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Ceramic water filter for point-of-use water treatment in developing countries: Principles, challenges and opportunities

Ceramic water filter for point-of-use water treatment in developing countries: Principles,... Front. Environ. Sci. Eng. 2020, 14(5): 79 https://doi.org/10.1007/s11783-020-1254-9 REVIEW ARTICLE Ceramic water filter for point-of-use water treatment in developing countries: Principles, challenges and opportunities 1,2 3 4 5 Haiyan Yang , Shangping Xu , Derek E. Chitwood , Yin Wang (✉) 1 SCNU Environmental Research Institute, Guangdong Provincial Key Laboratory of Chemical Pollution and Environmental Safety & MOE Key Laboratory of Theoretical Chemistry of Environment, South China Normal University, Guangzhou 510006, China 2 School of Environment, South China Normal University, University Town, Guangzhou 510006, China 3 Department of Geosciences, University of Wisconsin–Milwaukee, Milwaukee, WI 53211, USA 4 Department of Engineering, Dordt University, Sioux Center, IA 51250, USA 5 Department of Civil and Environmental Engineering, University of Wisconsin–Milwaukee, Milwaukee, WI 53211, USA HIGH LIGHTS GRAPHIC A BSTRA C T � CWF is a sustainable POU water treatment method for developing areas. � CWF manufacturing process is critical for its filtration performance. � Simultaneous increase of flow rate and pathogen removal is a challenge. � Control of pore size distribution holds promises to improve CWF efficiency. � Novel coatings of CWFs are a promising method to improve contaminant removal. ABSTRA CT AR TICL E I N F O Drinking water source contamination poses a great threat to human health in developing countries. Article history: Point-of-use (POU) water treatment techniques, which improve drinking water quality at the household level, offer an affordable and convenient way to obtain safe drinking water and thus can Received 14 January 2020 reduce the outbreaks of waterborne diseases. Ceramic water filters (CWFs), fabricated from locally Revised 18 March 2020 sourced materials and manufactured by local labor, are one of the most socially acceptable POU water Accepted 21 March 2020 treatment technologies because of their effectiveness, low-cost and ease of use. This review concisely summarizes the critical factors that influence the performance of CWFs, including (1) CWF Available online 16 May 2020 manufacturing process (raw material selection, firing process, silver impregnation), and (2) source water quality. Then, an in-depth discussion is presented with emphasis on key research efforts to Keywords: address two major challenges of conventional CWFs, including (1) simultaneous increase of filter flow rate and bacterial removal efficiency, and (2) removal of various concerning pollutants, such as viruses Point-of-use water treatment and metal(loid)s. To promote the application of CWFs, future research directions can focus on: (1) Ceramic water filter investigation of pore size distribution and pore structure to achieve higher flow rates and effective Bacterial removal pathogen removal by elucidating pathogen transport in porous ceramic and adjusting manufacture Surface modification parameters; and (2) exploration of new surface modification approaches with enhanced interaction Water quality between a variety of contaminants and ceramic surfaces. © The Author(s) 2020. This article is published with open access at link.springer.com and journal.hep. com.cn 2020 1 Introduction ✉ Corresponding author E-mail: wang292@uwm.edu World Health Organization (WHO) reported that 71% of Special Issue—Accounts of Aquatic Chemistry and Technology the world’s population used improved drinking water Research (Responsible Editors: Jinyong Liu, Haoran Wei & Yin Wang) 2 Front. Environ. Sci. Eng. 2020, 14(5): 79 sources that were free from microbiological and priority 2008; Clasen et al., 2015). CWFs, produced from a mixture chemical contamination in 2017 (World Health Organiza- of clay and sieved combustible material, can effectively tion, 2017,2019a). However, there are vast inequalities remove microorganisms and reduce turbidity from water among different countries and areas. For example, by gravity filtration through porous ceramic media. CWFs although 85% of world urban population could access can be fabricated into different shapes, such as disk, tube, quality drinking water that is free from contamination, the candle and pot (Fig. 1), which make them suitable for POU percentage is down to 53% in rural areas. A large effort is application in various situations. Silver impregnation has still needed in rural areas, especially in developing been conventionally applied to modify CWFs to enhance countries where pathogen contamination of drinking their performance, particularly for the removal of micro- water represents one of the greatest threats to human organisms. Silver-modified CWFs are effective at remov- health (World Health Organization, 2019b). What’s worse, in developing areas such as sub-Saharan Africa, Central and Southern Asia and Oceania, centralized water treat- ment and distribution systems are often unavailable due to political or socioeconomic factors (Peter-Varbanets et al., 2009; World Health Organization, 2019a). In these areas, point-of-use (POU) systems, which improve drinking water quality at the household level, represent a more affordable and effective way to produce water free from contamination. In practice, a POU technology should be effective, easy to operate, with low maintenance requirement, economic- ally viable, environmentally sustainable and sociocultu- rally acceptable. Chlorination with safe storage, combined coagulant-chlorine disinfection systems, solar disinfection, ceramic water filter (CWF) and biosand filter are examples of current POU technologies (Sobsey et al., 2008; Murphy et al., 2010; Kallman et al., 2011; Geremew et al., 2018). Based on a comprehensive comparison of these technol- ogies (Table 1), CWF is considered among the most practical and sustainable POU technologies with afford- able (highly cost effective) and low-maintenance (easy to Fig. 1 Schematic of different forms of ceramic water filters: (a) use, only periodic cleaning needed) features for household ceramic disk filter, (b) ceramic candle filter, (c) ceramic pot filter, (d) tubular ceramic filter. water treatment in developing countries (Sobsey et al., a) Table 1 Comparison of POU treatment technologies used in developing countries POU Water Water Overall d) e) Cost Ease to use Description b) c) technologies quality production score Chlorination 1 3 2 5 11 Hypochlorite liquid or tablets are used to inactivate pathogens in source water. Coagulation/ 2 2 1 2 7 Dry coagulant-flocculant and chlorine as tablets or sachets Chlorination are added to source water to inactive and settle down pathogens. Solar disinfec- 3 1 5 1 10 Source water is filled in polyethylene terephthalate (PET) or tion glass under sunlight, allowing UV and heat to inactivate pathogens. Ceramic water 5 2 4 4 15 Porous ceramic media (e.g., pot, disk, candle) with silver filter coating is used to filter pathogens from source water. Biosand filter 4 3 3 3 13 Biosand filter is adapted from slow sand filter cover with biofilm, removing pathogens using biological and physical processes. Note: a) The scoring system was used to compare POU technologies. For each criterion, a technology was ranked from best (with a score of 5) to worst (with a score of 1) and the performance score (5 to 1) represents its relative ranking; b) This score was based on their reduction in Diarrhea reported by Clasen et al. (2015); c) Water production was scored based on producing 20 L within 4 h of applying the treatment described by Sobsey et al. (2008). Due to the different water production methods for these POU technology, the score was only set to 1-3 with 1 for low, 2 for fair, and 3 for good performance; d) This score was based on their cost (dollars/L/year) calculated in Sobsey et al. (2008); e) This score was based on the steps of treatment processes and periodic maintenance. Haiyan Yang et al. Challenges for ceramic water filter technology 3 ing >99% of protozoan (Van Halem, 2006; Abebe et al., 2.1 Raw material selection 2015) and 90%–99.999% of bacteria (Brown and Sobsey, 2010) and thus have been evaluated for efficacy in 2.1.1 Clay source reducing rates of diarrheal and other waterborne diseases (Clasen et al., 2004; Morris et al., 2018). Unlike chlorine or Clay is the skeleton element of CWF, and its quality and thermal disinfection, CWFs do not significantly change the characteristics directly influence filter quality. Earthenware taste or temperature of the water. They are also very clay is most suitable for ceramic filter production due to its effective at reducing the turbidity of the water (Abebe porosity, availability, and low temperature needed for et al., 2016). With proper care as suggested by manufac- vitrification. In practice, clay physical properties (e.g., turers in the user manual, commercial ceramic pot filters, shrinkage, plasticity, workability), the persistent availabil- one of the most popular CWFs in the developing countries, ity of clay source, and the leachability of heavy metals/ have a potential service life of up to 5 years to effectively inorganic materials are some key parameters to be remove pathogens from raw water without the need for an considered for clay source selection (The Ceramics external energy source or consumable supplies (The Manufacturing Working Group, 2011). Details for clay Ceramics Manufacturing Working Group, 2011). So far, source test, selection and handling have been described in locally produced CWFs have been successfully applied in the instruction manuals of Potters for Peace (PFP) and more than 20 developing countries (e.g., Cambodia, Resource Development International (RDI) (Hagan et al., Guatemala) to improve the drinking water quality (Rayner 2009; The Ceramics Manufacturing Working Group, et al., 2013a). It was estimated that more than 4 million 2011). Although clay does not create pore structure people worldwide used CWFs to prepare daily safe directly, Oyanedel-Craver and Smith (2008) found that drinking water, which contributed to public health, social filters made from different clay sources showed varied and economic benefits (van der Laan et al., 2014). porosity, hydraulic conductivity and bacterial removal At the same time, CWFs have some disadvantages that efficiency. A recent study suggested that high content of limit their application. First, the relatively low water sand-rich clay may contribute to the decrease of mean pore filtration rates have limited their use. Improved CWF size of CWF and thus potentially influence filter flow rate designs that can simultaneously achieve both high flow and bacterial removal efficacy (Youmoue et al., 2017). rate and effective bacterial removal are desirable but Additionally, the inclusion of diatomaceous earth and remain a great challenge. In addition, CWFs often have MgO components in the CWFs were found to be beneficial limited capability of removing contaminants such as for viral removal (Michen et al., 2012; Michen et al., viruses and chemical pollutants (Yang et al., 2019a). 2013), suggesting that clay composition may also have Although the CWF technology has been gaining some influence on filter performance. growing research attentions and witnessing a wide application during the past decades, there have been few 2.1.2 Organic combustible material critical reviews on the progress of this technology. The objective of this account is to provide 1) a concise In the CWF manufacturing process, sieved organic summary of the progress of recent CWF research and combustible material, also known as burn-out material, is application with a particular focus on the critical factors mixed with raw clay for the purpose of creating a paste and processes that are closely related to CWF performance, which is subsequently pressed into various shaped filters. and 2) an overview of challenges and opportunities for The organic combustible materials are usually burnt out further improvement for CWF as a POU water treatment during the firing process to create porous structure that technology in developing areas. serve as both flow paths for water passage and reaction sites for the removal of various contaminants such as bacteria and virus. The combustible material thus has been 2 Critical factors for CWF performance identified as a critical parameter on the performance of CWFs. Sawdust and rice husk are the most common CWFs are fabricated from a mixture of clay and combustible materials used in manufacturing CWFs and a combustible materials (e.g., rice husks or sawdust) referred global-wide survey reported that 17 in 18 CWF production as burn-out materials. Filter porous structure is created as a factories used sawdust or rice husk as the combustible result of the burn-out during the firing process. As shown material (Rayner et al., 2013a). In addition, coffee husks, in Fig. 2, fabrication of conventional ceramic filter includes peanut husks and other organic materials that can be raw material selection and processing, mixing and pulverized to a suitable size can also be used as pressing, drying and firing, silver application, as well as combustible materials, depending on their cost and local quality testing. In this section, we focus on the processes availability. According to previous laboratory and field whose effects on filter effectiveness have been documented studies, the size and amount of combustible material have a by published reports and peer-reviewed references. strong influence on the filter performance (Kallman et al., 4 Front. Environ. Sci. Eng. 2020, 14(5): 79 Fig. 2 Ceramic water filter production flow chart. It was re-drawn based on the Ceramics Manufacturing Working Group (2011). 2011; van der Laan et al., 2014; Soppe et al., 2015; Rayner efficiency when the maximum firing temperature increased et al., 2017; Van Halem et al., 2017). For example, Van from 800°C to 950 °C, which may be attributed to the Halem et al. (2017) produced pot filters with a higher flow small increase of filter pore size with increasing maximum rate (2-6 times as regular pot filters) by increasing the rice firing temperature. Additionally, the incomplete combus- husk content from 24% to 31% (wt.% of dry clay mixture). tion of the organic materials could lead to the production of Combustible material with a larger particle size has also substances such as black carbon cores inside the filter been reported to introduce larger pore sizes of the resulting during the firing of commercial CWFs (The Ceramics CWF, which could facilitate a higher flow rate but often Manufacturing Working Group, 2011; Goodwin et al., resulted in lower microbial removal efficacy (Soppe et al., 2017). Currently, black CWF was reported to be fired in a 2015; Rayner et al., 2017). reductive atmosphere to improve its virus removal performance (Guerrero-Latorre et al., 2019). The potential 2.2 Firing process impact of the firing atmosphere on the performance of the resulting CWFs, especially for the removal of contami- The firing process of CWFs generally consists of seven nants other than pathogens, may warrant further investiga- stages that include water smoking (20°C–120°C), decom- tion in the future. position (120°C–350°C), burn-out combusts (350°C–450 °C), ceramic change (350°C–700°C), carbon burns out 2.3 Silver impregnation (700°C–900°C) and vitrification (< 1000°C) (The Cera- mics Manufacturing Working Group, 2011). A detailed recommendation of kiln construction and firing variables Silver is typically applied as an antimicrobial agent to has been provided by The Ceramics Manufacturing modify commercial CWFs after the firing process to Working Group (2011). Soppe et al. (2015) observed enhance the bacterial removal efficiency, and to prevent increased flow rate and slightly decreased microbe removal biofilm growth and recontamination after treatment. The Haiyan Yang et al. Challenges for ceramic water filter technology 5 primary mechanisms for bacterial removal using silver- 2.4 Source water quality impregnated CWFs include both physical filtration (e.g., size exclusion and straining) and chemical inactivation As CWFs are being used on a global scale for the production of safe drinking water for both household and/ (silver disinfection), and silver antibiotic activity occurs or small community, the source water quality can be during both the filtration process and the subsequent water substantially varied between different applications. While storage in the receptacle (van der Laan et al., 2014). variations in the performance of CWFs have been reported Previous studies reported that silver-impregnated CWFs from different areas or/and filter factories, these variations could exhibit up to 5 log reduction value (LRV) of bacterial were primarily attributed to the differences in local removal (i.e., 99.999%), while CWFs before silver materials and filter fabrication process, and the role of impregnation could only reach an average of 2 LRV (i.e., source water quality remains largely unknown. Among 99%) of bacterial removal (Oyanedel-Craver and Smith, various water quality factors, only limited studies have 2008; Rayner et al., 2013b). The amount of silver applied has been found to strongly influence the bacterial removal focused on turbidity and reported its negative impact on efficiency, and thus the silver dose needs to be delicately CWF performance (water production and/or flow rate) due controlled so that it is sufficient to ensure effective to pore blocking (Salvinelli and Elmore, 2015; Salvinelli et disinfection for a long time (Ren and Smith, 2013). al., 2016). For silver impregnated CWFs, both physical Therefore, understanding of factors governing the silver filtration and silver disinfection are considered important retention and release behavior from CWFs surface is pathogen removal mechanisms. Although water chemistry crucial to evaluate the performance and lifespan of silver- parameters (e.g., pH, ionic strength, presence of various impregnated CWFs. ions) may theoretically have relatively limited influence on Several factors have been reported to influence the silver pathogen removal via physical filtration, they can release behavior, including silver impregnation method, significantly affect the silver release behaviors, and thus silver type, and source water chemistry (Rayner et al., affect the filter performance and service lifespan. Detailed 2013b; Ren and Smith, 2013; van der Laan et al., 2014; and long-term laboratory and field studies are needed to Mittelman et al., 2015; Sullivan et al., 2017; Lyon-Marion further explore the performance of CWFs in various water et al., 2018). Ren and Smith (2013) compared three sources. methods to modify CWFs with silver nanoparticles (nAg) and found that the fire-in method where nAg was applied 3 Challenges and opportunities before firing significantly improved silver retention and decreased its release from CWFs, compared to the paint-on 3.1 Flow rate versus microbial removal efficiency and dipping methods, both of which were introducing nAg after the ceramic firing process. A few previous studies Flow rate and microbial removal efficiency are two reported that silver release from filters coated with silver performance indicators for CWFs. Filters that can maintain nitrate (Ag ) was greater than those coated with nAg, and the effluent silver concentration was dependent on the both high flow rates and effective microbial removal would amount of silver applied (Rayner et al., 2013b; Sullivan be particularly desirable for end users. As discussed in the et al., 2017). Additionally, source water chemistry, previous section, numerous attempts have been taken to including ionic strength, pH, ion valence, the presence of improve CWF fabrication (e.g., combustible material natural organic matter and free chlorine, has been found to processing) for the development of effective and fast- influence silver release (Rayner et al., 2013b; Mittelman flow CWFs. Table 2 summarizes the flow rate and et al., 2015; Sullivan et al., 2017; Lyon-Marion et al., microbial removal efficiency of CWFs reported in previous laboratory- and field-scale investigations. To allow for the 2018). Laboratory studies reported that the concentrations comparison of CWFs made in different shapes, the of leached silver in treated solutions were below the reported flow rates were converted to the “equivalent” drinking water standard (0.1 mg/L) (Rayner et al., 2013b; flow rates of a full-size ceramic pot filter with a frustum Mittelman et al., 2015). In practice, new silver-impreg- shape, based on the hydraulic method used in our previous nated CWFs are suggested by manufacturers to rinse with study (Yang et al., 2020). So far, the ceramic pot filter 2–3 pot full of water before use, which could lower the design (Fig. 1(c)), one of the most popular CWFs applied silver concentration in the effluent to levels below 0.1 mg/ in developing countries, can achieve a balance of 1–3 L/h L (The Ceramics Manufacturing Working Group, 2011). flow rate and ~2 LRV bacterial removal without silver Although silver impregnation shows improved bacterial impregnation (~4 LRV after silver impregnation). For a removal efficiency, there are concerns about the cost typical household size with 5 persons, at least 10 L of safe associated with the silver application, and the relatively drinking water would be needed on a daily basis (Ren short service life of conventional filters (3–5 years) et al., 2013). However, as sediment and particles clog the because the filters would become less effective when the filter pores, the flow rate decreases. Scrubbing the filter impregnated silver is depleted (Lantagne, 2001; Bielefeldt surface provides a temporary benefit but does not prevent et al., 2009). 6 Front. Environ. Sci. Eng. 2020, 14(5): 79 Table 2 Flow rate and bacterial removal efficiency of reported CWFs a) Laboratory/Field Flow rate Microbial removal Average pore size Reference/ source Porosity b) c) work (L/h) (LRV ) (mm) Yang et al. (2020) Laboratory 5.1–6.4 4.5 0.22 1.22 12.5–15.4 2.1 0.24 1.24 Oyanedel-Craver and Smith Laboratory ~2.6 3.0 (w/o Ag coating) 0.37 14.3 (2008) 4.0 (w/o Ag coating) ~1.7 2.9 (w/o Ag coating) 0.42 2.0 3.2 (w/o Ag coating) ~0.61 3.4 (w/o Ag coating) 0.39 8.2 3.8 (w/o Ag coating) Bielefeldt et al. (2009) Laboratory/Field 0.8–1.9 2.2–3.8 (w/o Ag coating) Not reported 3.2–4.2 (w/o Ag coating) van der Laan et al. (2014) Field 5.5–21.0 ~1 (w/o Ag coating) Not reported 2.55 2.5 (w/o Ag coating) Van Halem et al. (2017) Field 5–20 ~1.0 (w/o Ag coating) Not reported Soppe et al. (2015) Laboratory/Filed 7–23 2.1–2.9 (w/o Ag coating) Not reported d) PFP Field 1–3 ~2 (w/o Ag coating) Various from factory to factory Various from factory to factory e) RDIC Field 1.8–2.5 ~2 (w/o Ag coating) Note: a) Flow rate was adjusted for disk bottom diameter and water head to the full size of pot (Fig. 1(c)). Detail for adjustment can be found in Yang et al. (2020); b) LRV, log reduction value;c) Average pore diameter was obtained by the pore size distribution data measured by mercury intrusion porosimetry (MIP); d) Pottery for peace, a US-based non-profit working on ceramic water filter; e) Resource Development International-Cambodia, a non-profit working on ceramic water filter. long-term clogging. The development of filters that can a high flow rate; meanwhile, the presence of a sufficient achieve a higher flow rate while maintaining effective number of small pores is essential for achieving a microbial removal remains a challenge. Instigations into satisfactory bacterial removal efficiency. Results suggested manufacturing parameters that can be modified to achieve that fibrous materials like recycled paper fiber may both high flow rate and effective microbial removal are potentially be used as effective combustible materials for needed. CWF fabrication to create balanced and interconnected Pore size of CWFs is considered a critical parameter that pore structure. Meanwhile, Soppe et al. (2015) observed can affect both flow rate and microbial removal efficiency that average flow rate of CWFs could be increased by using (Soppe et al., 2015; Youmoue et al., 2017). CWFs were larger quantities of rice husks but without reducing the prepared using three combustible materials that include bacterial removal effectiveness. Further information and irregularly shaped rice husk, spherical/oval shaped starch analysis of the pore size distribution of these filters are and tubular shaped recycled paper fiber in our recent study needed to explore the critical factors for improved CWFs. (Yang et al., 2020). Because of the different shapes of An improved understanding of key factors that affect combustible materials, the fabricated CWFs had varied and control the pore size distribution can provide important pore size distribution patterns, resulting in substantially insights into guiding the design of CWFs with both high different flow rates and bacterial removal efficiencies. flow rate and bacterial removal. In an initial attempt, a Particularly, filters prepared using recycled paper fiber semiquantitative model was developed to explain the role showed improved performance with regards to both flow of pore size distribution on the design of optimal CWFs rate and bacterial removal. The best-performed filters (no that can balance effective microbial removal and adequate silver coating) exhibited (1) a fast equivalent flow rate of flow rate (Yang et al., 2020). With further information of 13.9 L/h while maintaining a >2 log bacterial removal pore network within porous ceramic using advanced efficiency (>99%), or (2) >4 log bacterial removal characterization and 3-dimensional imaging techniques, efficiency (>99.99%) with an equivalent flow rate of 5.9 development of more sophisticated pore-scale mathema- L/h, by simply tuning the amount of the recycled paper tical and numeric models in the future can be beneficial to fibers to 20% and 15% (i.e., with fiber-to-clay ratios of direct the design of ceramic filters with controlled pore size 20%:80% and 15%:85%), respectively. The improved distribution and improved performance. In addition, performance of filters prepared using recycled paper fibers manufacturing parameters other than combustible material could be attributed to the unique pore size distribution can affect the pore size distribution of CWFs. For instance, pattern that consists a good balance of both small and Oyanedel-Craver and Smith observed different pore size medium to large pores. Based on the classic Hagen- distributions for CWFs fabricated using three different Poiseuille law, medium to large pores are required to reach sourced clay materials, and filters prepared from redart clay Haiyan Yang et al. Challenges for ceramic water filter technology 7 had most abundance of small pores and thus highest could efficiently treat ~14500 and 3200 pore volumes of bacterial removal efficiency (Oyanedel-Craver and Smith, solutions highly contaminated with As(V) and As(III) 2008). A comprehensive investigation should be per- (~125 µg/L) below the drinking water standard (10 µg/L), formed in the future to determine the impact of individual respectively. The improved removal of As(V) and As(III) manufacturing parameters as well as their combined effect can be attributed to the formation of LaAsO surface on the pore size distribution of ceramic filters. It also precipitates and La involved inner-sphere surface com- should be mentioned that the flow rate of CWFs may plexes, respectively. Notably, the interaction between decrease after extended use in the field because of sediment chemical pollutants and La coating strongly depends on and/or particle clogging. Thus, the long-term performance the chemical composition and properties of the La coating, of ceramic filters prepared using improved recipes should which can be controlled by the coating process. Thus, a be evaluated under settings relevant to practical applica- delicate control of the La coating temperature is critical to tions. obtaining optimal La components that favor the removal of various chemical pollutants (Yang et al., 2019b). 3.2 Removal of various classes of contaminants Since most viruses hold negative charges in natural aquatic environments (pH 5–8), modification of ceramic Conventional CWFs are designed to primarily target the surface with positively charged chemical coatings has been removal of microbial pollutants with relatively large sizes applied to promote virus removal via enhanced electro- (e.g., bacteria, protozoa). Pore sizes of conventional CWFs static interaction (Table 3). Michen et al. (2013) reported are typically in the range of 0.1-100 mm (Oyanedel-Craver that ceramic filters amended with MgO increased the and Smith, 2008; Brown and Sobsey, 2010), and thus they removal of bacteriophages MS2 and PhiX174 by up to 4 are not able to retain small-sized pathogens (e.g., virus) LRV, while the filter performance decreased sharply after and commonly-found chemical pollutants (e.g., arsenic, 2000 pore volumes of treatment. Wegmann et al (2018 a, fluoride, chromate) through physical filtration. Addition- b). found that modification of ceramic microfilters with Zr ally, because of the negatively charged ceramic surface, (OH) and Y O greatly increased the isoelectric point x 2 3 CWFs with and without silver coating generally exhibit (pH ) of the filters from< 3.1 to 5.5–9 and 8–10, IEP low affinity with virus and negatively- and non-charged respectively. Thus, the Zr(OH) -and Y O -modified x 2 3 chemical pollutants, resulting in negligible capture of these ceramic filters showed remarkably increased removal pollutants through adsorption as well. For example, efficiency of MS2 (up to 7 LRV). Meanwhile, the authors previous studies reported that CWFs only had viral observed that the presence of humic acid negatively impact removal efficiencies in the range of 0.21 to 1.6 LRV in the filter performance, and the virus removal efficiency long-term testing (Brown and Sobsey, 2010; Salsali et al., also significantly decreased after extended operation 2011; van der Laan et al., 2014), with no significant (Wegmann et al., 2008b). In addition to virus removal, it differences between silver-impregnated and non-impreg- is worth mentioning that some novel coatings such as nano nated CWFs (van der Laan et al., 2014). The low removal TiO and nano ZnO have also been applied as alternative efficiency for virus and various chemical pollutants antimicrobial agents to silver for ceramic modification to represents a major challenge for conventional CWFs. improve the cost-effectiveness of bacterial removal in the Chemical pollutants are generally removed through the laboratory-scaled studies (Lucier et al., 2017; He et al., interaction (e.g., adsorption, surface precipitation) with 2018; Huang et al., 2018). surface of (modified) CWFs. To improve the removal of As more and more low-cost, environmentally benign chemical pollutants, a practical approach is to modify the and novel coatings are being explored for ceramic surface ceramic surface with desired coatings to enhance the modification, it holds promises to develop modified CWFs affinity with the target pollutants. For instance, ceramic with improved performance for the removal of various disk filter modified with ferric iron was developed to small-sized chemical and microbial pollutants. Since the improve arsenic removal, and the filter performance was properties of surface coatings and thus their interaction strongly affected by the ferric iron loading (Robbins et al., with pollutants are strongly influenced by water chemistry 2014). Recently, lanthanum (La), an abundant rare earth parameters, further investigations are required to determine element with relatively low cost, was applied in our study the long-term filter performance in both laboratory and as a novel and effective coating for ceramic materials to field scales under various source waters. The safety and target the removal of negatively-charged arsenate (As(V)) leaching behavior of the novel coatings should also be and non-charged arsenite (As(III)) (Yang et al., 2019a). carefully evaluated before field application. The La-coated ceramic material showed substantially enhanced capture of both As(V) and As(III) with the 4 Conclusions sorption capacities of 24.8 and 10.9 mg/g, respectively. On the contrary, negligible adsorption of As(V) or As(III) was CWFs, produced from local-sourced clay and combustible observed for bare ceramic material without La coating. material is an affordable, effective, low-maintenance and Furthermore, a prototype La-coated ceramic disk filter 8 Front. Environ. Sci. Eng. 2020, 14(5): 79 Table 3 Studies exploring CWFs surface modification besides silver impregnation a) Modification Modification method Target contaminants Removal efficacy/capacity References component b) Nano TiO Painted-on Escherichia coli >90% He et al. (2018) ZnO Painted-on Escherichia coli 2.19–2.97 LRV Huang et al. (2018) c) TPA Painted-on Escherichia coli 6.24 LRV Zhang and Oyanedel-Craver (raw filter 4.34 LRV) (2013) 3+ Iron oxide Submersed in Fe solution! baked Arsenite/ Treating 49–1619 bed volumes of arsenic- Robbins et al. (2014) at 110°C (4h)! 550°C (3h) As(III) contaminated solution under 10 mg/L Arsenate/ As(V) 3+ Lanthanum Submersed in La solutio! ther- Arsenite/ Treating ~3200 pore volumes of As(III)- Yang et al. (2019a,b) components mally treated for 3h As(III) contaminated solution under 10 mg/L Arsenate/ Treating ~14500 pore volumes of As(V)- As(V) contaminated solution under 10 mg/L Chromate/ 13 mg/g Cr(VI) Virus (MS2) >5LRV Y O Submersed in Y O colloids! dried virus (MS2) Up to 6.5 LRV Wegmann et al. (2008a) 2 3 2 3 at 80°C (12h)! calcined at 500°C– 1040°C (1h) Zr(OH) Submersed in Zr(OH) colloids! virus (MS2) 6.2–6.6 LRV (pH5) Wegmann et al. (2008b) x x dried at 150°C (12h)! calcined at 4.0–6.9 LRV (pH7) 250/300/400°C (1h) 3.7–7.4 LRV (pH9) d) MgO Fired-in virus (MS2, PhiX174) 0.3–4.7 LRV (MS2) Michen et al. (2013) 0–4 LRV (PhiX174) Note: a) Removal efficacy/capacity: removal capacity was used to indicate CWFs performance for chemical pollutant removal; b) Painted-on: using a paint brush to paint the coating chemical solution to filter surface, then air-dried; c) TPA: poly (trihydroxysilyl) propyldimethyloctadecyl ammonium chloride; d) Fired-in: coating chemical was added to clay mixture before firing. Wisconsin Milwaukee Catalyst Grant (MIL113501). The authors declare no sustainable technology appropriate for POU household competing financial interest. water treatment in developing areas. This account focuses on the critical factors for CWFs performance, including Open Access This article is licensed under a Creative Commons manufacturing processes and source water quality. Two Attribution 4.0 International License, which permits use, sharing, adaptation, technical challenges that limit the further application of distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the CWFs are discussed in detail, including simultaneous Creative Commons licence, and indicate if changes were made. The images improvement of filter flow rate and bacterial removal, and or other third party material in this article are included in the article’s Creative efficient removal of various chemical and microbial Commons licence, unless indicated otherwise in a credit line to the material. pollutants. Great efforts from researchers and CWFs If material is not included in the article’s Creative Commons licence and your manufacturers have been taken to (1) improve CWFs intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To with both high flow rate and effective bacterial removal by view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. optimizing pore properties during the filter manufacturing processes, as well as (2) enhance CWFs removal efficacy of various waterborne contaminants by novel chemical References modification on ceramic surface. According to the brief summary of progress for CWFs research and application in Abebe L S, Chen X, Sobsey M D (2016). 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Best practice Milwaukee. She currently joined South recommendations for local manufacturing of ceramic pot filters for China Normal University. Her research household water treatment. Atlant, GA, USA: Ceramics Manufac- focuses on the sustainable point-of-use turing Working Group water treatment and the fate of colloidal van der Laan H, Van Halem D, Smeets P, Soppe A, Kroesbergen J, contaminants. Wubbels G, Nederstigt J, Gensburger I, Heijman S (2014). Bacteria and virus removal effectiveness of ceramic pot filters with different silver applications in a long term experiment. Water Research, 51: Shangping Xu received his B.S. degree 47–54 from Peking University, and M.S and Van Halem D (2006). Ceramic silver impregnated pot filters for Ph.D. degrees from Princeton University. household drinking water treatment in developing countries. Delft, He is currently an Associate Professor at Netherlands: Delft University of Technology the University of Wisconsin – Milwaukee. Van Halem D, Van Der Laan H, Soppe A, Heijman S (2017). High flow His research interests include the transport ceramic pot filters. Water Research, 124: 398–406 of contaminant within the environment, Wegmann M, Michen B, Graule T (2008a). Nanostructured surface effects of global climate change on water modification of microporous ceramics for efficient virus filtration. resources and the development of water Journal of the European Ceramic Society, 28(8): 1603–1612 treatment techniques. Wegmann M, Michen B, Luxbacher T, Fritsch J, Graule T (2008b). Modification of ceramic microfilters with colloidal zirconia to Derek E Chitwood has a B.S. degree in promote the adsorption of viruses from water. Water Research, 42 aerospace engineering and Ph.D. in envir- (6–7): 1726–1734 onmental engineering, both from the Uni- World Health Organization (2017). Progress on drinking water sanitation versity of Southern California. He spent 18 and hygiene: 2017 update and SDG baselines. Geneva: World Health years working in rural southwest China. Organization His research focus is studying mountain World Health Organization (2019a). Progress on household drinking spring water quality and point of use water water, sanitation and hygiene 2000–2017: Special focus on inequal- filters for rural peoples in developing ities. Geneva: World Health Organization communities. World Health Organization (2019b). Drinking-water Fact sheet. New York: World Health Organization Yang H, Min X, Xu S, Bender J, Wang Y (2020). Development of Yin Wang received his B.S. degree from effective and fast-flow ceramic porous media for point-of-use water Peking University, and M.S. and Ph.D. treatment: Effect of pore size distribution. ACS Sustainable degrees from Washington University in St. Chemistry & Engineering, 8(6): 2531–2539 Louis. He is currently an Assistant Profes- Yang H, Min X, Xu S, Wang Y (2019a). Lanthanum(III)-coated ceramics sor at the University of Wisconsin – as a promising material in point-of-use water treatment for arsenite Milwaukee. His research focuses on the and arsenate removal. ACS Sustainable Chemistry & Engineering, 7 development of efficient and sustainable (10): 9220–9227 solutions to address water-related grand Yang H, Wang Y, Bender J, Xu S (2019b). Removal of arsenate and challenges. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Frontiers of Environmental Science & Engineering Springer Journals

Ceramic water filter for point-of-use water treatment in developing countries: Principles, challenges and opportunities

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10.1007/s11783-020-1254-9
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Abstract

Front. Environ. Sci. Eng. 2020, 14(5): 79 https://doi.org/10.1007/s11783-020-1254-9 REVIEW ARTICLE Ceramic water filter for point-of-use water treatment in developing countries: Principles, challenges and opportunities 1,2 3 4 5 Haiyan Yang , Shangping Xu , Derek E. Chitwood , Yin Wang (✉) 1 SCNU Environmental Research Institute, Guangdong Provincial Key Laboratory of Chemical Pollution and Environmental Safety & MOE Key Laboratory of Theoretical Chemistry of Environment, South China Normal University, Guangzhou 510006, China 2 School of Environment, South China Normal University, University Town, Guangzhou 510006, China 3 Department of Geosciences, University of Wisconsin–Milwaukee, Milwaukee, WI 53211, USA 4 Department of Engineering, Dordt University, Sioux Center, IA 51250, USA 5 Department of Civil and Environmental Engineering, University of Wisconsin–Milwaukee, Milwaukee, WI 53211, USA HIGH LIGHTS GRAPHIC A BSTRA C T � CWF is a sustainable POU water treatment method for developing areas. � CWF manufacturing process is critical for its filtration performance. � Simultaneous increase of flow rate and pathogen removal is a challenge. � Control of pore size distribution holds promises to improve CWF efficiency. � Novel coatings of CWFs are a promising method to improve contaminant removal. ABSTRA CT AR TICL E I N F O Drinking water source contamination poses a great threat to human health in developing countries. Article history: Point-of-use (POU) water treatment techniques, which improve drinking water quality at the household level, offer an affordable and convenient way to obtain safe drinking water and thus can Received 14 January 2020 reduce the outbreaks of waterborne diseases. Ceramic water filters (CWFs), fabricated from locally Revised 18 March 2020 sourced materials and manufactured by local labor, are one of the most socially acceptable POU water Accepted 21 March 2020 treatment technologies because of their effectiveness, low-cost and ease of use. This review concisely summarizes the critical factors that influence the performance of CWFs, including (1) CWF Available online 16 May 2020 manufacturing process (raw material selection, firing process, silver impregnation), and (2) source water quality. Then, an in-depth discussion is presented with emphasis on key research efforts to Keywords: address two major challenges of conventional CWFs, including (1) simultaneous increase of filter flow rate and bacterial removal efficiency, and (2) removal of various concerning pollutants, such as viruses Point-of-use water treatment and metal(loid)s. To promote the application of CWFs, future research directions can focus on: (1) Ceramic water filter investigation of pore size distribution and pore structure to achieve higher flow rates and effective Bacterial removal pathogen removal by elucidating pathogen transport in porous ceramic and adjusting manufacture Surface modification parameters; and (2) exploration of new surface modification approaches with enhanced interaction Water quality between a variety of contaminants and ceramic surfaces. © The Author(s) 2020. This article is published with open access at link.springer.com and journal.hep. com.cn 2020 1 Introduction ✉ Corresponding author E-mail: wang292@uwm.edu World Health Organization (WHO) reported that 71% of Special Issue—Accounts of Aquatic Chemistry and Technology the world’s population used improved drinking water Research (Responsible Editors: Jinyong Liu, Haoran Wei & Yin Wang) 2 Front. Environ. Sci. Eng. 2020, 14(5): 79 sources that were free from microbiological and priority 2008; Clasen et al., 2015). CWFs, produced from a mixture chemical contamination in 2017 (World Health Organiza- of clay and sieved combustible material, can effectively tion, 2017,2019a). However, there are vast inequalities remove microorganisms and reduce turbidity from water among different countries and areas. For example, by gravity filtration through porous ceramic media. CWFs although 85% of world urban population could access can be fabricated into different shapes, such as disk, tube, quality drinking water that is free from contamination, the candle and pot (Fig. 1), which make them suitable for POU percentage is down to 53% in rural areas. A large effort is application in various situations. Silver impregnation has still needed in rural areas, especially in developing been conventionally applied to modify CWFs to enhance countries where pathogen contamination of drinking their performance, particularly for the removal of micro- water represents one of the greatest threats to human organisms. Silver-modified CWFs are effective at remov- health (World Health Organization, 2019b). What’s worse, in developing areas such as sub-Saharan Africa, Central and Southern Asia and Oceania, centralized water treat- ment and distribution systems are often unavailable due to political or socioeconomic factors (Peter-Varbanets et al., 2009; World Health Organization, 2019a). In these areas, point-of-use (POU) systems, which improve drinking water quality at the household level, represent a more affordable and effective way to produce water free from contamination. In practice, a POU technology should be effective, easy to operate, with low maintenance requirement, economic- ally viable, environmentally sustainable and sociocultu- rally acceptable. Chlorination with safe storage, combined coagulant-chlorine disinfection systems, solar disinfection, ceramic water filter (CWF) and biosand filter are examples of current POU technologies (Sobsey et al., 2008; Murphy et al., 2010; Kallman et al., 2011; Geremew et al., 2018). Based on a comprehensive comparison of these technol- ogies (Table 1), CWF is considered among the most practical and sustainable POU technologies with afford- able (highly cost effective) and low-maintenance (easy to Fig. 1 Schematic of different forms of ceramic water filters: (a) use, only periodic cleaning needed) features for household ceramic disk filter, (b) ceramic candle filter, (c) ceramic pot filter, (d) tubular ceramic filter. water treatment in developing countries (Sobsey et al., a) Table 1 Comparison of POU treatment technologies used in developing countries POU Water Water Overall d) e) Cost Ease to use Description b) c) technologies quality production score Chlorination 1 3 2 5 11 Hypochlorite liquid or tablets are used to inactivate pathogens in source water. Coagulation/ 2 2 1 2 7 Dry coagulant-flocculant and chlorine as tablets or sachets Chlorination are added to source water to inactive and settle down pathogens. Solar disinfec- 3 1 5 1 10 Source water is filled in polyethylene terephthalate (PET) or tion glass under sunlight, allowing UV and heat to inactivate pathogens. Ceramic water 5 2 4 4 15 Porous ceramic media (e.g., pot, disk, candle) with silver filter coating is used to filter pathogens from source water. Biosand filter 4 3 3 3 13 Biosand filter is adapted from slow sand filter cover with biofilm, removing pathogens using biological and physical processes. Note: a) The scoring system was used to compare POU technologies. For each criterion, a technology was ranked from best (with a score of 5) to worst (with a score of 1) and the performance score (5 to 1) represents its relative ranking; b) This score was based on their reduction in Diarrhea reported by Clasen et al. (2015); c) Water production was scored based on producing 20 L within 4 h of applying the treatment described by Sobsey et al. (2008). Due to the different water production methods for these POU technology, the score was only set to 1-3 with 1 for low, 2 for fair, and 3 for good performance; d) This score was based on their cost (dollars/L/year) calculated in Sobsey et al. (2008); e) This score was based on the steps of treatment processes and periodic maintenance. Haiyan Yang et al. Challenges for ceramic water filter technology 3 ing >99% of protozoan (Van Halem, 2006; Abebe et al., 2.1 Raw material selection 2015) and 90%–99.999% of bacteria (Brown and Sobsey, 2010) and thus have been evaluated for efficacy in 2.1.1 Clay source reducing rates of diarrheal and other waterborne diseases (Clasen et al., 2004; Morris et al., 2018). Unlike chlorine or Clay is the skeleton element of CWF, and its quality and thermal disinfection, CWFs do not significantly change the characteristics directly influence filter quality. Earthenware taste or temperature of the water. They are also very clay is most suitable for ceramic filter production due to its effective at reducing the turbidity of the water (Abebe porosity, availability, and low temperature needed for et al., 2016). With proper care as suggested by manufac- vitrification. In practice, clay physical properties (e.g., turers in the user manual, commercial ceramic pot filters, shrinkage, plasticity, workability), the persistent availabil- one of the most popular CWFs in the developing countries, ity of clay source, and the leachability of heavy metals/ have a potential service life of up to 5 years to effectively inorganic materials are some key parameters to be remove pathogens from raw water without the need for an considered for clay source selection (The Ceramics external energy source or consumable supplies (The Manufacturing Working Group, 2011). Details for clay Ceramics Manufacturing Working Group, 2011). So far, source test, selection and handling have been described in locally produced CWFs have been successfully applied in the instruction manuals of Potters for Peace (PFP) and more than 20 developing countries (e.g., Cambodia, Resource Development International (RDI) (Hagan et al., Guatemala) to improve the drinking water quality (Rayner 2009; The Ceramics Manufacturing Working Group, et al., 2013a). It was estimated that more than 4 million 2011). Although clay does not create pore structure people worldwide used CWFs to prepare daily safe directly, Oyanedel-Craver and Smith (2008) found that drinking water, which contributed to public health, social filters made from different clay sources showed varied and economic benefits (van der Laan et al., 2014). porosity, hydraulic conductivity and bacterial removal At the same time, CWFs have some disadvantages that efficiency. A recent study suggested that high content of limit their application. First, the relatively low water sand-rich clay may contribute to the decrease of mean pore filtration rates have limited their use. Improved CWF size of CWF and thus potentially influence filter flow rate designs that can simultaneously achieve both high flow and bacterial removal efficacy (Youmoue et al., 2017). rate and effective bacterial removal are desirable but Additionally, the inclusion of diatomaceous earth and remain a great challenge. In addition, CWFs often have MgO components in the CWFs were found to be beneficial limited capability of removing contaminants such as for viral removal (Michen et al., 2012; Michen et al., viruses and chemical pollutants (Yang et al., 2019a). 2013), suggesting that clay composition may also have Although the CWF technology has been gaining some influence on filter performance. growing research attentions and witnessing a wide application during the past decades, there have been few 2.1.2 Organic combustible material critical reviews on the progress of this technology. The objective of this account is to provide 1) a concise In the CWF manufacturing process, sieved organic summary of the progress of recent CWF research and combustible material, also known as burn-out material, is application with a particular focus on the critical factors mixed with raw clay for the purpose of creating a paste and processes that are closely related to CWF performance, which is subsequently pressed into various shaped filters. and 2) an overview of challenges and opportunities for The organic combustible materials are usually burnt out further improvement for CWF as a POU water treatment during the firing process to create porous structure that technology in developing areas. serve as both flow paths for water passage and reaction sites for the removal of various contaminants such as bacteria and virus. The combustible material thus has been 2 Critical factors for CWF performance identified as a critical parameter on the performance of CWFs. Sawdust and rice husk are the most common CWFs are fabricated from a mixture of clay and combustible materials used in manufacturing CWFs and a combustible materials (e.g., rice husks or sawdust) referred global-wide survey reported that 17 in 18 CWF production as burn-out materials. Filter porous structure is created as a factories used sawdust or rice husk as the combustible result of the burn-out during the firing process. As shown material (Rayner et al., 2013a). In addition, coffee husks, in Fig. 2, fabrication of conventional ceramic filter includes peanut husks and other organic materials that can be raw material selection and processing, mixing and pulverized to a suitable size can also be used as pressing, drying and firing, silver application, as well as combustible materials, depending on their cost and local quality testing. In this section, we focus on the processes availability. According to previous laboratory and field whose effects on filter effectiveness have been documented studies, the size and amount of combustible material have a by published reports and peer-reviewed references. strong influence on the filter performance (Kallman et al., 4 Front. Environ. Sci. Eng. 2020, 14(5): 79 Fig. 2 Ceramic water filter production flow chart. It was re-drawn based on the Ceramics Manufacturing Working Group (2011). 2011; van der Laan et al., 2014; Soppe et al., 2015; Rayner efficiency when the maximum firing temperature increased et al., 2017; Van Halem et al., 2017). For example, Van from 800°C to 950 °C, which may be attributed to the Halem et al. (2017) produced pot filters with a higher flow small increase of filter pore size with increasing maximum rate (2-6 times as regular pot filters) by increasing the rice firing temperature. Additionally, the incomplete combus- husk content from 24% to 31% (wt.% of dry clay mixture). tion of the organic materials could lead to the production of Combustible material with a larger particle size has also substances such as black carbon cores inside the filter been reported to introduce larger pore sizes of the resulting during the firing of commercial CWFs (The Ceramics CWF, which could facilitate a higher flow rate but often Manufacturing Working Group, 2011; Goodwin et al., resulted in lower microbial removal efficacy (Soppe et al., 2017). Currently, black CWF was reported to be fired in a 2015; Rayner et al., 2017). reductive atmosphere to improve its virus removal performance (Guerrero-Latorre et al., 2019). The potential 2.2 Firing process impact of the firing atmosphere on the performance of the resulting CWFs, especially for the removal of contami- The firing process of CWFs generally consists of seven nants other than pathogens, may warrant further investiga- stages that include water smoking (20°C–120°C), decom- tion in the future. position (120°C–350°C), burn-out combusts (350°C–450 °C), ceramic change (350°C–700°C), carbon burns out 2.3 Silver impregnation (700°C–900°C) and vitrification (< 1000°C) (The Cera- mics Manufacturing Working Group, 2011). A detailed recommendation of kiln construction and firing variables Silver is typically applied as an antimicrobial agent to has been provided by The Ceramics Manufacturing modify commercial CWFs after the firing process to Working Group (2011). Soppe et al. (2015) observed enhance the bacterial removal efficiency, and to prevent increased flow rate and slightly decreased microbe removal biofilm growth and recontamination after treatment. The Haiyan Yang et al. Challenges for ceramic water filter technology 5 primary mechanisms for bacterial removal using silver- 2.4 Source water quality impregnated CWFs include both physical filtration (e.g., size exclusion and straining) and chemical inactivation As CWFs are being used on a global scale for the production of safe drinking water for both household and/ (silver disinfection), and silver antibiotic activity occurs or small community, the source water quality can be during both the filtration process and the subsequent water substantially varied between different applications. While storage in the receptacle (van der Laan et al., 2014). variations in the performance of CWFs have been reported Previous studies reported that silver-impregnated CWFs from different areas or/and filter factories, these variations could exhibit up to 5 log reduction value (LRV) of bacterial were primarily attributed to the differences in local removal (i.e., 99.999%), while CWFs before silver materials and filter fabrication process, and the role of impregnation could only reach an average of 2 LRV (i.e., source water quality remains largely unknown. Among 99%) of bacterial removal (Oyanedel-Craver and Smith, various water quality factors, only limited studies have 2008; Rayner et al., 2013b). The amount of silver applied has been found to strongly influence the bacterial removal focused on turbidity and reported its negative impact on efficiency, and thus the silver dose needs to be delicately CWF performance (water production and/or flow rate) due controlled so that it is sufficient to ensure effective to pore blocking (Salvinelli and Elmore, 2015; Salvinelli et disinfection for a long time (Ren and Smith, 2013). al., 2016). For silver impregnated CWFs, both physical Therefore, understanding of factors governing the silver filtration and silver disinfection are considered important retention and release behavior from CWFs surface is pathogen removal mechanisms. Although water chemistry crucial to evaluate the performance and lifespan of silver- parameters (e.g., pH, ionic strength, presence of various impregnated CWFs. ions) may theoretically have relatively limited influence on Several factors have been reported to influence the silver pathogen removal via physical filtration, they can release behavior, including silver impregnation method, significantly affect the silver release behaviors, and thus silver type, and source water chemistry (Rayner et al., affect the filter performance and service lifespan. Detailed 2013b; Ren and Smith, 2013; van der Laan et al., 2014; and long-term laboratory and field studies are needed to Mittelman et al., 2015; Sullivan et al., 2017; Lyon-Marion further explore the performance of CWFs in various water et al., 2018). Ren and Smith (2013) compared three sources. methods to modify CWFs with silver nanoparticles (nAg) and found that the fire-in method where nAg was applied 3 Challenges and opportunities before firing significantly improved silver retention and decreased its release from CWFs, compared to the paint-on 3.1 Flow rate versus microbial removal efficiency and dipping methods, both of which were introducing nAg after the ceramic firing process. A few previous studies Flow rate and microbial removal efficiency are two reported that silver release from filters coated with silver performance indicators for CWFs. Filters that can maintain nitrate (Ag ) was greater than those coated with nAg, and the effluent silver concentration was dependent on the both high flow rates and effective microbial removal would amount of silver applied (Rayner et al., 2013b; Sullivan be particularly desirable for end users. As discussed in the et al., 2017). Additionally, source water chemistry, previous section, numerous attempts have been taken to including ionic strength, pH, ion valence, the presence of improve CWF fabrication (e.g., combustible material natural organic matter and free chlorine, has been found to processing) for the development of effective and fast- influence silver release (Rayner et al., 2013b; Mittelman flow CWFs. Table 2 summarizes the flow rate and et al., 2015; Sullivan et al., 2017; Lyon-Marion et al., microbial removal efficiency of CWFs reported in previous laboratory- and field-scale investigations. To allow for the 2018). Laboratory studies reported that the concentrations comparison of CWFs made in different shapes, the of leached silver in treated solutions were below the reported flow rates were converted to the “equivalent” drinking water standard (0.1 mg/L) (Rayner et al., 2013b; flow rates of a full-size ceramic pot filter with a frustum Mittelman et al., 2015). In practice, new silver-impreg- shape, based on the hydraulic method used in our previous nated CWFs are suggested by manufacturers to rinse with study (Yang et al., 2020). So far, the ceramic pot filter 2–3 pot full of water before use, which could lower the design (Fig. 1(c)), one of the most popular CWFs applied silver concentration in the effluent to levels below 0.1 mg/ in developing countries, can achieve a balance of 1–3 L/h L (The Ceramics Manufacturing Working Group, 2011). flow rate and ~2 LRV bacterial removal without silver Although silver impregnation shows improved bacterial impregnation (~4 LRV after silver impregnation). For a removal efficiency, there are concerns about the cost typical household size with 5 persons, at least 10 L of safe associated with the silver application, and the relatively drinking water would be needed on a daily basis (Ren short service life of conventional filters (3–5 years) et al., 2013). However, as sediment and particles clog the because the filters would become less effective when the filter pores, the flow rate decreases. Scrubbing the filter impregnated silver is depleted (Lantagne, 2001; Bielefeldt surface provides a temporary benefit but does not prevent et al., 2009). 6 Front. Environ. Sci. Eng. 2020, 14(5): 79 Table 2 Flow rate and bacterial removal efficiency of reported CWFs a) Laboratory/Field Flow rate Microbial removal Average pore size Reference/ source Porosity b) c) work (L/h) (LRV ) (mm) Yang et al. (2020) Laboratory 5.1–6.4 4.5 0.22 1.22 12.5–15.4 2.1 0.24 1.24 Oyanedel-Craver and Smith Laboratory ~2.6 3.0 (w/o Ag coating) 0.37 14.3 (2008) 4.0 (w/o Ag coating) ~1.7 2.9 (w/o Ag coating) 0.42 2.0 3.2 (w/o Ag coating) ~0.61 3.4 (w/o Ag coating) 0.39 8.2 3.8 (w/o Ag coating) Bielefeldt et al. (2009) Laboratory/Field 0.8–1.9 2.2–3.8 (w/o Ag coating) Not reported 3.2–4.2 (w/o Ag coating) van der Laan et al. (2014) Field 5.5–21.0 ~1 (w/o Ag coating) Not reported 2.55 2.5 (w/o Ag coating) Van Halem et al. (2017) Field 5–20 ~1.0 (w/o Ag coating) Not reported Soppe et al. (2015) Laboratory/Filed 7–23 2.1–2.9 (w/o Ag coating) Not reported d) PFP Field 1–3 ~2 (w/o Ag coating) Various from factory to factory Various from factory to factory e) RDIC Field 1.8–2.5 ~2 (w/o Ag coating) Note: a) Flow rate was adjusted for disk bottom diameter and water head to the full size of pot (Fig. 1(c)). Detail for adjustment can be found in Yang et al. (2020); b) LRV, log reduction value;c) Average pore diameter was obtained by the pore size distribution data measured by mercury intrusion porosimetry (MIP); d) Pottery for peace, a US-based non-profit working on ceramic water filter; e) Resource Development International-Cambodia, a non-profit working on ceramic water filter. long-term clogging. The development of filters that can a high flow rate; meanwhile, the presence of a sufficient achieve a higher flow rate while maintaining effective number of small pores is essential for achieving a microbial removal remains a challenge. Instigations into satisfactory bacterial removal efficiency. Results suggested manufacturing parameters that can be modified to achieve that fibrous materials like recycled paper fiber may both high flow rate and effective microbial removal are potentially be used as effective combustible materials for needed. CWF fabrication to create balanced and interconnected Pore size of CWFs is considered a critical parameter that pore structure. Meanwhile, Soppe et al. (2015) observed can affect both flow rate and microbial removal efficiency that average flow rate of CWFs could be increased by using (Soppe et al., 2015; Youmoue et al., 2017). CWFs were larger quantities of rice husks but without reducing the prepared using three combustible materials that include bacterial removal effectiveness. Further information and irregularly shaped rice husk, spherical/oval shaped starch analysis of the pore size distribution of these filters are and tubular shaped recycled paper fiber in our recent study needed to explore the critical factors for improved CWFs. (Yang et al., 2020). Because of the different shapes of An improved understanding of key factors that affect combustible materials, the fabricated CWFs had varied and control the pore size distribution can provide important pore size distribution patterns, resulting in substantially insights into guiding the design of CWFs with both high different flow rates and bacterial removal efficiencies. flow rate and bacterial removal. In an initial attempt, a Particularly, filters prepared using recycled paper fiber semiquantitative model was developed to explain the role showed improved performance with regards to both flow of pore size distribution on the design of optimal CWFs rate and bacterial removal. The best-performed filters (no that can balance effective microbial removal and adequate silver coating) exhibited (1) a fast equivalent flow rate of flow rate (Yang et al., 2020). With further information of 13.9 L/h while maintaining a >2 log bacterial removal pore network within porous ceramic using advanced efficiency (>99%), or (2) >4 log bacterial removal characterization and 3-dimensional imaging techniques, efficiency (>99.99%) with an equivalent flow rate of 5.9 development of more sophisticated pore-scale mathema- L/h, by simply tuning the amount of the recycled paper tical and numeric models in the future can be beneficial to fibers to 20% and 15% (i.e., with fiber-to-clay ratios of direct the design of ceramic filters with controlled pore size 20%:80% and 15%:85%), respectively. The improved distribution and improved performance. In addition, performance of filters prepared using recycled paper fibers manufacturing parameters other than combustible material could be attributed to the unique pore size distribution can affect the pore size distribution of CWFs. For instance, pattern that consists a good balance of both small and Oyanedel-Craver and Smith observed different pore size medium to large pores. Based on the classic Hagen- distributions for CWFs fabricated using three different Poiseuille law, medium to large pores are required to reach sourced clay materials, and filters prepared from redart clay Haiyan Yang et al. Challenges for ceramic water filter technology 7 had most abundance of small pores and thus highest could efficiently treat ~14500 and 3200 pore volumes of bacterial removal efficiency (Oyanedel-Craver and Smith, solutions highly contaminated with As(V) and As(III) 2008). A comprehensive investigation should be per- (~125 µg/L) below the drinking water standard (10 µg/L), formed in the future to determine the impact of individual respectively. The improved removal of As(V) and As(III) manufacturing parameters as well as their combined effect can be attributed to the formation of LaAsO surface on the pore size distribution of ceramic filters. It also precipitates and La involved inner-sphere surface com- should be mentioned that the flow rate of CWFs may plexes, respectively. Notably, the interaction between decrease after extended use in the field because of sediment chemical pollutants and La coating strongly depends on and/or particle clogging. Thus, the long-term performance the chemical composition and properties of the La coating, of ceramic filters prepared using improved recipes should which can be controlled by the coating process. Thus, a be evaluated under settings relevant to practical applica- delicate control of the La coating temperature is critical to tions. obtaining optimal La components that favor the removal of various chemical pollutants (Yang et al., 2019b). 3.2 Removal of various classes of contaminants Since most viruses hold negative charges in natural aquatic environments (pH 5–8), modification of ceramic Conventional CWFs are designed to primarily target the surface with positively charged chemical coatings has been removal of microbial pollutants with relatively large sizes applied to promote virus removal via enhanced electro- (e.g., bacteria, protozoa). Pore sizes of conventional CWFs static interaction (Table 3). Michen et al. (2013) reported are typically in the range of 0.1-100 mm (Oyanedel-Craver that ceramic filters amended with MgO increased the and Smith, 2008; Brown and Sobsey, 2010), and thus they removal of bacteriophages MS2 and PhiX174 by up to 4 are not able to retain small-sized pathogens (e.g., virus) LRV, while the filter performance decreased sharply after and commonly-found chemical pollutants (e.g., arsenic, 2000 pore volumes of treatment. Wegmann et al (2018 a, fluoride, chromate) through physical filtration. Addition- b). found that modification of ceramic microfilters with Zr ally, because of the negatively charged ceramic surface, (OH) and Y O greatly increased the isoelectric point x 2 3 CWFs with and without silver coating generally exhibit (pH ) of the filters from< 3.1 to 5.5–9 and 8–10, IEP low affinity with virus and negatively- and non-charged respectively. Thus, the Zr(OH) -and Y O -modified x 2 3 chemical pollutants, resulting in negligible capture of these ceramic filters showed remarkably increased removal pollutants through adsorption as well. For example, efficiency of MS2 (up to 7 LRV). Meanwhile, the authors previous studies reported that CWFs only had viral observed that the presence of humic acid negatively impact removal efficiencies in the range of 0.21 to 1.6 LRV in the filter performance, and the virus removal efficiency long-term testing (Brown and Sobsey, 2010; Salsali et al., also significantly decreased after extended operation 2011; van der Laan et al., 2014), with no significant (Wegmann et al., 2008b). In addition to virus removal, it differences between silver-impregnated and non-impreg- is worth mentioning that some novel coatings such as nano nated CWFs (van der Laan et al., 2014). The low removal TiO and nano ZnO have also been applied as alternative efficiency for virus and various chemical pollutants antimicrobial agents to silver for ceramic modification to represents a major challenge for conventional CWFs. improve the cost-effectiveness of bacterial removal in the Chemical pollutants are generally removed through the laboratory-scaled studies (Lucier et al., 2017; He et al., interaction (e.g., adsorption, surface precipitation) with 2018; Huang et al., 2018). surface of (modified) CWFs. To improve the removal of As more and more low-cost, environmentally benign chemical pollutants, a practical approach is to modify the and novel coatings are being explored for ceramic surface ceramic surface with desired coatings to enhance the modification, it holds promises to develop modified CWFs affinity with the target pollutants. For instance, ceramic with improved performance for the removal of various disk filter modified with ferric iron was developed to small-sized chemical and microbial pollutants. Since the improve arsenic removal, and the filter performance was properties of surface coatings and thus their interaction strongly affected by the ferric iron loading (Robbins et al., with pollutants are strongly influenced by water chemistry 2014). Recently, lanthanum (La), an abundant rare earth parameters, further investigations are required to determine element with relatively low cost, was applied in our study the long-term filter performance in both laboratory and as a novel and effective coating for ceramic materials to field scales under various source waters. The safety and target the removal of negatively-charged arsenate (As(V)) leaching behavior of the novel coatings should also be and non-charged arsenite (As(III)) (Yang et al., 2019a). carefully evaluated before field application. The La-coated ceramic material showed substantially enhanced capture of both As(V) and As(III) with the 4 Conclusions sorption capacities of 24.8 and 10.9 mg/g, respectively. On the contrary, negligible adsorption of As(V) or As(III) was CWFs, produced from local-sourced clay and combustible observed for bare ceramic material without La coating. material is an affordable, effective, low-maintenance and Furthermore, a prototype La-coated ceramic disk filter 8 Front. Environ. Sci. Eng. 2020, 14(5): 79 Table 3 Studies exploring CWFs surface modification besides silver impregnation a) Modification Modification method Target contaminants Removal efficacy/capacity References component b) Nano TiO Painted-on Escherichia coli >90% He et al. (2018) ZnO Painted-on Escherichia coli 2.19–2.97 LRV Huang et al. (2018) c) TPA Painted-on Escherichia coli 6.24 LRV Zhang and Oyanedel-Craver (raw filter 4.34 LRV) (2013) 3+ Iron oxide Submersed in Fe solution! baked Arsenite/ Treating 49–1619 bed volumes of arsenic- Robbins et al. (2014) at 110°C (4h)! 550°C (3h) As(III) contaminated solution under 10 mg/L Arsenate/ As(V) 3+ Lanthanum Submersed in La solutio! ther- Arsenite/ Treating ~3200 pore volumes of As(III)- Yang et al. (2019a,b) components mally treated for 3h As(III) contaminated solution under 10 mg/L Arsenate/ Treating ~14500 pore volumes of As(V)- As(V) contaminated solution under 10 mg/L Chromate/ 13 mg/g Cr(VI) Virus (MS2) >5LRV Y O Submersed in Y O colloids! dried virus (MS2) Up to 6.5 LRV Wegmann et al. (2008a) 2 3 2 3 at 80°C (12h)! calcined at 500°C– 1040°C (1h) Zr(OH) Submersed in Zr(OH) colloids! virus (MS2) 6.2–6.6 LRV (pH5) Wegmann et al. (2008b) x x dried at 150°C (12h)! calcined at 4.0–6.9 LRV (pH7) 250/300/400°C (1h) 3.7–7.4 LRV (pH9) d) MgO Fired-in virus (MS2, PhiX174) 0.3–4.7 LRV (MS2) Michen et al. (2013) 0–4 LRV (PhiX174) Note: a) Removal efficacy/capacity: removal capacity was used to indicate CWFs performance for chemical pollutant removal; b) Painted-on: using a paint brush to paint the coating chemical solution to filter surface, then air-dried; c) TPA: poly (trihydroxysilyl) propyldimethyloctadecyl ammonium chloride; d) Fired-in: coating chemical was added to clay mixture before firing. Wisconsin Milwaukee Catalyst Grant (MIL113501). The authors declare no sustainable technology appropriate for POU household competing financial interest. water treatment in developing areas. This account focuses on the critical factors for CWFs performance, including Open Access This article is licensed under a Creative Commons manufacturing processes and source water quality. Two Attribution 4.0 International License, which permits use, sharing, adaptation, technical challenges that limit the further application of distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the CWFs are discussed in detail, including simultaneous Creative Commons licence, and indicate if changes were made. The images improvement of filter flow rate and bacterial removal, and or other third party material in this article are included in the article’s Creative efficient removal of various chemical and microbial Commons licence, unless indicated otherwise in a credit line to the material. pollutants. Great efforts from researchers and CWFs If material is not included in the article’s Creative Commons licence and your manufacturers have been taken to (1) improve CWFs intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To with both high flow rate and effective bacterial removal by view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. optimizing pore properties during the filter manufacturing processes, as well as (2) enhance CWFs removal efficacy of various waterborne contaminants by novel chemical References modification on ceramic surface. According to the brief summary of progress for CWFs research and application in Abebe L S, Chen X, Sobsey M D (2016). 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Best practice Milwaukee. She currently joined South recommendations for local manufacturing of ceramic pot filters for China Normal University. Her research household water treatment. Atlant, GA, USA: Ceramics Manufac- focuses on the sustainable point-of-use turing Working Group water treatment and the fate of colloidal van der Laan H, Van Halem D, Smeets P, Soppe A, Kroesbergen J, contaminants. Wubbels G, Nederstigt J, Gensburger I, Heijman S (2014). Bacteria and virus removal effectiveness of ceramic pot filters with different silver applications in a long term experiment. Water Research, 51: Shangping Xu received his B.S. degree 47–54 from Peking University, and M.S and Van Halem D (2006). Ceramic silver impregnated pot filters for Ph.D. degrees from Princeton University. household drinking water treatment in developing countries. Delft, He is currently an Associate Professor at Netherlands: Delft University of Technology the University of Wisconsin – Milwaukee. Van Halem D, Van Der Laan H, Soppe A, Heijman S (2017). High flow His research interests include the transport ceramic pot filters. Water Research, 124: 398–406 of contaminant within the environment, Wegmann M, Michen B, Graule T (2008a). Nanostructured surface effects of global climate change on water modification of microporous ceramics for efficient virus filtration. resources and the development of water Journal of the European Ceramic Society, 28(8): 1603–1612 treatment techniques. Wegmann M, Michen B, Luxbacher T, Fritsch J, Graule T (2008b). Modification of ceramic microfilters with colloidal zirconia to Derek E Chitwood has a B.S. degree in promote the adsorption of viruses from water. Water Research, 42 aerospace engineering and Ph.D. in envir- (6–7): 1726–1734 onmental engineering, both from the Uni- World Health Organization (2017). Progress on drinking water sanitation versity of Southern California. He spent 18 and hygiene: 2017 update and SDG baselines. Geneva: World Health years working in rural southwest China. Organization His research focus is studying mountain World Health Organization (2019a). Progress on household drinking spring water quality and point of use water water, sanitation and hygiene 2000–2017: Special focus on inequal- filters for rural peoples in developing ities. Geneva: World Health Organization communities. World Health Organization (2019b). Drinking-water Fact sheet. New York: World Health Organization Yang H, Min X, Xu S, Bender J, Wang Y (2020). Development of Yin Wang received his B.S. degree from effective and fast-flow ceramic porous media for point-of-use water Peking University, and M.S. and Ph.D. treatment: Effect of pore size distribution. ACS Sustainable degrees from Washington University in St. Chemistry & Engineering, 8(6): 2531–2539 Louis. He is currently an Assistant Profes- Yang H, Min X, Xu S, Wang Y (2019a). Lanthanum(III)-coated ceramics sor at the University of Wisconsin – as a promising material in point-of-use water treatment for arsenite Milwaukee. His research focuses on the and arsenate removal. ACS Sustainable Chemistry & Engineering, 7 development of efficient and sustainable (10): 9220–9227 solutions to address water-related grand Yang H, Wang Y, Bender J, Xu S (2019b). Removal of arsenate and challenges.

Journal

Frontiers of Environmental Science & EngineeringSpringer Journals

Published: Oct 1, 2020

Keywords: Point-of-use water treatment; Ceramic water filter; Bacterial removal; Surface modification; Water quality

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