Downloaded from ascelibrary.org by NATIONAL INSTITUTE OF TECHNOLOGY on 06/26/14. Copyright ASCE. For personal use only; all rights reserved. Porosity, Flow, and Filtration Characteristics of Frustum-Shaped Ceramic Water Filters Ismaiel Yakub, Ph.D. 1; Anand Plappally, Ph.D. 2; Megan Leftwich, Ph.D. 3; Karen Malatesta, Ph.D. 4; Katie C. Friedman 5; Sam Obwoya, Ph.D. 6; Francis Nyongesa, Ph.D. 7; Amadou H. Maiga, Ph.D., M.ASCE 8; Alfred B. O. Soboyejo, Ph.D. 9; Stefanos Logothetis 10; and Wole Soboyejo, Ph.D. 11 Abstract: This paper presents the results of an experimental study of the effects of porosity on the flow rate and Escherichia coli (E. coli) filtration characteristics of porous ceramic water filters (CWFs) prepared without a coating of silver. Clay-based CWFs were fabricated by sintering composites of redart clay and fine woodchips (sawdust) in three different proportions by volume, viz: 50∶50, 65∶35, and 75∶25. Sintering the greenware below 1,000°C produced reddish colored pot of three different degrees of porosity and micro- and nanoscale pores, which are the key to efficient filtration. The porosities and pore size distribution frequencies of the sintered clay ceramics were characterized using mercury intrusion porosimetry (MIP). The porosity of the CWFs ranged from ∼36% to ∼47% and increased with increasing sawdust content in a linear fashion, and the pore size varied from ∼10 nm to ∼100 μm. The volume flow rates of water through the CWFs were investigated by measuring the cumulative amount of water flow as a function of time. The flow rate was found to increase with increasing porosity of the CWFs. The effective intrinsic permeabilities of the CWFs were then obtained from Darcy fits to the flow rate data. These were compared with values obtained using the Katz-Thompson method. Both approaches gave comparable results of permeability between ∼1 millidarcy to ∼50 millidarcy. The tortuosity of the CWFs was found from Hager’s equation to range from ∼10 to ∼60. In general, while the permeability of the CWFs decreased with increasing clay content, tortuosity increased with increasing clay content. The CWFs removed E. coli from aqueous suspension very efficiently with average log reduction values between 5.7–6.4. The implications and limitations of the results are discussed for the effective filtration of water in the developing world. DOI: 10.1061/(ASCE)EE.1943-7870 .0000669. © 2013 American Society of Civil Engineers. CE Database subject headings: Filters; Flow rates; Permeability; Filtration; Porosity. Author keywords: Ceramic water filters; Porosimetry; Flow rate; Intrinsic permeability; Tortuosity; E. coli filtration. Introduction Although clean water and effective sanitation are essential to good health and wellbeing, they are not available globally. Around 884 million people in the world do not have access to clean drinking water (WHO/UNICEF 2008). Consequently, about 5,000 people die each day from preventable waterborne diseases such as diarrhea, dysentery, poliomyleitis, typhoid, ascariasis, and leptospirosis (Kosek et al. 2003; Lantagne 2002; Wenhold and Faber 2009). These diseases greatly increase the mortality rates and the disease burden in developing countries, especially in children younger than 5 years old (UNICEF/WHO 2009). In 1981, in an effort to address health and mortality issues associated with the consumption of contaminated water, the Inter-American Bank organized a competition to promote the development of an affordable filter that could remove bacteria while enabling sufficient water flow for the consumption needs of families in developing countries. This stimulated Dr. Fernando Mazariegos of the Central American Research Institute-Guatemala (ICAITI) to make the first frustum-shaped ceramic water filter with 1 Lecturer, Dept. of Mechanical and Aerospace Engineering and the Keller Center for Innovation in Engineering Education, Princeton Univ., Princeton, NJ 08544. E-mail: iyakub@princeton.edu 2 Assistant Professor, Indian Institute of Technology Jodhpur (IITJ), Rajasthan 342011, India; formerly Graduate Student, Dept. of Food, Agricultural and Biological Engineering, Ohio State Univ., 590 Woody Hayes Dr., Columbus, OH 43210. E-mail: anand.plappally@gmail.com 3 Assistant Professor, Dept. of Mechanical and Aerospace Engineering, GeorgeWashingtonUniv.,Washington,DC20052.E-mail:mleftwich@gwu.edu 4 Lecturer and Research Specialist II, Dept. of Mechanical and Aerospace Engineering,PrincetonUniv.,Princeton,NJ08544.E-mail:kmalates@princeton.edu Note. This manuscript was submitted on February 15, 2012; approved on October 18, 2012; published online on October 19, 2012. Discussion period open until December 1, 2013; separate discussions must be submitted for individual papers. This paper is part of the Journal of Environmental Engineering, Vol. 139, No. 7, July 1, 2013. © ASCE, ISSN 0733-9372/2013/7-986-994/$25.00. 5 Undergraduate Student, Dept. of Chemical Engineering, Princeton Univ., Princeton, NJ 08544. E-mail: kcfriedman@alumni.princeton.edu 6 Associate Professor, Dept. of Physics, Kyambogo Univ., P.O. Box 1, Kampala, Uganda. E-mail: ksobwoya@yahoo.co.uk 7 Senior Lecturer, Dept. of Physics, Univ. of Nairobi, P.O. Box 3019700100, Nairobi, Kenya. E-mail: fnyongesa@unonbi.ac.ke 8 Professor, Dept. of Water and Sanitation, International Institute for Water and Environmental Engineering (2iE), Ouagadougou, Burkina Faso. E-mail: amadou.hama.maiga@2ie-edu.org 9 Professor, Dept. of Food, Agricultural and Biological Engineering, Ohio State Univ., 590 Woody Hayes Dr., Columbus, OH 43210. E-mail: soboyejo.2@osu.edu 10 Student, Montgomery High School, Skillman, NJ 08558. E-mail: stob50@myway.com 11 Professor, Dept. of Mechanical and Aerospace Engineering, Princeton Univ., Princeton, NJ 08544 (corresponding author). E-mail: soboyejo@ princeton.edu 986 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 J. Environ. Eng. 2013.139:986-994. Downloaded from ascelibrary.org by NATIONAL INSTITUTE OF TECHNOLOGY on 06/26/14. Copyright ASCE. For personal use only; all rights reserved. a colloidal silver coating (PFP 2008). The filter relies on gravity for flow of water, and size exclusion for the removal of bacteria and other pathogens from water, while the colloidal silver is antimicrobial (Lantagne 2002; Yakub and Soboyejo 2012). Subsequently, the Medical Assistance Program trained potters in Ecuador and ICAITI, and a number of nongovernmental organizations (NGO’s), started to make the filters (Lantagne 2002). In 1998, Potters For Peace (PFP) standardized the Mazariegos design, with the development of a mold and a press. Mass production of these filters, named Filtròn, started in 1999 in Nicaragua (PFP 2008), where PFP developed a new strategy of helping poor communities across the world to establish water filter factories that make and sell the filters to rural and urban populations in developing countries (Donachy 2004; PFP 2008). Since 1999, PFP has helped to establish water filter factories in over 20 countries, including: Mexico, Cambodia, Haiti, Guatemala, El Salvador, Pakistan, Sudan, Kenya, Benin, Ghana and Nigeria (Brown et al. 2009; Dies 2003; Donachy 2004; Hwang 2003; Lantagne 2002; Lantagne et al. 2010; Lee 2009; Oyanedel-Craver and Smith 2008; PFP 2008; Swanton 2008; Van Halem 2006). In an effort to enhance the availability of the filters across borders in the developing world, the filter has not been patented. Although there are other methods for the treatment of water in developing countries [boiling, pasteurization, chlorination, flocculation disinfection, solar disinfection, biosand filter (Clasen et al. 2007; Fewtrell et al. 2005; Sobsey et al. 2008)], point-of-use filtration is one of the most promising solutions available (Sobsey et al. 2008). Ceramic water filters (CWFs) are especially appealing because of their low cost, ease of fabrication and use, and their ability to filter out bacteria from water very effectively. CWFs are also attractive because they represent a sustainable solution with the potential for large scale adoption. So far, about 500,000 people in the developing world have adopted some form of porous ceramic filter technology (Sobsey et al. 2008). Clay-based ceramic water filters (CWFs) are usually produced by mixing of clay, sawdust (woodchips) and water. Other combustible organic materials, such as rice husk, coffee husk, or flour can also be used (Oyanedel-Craver and Smith 2008). After drying and firing, the CWFs are usually coated with a layer of colloidal silver (PFP 2008), which is used because of its antimicrobial activity. By careful control of the clay and woodchip mixtures proportions, as well as the processes that are used for the fabrication of CWFs, initial water flow rates of around 2 L outflow, in the first hour, have been shown consistently to result in the removal of many microbes and pathogens in water (Brown et al. 2008; Lantagne 2002; Lantagne et al. 2010; PFP 2008). The flow rate, which may depend on the water turbidity conditions, decreases gradually with increasing filter use during the 2–3 year recommended lifetime of the CWFs. This is due to the accumulation of solids on the inner surface and gradual blockage of the pores by trapped contaminants (Brown et al. 2008). During use, PFP recommends that the CWF be cleaned by fire heating, scrubbing and brushing to remove caked impurities that form with time. This cleaning procedure, however, is currently under debate because of the high risk of recontaminating treated water (personal communication, Reviewer #1, Jour Environ Eng). Furthermore, the clay-water mixture has a rheological property, which not only permits shaping, but also allows for doping with other materials. Doping can potentially give the CWFs the robustness required for the removal of other contaminants besides microbes from water. For instance, the removal of chemical contaminants, such as arsenic, iron, and fluoride has also been reported (Dies 2003; Friedman 2010; Yakub and Soboyejo 2013). Furthermore, it has also been shown that with the right amount of iron oxide doping, the CWFs are capable of removing viruses (Brown and Sobsey 2009; Tsao 2011). Although porous CWFs have been used successfully in the field (Albert et al. 2010; Brown et al. 2009; Dies 2003; Hwang 2003; Lantagne 2002; Lee 2009; Oyanedel-Craver and Smith 2008; PFP 2008; Swanton 2008; Van Halem 2006) for over a decade, scientific understanding of the effects of porosity on the water flow rate and microbial filtration efficiency is still very limited. There is, therefore, a need for scientific studies of the effects of porosity on the water filtration properties of CWFs. This paper presents the results of an experimental study of the effects of porosity on water flow and filtration characteristics of CWFs. that were fabricated without a coating of colloidal silver. Silver was not applied to the CWFs so that the effect of CWF structure on flow rate and the removal of microbial contaminants could be assessed in the absence of the antimicrobial effect of silver. The effects of porosity are explored using ceramics produced from different, well-controlled mixtures of redart clay and wood chips. The effective permeabilities of the CWFs were determined from fits to the Darcy equation and also by using the Katz and Thompson method. The tortuosity of the CWFs was found using Jörgen Hager’s equation. The efficiencies of bacterial removal (filtration) were elucidated for two of the three CWFs. The implications of the results are then discussed for applications of CWFs in the developing world. Experimental Materials and Processing CWFs were made by mixing clay (Cedar Heights Redart Airfloated Clay, Pittsburgh, PA) composed of illite and kaolinite clays with sawdust consisting of 80% oak and 20% Spanish cedar (Hamilton Building Supplies, Trenton, NJ). In order to produce filters with different porosities, three different proportions of clay to sawdust by volume (50∶50, 65∶35, and 75∶25) were used to produce the CWFs. Prior to mixing, the sawdust was manually sieved using 35–1,000 mesh wire sieves. The initial blending of sieved sawdust and clay was then done manually to ensure a thorough mixing and to avoid the formation of clustered pores. The dry mixture of clay and sawdust was transferred to an industrial mixer (Model A-200, The Hobart Manufacturing Company, Troy, OH) and thoroughly mixed again for approximately 5 minutes before the addition of water. Half (0.9 L) of the required amount of water (usually 1.8–2 L for a 50∶50 CWF) was gradually added during mixing, and the mixture was again blended for 5–10 minutes. Next, half of the remaining water (one quarter of the total amount or 0.45 L) was added, and the mixture was mixed for another 5–10 minutes. The remaining water (approximately 0.45 L) was added in small increments until the mixture began to coalesce into large clumps and no longer adhered to the walls of the mixer. No additional water was added after coalescence occurred. Approximately 12 pounds of the blended mixture were then manually formed into a ball, which was pressed tightly together so that no cracks were visible on its surface. This ball was compacted in a two-piece aluminum mold that was covered with a 49.21 L (13 gallon) plastic bag to prevent the greenware from sticking to the walls of the mold during pressing. The blended mixture was placed into the female part of the mold before applying a pressure of 140 kPa (20 psi) to the male part of the mold using a 50 ton hydraulic press (TRD55002, Torin Jacks, Inc., Ontario, Canada). JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 / 987 J. Environ. Eng. 2013.139:986-994. Downloaded from ascelibrary.org by NATIONAL INSTITUTE OF TECHNOLOGY on 06/26/14. Copyright ASCE. For personal use only; all rights reserved. After pressing, the greenwares were dried in laboratory air at a temperature of about 25°C and a relative humidity of about 40%. The time required for drying the greenwares varied between 5 and 8 days, depending on the mixture ratio of sawdust to clay. After drying, the greenwares were sintered in a gas kiln (Ceramics Art Department, Princeton University, Princeton, NJ). This involved the preheating of the greenwares to 450–550°C for 3 h (to burn off the sawdust), followed by heating to the sintering temperature of 955°C in the same gas kiln. The initial heating rate of 50°C per hour was increased to 100°C per hour beyond a furnace temperature of 200°C. The greenwares were sintered for 5 h at a peak temperature of about 955°C. They were then furnace cooled in air to room temperature. The frustum-shaped CWFs consisted of two sections, the base (or disc part) and the side. The disc had a radius of ∼91.5 mm and a thickness of ∼15 mm. The side had an interior slanted height of ∼240 mm and a thickness of ∼10 mm. The CWFs had an interior depth of ∼237 mm and hence have a capacity of about 10 L. After cooling, the porosities of the CWFs were characterized using mercury intrusion porosimetry (MIP). The flow rates and bacterial filtration characteristics of the filters were also determined. A standard PFP reference CWF, produced by Potters for Peace in Managua, Nicaragua, was used as a reference in the porosimetry and flow tests. The PFP reference CWF was made from a 60∶40 mixture of clay to sawdust by volume. This was also coated with colloidal silver, and had a flow rate of between ∼1–2 L per hour. This flow rate was estimated by measuring the volume of water that drained from an initially full filter over a period of 1 h. A more accurate dynamic method of measuring the water flow rate will be presented in section 2.4 below. Fig. 1. Schematic of a CWF with key variables labeled; the four sites from which samples for the MIP analyses were obtained are demarcated with black rectangles Engineering Incorporated, Stamford, CT). The load cell was connected to a LabView card (NI PCI-6259, National Instruments, Austin, TX) via a 68-pin digital and trigger I/O terminal block (CB-68LP, National Instruments, Austin, TX) to LabView software (Version 8.0, National Instruments, Austin, TX) that recorded the mass of the water as it flowed into the collection bucket. At the start of each experiment, a water-saturated CWF was filled completely with approximately 10 Liters of purified water and covered with a plastic lid. The water was allowed to drain passively from the CWF being tested. No additional water was added to the CWF during the experiment. In this way, time durations between 3 and 21 days were required to drain the range of CWFs examined in this study. Modeling Porosity and Pore Size Distribution The porosities of the CWFs were characterized using mercury intrusion porosimetry (MIP). MIP measurements were carried out in a MicroMetrics Autopore III 9400 analyzer (MicroMetrics, Norcross, GA). The two-stage MIP experiments were used to characterize the nano- and the microscale pore size distributions. The MIP tests were performed by filling up a penetrometer, with a stem volume of 1.131 mL, with pieces of ceramics with dimensions of ∼3 mm × 3 mm × 3 mm that were cut from the three different CWFs (50∶50, 65∶35 and 75∶25) and also the PFP reference CWF. Samples were cut from four different locations in each CWF [the base and three locations (the top, middle, bottom) on the side of the filter], as shown in Fig. 1. Water Flow Experiments Prior to the water flow experiments, the CWFs were saturated by complete submersion in a vat containing purified water (Model D8611, Barnstead/Thermolyne, Hampton, NH) for about 12 hours to remove internal air bubbles. The flow rates of purified water were then measured through the three CWFs (50∶50, 65∶35, and 75∶25). Flow rates across the PFP reference filter CWF were also determined as a control. The flow rates were obtained by measuring the volume of water discharged from the CWFs as a function of time. The flow measurement system was enclosed in a plastic container to minimize possible contamination and mass loss by evaporation. The CWF being tested ceramic pot was first placed into a plastic receptacle that was fitted with a large plastic funnel. The filter and receptacle-funnel were suspended above an empty collection bucket that rested on a load cell (Model LSC 7000-50, Omega An analytical hydrodynamic model was developed to describe the flow of water through the filters due to the effect of gravity. Flow continued until the filter had emptied to levels where there was not enough pressure head to overcome the resistance of the membrane/filter. Fig. 1 is a schematic of the water filter with the important variables labeled. Flow through the porous filters was assumed to follow Darcy’s Law, which is given by (Bear 1972) Q¼ κA Δp μL ð1Þ where Q = flow rate; κ = permeability of the material; A = surface area; L = thickness of the material; μ = dynamic viscosity of the fluid; and Δp = pressure difference from the top to the bottom of the surface. In this case, the bottom and sides of the filter are considered separately (denoted Qb and Qs , respectively) and the corners are neglected. The pressure change between the surfaces is equal to the hydrostatic pressure of the water (as the flow is very slow, a quasi-steady approximation is appropriate). For the flow through the bottom, the change in pressure from the inside bottom surface to the outside bottom surface (the distance of porous media through which the water flows) is equal to the hydrostatic pressure of the fluid at that time. This is given by Δp ¼ ρghðtÞ ð2Þ where hðtÞ = height of water above the base of the filter at any given time; ρ = density of water; and g = acceleration due to gravity. Eq. (2) is substituted into Eq. (1). Additionally, the area of the bottom of the filter (A ¼ πr2o , where ro is the radius of the base of 988 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 J. Environ. Eng. 2013.139:986-994. the filter) is the surface area that the flow is acting on; the thickness of permeable material is L ¼ tb ; and the permeability is considered to be constant. Thus, the flow rate through the bottom of the filter is given by Downloaded from ascelibrary.org by NATIONAL INSTITUTE OF TECHNOLOGY on 06/26/14. Copyright ASCE. For personal use only; all rights reserved. Qb ¼ κ πr20 ρghðtÞ μ tb ð3Þ On the sides of the filter, the pressure is a function of the position y and is given by Δp ¼ ρgðhðtÞ − y). The area of the filter is also a function of y. The radius changes along the filter height of the filter and is expressed as rðyÞ ¼ r0 þ y tan θ. The permeability coefficient is again considered to be constant and to be the same as on the bottom of the filter. Thus, the flow rate through the side of the filter is given by Z hðtÞ κ πρgðhðtÞ − yÞ Qs ¼ 2ðr0 þ y tan θÞdy ð4Þ μ ts 0 Integration of Eq. (4) gives: Qs ¼ 
κ πρg2h2 ðtÞ r0 hðtÞ − tan θ μ ts 3 2 ð5Þ Furthermore, by adding Eqs. (3) and (5), the following expression for the total mass flow rate is obtained for the total mass flow rate, Q:
2  κ r r hðtÞ 2h2 ðtÞ − tan θ ð6Þ Q ¼ πρghðtÞ 0 þ 0 tb μ ts 3ts Thus, an expression is derived for the flow rate through the CWF, as a function of the height of water in the CWF. The values of hðtÞ were found from the following expression for the volume of water, VðtÞ, contained in the frustum-shaped CWF at any given time t:
 hðtÞ3 tan2 θ VðtÞ ¼ π R2 hðtÞ þ RhðtÞ2 tan θ þ ð7Þ 2 The values of κ were used to fit Eq. (6) to the experimental measurements of flow rate that were obtained using the methods described earlier in section 2.3. Permeability The permeability of the porous CWFs was calculated using two methods. The first method involved fitting flow rate data to an equation derived from the Darcy’s equation (see section on Modeling). The second method for determining permeability was based on data obtained from MIP measurements. The equation, given below, was derived by Katz and Thompson (Katz and Thompson 1987) and has been used to find the permeability of a wide range of porous materials (Crowley et al. 2004; Eldieb and Hooton 1994; Garboczi and Bentz 2001; Hooton et al. 2001; Van Halem 2006) k¼ 1 2 Lmax L ∅SðLmax Þ 89 max Lc ð8Þ where k (darcy) = intrinsic permeability; Lmax (μm) = pore size at which conductance is maximum; Lc (μm) = pore breakthrough size, which is determined from the mercury depression curve; ∅ = porosity of the filter; and SðLmax Þ = fractional volume of connected pore space composed of pore widths of size Lmax and larger. Tortuosity Hager (Hager 1998) has derived an expression for material permeability in which pores are treated as bundle of capillary tubes of varying sizes. By rewriting the equation of the permeability and making tortuosity the subject of the formula, we have sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Z ζ¼r c;max ρS 2 ζ fv ðζÞdζ ð9Þ τ¼ 24kð1 þ ρS V tot Þ ζ¼rc;min where τ = tortuosity; ρs ðg=mLÞ = skeletal density; kðm2 Þ) = permeability; V tot ðmL=gÞ = the total pore volume of the material; ζ¼r and ∫ ζ¼rc;max ζ 2 fv ðζÞdζ = the pore volume distribution by pore size. c;min Except for the tortuosity, all these parameters can be obtained from the mercury intrusion porosimetry tests. Since mercury cannot intrude into small micropores, a more accurate value of tortuosity is obtained by using the value of skeletal density obtained by gas pycnometry (Webb 2001). In this paper the skeletal density obtained from helium pycnometry method was used (Yakub et al. 2012). Moreover, the tortuosity for each CWF type was computed using both the value of permeability obtained from the Katz and Thompson (K-T) method and from the Darcy fits (see section on Modeling). E. coli Removal Experiments E. coli filtration experiments were performed on CWFs with volume ratios of clay to sawdust of 50∶50 and 65∶35. To determine the removal efficiencies, 10–20 milliliter (mL) cultures of the nonpathogenic E. coli K-12 strain W3110 [(Bachmann 1972) obtained from N. Ruiz, The Ohio State University] were grown in Miller’s LB Broth (Miller 1972) at 37°C for 18–24 hrs. The growth was completed with vigorous aeration, either by shaking at approximately 200–220 revolutions per minute (Model G-24 Incubator Shaker, New Brunswick Scientific), or by stirring using a digital stirrer/hotplate (Model 735-HPS, VWR, West Chester, PA). Four milliliters of the stationary phase culture were thoroughly mixed into 4 L of sterile, purified water, producing a prefiltrate suspension containing approximately 106 to 107 cells=mL, which is similar to the concentration used by Bielefeldt et al. (2009), and also to the maximum bacterial concentration determined for surface drinking water in South Africa (Obi et al. 2003). This concentration of E. coli is approximately 1,000-fold greater than the density rated as very high risk for drinking water quality guidelines of World Health Organization (WHO) for rural drinking water supplies (WHO 2011a). The entire 4 L of prefiltrate was poured rapidly into a water-saturated CWF, and sample of 10 mL was removed. A volume of 3–4 L of the filtrate was collected in a 18.93 L (5-gallon) plastic bucket lined with a sterile plastic bag after approximately 24 hr of filtration, and a sample of 50–100 mL was removed. The numbers of viable cells (colony-forming units, CFU) in the samples of the prefiltrate and filtrate suspensions were determined by appropriate dilution of these samples into sterile purified water followed by plating onto Miller’s LB agar (Miller 1972). Additional experiments confirmed that the viability (CFU) of the W3110 strain of E. coli in sterile purified water decreased less than 0.17 LRV during the 24 hr duration of filtration (unpublished results). The number of colonies (CFU) in the prefiltrate and filtrate was counted after overnight incubation at 37°C. When the viable count (CFU) of the filtrate was low (≤ 5 CFU=mL), the cells present in larger filtrate samples (10–100 mL) were collected using sterile filtration assemblies (MVHAWG124, Millipore Corp., Billerica, MA). After filtration, the membrane filter was then removed from the filtration assembly, placed directly onto Miller’s LB agar and JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 / 989 J. Environ. Eng. 2013.139:986-994. Downloaded from ascelibrary.org by NATIONAL INSTITUTE OF TECHNOLOGY on 06/26/14. Copyright ASCE. For personal use only; all rights reserved. Fig. 2. Pore size distribution for samples taken from the bases of (a) PFP filter; (b) 50∶50 filter; (c) 65∶35 filter; (d) 75∶25 filter incubated overnight at 37°C. To decontaminate CWFs between experiments, the CWFs were rinsed thoroughly with purified water and dried in full sunlight for 5–8 hrs to mimic conditions in the field. The efficiency of E. coli filtration is expressed both as a (Mwabi et al. 2012) and a log reduction value (Brown and Sobsey 2009). Results and Discussion Porosity and Pore Size Distribution The pore size distribution results obtained for CWFs with clay to sawdust ratios of 50∶50, 65∶35, and 75∶25 are compared with those from the base of a PFP reference filter (60∶40) in Figs. 2(a–d). All CWFs tested had a range of pore sizes between the nano- and micron-scales. The fact that each CWF has both nano- and micron-scale porosities is important because the micron-scale pores can trap larger microbes and multicellular organisms, while the nanoscale pores could potentially trap viruses. A unimodal distribution of pore sizes was observed in the PFP reference filter and the 50∶50 CWF [Figs. 2(a and b)]. In contrast, the 65∶35 and 75∶25 CWFs had bimodal micro- and nanoscale pore size distributions [Figs. 2(c and d)]. In all cases, most of the pores in the three different CWFs were between 0.05 and 1 μm in size. This is smaller than the typical sizes of most bacteria and nonviral pathogens, which are usually ∼1–3 μm in size (Weart et al. 2007; Willey et al. 2008). All three CWFs should, therefore, be effective in removing bacteria, as well as larger cells such as helminth ova, which are typically between 10 μm and a few hundred microns in size (Crompton and Joyner 1980). The presence of a significant fraction of nanoscale pores is also of some importance, since it raises the possibility of using CWFs in the nanoscale filtration of viruses, which are typically between 10–100 nm in size (Willey et al. 2008). One way of confirming that the filter is capable of removing viruses by particle size occlusion is the use of a surrogate in the form of fluorescent labeled polystyrene beads (Bales et al. 1997; Bielefeldt et al. 2010; Dai and Hozalski 2003; Hendricks et al. 2005). It has been shown that the removal of microspherical beads from water by CWF decreases with decreasing bead size (Bielefeldt et al. 2010). This means that to effectively filter out viruses from water the CWFs cannot rely on size occlusion alone. One solution that is currently been explored is to doping the CWFs with materials that have affinity for viruses (Brown and Sobsey 2009; Tsao 2011). The porosities determined from the MIP analyses showed that the porosity of the base of the CWF (Fig. 1) increased with increasing volume fraction of sawdust (Fig. 3). Similarly, the porosity of each of the three samples extracted from the top, middle, and bottom of the sides of the CWFs (Fig. 1) also increased with the volume fraction of sawdust. Consequently, the average porosity obtained from all four samples (from each specific CWF) increased with increasing volume fraction of sawdust (in Fig. 3). Fig. 3. Dependence of porosity on the volume fraction of sawdust used in filter fabrication; solid line represents the CWF base porosity levels, and the dash line is the overall average porosity with sample taken from CWF sides and base 990 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 J. Environ. Eng. 2013.139:986-994. Furthermore, by taking samples from different regions of the CWF, porosimetry tests should give an indication of how homogenous the CWF is, since the pressure used in the manufacturing process is not equal over the height of the filter (Van Halem 2006). With a standard deviation of ∼5.52%, ∼4.88% and ∼3.92%, for CWFs with volume fraction of sawdust of 25, 35, and 50 respectively, the filter can be said to be homogenous. Downloaded from ascelibrary.org by NATIONAL INSTITUTE OF TECHNOLOGY on 06/26/14. Copyright ASCE. For personal use only; all rights reserved. Flow Rate Measurements, Permeability, and Tortuosity The 24 hour discharge of water from the CWFs is presented in Fig. 4. While the pore size and pore size distribution of a filter are important in determining the filter’s efficacy at removing particulates from water, the porosity and permeability are important in determining the rate of fluid flow through the filter. The most porous of the filters studied, the 50∶50 CWF, exhibited the fastest discharge, followed by the PFP reference filter. The slowest discharge rates, for both the short- and long-term, were associated with the 75∶25 CWF. Therefore, the rate of water discharge by CWF increases with porosity, or in other words with the volume fraction of sawdust used in making the filter. The flow rates obtained for the 50∶50 CWF were between ∼1.3 and 2 L=hr during the first two hours, which is the range that is typical of PFP filters. Flow rates for the 65∶35 and 75∶25 CWFs were well below this level (Fig. 4). It is important to note that the amount of water through the 50∶50 CWF approached an asymptote, as the pressure head decreased with increasing flow. Hence, the flow-time plots became increasingly nonlinear with increasing flow, as shown in Fig. 4. This asymptotic behavior was recently mathematically simulated with a stochastic birth process model specific to respective amounts of organic raw material used in the manufacturing of these filters (Plappally et al. 2009). However, asymptotic regimes were not observed for the 65∶35 and 75∶25 CWFs (Fig. 4), since the amount of flow was low, even after 24 hours. In these cases, the pressure head is insufficient to drive a significant amount of flow through the porous CWFs, as the water level reduced. Figs. 5(a–c)shows how the measured volume flow rate of the different filters changes with time. Darcy fits are also presented to show how well the measured flow rate data fit the theory. The effective intrinsic permeabilities obtained for each of the CWFs from the Darcy fit (described in section 2.4) are plotted against the volume fraction of sawdust in Fig. 6. This shows that the effective permeabilities of the filters increase with increasing volume fraction of sawdust. Fig. 7(a) shows the comparison between the permeability obtained from the Darcy fit to that obtained using the K-T method. For a particular filter composition the variability in the value of permeability obtained using the K-T approach is largely due to the variation of the porosity of the filter with respect to the location from which the sample was taken for the mercury porosimetry test, and also in part due to Lc , Lmax and SðLmax Þ [see Eq. (1)]. The variability in the permeability obtained from the Darcy fit may be attributed to pores opening and clogging during multiple flow experiments and possibly the effect of the degree of saturation of the CWF prior to testing. The permeabilities of the filters studied (obtained by both methods) were found to be between ∼10−15 m2 to ∼5.0 × 10−14 m2 (i.e., ∼1 millidarcy to ∼50 millidarcy), which means the filters can be classified as semipervious (Bear 1972). The value of the permeability is of the same order as that reported by van Halem (2006). The difference may be due to the choice of inflexion point (Webb 2001) and the effects of production variables during fabrication of the CWFs (Lantagne et al. 2010). Fig. 4. Plots of effluent discharge as a function of time for one day-long (24 hour) effluent discharge Fig. 6. Dependence of the effective CWF intrinsic permeability on the volume percentage of sawdust Fig. 5. Plots of volume flow rate against time—comparisons of experimental data and Darcy fits: (a) 50∶50: CWF; (b) 65∶35 CWF; (c) 75∶25 CWF (note: 1 m3 =s ¼ 3.6 × 106 L=h) JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 / 991 J. Environ. Eng. 2013.139:986-994. Downloaded from ascelibrary.org by NATIONAL INSTITUTE OF TECHNOLOGY on 06/26/14. Copyright ASCE. For personal use only; all rights reserved. the permeability obtained from Darcy fit and K-T method, respectively) are presented in Fig. 7(b). In general, the tortuosity is found to increase with increasing clay content. The material tortuosity ranged from ∼10 to ∼60. A tortuosity value of, say, 10 means that in other for a particle (traveling with the water) to get through the CWF it must travel an effective length 10 times the actual length (or thickness) of the CWF. It is should be noted that not all of the channels of the filter have the same effective length. Some effective lengths may be shorter or longer than the ‘average’ effective lengths, and, hence, compositionally different CWFs may have a large fraction of similar effective lengths. The overlapping error bars in Fig. 7(b) partially testify to this fact. This may also explain why there is no significant difference in the filtration efficiencies of the CWFs studied, despite the fact that the filters have different porosities (see section 3.3 on bacteria filtration). Furthermore, the 50∶50 filter, which has a composition similar to filters used in the field, has a tortuosity value that is comparable to that reported by van Halen (2006). Bacterial Filtration Fig. 7. Bar chart: (a) comparing the permeability obtained from Darcy fit with the K-T method; (b) showing the tortuosity of the CWFs; the tortuosity was found by combining the Jörgen Hager approach with (1) the permeability obtained from the Darcy fit; (2) the permeability obtained using the K-T method; the skeletal density used was obtained using helium pycnometer While the level of permeability of a porous ceramics gives a measure of the relative ease at which water (or any other fluid) will flow through it, the tortuosity gives an indication of the chances of capture of contaminants, such as E. coli bacteria that are carried along with the water by processes such as adsorption, geometrical occlusion and sedimentation. The results for the tortuosity of the CWFs found by using the Jörgen Hager equation (combined with Following the flow rate tests, E. coli filtration experiments were performed on the two CWFs with the fastest flow rates, the 50∶50 CWF and the 65∶35 CWF. Each of these two CWFs were tested twice using a 4 L prefiltrate with a high E. coli density as described in section 2.7. Both CWFs tested were highly efficient at removing E. coli from 4 L aqueous suspensions (Table 1), having high average filtration percentages (99.9996 and 99.9999%) and also high average LRVs of 5.67 and 6.36, respectively. Thus, the removal efficiency of the 65∶35 CWF was slightly greater than that of the 50∶50 CWF. Both CWFs meet the WHO standard for water treatment (WHO 2011b) that no E. coli nor coliform bacteria should be detectable in 100 mL of drinking water. In comparison, the filtration efficiency of the PFP filter has been reported to be approximately 99.99% with LRV of ∼2–6 (Lantagne 2002; Oyanedel-Craver and Smith 2008; Plappally et al. 2009; Sobsey et al. 2008). Following the first set of filtration experiments, a second set of experiments was performed on the same CWFs to explore possible performance differences as a function of increased CWF use. Only small differences in filtration efficiencies were observed between the first and the second sets of tests. These results suggest that CWF pores did not become significantly clogged or plugged during the first test because of the high E. coli density in the prefiltrate suspensions. Further work is clearly needed to study the dependence of flow and filtration characteristics of the CWFs on the number of filtration cycles. In addition to tests using a prefiltrate volume of 4 L, a prefiltrate volume of 8 L was also used to determine whether the efficiency of filtration would be affected by the volume of the prefiltrate. Base on a study of two different 50∶50 CWFs, the average LRV did not change significantly using an 8 L prefiltrate compared to a 4 L prefiltrate (LRV4 L ¼ 5.67  2.50 and LRV8 L ¼ 4.61  0.53). Table 1. E. coli Filtration Efficiency Obtained for Filters with Different Clay to Sawdust Volume Ratios Using 4 L and 8 L Prefiltrates Volume fraction (clay : sawdust) 50∶50 50∶50 65∶35 65∶35 Prefiltrate volume 4 4 4 4 L L L L Percentage E. coli removal 99.99 99.99 99.99 99.99 Average percent ± range 99.99  0.00 99.99  0.00 LRV E. coli removal 8.17 3.16 5.82 6.9 Average LRV ± range 5.67  2.50 6.36  0.54 Note: Results are expressed as percentages (Mwabi et al. 2012) and also as the Log10 reduction value (Brown and Sobsey 2009). 992 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JULY 2013 J. Environ. Eng. 2013.139:986-994. Experiment number 1 2 1 2 Downloaded from ascelibrary.org by NATIONAL INSTITUTE OF TECHNOLOGY on 06/26/14. Copyright ASCE. For personal use only; all rights reserved. Implications The implications of the above results are quite significant. First, they suggest that point-of-use CWFs with different porosities can be used to filter out most of the bacterial pathogens in water. With E. coli removal rates of approximately 99.9%, the use of the CWFs can contribute significantly to the removal of microbial pathogens from drinking water in the developing world, where about 5,000 people die every day from the effects of consuming contaminated water. The porosimetry data also provide some useful insights into how the different ranges of pore sizes can contribute to the trapping of bacteria (Fig. 2). Based on the current results, the range of nanoand micron-scale pores can trap single-celled and multicellular organisms that cause water-borne diseases. However, the nanoscale pores may not be sufficient to trap viruses that have sizes of about 10–30 nm. This suggests a need for adsorbing surfaces that attract viruses during flow through the CWFs. Further work is clearly needed to develop such surfaces. The successful fitting of Darcy’s equation to most of the flow data suggests that much of the flow through the CWFs is well described by continuum flow through porous membranes. However, experiments carried out with filters that were not presoaked with water (results not shown) suggest some discrepancies between the measured and the fitted data during the initial stages of the flow. The discrepancies between the initial flow data are attributed to the initial transient flow required for the transport of water from the inner to the outer surface of the CWF. Once this occurs (or if the filter is presoaked), the conditions for continuum flow appear to be established, and the data is well described by Darcy’s equation [Figs. 5(a–c)]. Furthermore, during the latter stages of the experiments, the hydraulic pressures are insufficient to drive the fluid flow through the porous ceramic walls. Under such conditions, the flow rates decrease, and the flow asymptotes. Before closing, it is important to note that the flow data presented in Figs. 4 and 5 are highly nonlinear. Hence, the simple use of initial flow rate data in CWF quality control provides only a limited perspective of the CWF flow and filtration characteristics. At best, these represent average flow rates during the initial stages. Moreover, studies on the flow rates of the CWFs that have been performed using less sophisticated equipment such as stopwatches, graduated cylinders and/weight scales (Lantagne et al. 2010; Van Halem 2006). This is more or less a single snapshot flow approach and does not give an accurate picture of the flow characteristics of a CWF. Better accuracy was achieved in this paper with the aid of an automated flow measuring device. This method should be complemented with bacterial filtration testing, especially when different batches or sources of clay are used in the fabrication of CWFs. For the greatest precision, the effective permeabilities should be extracted from the measured flow data, and also used as quality control measures at CWFs factories. However, this may not be practical in rural environments in developing countries, where the CWFs are produced by people with limited capacity to analyze fluid flow data. This suggests a need for simple software and materials process design charts that combine the theories presented in this paper into guidelines useful to CWF producers in the developing world. These are clearly some of the challenges for future work. Conclusions This paper presents the results of an experimental study of the water flow and E. coli removal efficacy of porous semipervious CWFs produced by the sintering of well controlled mixtures of redart clay and sawdust. The flow was well described by Darcy’s law and continuum theory. The permeability obtained from the Darcy fit is comparable to that obtained using the Katz and Thompson method. The porosity, intrinsic permeability and overall flow rates increase with increasing volume fraction of sawdust. The tortuosity, however, decreases with increasing volume fraction of sawdust. An optimum flow rate of ∼2 Liters per hour was obtained from the CWFs with a sawdust volume fraction of 50%. This filter also removed more than 99.96% or 5.67 LRV of E. coli from aqueous suspensions. However, the filtration efficiency did not change significantly with volume fraction of sawdust. The E. coli removal was attributed largely to the geometrical occlusion provided by micron-scale pores and quite possibly by adsorption due to the high tortuosity and value. Acknowledgments This work was supported by grants from the National Science Foundation (DMR 0231418) and The Grand Challenges Program at Princeton University. 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