samples over this assessment are summarised in Fig. 3. E. coli log removal rates are summarised in Table 3. Over Runs 1, 3, 4, 6 and 8, where raw sewage-spiked pond water was used as test water, the outflow concentrations varied temporally and differed between designs although E. coli inflow concentrations remained almost constant (20,000–32,000 MPN/100 mL). All filter designs showed very good removal during Run 1 due to empty bed filtration. Sand only columns (S) showed 1.6 log removal during Run 3, which was higher than the new filter designs. Within the hydraulic residence time 18 min in the former, adsorption and straining were believed to be the main processes for bacterial removal. Both finer particle size and slower infiltration rate of the former might contribute to the observed better bacterial retention

Y.L. Li et al. / Journal of Hazardous Materials 273 (2014) 222–230

229

Table 3 Mean E. coli log removal by five sand filter designs over runs (standard deviation in parenthesis). Filter design

Run 1

Run 2

Run 3

Run 4

Run 5

Run 6

Run 7

Run 8

Total

T TT MM TM S

0.75 (0.14) 2.30 (0.18) 2.04 (0.05) 2.44 (0.32) 2.16 (1.34)

−0.69 (0.06) 1.43 (0.17) 0.45 (0.16) 1.23 (0.30) 0.92 (0.87)

0.52 (0.13) 0.79 (0.04) 0.90 (0.15) 1.14 (0.09) 1.56 (0.07)

0.34 (0.04) 0.61 (0.07) 0.78 (0.14) 0.80 (0.07) 0.55 (0.62)

−0.84 (0.17) −0.11 (0.11) −0.47 (0.11) 0.12 (0.13) −0.28 (0.26)

0.77 (0.05) 1.18 (0.02) 1.01 (0.05) 1.26 (0.19) 1.00 (0.01)

−1.60 (0.22) −1.11 (0.12) −1.59 (0.06) −0.84 (0.09) −1.61 (0.17)

0.96 (0.09) 1.31 (0.06) 1.26 (0.04) 1.67 (0.24) 1.12 (0.17)

0.03 (0.91) 0.80 (1.00) 0.55 (1.07) 0.98 (0.96) 0.68 (1.21)

Note: Shaded runs using sewage-spiked pond water and open runs using pond water without sewage.

in the systems. These sand filters, however, showed poorer removal in the next run (Run 4; 0.6 log) (a dosing event applied immediately after Run 3, while filters were still inundated by the previous event). It is believed that adsorption sites on sand media can become saturated during continuous dosing, leading to breakthrough of pollutants. The performance of the new filter designs, especially those with ZCuCuO180, was less affected by such a challenging dosing event despite only 2 min contact time between test water and antibacterial media ZCuCuO180, indicating the superior benefits of inactivation and adsorption capacity of ZCuCuO180. Under restricted infiltration rate (160 mm/h, Runs 6–8), all new filter designs showed improved performance. TM columns performed better than TT columns but significantly so than all other designs (p < 0.05). Despite the less favourable operational conditions (high hydraulic loading – 15 pore volumes and only 9 min of effective contact time) than those used in the pure media assessment, 1.7 log E. coli removal was achieved by TM columns, whereas only 1.1 log removal by the S columns. Remobilisation of removed bacteria under intermittent flows could have further contributed to bacteria outflow concentrations from S columns [9], yet not so in this study due to the intermittent flushing of filters with sewagefree pond water as specified in Table 1. TM systems showed effective antibacterial efficiency between dosing events; only 120 MPN/100 mL of E. coli were detected in effluent samples when the filters were rinsed with sewagefree pond water (Run 7). This TM system has been carefully designed to provide optimal removal during inundation/dosing events (via the ZCuCuO180 media), together with inactivation of the trapped microbes between events (via both ZCu400 and ZCuCuO180 media). Indeed, as explained in earlier sections, the new media ZCu400 showed mainly inactivation efficiency during dry period yet limited removal efficiency during rain events, while ZCuCuO180 showed both instantaneous removal during rain events and inactivation during dry periods. Since top layers are contaminated first with highest levels of bacteria recorded within a filter [28], using ZCu400 as a top media layer could maximise their inactivation in-between rain events. Placing ZCuCuO180 as a lower layer could achieve treatment of bacteria escaping from top layers during dosing events. The mean (min, max) copper concentrations from T, TT, MM, TM were 32 (16, 44), 11 (10, 11), 21 (13, 32), 9 (7, 12) ␮g/L respectively. Those out of TT and TM designs were well below the copper level in untreated urban stormwater 55 (10, 140) ␮g/L and comparable to the level of Cu in treated stormwater after biofiltration [6,29]. Future testing over longer operational time is needed to ensure stability over the lifetime of typical stormwater filtration systems.

4. Conclusions Seven types of antibacterial media were developed via calcination of ZCu and in situ Cu(OH)2 coating on ZCu. The effectiveness of these media on stormwater treatment, especially with high salinity, was examined. It was found that:

• Copper leaching from ZCu was reduced by more than 97% in test water of salinity 250 ␮S/cm through in situ Cu(OH)2 coating and calcination. • ZCuCuO1800.3 prepared by in situ Cu(OH)2 coating on ZCu0.3 followed by heat treatment at 180 ◦ C showed stable E. coli removal about 2 log despite large range of salinity levels from 100 to 500 ␮S/cm, whereas ZCu0.3 showed varied removal from 1.2 to 3.3 log. • ZCu0.3 calcined at 400 ◦ C (ZCu4000.3 ) showed excellent inactivation of removed E. coli during a short drying period (24 h). • New sand filters having a layer of ZCu400 at the top for inactivation of bacteria trapped in large numbers in this area, and ZCuCuO180 in the middle for removal of bacteria that passed through the top layer during events, showed optimal bacterial removal and inactivation efficiency; while copper leaching from these filters was only 9 ␮g/L.

Acknowledgements CRC for Water Sensitive Cities is acknowledged for supporting this study. Louisa John-Krol is greatly acknowledged for editing English. Active support from Hiba Bisada, Josh J. Kamil, Christelle Schang, Peter Kolotelo, Richard Williamson, Frank Winston, Catherine Osborne, Javier Neira Bravo, and Tracey Pham are greatly acknowledged with gratitude.

References [1] T.H.F. Wong, R. Allen, R.R. Brown, A. Deletic, L. Gangadharan, W. Gernjak, C. Jakob, P. Johnstone, M. Reeder, N. Tapper, G. Vietz, C.J. Walsh, in: T.H.F. Wong (Ed.), Blueprint2013 – Stormwater Management in a Water Sensitive City, Cooperative Research Centre for Water Sensitive Cities, Melbourne, Australia, 2013. [2] T.D. Fletcher, A. Deletic, V.G. Mitchell, B.E. Hatt, Reuse of urban runoff in Australia: a review of recent advances and remaining challenges, J. Environ. Qual. 37 (2008) S116–S127. [3] E. Daly, P. Kolotelo, C. Schang, C.A. Osborne, R. Coleman, A. Deletic, D.T. McCarthy, Escherichia coli concentrations and loads in an urbanised catchment: the Yarra River, Australia, J. Hydrol. 497 (2013) 51–61. [4] D.T. McCarthy, J.M. Hathaway, W.F. Hunt, A. Deletic, Intra-event variability of Escherichia coli and total suspended solids in urban stormwater runoff, Water Res. 46 (2012) 6661–6670. [5] G.I. Chandrasena, A. Deletic, J. Ellerton, D.T. McCarthy, Evaluating Escherichia coli removal performance in stormwater biofilters: a laboratoryscale study, Water Sci. Technol. 66 (2012) 1132–1138. [6] NRMMC, EPHC, NHMRC, Australian Guidelines for Water Recycling (Phase 2): Stormwater Harvesting and Reuse, Natural Resource Management Ministerial Council, Environment Protection and Heritage Council, National Health and Medical Research Council, Canberra, Australia, 2009. [7] J.M. Hathaway, W.F. Hunt, A.K. Graves, J.D. Wright, Field evaluation of bioretention indicator bacteria sequestration in Wilmington, North Carolina, J. Environ. Eng. 137 (2011) 1103–1113. [8] Y.L. Li, A. Deletic, L. Alcazar, K. Bratieres, T.D. Fletcher, D.T. McCarthy, Removal of Clostridium perfringens, Escherichia coli and F-RNA coliphages by stormwater biofilters, Ecol. Eng. 49 (2012) 137–145. [9] M. Auset, A.A. Keller, F. Brissaud, V. Lazarova, Intermittent filtration of bacteria and colloids in porous media, Water Resour. Res. 41 (2005). [10] S. Pal, J. Joardar, J.M. Song, Removal of E. coli from water using surface-modified activated carbon filter media and its performance over an extended use, Environ. Sci. Technol. 40 (2006) 6091–6097.

230

Y.L. Li et al. / Journal of Hazardous Materials 273 (2014) 222–230

[11] Z. Milan, C. de las Pozas, M. Cruz, R. Borja, E. Sanchez, K. Ilangovan, Y. Espinosa, B. Luna, The removal of bacteria by modified natural zeolites, J. Environ. Sci. Health A: Toxic/Hazard. Subst. Environ. Eng. 36 (2001) 1073–1087. [12] J. Hrenovic, J. Milenkovic, T. Ivankovic, N. Rajic, Antibacterial activity of heavy metal-loaded natural zeolite, J. Hazard. Mater. 201 (2012) 260–264. [13] D. Stojakovic, J. Hrenovic, M. Mazaj, N. Rajic, On the zinc sorption by the Serbian natural clinoptilolite and the disinfecting ability and phosphate affinity of the exhausted sorbent, J. Hazard. Mater. 185 (2011) 408–415. [14] Y.L. Li, A. Deletic, D.T. McCarthy, Removal of E. coli from urban stormwater using antimicrobial-modified filter media, J. Hazard. Mater. 271 (2014) 73–81. [15] NRMMC–EPHC–NHMRC, Australian Guidelines for Water Recycling: Augmentation of Drinking Water Supplies, 2008. [16] S.B. Wang, Z.H. Zhu, Characterisation and environmental application of an Australian natural zeolite for basic dye removal from aqueous solution, J. Hazard. Mater. 136 (2006) 946–952. [17] K. Atsumi, T. Saito, M. Komori, Process for producing an antibacterial ceramic material, U.S. Patent, Kabushiki Kaisha Sangi, Tokyo, Japan, 1990. [18] FAWB, Guidelines for Soil Filter Media in Bioretention Systems, Facility for Advancing Water Biofiltration Monash University, Melbourne, Australia, 2009. [19] A.P. Davis, M. Shokouhian, H. Sharma, C. Minami, Laboratory study of biological retention for urban stormwater management, Water Environ. Res. 73 (2001) 5–14. [20] B.E. Hatt, T.D. Fletcher, A. Deletic, Hydraulic and pollutant removal performance of fine media stormwater filtration systems, Environ. Sci. Technol. 42 (2008) 2535–2541.

[21] R.E. Kirk, Statistics: An Introduction, 5th ed., Thomson/Wadsworth, Belmont, CA, 2008. [22] M.A. Stylianou, M.P. Hadjiconstantinou, V.J. Inglezakis, K.G. Moustakas, M.D. Loizidou, Use of natural clinoptilolite for the removal of lead, copper and zinc in fixed bed column, J. Hazard. Mater. 143 (2007) 575–581. [23] H.S. Sherry, Ion exchange properties of zeolite. 3. Rare earth ion exchange of synthetic faujasites, J. Colloid Interface Sci. 28 (1968) 288–295. [24] ANZECC/ARMCANZ, Australian and New Zealand Guidelines for Fresh and Marine Water Quality, in: Australian and New Zealand Environmental Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand, 2000. [25] VGG, Victoria Government Gazette S107 – Variation to State Environment Protection Policies (Waters of Victoria), Craftsman Press Pty. Ltd., Victoria, Australia, 2003. [26] T.K. Stevik, K. Aa, G. Ausland, J.F. Hanssen, Retention and removal of pathogenic bacteria in wastewater percolating through porous media: a review, Water Res. 38 (2004) 1355–1367. [27] S.A. Bradford, J. Simunek, S.L. Walker, Transport and straining of E. coli O157: H7 in saturated porous media, Water Resour. Res. 42 (2006). [28] L. Zhang, E.A. Seagren, A.P. Davis, J.S. Karns, The capture and destruction of Escherichia coli from simulated urban runoff using conventional bioretention media and iron oxide-coated sand, Water Environ. Res. 82 (2010) 701–714. [29] G.T. Blecken, Y. Zinger, A. Deletic, T.D. Fletcher, M. Viklander, Impact of a submerged zone and a carbon source on heavy metal removal in stormwater biofilters, Ecol. Eng. 35 (2009) 769–778.