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Comparison of Three Household Water Treatment Technologies in San Mateo Ixtatán, Guatemala Jonathan E. Mellor1 ; Erin Kallman2 ; Vinka Oyanedel-Craver, A.M.ASCE3 ; and James A. Smith, F.ASCE4 Abstract: Silver-impregnated ceramic water filters (CWFs) are a simple and sustainable low-cost technology that has shown promise in improving household drinking water quality and reducing incidences of early childhood diarrhea in a variety of settings. Despite this promise, lower reservoir contamination is thought to be a contributing factor to the decline in the effectiveness being seen over time. A novel silverimpregnated ceramic torus that can be placed in the lower reservoir was designed to minimize this contamination. This study uses a one-year randomized trial to compare the relative effectiveness of the CWF þ torus design with a standard CWF and point-of-use chlorination. The effectiveness of each technology was measured at project inception and subsequently after six and 12 months. Results indicate that the toruses, as designed, are not able to consistently maintain lower-reservoir silver concentrations above those of the simple CWF design and are hence unable to prevent contamination. Furthermore, after six months, only 65% of households that used point-of-use chlorination maintained sufficient chlorine levels above the 0.2 mg=L needed to be effective. All three technologies showed statistically equivalent log removal efficiencies for total coliform bacteria and all three declined in effectiveness over the first six months. Combined average log removal efficiencies for all three technologies ranged from 2.22 0.21 initially but declined to 1.45 0.35 after six months and to 1.42 0.29 after one year. DOI: 10.1061/(ASCE)EE.1943-7870.0000914. © 2014 American Society of Civil Engineers. Introduction Worldwide, there are an estimated 780 million people who lack access to improved water sources and an additional 2.2 billion without consistent access to microbiologically safe water (Onda et al. 2012). This poor access is a leading cause of death for 842,000 children a year who suffer from poor access to water, sanitation, and hygiene (WASH) services (Pruss-Ustun et al. 2014). Even when a community has a clean water source, water is frequently contaminated after collection or treatment (Wright et al. 2004) by a myriad of contamination sources as well as biological regrowth (Mellor et al. 2013). This is a particular problem for residents who must travel long distances to collect water (Mellor et al. 2012b) and thus store their water for extended periods. One point-of-use (POU) technology that has shown promise at improving household drinking water quality is a ceramic water filter (CWF). CWFs consist of a porous ceramic pot-shaped filter that purifies water using size exclusion and an antimicrobial silver nano-particle coating. Contaminated water can be poured into the top from where it gradually percolates down through the ceramic and is collected in a plastic lower reservoir. CWFs have been shown to be highly effective at removing bacteria in both laboratory (97.8–100% removal) (Oyanedel-Craver and Smith 2008) and field environments (∼90% average removal) (Kallman et al. 2011) and reducing diarrhea incidences both generally (Hunter 2009) as well as in human immunodeficiency virus-positive adult populations (Abebe et al. 2014). CWFs are also likely able to remove viruses and protozoa, but with lower and higher respective effectiveness (Bielefeldt et al. 2010). Indeed, recent work has shown Cryptosporidium parvum removal to be 99.2% through ceramic disks (Abebe 2013). Although these are positive results, some researchers have recently questioned the ability of ceramic filters and other pointof-use interventions to reduce diarrhea incidence especially over the long term. Hunter (2009) found that the effectiveness of point-of-use interventions as a means of diarrhea reduction declines with follow-up duration and that blinded studies indicate lower effectiveness. Others have suggested that household water treatment technologies should not be promoted widely until further research is conducted (Schmidt and Cairncross 2009). One possible cause of the long-term decline in effectiveness is biological buildup that has been shown to be present in household water storage containers (Jagals et al. 2003) (which are similar in form and function to the lower reservoir of CWFs). This, along with biological regrowth, may be a significant contributing factor to early-childhood diarrhea incidence in South Africa (Mellor et al. 2012a). In fact, previous results obtained by the authors have suggested that this biological buildup coupled with poor cleaning regimes might be significant factors in CWFs’ declining effectiveness (Mellor et al. 2014). CWFs effectively remove bacteria both through size exclusion and the antimicrobial action of colloidal silver or silver nitrate that is painted on or infused into the ceramic. The bactericidal properties of the applied silver are dependent on the applied mass of colloidal silver (Oyanedel-Craver and Smith 2008) and the retention of silver in the filter (Ren et al. 2013). The bacterial growth inhibition by silver is dependent on the silver dose applied to the CWF (Rayner et al. 2013) and the number of bacteria present (Sondi and Salopek-Sondi 2004). 1 Postdoctoral Research Associate, Dept. of Chemical and Environmental Engineering, Yale Climate and Energy Institute, Yale Univ., P.O. Box 208286, New Haven, CT 06520. E-mail: firstname.lastname@example.org 2 Engineer, Dept. of Civil and Environmental Engineering, Univ. of Virginia, P.O. Box 40072, Charlottesville, VA 22904-4742. 3 Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of Rhode Island, Bliss Hall 213, Kingston, RI 22881. E-mail: email@example.com 4 Professor, Dept. of Civil and Environmental Engineering, Univ. of Virginia, P.O. Box 40072, Charlottesville, VA 22904-4742 (corresponding author). E-mail: firstname.lastname@example.org Note. This manuscript was submitted on November 11, 2013; approved on October 16, 2014; published online on November 18, 2014. Discussion period open until April 18, 2015; separate discussions must be submitted for individual papers. This paper is part of the Journal of Environmental Engineering, © ASCE, ISSN 0733-9372/04014085(7)/$25.00. © ASCE 04014085-1 J. Environ. Eng. J. Environ. Eng. 2015.141. Downloaded from ascelibrary.org by Northern Arizona Univ on 09/09/15. Copyright ASCE. For personal use only; all rights reserved. The safe water system (SWS) is another common point-of-use water treatment system developed by the U.S. Centers for Disease Control and Prevention (Mintz et al. 1995). It combines household water treatment and safe water storage. Treatment is typically done using sodium hypochlorite (NaOCl) added to household drinking water. Free chlorine residual should be between 0.2 and 2.0 mg=L (Lantagne 2008). The safe water storage containers have small openings to prevent contamination and a spigot to allow easy access. Attempting to address the problem of lower-reservoir recontamination, members of the nonprofit organization Potters for Peace proposed placing a ceramic torus painted with colloidal silver (Fig. 1) in the lower reservoir of the filter system (Fig. 2). It was hypothesized that the release of silver ions from the colloidal silver-impregnated torus in the lower reservoir would help to prevent microbial regrowth and biofilm formation. To test this claim, the microbial effectiveness of three technologies was compared concurrently in the same community over the course of a year: a CWF; a CWF with the torus placed in the lower reservoir; and the safe water system. Therefore, the overarching goal of this study was to compare the longitudinal microbial effectiveness of the three technologies over a year-long period while testing the novel silver-impregnated torus’s ability to slowly release silver thus improving the water quality in the lower reservoir. This goal was achieved through the recruitment of 107 participants from San Mateo Ixtatán, Guatemala, who were randomly divided into three study groups.
Each group received one intervention. Methods Community Setting and Cohort The study was undertaken in the community of San Mateo Ixtatán in the Guatemalan highlands. Access to suitable WASH infrastructure is severely limited and diarrhea is common in this region making it a leading cause of death among children in Guatemala (Guatemala 2002). San Mateo Ixtatán is the poorest community in the poorest department of Huehuetenango and has a population of approximately 30,000. Although the community has an extensive spring-fed water distribution system, the water is not treated and is of poor quality (Kallman et al. 2011). This system is the only source of drinking water and is used by all members of the community with the exception of the occasional use of bottled water. In June 2009, 107 participants were recruited to participate in the study and were randomly assigned to one of three groups of approximately equal size as shown in Fig. 3. Participants were invited through flyers and radio announcements. Technology assignments were done via a rotation where the first person in line got a CWF, the second a CWF þ torus, the third the SWS, etc. The SWS group received their first chlorine bottles free of charge and could purchase dosed chlorine bottles from a local distributor for approximately 25 quetzales (≈US$3.14) for a six-month supply. Instructions were given to the participants about operation and maintenance of their technologies at project inception. However, there was no systematic attempt to assesses actual compliance or maintenance practices. Study attrition was mostly due to participants not being present during the Period 2 and 3 sampling rounds. Two attempts were made at each household to find participants. Institutional review board approval was obtained for this study from the University of Virginia. Torus Fabrication The toruses were fabricated in San Mateo Ixtatán using a method similar to the one described by Kallman et al. (2011) to fabricate CWFs, however they were hand-molded instead of pressed. In brief, approximately 60 lb (27.2 kg) of locally collected clay is combined with 8–10 lb (3.6–4.5 kg) of sieved sawdust. Once mixed, 10 L of water is added and the toruses are molded by hand. They are then allowed to air-dry for 8 days, after which time they are fired at a temperature of 800°C. The temperature was slowly increased from ambient by 75°C=h for 4 h and then by 150°C=h until the maximum temperature was reached. They are then hand-painted with approximately 23 mL of 200 ppm silver nano-particle solution and allowed to dry. The finished torus mean mass was 175 g with a range of 158–213 g. Fig. 1. Silver-impregnated torus investigated in this study (image by James Smith) Torus Ceramic Water Filter Water Level Lower Reservoir Fig. 2. Ceramic water filter plus silver-infused ceramic torus (CWF þ torus) configuration; the ceramic water filter is placed inside the lower reservoir as shown; the torus is then placed underwater at the bottom of the reservoir where it can inhibit biological growth © ASCE 04014085-2 J. Environ. Eng. J. Environ. Eng. 2015.141. Downloaded from ascelibrary.org by Northern Arizona Univ on 09/09/15. Copyright ASCE. For personal use only; all rights reserved. Analytical Methods For each household sampling event, water samples were collected from the household tap (which was the source water) and from the spigot of the CWF or SWS. Sampling took place in June 2009, January 2010, and June 2010, which will hereafter be referred to as Periods 1, 2, and 3, respectively. Period 1 sampling was conducted within approximately two days of a household receiving their filter. Silver concentration was measured in the field each time using a Hach DR/4000 spectrophotometer and the Hach 8120 silver colorimetric method (Hach 2003) (Hach Company, Loveland, Colorado). The detection limits for that method are 0.02–0.70 mg=L. Free chlorine levels were measured during Periods 1 and 2 using Hach Chlorine Color Disk Test Kits Model CN-66 (Hach Company, Loveland, Colorado). The effective range is 0.1–3.4 mg=l Cl2. The samples taken from each household at each period were tested for total coliform bacteria during each of the three visits and E. coli bacteria during the last two visits using standard membrane filtration methods. All samples were collected in sterile, 250 mL plastic bottles and stored in a cooler with ice during transport to the field laboratory. All samples were tested within eight hours of collection. The selective media m-ColiBlue24 (Millipore, Billerica, Massachusetts) was not available during the first sampling period necessitating the use of m-Endo Total Coliform Broth (Millipore, Billerica, Massachusetts), which does not differentiate between E. coli and total coliforms. m-ColiBlue24 was used for the subsequent periods. The membrane filtration protocol is as follows. A hand pump was first used to pass 100 mL of each undiluted sample through a sterilized 0.45 μm membrane filter (Fisher Brand, Pittsburgh). The filter was then placed in a petri dish containing m-ColiBlue24 or m-Endo total coliform broth and incubated at 35°C for 24 h in a portable incubator (Millipore, Billerica, Massachusetts). Colonies were then counted and reported as CFU=100 mL. Plates with too many colonies to count were recorded as having 2,000 CFU= 100 mL. Daily boiled water samples all had zero colonies. Statistical Methods Mean differences in silver concentrations over time (withinsubjects effects) and between technologies (between-subjects effects) were assessed using repeated-measures analysis of variance (ANOVA). Shapiro-Wilk’s tests of normality, Levene’s test for equality of error variances, Box’s test of equality of covariance and Mauchly’s test for sphericity were all conducted on the data to test if ANOVA assumptions were met. A paired t-test was conducted to assess the chlorine concentration changes. Log reduction values were calculated from the influent and effluent water samples and used in the subsequent analyses of the microbial concentrations. One-way ANOVA analyses were conducted at each time period to assess the equity of the influent water for the three technologies and two bacteria types. The microbial effectiveness of the three technologies was assessed using a repeated-measures ANOVA in an identical manner to the silver concentrations. Pearson correlation coefficients were calculated to see if effluent water silver concentrations were correlated to the log removal efficiency of the filters. Finally, Student’s t-test were conducted to assess the difference in log removal rates between households with and without sufficient chlorine during Period 2. F-tests were used to assess variance for t-test analyses. IBS SPSS statistical analysis software version 21.0 (IBM SPSS, Chicago, 2011) as well as Microsoft Excel were used for all analyses. All tests were conducted with a significance level of 0.05. Results Fig. 4 presents mean silver concentrations (with 95% confidence intervals) in treated water for the CWF and CWF þ torus interventions for each of the three sampling periods. All samples were 107 Initial Participants 37 CWF 36 CWF + Torus 34 SWS Period 1 (June 2009) 21 CWF 25 CWF + Torus 19 SWS Period 3 (June 2010) 27 CWF 26 CWF + Torus 20 SWS Period 2 (Jan 2010) Fig. 3. Three-product study design; 107 initial participants were recruited in June 2009 and were approximately evenly divided randomly into three study arms; there was significant dropout during the subsequent follow-up visits Sampling Period [Ag] mg/l 123 CWF CWF + Torus 0.02 0.03 0.04 0.05 Fig. 4. Silver concentration for the three time periods sampled; repeated-measures ANOVA analyses indicated there was no variation with either time or between technologies; error bars indicate 95% CI © ASCE 04014085-3 J. Environ. Eng. J. Environ. Eng. 2015.141. Downloaded from ascelibrary.org by Northern Arizona Univ on 09/09/15. Copyright ASCE. For personal use only; all rights reserved. below the World Health Organization (WHO) recommendation for silver concentration, which is 0.1 mg=L (WHO 2011). Although apparent mean differences
in effluent water silver concentration were seen over time (Fig. 4), these differences were not statistically significant according to the repeated measures ANOVA analysis [Fð2; 34Þ ¼ 0.812, p ¼ 0.412]. Likewise, there were no significant differences between technologies [Fð1; 17Þ ¼ 0.160, p ¼ 0.694]. Chlorine concentrations fell from Period 1 to Period 2 for households that received the safe water system, however, this result was not statistically significant according to a paired t-test (p ¼ 0.125). During the first sampling round, chlorine concentration was 1.43 mg=L on average with 100% (n ¼ 34) having more than 0.2 mg=L. However, during the Period 2 sampling only 65% (n ¼ 20) had levels of 0.2 mg=L or higher although the average rose to 2.66 mg=L. This is due to the fact that 20% of households had concentrations in excess of 5 mg=L, indicating a minority of households were possibly overchlorinating. Table 1 summarizes mean tap (influent) water quality for the three technologies, two bacteria types, and three sampling periods. According to one-way ANOVA analyses, mean tap water samples for both E. coli and total coliform bacteria were statistically equivalent for all three technologies for Periods 2 and 3. However, they were not equivalent for Period 1 (p ¼ 0.037) as is shown in Table 1. Mean log reduction values for all three technologies over the sampling periods for the two bacteria types are shown in Figs. 5 and 6, while the same data is displayed as boxplots in Fig. 7. Combined average log removal efficiencies for all three technologies for total coliform bacteria ranged from 2.22 0.21 initially but declined to 1.45 0.35 after six months and to 1.42 0.29 after 1 year. Total coliform removal declined between Period 1 and Period 3 from 2.20 to 1.18 for CWF, from 2.10 to 1.48 for the CWF þ torus design, and from 2.37 to 1.60 for the SWS. E. coli removal changed from the Period 2 to Period 3 sampling rounds from 1.45 to 1.37 for the CWFs, from 1.51 to 1.87 for the CWF þ torus design and from 1.26 to 1.05 for the SWS. A repeated-measures ANOVA of all three technologies indicated that there were mean differences over time [Fð2; 92Þ ¼ 12.410, p < 0.001], but not between the technologies [Fð2; 46Þ ¼ 0.417, p ¼ 0.661] for total coliform bacteria. Pairwise comparisons identified mean differences between Period 1 and Period 2 (p < 0.001), but not between Period 2 and Period 3 (p ¼ 1.000). However, there was no similar temporal decline for E. coli [Fð1; 32Þ ¼ 0.008, p ¼ 0.930] nor was there a difference between technologies for E. coli [Fð2; 32Þ ¼ 1.409, p ¼ 0.259]. The lack of temporal decline for E. coli is likely due to the lack of data for Period 1. There was also no correlation between silver concentration in the effluent water and log reduction values for total coliform (R ¼ 0.08) or E. coli (R ¼ 0.087). Results are also displayed in terms of the WHO risk categories of <1, 1–10, 10–100, 100–1,000 and >1,000 CFU=100 mL in Figs. 8 and 9. There plots are largely consistent with the previous results and indicate that the effectiveness of all three technologies declines between Periods 1 and 2, but remains relatively constant between Periods 2 and 3. There are few differences between the three technologies. Finally, the log removal rates for households with and without sufficient residual chlorine during the Period 2 sampling was compared. Households with chlorine concentrations of 0.2 mg=L had Table 1. Mean Tap/Influent Water Quality for the Three Sampling Periods for Each of the Three Technologies and Two Bacteria Types; P-Values are the Result of One-Way ANOVA Analyses to Compare Means; Results Indicate that Means are Statistically Equivalent Except for the First Sampling Period Sampling period Bacteria type Mean tap (influent) water quality (CFU=100 mL) CWF CWF þ torus SWS p-Value 1 TC 465 499 914 0.037 2 TC 525 773 651 0.746 2 EC 352 328 255 0.886 3 TC 798 632 328 0.333 3 EC 269 221 66 0.405 Note: EC = E. Coli; TC = total coliform. Total Coliform Sampling Period Log Reduction 0.5 1.0 1.5 2.0 2.5 123 CWF CWF + Torus SWS Fig. 5. Mean log reduction over the three sampling periods for total coliform bacteria; repeated-measure ANOVA test indicates temporal decline for total coliform bacteria between Periods 1 and 2; no significant differences were found between technologies; plot is consistent with a temporal decline in effectiveness, but limited differences between technologies; error bars indicate 95% CI E. Coli Sampling Period Log Reduction 0.5 1.0 1.5 2.0 2.5 2 3 CWF CWF + Torus SWS Fig. 6. Mean log reduction over the two sampling periods for E. coli bacteria; there was no temporal decline for E. coli and no significant difference between technologies; the lack of temporal decline is likely due to the fact that no E. coli data was taken for Period 1; error bars indicate 95% CI © ASCE 04014085-4 J. Environ. Eng. J. Environ. Eng. 2015.141. Downloaded from ascelibrary.org by Northern Arizona Univ on 09/09/15. Copyright ASCE. For personal use only; all rights reserved. significantly higher log removal rates for total coliform bacteria than those that did not (1.79 versus 0.41, p ¼ 0.022) (Student’s t-test). However, log removal rates were equivalent for E. coli bacteria (1.09 versus 1.09, p ¼ 0.936) (Student’s t-test). Discussion This paper reports on a randomized trial of the longitudinal field effectiveness of three point-of-use water treatment systems. To the best of the authors’ knowledge, this is the first concurrent, comparative study of these three technologies. Results indicate that all three technologies decline in effectiveness over the first six months and that the toruses, as designed, are not sufficient to improve performance over the simple CWF. However, it is also noteworthy that the microbial effectiveness decline appears to level off after six months for all three technologies. Furthermore, it is evident that chlorination adherence falls precipitously during the first six months, which is something that can affect its suitability as a sustainable point-of-use intervention. The longitudinal declines in CWF effectiveness are consistent with those reported previously from a study conducted in South Africa (Mellor et al. 2014) and are likewise consistent with that of Hunter (2009) who found that longer duration studies of POU devices showed decreased effectiveness at reducing diarrhea. The high variability and negative log reduction values have likewise been seen by Brown (2007) who found that 17% of CWF effluent samples had higher E. coli concentrations in the treated water compared to the influent water. One reason for the apparent effectiveness declines seen in the CWF and the CWF þ torus designs may be the depletion of the silver in the filters. As noted by Ren et al. (2013), silver nanoparticles are relatively mobile through a ceramic porous media, and colloidal silver solutions that are painted onto porous ceramic filters (as they were in this case) result in silver nanoparticle release into the treated water at a rate much faster than filters fabricated by firing the nanosilver into the filter. The relative ineffectiveness of the torus design is surprising. If designed properly, such a technology should reduce the biofilm buildup quantified by others (Jagals et al. 2003) and help to mitigate regrowth in such settings (Mellor et al. 2013). One possibility is that the reservoir silver concentrations, which ranged from ∼15 to 45 ppb (Fig. 4), were insufficient to deactivate high concentrations of bacteria. However, prior research indicates that ∼2 log reduction occurs in about 30 min at concentrations down to at least 50 ppb (Jung et al. 2008). In the filter design, the microbes pass through the pores of the silver-impregnated ceramic, which forces them into very close contact with the silver-impregnated pore surfaces. This is apparently not occurring as efficiently in the toruses because the water is not forced through the torus. It is possible that the toruses could be more effective if they were painted with higher concentrations of colloidal silver, had
different pore sizes, or had a different geometry. However, the Pearson’s test indicates that there is no Log Reduction -2 -1 0 1 2 3 4 5 TC Period 1 TC Period 2 TC Period 3 EC Period 2 EC Period 3 SWS Technology CWF + Torus CWF Fig. 7. Boxplots of log reductions for the three sampling periods for each of the three technologies and two bacteria types [EC (E. coli) and TC (total coliform)] CWF CWF+Torus SWS CWF CWF+Torus SWS CWF CWF+Torus SWS Percent (%) 0 20 40 60 80 100 Period 1 Period 2 Period 3 Risk Category (CFU/100ml) <1 1-10 10-100 100-1000 >1000 Fig. 8. Bar plots showing the percent of samples with total coliform concentrations in WHO risk categories at the three periods; results indicate that water quality declines between Period 1 and Period 2 for all three technologies but remains relatively constant between Period 2 and Period 3; there is minimal difference between the three technologies © ASCE 04014085-5 J. Environ. Eng. J. Environ. Eng. 2015.141. Downloaded from ascelibrary.org by Northern Arizona Univ on 09/09/15. Copyright ASCE. For personal use only; all rights reserved. relationship between silver concentrations and log reduction values. It is also notable that silver concentrations showed little variation with time or between the two CWF configurations. The one exception to this was during the second sampling period when the torus design appears to have had higher levels of silver. This could have led to the statistically insignificant mean increase in log reduction for that period for both total coliform (1.42 versus 1.59, p ¼ 0.636) and E. coli (1.41 versus 1.53, p ¼ 0.738) for the CWF versus CWF þ torus designs, respectively. Another possibility is that reservoir contamination was not due to contamination from the reservoir itself, but rather from the ceramic filter walls or pores as was found by others in a controlled laboratory experiment (Bielefeldt et al. 2009). Also, the detection range of the chosen method (Hach 8120) is 0.02–0.70 mg=L. Since the silver concentrations were near the lower detection limit, there might be uncertainty in those measurements (Hach 2003). It is important to note that this does not mean that lower-reservoir biofilm buildup is not occurring, or that silver-impregnated toruses are infective generally, but it does call for an improved torus design and further research to pinpoint the source of the recontamination. The mean log reduction values seen for the CWFs in this study were 2.20 0.30 initially and declined to 1.34 0.65 and to 1.18 0.49. This makes these filters comparable to those reported previously by Brown (2007) who found log reduction values of ∼2. A recent study that compared the effectiveness of chlorination with a silver-coated porous ceramic candle element found a mean log reduction value of 1.21 for households provided WaterGuard (a dilute hypochlorite solution) while the ceramic candles provided a log reduction of only 0.91 (Albert et al. 2010). Likewise, the 73% reduction in the number of households with detectable levels of E. coli before and after chlorination for Period 2 was consistent with a major meta-analysis that found an 80% reduction in the proportion of stored water samples with detectable E. coli (Arnold and Colford 2007) after chlorination interventions. Although the percentage in this study declined to 58% during Period 3, the Arnold and Colford meta-analysis relied on studies that had a median length of only 30 weeks. Finally, it is worth noting that if the log reduction values measured in the current study were 3 or better, it could lead to improved outcomes (Mellor et al. 2012a; Enger et al. 2012). However, given that the influent water frequently had less than 1,000 CFU=100 mL of bacterial contamination, the reported log reduction values might be higher if the influent water were more highly contaminated. The use of silver in water treatment technologies is not without risk, however, this risk is minimal and the WHO has not established a firm limit due to inadequate data (WHO 2011). Indeed, the WHO suggests that the only known risk for silver ingestion is argyria, which is a condition that discolors the skin and hair. To prevent this, the WHO recommends a lifetime limit of 10 g of silver. Based on this limit, the WHO recommends that silver concentrations of 0.1 mg=L can be tolerated for 70 years without any health risk (WHO 2011). The WHO limit for chlorine is 5 mg=L and there are no specific adverse health effects that have been observed (WHO 2011). The current study had a number of limitations that warrant discussion. First of all, the nonuniform influent water supplies may have biased some of the results. Secondly, the torus design did not generally increase the silver concentration in the lower reservoir, which is likely why it proved to be equally as effective at removing bacteria as the filter only design. This does not mean that the torus cannot be effective, or that biofilm layer buildup is not occurring, but it does mean that the torus needs to be redesigned to increase efficiency. That improved design should then be tested in future laboratory and field trials. Also, systematic baseline demographic data about socioeconomic status or other covariates was not conducted. Such factors might have affected the use and maintenance of the three technologies. The fact that the participants using chlorine had to purchase chlorine bottles every six months, while the filter users did not have any additional costs, might have biased the results. However, the periodic purchase of chlorine is realistic for that method. Finally, the high drop-out rate might have biased the results and lowered their statistical power. Conclusions The first randomized trial has been conducted to study the relative microbial effectiveness of three different point-of-use water CWF CWF+Torus SWS CWF CWF+Torus SWS Percent (%) 0 20 40 60 80 100 Period 2 Period 3 Risk Category (CFU/100ml) <1 1-10 10-100 100-1000 >1000 Fig. 9. Bar plots showing the percent of samples with E. coli concentrations in WHO risk categories at the two periods © ASCE 04014085-6 J. Environ. Eng. J. Environ. Eng. 2015.141. Downloaded from ascelibrary.org by Northern Arizona Univ on 09/09/15. Copyright ASCE. For personal use only; all rights reserved. treatment technologies in the developing-world community of San Mateo Ixtatán, Guatemala. The POU technologies studied were chlorination (i.e., the safe water system advocated by the Centers for Disease Control and Prevention), CWFs, and CWFs with a ceramic torus impregnated with silver placed in the lower filter reservoir. Surprisingly, the CWF þ torus design did not significantly increase silver concentrations in lower reservoirs as expected. Furthermore, the percent of households in the chlorination group with adequate residual chlorination dropped from 100 to 65% between Periods 1 and 2 of the study. Log removal efficiency was highly variable and declined over the first six months with all three technologies, and there were no statistically significant differences seen between the three technologies in terms of microbial removal efficiency. These results highlight the need for further study into the causes of lower-reservoir contamination in CWFs and ways to remedy this problem. Improved silver-impregnated ceramic toruses might be an effective technology, but further research is needed to improve their design. Acknowledgments The authors thank Beth Neville Evans and the Ixtatán Foundation for assistance in participant enrollment and logistical field support. We would also like to thank Dr. Relana Pinkerton for her assistance with some of our methods. This work was supported by the National Science Foundation (CBET 651996). It was also developed under STAR Fellowship Assistance Agreement no. FP91728601 awarded by the U.S. Environmental Protection Agency (USEPA). It has not been formally reviewed by USEPA. The views expressed in this publication are solely those of the authors, and USEPA does not endorse any products or commercial services mentioned in this publication. References Abebe, L. (2013). “Silver-impregnated ceramic-water
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