key: cord-0809582-z7iowcgj authors: Zhao, Jinghan; Yan, Peihua; Snow, Benjamin; Santos, Rafael M.; Chiang, Yi Wai title: Micro-structured copper and nickel metal foams for wastewater disinfection: proof-of-concept and scale-up date: 2020-06-13 journal: Process Saf Environ Prot DOI: 10.1016/j.psep.2020.06.013 sha: f060992e16c611a724551c6b62b58ccfdcdf183d doc_id: 809582 cord_uid: z7iowcgj It is necessary to disinfect treated wastewater prior to discharge to reduce exposure risks to humans and the environment. The currently practiced wastewater disinfection technologies are challenged by toxic by-products, chemicals and energy demand, a range of effectiveness limitations, among other concerns. An effective, eco-friendly, and energy-efficient alternative disinfection technique is desirable to modernize and enhance wastewater treatment operations. Copper and nickel micro-structured metal foams, and a conventional copper mesh, were evaluated as disinfecting surfaces for treating secondary-treated wastewater contaminated with coliform bacteria. The micro-structured copper foam was adopted for scale-up study, due to its stable and satisfactory bactericidal performance obtained over a wide range of bacterial concentrations and metal-to-liquid ratios. Three scales of experiments, using two types of reactor designs, were performed using municipal wastewater to determine the optimal scale-up factors: small lab-scale batch reactor, intermediate lab-scale batch reactor, and pilot-scale continuous tubular reactor experiments. The performance was evaluated with the aim of minimizing metal material requirement with respect to bactericidal efficiency and leaching risks at all scales. Copper foam, at or above optimal conditions, consistently inactivated over 95% of total coliforms, fecal coliforms and E.coli in wastewater at various scales, and leachate copper concentrations were determined to be below Canadian guideline values for outfall. This study successfully implemented the “structure” strategy of process intensification, and opens up the possibility to apply micro-structured copper foam in a range of other water disinfection systems, from pre-treatment to point-of-use, and should thus become a topic of further research. Disinfection is one of the indispensable processes to inactivate disease-causing microorganisms in wastewater treatment plants (WWTPs) effluent prior to outfall. The most widely used disinfection technologies are chlorination and ultraviolet (UV) irradiation (Leong et al., 2008) . However, the challenges faced with these two techniques are raising concerns, such as the disinfection byproducts (DBPs) generation after chlorination, and the energy-consumption and influent turbidity limitations associated with the performance of UV irradiation (Du et al., 2017; Liu and Zhang, 2006) . There are several other disinfection technologies available that are less commonly applied in WWTPs, such as ozonation and reverse osmosis. The limited number of applications are due to J o u r n a l P r e -p r o o f high capital and/or operating costs, ineffective performance for specific pathogens, or by-products concerns (Chidambaranathan and Balasubramanium, 2019) . Accordingly, there is still an essential need for an alternative disinfection technology that can achieve high disinfection performance while being eco-friendly, energy-saving, simple to run, and economically viable. Microbial contact with metal surfaces is considered to be an alternative form of disinfection technology in sanitary applications. The U.S. Environmental Protection Agency has recognized copper as a solid antimicrobial material that could be applied in facilities with a high sanitary requirement such as hospitals (Colin et al., 2020 (Colin et al., , 2018 Molteni et al., 2010) . Most recently, copper surfaces have even been shown to be capable of combating the spread of the coronavirus, including COVID-19 (van Doremalen et al., 2020; Warnes et al., 2015) . Bright et al. (2009) studied the antiviral effect of zeolites loaded with metal ions, and tested copper due to is toxic to many microorganisms as a result of its ability to block functional groups on proteins and inactive enzymes, thus preventing both enveloped and unenveloped viruses from being able to enter human cells. Such zeolite powders could be added or bonded to materials such as plastics, paints, synthetic fabrics, and steel surfaces. For water and wastewater applications, there are two forms of metal materials that have been considered for disinfection: metal nanoparticles and metal surfaces (Chakraborty et al., 2017; Cui et al., 2012) . The antimicrobial properties of metal nanoparticles have been widely studied and enunciated in the applications of water disinfection as an alternative water and wastewater treatment solution (Cui et al., 2012; Dankovich and Smith, 2014; Lv et al., 2018) . Metal nanoparticles, of appropriate antimicrobial composition, have been demonstrated to be highly effective on microorganism inactivation due to their high specific surface area. Lv et al. (2018) reported that the antibacterial efficiency of copper nanoparticles could reach 94.3±0.1% after a 12-hour treatment cycle with an initial Escherichia coli (E.coli) concentration at 10 5 colony-forming unit (CFU)/ml. In another study, silver nanoparticles were embedded in a blotter paper to inactivate bacteria while water percolated through (Dankovich and Gray, 2011) . The result supported the disinfection potential of metal nanoparticles since a log 7.6 ± 1.3 reduction of E.coli was achieved in 10 minutes percolation compared to the 10 9 CFU/ml influent concentration. However, the complex and costly synthesis processes, tendency to easily aggregate, and low reusability of nanoparticles have limited their applicability in large-scale applications (Dizaj et al., 2014; Musarrat et al., 2010; Villaseñor J o u r n a l P r e -p r o o f and Ríos, 2018). The potential impacts of metal nanoparticles in the environment and on human health is another concern for its application in water and wastewater treatment (Kim et al., 2013; Scown et al., 2010) . The use of metal surfaces as bactericides has its origin in the search for materials to be incorporated into object surfaces to prevent microbes. A typical setting for this application is hospitals (Michels et al., 2009) , food processing facilities (Wilks et al., 2005) and other public areas (e.g. public transport) where people are susceptible to topically contacting pathogens. The main antimicrobial mechanisms are attributed to cell damage when bacteria come into contact with metal surfaces, leading to rapid cell death, are cell membrane rupture, protein disruption and Fenton-type reaction. For water treatment application, Varkey and Dlamini (2012) used copper mesh inside clay and sawdust filtration pots, meant for small-scale applications, and observed complete elimination of E. coli bacteria. Varkey (2010) also tested thin metal plates immersed in contaminated water for E. coli inactivation, and found that copper, silver and zinc had higher rates of destroying the bacteria than aluminium, tin and silicon, and that greater surface area led to faster coliform elimination. However, the studies on the application of metal surfaces are limited and no experimental data are available on either wastewater treatment or larger scale experiments beyond laboratory experimentation, which is insufficient to demonstrate the wastewater disinfection potential of metal surfaces. The present study was carried out to determine the practicality of micro-structured metal foams, similar to those used in hybrid-car battery technology, to disinfect wastewater treatment effluent, as a process intensification (PI) approach. The use of micro-structure foams for wastewater disinfection constitutes an application of the "structure" domain of PI, with the objective to "maximize the effectiveness of molecular events" (Santos and Van Gerven, 2011) ; in wastewater disinfection the molecules are the pathogens, and the events become the pathogen-metal contact. Such innovative water disinfection technology could supplant or work in conjunction with commonly used sterilization processes found in wastewater or drinking water treatment plants. Metal foams have high (>95%) porosity, which minimizes pressure losses as water travels through it, while having high surface area for effective contact between the metallic surfaces and the flowing water. Such foams are also metallurgically stable and resist dissolution in water. These properties should allow the development of a continuous flow-through bed treatment system that J o u r n a l P r e -p r o o f can be compact and long-lasting, while having little impact on water flow or composition. The main purpose of this study was to examine the bactericidal efficiency of metal foams at lab-and pilot-scales fed with municipal WWTP secondary-treatment effluent. A scale-up study was conducted to investigate the efficiency of the metal foams while treating a large volume of water over extended periods of time, and the scale-up factors were determined for further technology development. Metal leaching risk was also assessed to evaluate the potential impacts on environmental and human health. The nickel foam and copper foam materials were supplied by Cnem Corporation (Mississauga, Ontario, Canada). The density, porosity and pore density of copper foam were, respectively, 215 kg/m 3 , 95% and 110 ppi (pores per linear inch). For the nickel foam, these values were 292 kg/m 3 , 95%, and 90 ppi. Fig. 1 illustrates the micro-structure of the metal foams observed using a digital microscope (VHX-5000, Keyence). The metal struts form a mesh structure with fully open and interconnected pores. For comparison, a common copper mesh was also used, supplied by Bird B Gone (Irvine, California, USA) for household usage, and the density, porosity, and pore density the copper mesh were determined to be 197 kg/m 3 , 90%, and 4.6 ppi. E.coli CMF-Sh1 strain (ATCC ® 43651™) was purchased from ATCC (Manassas, Virginia, USA). The E.coli strain was activated by inoculating it into autoclaved agar medium and incubating it under 37±2℃ for 24 hours following the "ATCC ® bacterial culture guide" (ATCC, 2015) . The J o u r n a l P r e -p r o o f typical E.coli concentrations found in treated wastewater prior to disinfection in published literature were reviewed and classified into three levels: low concentration (Boutilier et al., 2009; Wen et al., 2004) , moderate concentration (Lazarova et al., 1999; Pérez et al., 2010) , and high concentration (Le-Thi et al., 2017; Wang et al., 2007) . The dosage of the bacterial stock solution was adjusted in order to achieve low concentration (10 2 -10 3 CFU/ml), moderate concentration (10 3 -10 4 CFU/ml) and high concentration (10 6 -10 8 CFU/ml) in autoclaved water. Water used to prepare the bacterial solutions was previously treated by Milli-Q® IQ 7003/05/10/15 water purification system. The bacterial solutions thus prepared were used for bactericidal performance comparison of metal materials in lab-scale testing. Killing rate (KR) was determined by Eqn. (1): where C0 (CFU/ml) and Ct (CFU/ml) are the bacterial counts of solutions before and after metal exposure, respectively. Three reactor scales (small lab-scale, intermediate lab-scale, and pilot-scale) were used for testing the scale-up potential of the conceptualized process. Bactericidal efficiency, retention time and metal material mass were considered to evaluate performance. The small lab-scale reactor was designed, as demonstrated in Fig. 2a , to model a continuous stirred tank reactor (CSTR). Copper foam, copper mesh, and nickel foam were used to construct reactors. The metal materials were shaped into three layers of cylinders with increasing diameters and stabilized by thin copper wire to maintain an equal gap between the concentric cylinders. The metal materials were placed within beakers of two sizes (600 ml and 2000 ml) containing between 300 ml and 2000 ml of bacterial solution or of real secondary-treatment effluent from the Guelph WWTP. The metal mass of the three tested materials was kept constant at 5.5g for all tests. Air pumps (Tetra 77847) were applied to supply air bubbling throughout the experiment in each reactor to achieve aeration and agitation. The experiments were conducted under room temperature. Control group (C), copper foam group (CF), copper mesh group (CM), and nickel foam group (NF) were set in the beakers with no material, copper foam material, copper mesh material, and nickel foam material, respectively. Treated solution samples were collected after 0. To achieve the goals of space-saving and higher efficiency, the plug flow reactor model, under continuous operation, was applied in the pilot-scale experiment for further scale-up study. Plug flow reactors are ideal for continuous processing and ensure tight residence time distribution in a reduced reactor volume compared to CSTR design. The pilot-scale setup is illustrated in Fig. 2c as was operated at the Guelph WWTP piloting facility, which continuously receives wastewater streams for testing directly from the full-scale municipal plant. The length of straight PVC pipes was 6.16 m in total, and connected by six 4-inch-diameter PVC elbows to create three horizontal legs stacked over each other. The reactor was fed with secondary treatment effluent (sampled for each experiment, and analyzed for total coliform, fecal coliform, and E. coli) from an overhead tank continuously, and a constant fluid height was maintained at the inlet vertical leg to ensure constant pressure head to drive the flow at a constant flow rate. The inflow rate was measured at is 2.07 ± 0.04 L/min. The flow outlet was located above the height of the highest horizontal leg, and the horizontal legs where stacked at different heights, to ensure equal residence time for all J o u r n a l P r e -p r o o f flow exiting the reactor. The total liquid volume at any given time within the pilot-scale reactor was 58 L, resulting is a total residence time (from inflow to outflow) of 28.0 ± 0.5 min. Four valves were installed along the length of the reactor for periodically collecting treated wastewater samples. Circular-cut portions of copper foam sheets (selected based on its performance in labscale tests) were stacked into cylinder-shaped stacks that snuggly fit inside the PVC pipes to evenly occupy the cross-sectional flow area, and the wastewater percolated through eight foam sections The membrane filtration technique was adopted from the procedure outlined in the EPA Methods 9132, 1603 and 1604 (U.S. EPA, 2002a EPA, , 2002b EPA, , 1986 Water samples were also monitored for pH, and dissolved oxygen (DO), in triplicates. The statistical difference of monitoring parameters and KR values between various treatment groups in the lab-scale experiments was assessed by one-way ANOVA. P<0.05 was used to determine the significant level. The loss of metal ions to the treated wastewater after metal material contact was assessed by The KR of different treatment groups were determined under three different levels of bacterial concentration in Milli-Q water solutions (low concentration, moderate concentration, and high concentration), using the small lab-scale CSTR-style reactor and the prepared bacterial solutions. The bactericidal results are presented in Figs. 3a, 3b, and 3c, respectively. J o u r n a l P r e -p r o o f The initial bacterial count of the low concentration solution was at 202 ± 15 CFU/ml. The control group had a decreasing bacterial concentration with time due to these bacterial solutions being prepared by heavily diluting bacterial stock solution using autoclaved Milli-Q water. Thus, all bacterial solutions contained minimal amount of nutrients to support the initial bacterial population. What is noticed, however, it that the death rate was drastically sped up by the metal materials. All the mental materials could achieve a 99.0% or greater KR at 0.5 h of retention time. The effectiveness of the metal materials is further supported by the small SD values for those samples, versus the control, which shows that copper and nickel metal contact invariably kills E. coli. For the moderate bacterial concentration experiments, the average bacterial concentration was 3,600 ± 361 CFU/ml. Here, as illustrated in Fig. 3b Considering the economic feasibility, copper and nickel were selected in this study among other metals with antibacterial potencies, such as silver and gold. The antibacterial properties of metal copper have gained momentum since 2008, when the U.S EPA registered over 350 copper alloys as antimicrobial (Wei et al., 2014) . Comparatively, there are only several studies available on investigating the disinfection efficiency of nickel; however, it has been principally used to facilitate the effectiveness of materials on disinfection, especially those with photocatalytic properties (Applerot et al., 2012; Kruk et al., 2015) . From the small lab-scale results, it is clear that three metal materials behave differently with respect to bactericidal performance, depending on retention time and bacterial load. Copper foam and copper mesh exhibited higher bactericidal efficiency compared to nickel foam. In turn, copper foam was substantially more effective under high bacterial concentration due to its micro-structured pores, resulting in greater pore density, and J o u r n a l P r e -p r o o f thus increased surface area for contact between the metal, the flowing water, and the suspended pathogens. The mechanism underlying the antibacterial activities of copper and nickel has been a subject of continued investigation. Some studies indicate that the antibacterial activities of nickel and copper are attributable to metal-induced reactive oxygen species (ROS) generated through Fenton-like reactions (Eqn 2-5) (Applerot et al., 2012; Grass et al., 2011; Wan et al., 2017) . ROS is part of the oxygen metabolization during bacterial growth (Brynildsen et al., 2013) , and under natural circumstances cells can detoxify ROS spontaneously through enzymes (Hanukoglu, 2006) . Metal materials are postulated to accelerate the generation of ROS, and the excess ROS leads to oxidative damage of cell membranes and DNA, which eventually kill the microorganisms (Wan et al., 2017) . The DO level in the small lab-scale experiments was kept at saturation by aeration for all treatment groups throughout the experiments, and no significant difference (P=0.98) was observed; thus, sufficient oxygen was available for Fenton reactions to proceed. However, sufficient reduced iron is needed to catalyze the production of hydroxyl radicals, which was not present in these experiments in any detectable amount (i.e. below ICP-MS detection limit). Nickel-induced Fenton-like reaction (Wan et al., 2017) : Copper-induced Fenton-like reaction (Grass et al., 2011) : Another hypothesis of the metal-induced disinfection mechanism in water is that bacterial activity on metal surfaces releases metal ions to the solution. The liberated metal ions change the redox potential, neutralize negative charges of the bacterial cell membrane and further disrupt the membrane, which eventually leads to lipid oxidation, DNA lesion, and protein denaturation (Cioffi et al., 2005; Gabrielyan et al., 2016; Hong et al., 2012 and Mycobacterium fortuitum, and found that copper did not significantly reduce S. Typhimurium and E. coli on its own, while 2.80-log10 reductions were observed for the other two species. Based on the metal comparison trial results, the copper foam was applied for an optimization study that aimed to minimize the material used to disinfect real wastewater in an acceptable time period. The experiments were conducted with 5.5g of copper foam to treat adjustable volumes of wastewater, which includes 300 ml, 550 ml, 600 ml, 650 ml, 750 ml, 850 ml, 950 ml, 1000 ml, 1500 ml, and 2000 ml. Therefore, the treatment ratios (the mass of metal material to the volume of wastewater) were 1.80 g/100ml, 1.00 g/100ml, 0.92 g/100ml, 0.85 g/100ml, 0.73 g/100ml, 0.65 g/100ml, 0.58 g/100ml, 0.55 g/100ml, 0.37 g/100ml, and 0.28 g/100ml, respectively. Total coliform, fecal coliform, and E.coli were quantified for evaluating the bactericidal performance. guidelines (Tobin and Ward, 1984) . Therefore, E.coli was the main parameter in this study for determining the optimal treatment ratio. Retention time and KR were considered to be the decisive factors for metal-to-liquid ratio optimization. For meeting the Canadian recreational water quality guidelines, a KR at 96% is the minimum requirement based on the average E.coli concentration at the Guelph WWTP. The KR as a function of retention time of the three different bacteria groups under the same treatment ratios had slight differences; however, these differences are within an expected variability range for bacterial analysis (within ± 5% differences), except for the lowest ratio. At 0.28 g/100ml ratio, the three different bacteria groups showed a significant difference in KR, likely due to insufficient contact of liquid with metal surface. The minimum required KR (96%) was achieved with the treatment ratios higher than 0.85 g/100ml within 0.5 h. With 0.73 g/100ml, 0.65 g/100ml, and 0.58 g/100ml ratio, the disinfection was not effective in short retention time. The final KR of the 0.37 g/100ml and 0.28 g/100ml groups could reach 96% and 95% at 4 h retention time. Comparatively, the 1.10 g/100ml, 1.00 g/100ml, 0.92 g/100ml, and 0.85 g/100ml treatment ratios had a better killing performance with shorter treatment duration. As the KR values of the four groups were similar during the experiment conducted 0.85 g/100ml was identified as the optimal treatment ratio for the small-scale CSTR trials due to lower material requirement per unit volume of treated wastewater. However, it was desired to know if this optimum ratio would hold when the reactor volume was significantly increased, as discussed in the next section. To investigate the scalability of the copper foam treatment process, intermediate-scale laboratory CSTR experiments were first conducted. The optimal metal-to-liquid ratio was carried over from the small-scale optimization, but higher ratios were also tested to investigate whether the kinetics of the KR would change at larger scale. The KR values over time of two different treatment ratios are compared in Fig. 5 for total coliform, fecal coliform, and E. coli. From Figs. 6a, 6b and 6c, several process behaviours can be observed. First, the first samples collected at 30 min had considerably higher bacterial load than the next sample collected at 1 h. This means that the reactor had not reached steady-state performance in the first thirty minutes, which is just over one residence time. By the one-hour mark, and over two reactor volumes of flow having passed each sampling valve, it is more likely that the performance of a plug flow reactor has reached steady-state. Second, the inflow bacterial load varied throughout the experiment, and this oscillatory behavior carried through to all sampling valves, except for being attenuated at valve 4 given that at the outflow the bacterial load was either null or small (≤ 60 CFU/ml total coliforms, and ≤ 16 CFU/ml E. coli). In fact, the other temporal trend to be seen is the bacterial performance over run time, to detect any decline in performance. This can be quantitatively assessed by fitting straight-line equations to the bacterial count at each valve, and noting the slope of each equation for signs of improving (negative slope) or declining (positive slope) performance. The slopes for valves 1 to 4, ignoring the first sample before steady-state is deemed to have been reached, respectively were: 0.559, 2.467, -0.767, and 0.057 CFU/(ml·h). Thus, performance slightly decline over time at valves 1 and 2, and improved or remained stable at valves 3 and 4. Notably, the inflow wastewater trended upwards during the 12-hour experiment (slope 5.993 CFU/(ml·h)), hence it can be concluded that copper foam performance was stable for the run duration tested. Third, the bactericidal performance consistently improved from valves 1 to 4; it should be noted again that values at a given point in time should not be compared, since the fluid plugs at each location originate from different fluid plugs entering the reactor, which in turn had variable bacterial load. Hence, the best way to confirm that bactericidal performance is proportional to residence time is to take 12-hour mean bacterial counts at each valve and generate 12-hour averaged KR values (see Table 1 ). J o u r n a l P r e -p r o o f In all cases listed in Table 1 , the bacterial counts gradually reduce as the wastewater travels down the tubular reactor and passes through multiple metal foam stacks (two between each sample), resulting in gradually increasing KR along the reactor length. KR values for fecal coliform and E. coli were similar at each valve, while total coliforms KR lagged but caught up by the outflow. This suggests that certain bacterial species that appear in the total coliforms count are more resistant to copper contact, but eventually are impacted. Lastly, it should be noted that the residence time of the pilot-scale tubular reactor was optimized in preliminary trials (the piloting experiments were part of a long campaign of reactor testing and optimization), so residence time, superficial liquid velocity and reactor length, metal-to-liquid ratio, and metal foam positioning and packing within the reactor, are critical design parameters to achieving desired performance. There is an abundance of studies that have investigated the potency of metal materials on antibacterial applications in a laboratory-scale; however, the feasibility of their scaled-up application remains scarce (Baruah et al., 2016; Maguire-Boyle et al., 2012; Pulit et al., 2013) . The present study tested the efficiency of copper foam treating municipal wastewater as well as its stability treating a larger amount of wastewater continuously. Treating municipal wastewater is challenging with many design and performance factors involved, and further technology development is warranted, particularly to test long-term performance and foam cleaning requirements. To be noted from the present study, the plug flow tubular reactor significantly reduced the dead zone volumes, versus typical continuous processing units commonly used in water treatment applications, and improved metal/liquid contact efficiency, versus batch reactors. It is thus recommended that in large-scale applications the tubular reactor design be considered to improve disinfection efficiency. There was no energy supply throughout the pilot-scale experiment, since the water flow was sustained by gravity, which is attractive for large water treatment systems. In fact, a tubular reactor can also be driven by syphoning action when a height different between inflow and outflow is not possible. This scale-up study showed a promising result of the application of copper foam in wastewater disinfection since it not only demonstrated the prominent antibacterial capability of copper foam but also the low energy consumption and high operating flexibility. Material characterization was conducted at the end of the experiment to examine the durability of copper foam and to have a better understanding of its interaction with microorganisms (as shown in Fig. 7) . It is shown that the interconnected porous structure of copper foam was stable throughout the experiment, which further supports its robustness and stability. However, it is visible that the partial oxidation of copper occurred under the aerobic condition. Though the effects of oxidized copper on the antibacterial performance were not observed and the performance proved stable throughout this study, the potential inhibition of the antibacterial activities in long-term applications still needs to be considered. According to the study of Akhavan and Ghaderi (2010) , copper oxide nanoparticles showed a lower inactivation on E.coli compared to intrinsic copper. Therefore, the occasional treatment of surface oxidized copper layer may be needed. Dissolution of copper oxide layer by immersing in mineral acid may be one of the treatment solutions (Habbache et al., 2009) ; however, it might shorten the longevity of copper foam. The removal of copper oxide will reduce copper mass, and the durability and antimicrobial performance of copper foam may be accordingly impacted. This needs to be determined experimentally in subsequent long-term testing. Another potential treatment strategy is to reduce the oxidized copper to intrinsic copper (Kirsch and Ekerdt, 2001) . The reduction may be achieved by using hydrogen, carbon monoxide, etc. This process may need external heating to accelerate the process and that could potentially impact the mechanical strength of the material, and lead to the structural change of the microstructured pores. The antibacterial effects of oxidized copper and detailed long-term maintenance strategy of copper foam will need to be addressed in future work. J o u r n a l P r e -p r o o f In addition to the concern of oxidized copper, biofilms and fouling should also be considered for long-term applications, especially treating wastewater having a high load of residual solids and microorganisms in the tertiary treatment effluent. The maintenance strategies can likely rely on periodic chemical backwash to dislodge foulants and dissolve scale and biofilms. According to the study of Mikolay et al. (2010) on metallic copper surfaces, a solution containing glucoprotamin can be used as a cleaning detergent, which itself is a disinfectant against microorganisms (Zeitler and Rapp, 2014) . The study showed that the cleaned surfaces had lower survival of microorganisms compared to the uncleaned surfaces. Harrison et al. (2008) found that the quaternary ammonium compound can work synergetically with ionic copper to eradicate the biofilm of E. coli, Staphylococcus aureus, Salmonella enterica, and Pseudomonas fluorescens. Therefore, chemical detergents may be adopted for cleaning metal foams to eradicate biofilm and facilitate the antibacterial activities, and this should be subject of further study before full-scale implementation. The ICP-MS analysis showed that the prepared bacterial solutions, before metal foam treatment, had copper and nickel concentrations below the detection limit (0.1 µg/L). Fig. 8a In the pilot-scale experiment, the initial copper concentration in the influent was 0.29 ± 0.01 µg/L. Fig. 8c shows the trend of total copper concentration with increased run time at each valve location. The leaching concentration in treated water was positively correlated to reactor length (i.e. increasing from closest to inflow, valve 1, to closest to outflow, valve 4). Thus, as wastewater passes through each foam stack, it picks up copper. However, the highest outflow value recorded was 2.1 µg/L, and the outflow concentration was relatively stable over the course of the experiment, with some variability attributable to wastewater compositional changes. The continuous flushing in the pilot-scale experiment did not impact the leaching risk, and the copper concentration was maintained within a safe range for treated wastewater outfall. This study was carried out to investigate the bactericidal properties of micro-structured metal materials. The tested metal materials could successfully kill wastewater bacteria at different extents and kinetics. Among nickel foam, copper mesh, and copper foam, the copper foam had the best bactericidal performance. The copper foam was applied in a scale-up study, that went from small to intermediate lab-scale, and subsequently to pilot-scale. Based on optimal conditions at each scale, the scale-up factor from small lab-scale to pilot-scale in terms of reactor volume was 89, while that in terms of foam mass was 211. The intermediate lab-scale experiment led to an adjustment in the optimal metal-to-liquid ratio, and thus the two different scale-up factors used, form 0.85 g/100ml to 2.00 g/100ml. The optimization target used was a minimum KR value of 95% for total coliforms, fecal coliforms and E. coli. At pilot-scale, a continuous plug flow tubular reactor proved successful in treating multiple times (25.7) the amount of wastewater inflow compared to the reactor volume during a 12-hour run with 28.0 min residence time. The satisfactory performance reached (96% KR) was attributed to the reactor design, the operating parameters, the metal foam packing, and the metal foam micro-structure. Thus, the implementation of the "structure" PI strategy was deemed successful up to the scale tested, and further development just focus on ensuring the technical and economical feasibility to disinfect secondary or tertiary J o u r n a l P r e -p r o o f wastewater, including requirements and strategies for foam maintenance and estimation of longterm foam durability. Copper leaching risk was assessed at all scales, and the leachates were determined to be below Canadian guideline values, thus unlikely to pose outfall concerns to the environment. The copper foam not only showed significant antibacterial activity, but also can overcome the drawbacks of currently practiced wastewater disinfection technologies, including the elimination of chemical use, the avoidance of DBPs, not detrimentally being affected by turbidity, among other advantages. Micro-structured copper foam application for water disinfection are likely extendable to other uses, such as point-of-use treatment of recreational water or irrigation water, or pre-treatment of well water or lake water, and thus can become an important topic of future research. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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