Membrane Distillation Proess for Pure Water and Removal of Arseni M. S. tesis for te International Master s Program in Applied Environmental Measurement Teniques Asiq Moinul Islam Supervisor: Asso. Prof. Börje Gevert Department of Materials and Surfae Cemistry (Applied Surfae Cemistry) Calmers University of Tenology, Gotenburg, Sweden. Examiner: Prof. Krister Holmberg Department of Materials and Surfae Cemistry (Applied Surfae Cemistry) Calmers University of Tenology, Gotenburg, Sweden. Co-supervisor: Henrik Dolfe Sarab Development AB Stokolm, Sweden i
Aknowledgement I am very grateful to my supervisor Assoiate Professor Börje Gevert and Aapo Sääsk, CEO of HVR Water Purifiation AB, for oosing me for tis tesis work and teir ontinued support for te suessful ompletion of it. I am also tankful to my examiner Professor Kristen Holmberg for is all-out support. I am also grateful to my o-supervisor Henrik Dolfe for making tings more understandable to me, allowing me to ave ontats wit people from various institutions. I would like to tank Assoiate Professor Prosun Battaarya and Professor Gunnar Jaks from KTH (Royal Institute of Tenology), Professor Marie Vater from Karolinska Institute in Stokolm for teir important advies. I am also grateful to Professor Kazi Matin Amed from University of Daka, Professor A. B. M. Badruzzaman and Assoiate Professor Asraf Ali from BUET (Banglades University of Engineering & Tenology) for being so elpful during my tesis work in Banglades. I would like to tank te Civil Engineering Department of BUET for allowing me to do experiments in te Environmental Engineering Laboratory. I would like to tank Cristin Dalberg from Högsby kommun in Sweden for sending te arseni ontaminated water sample. I must tank te people of Sarab Development AB for reating a friendly working environment. ii
Abstrat Safe drinking water is very important for uman beings. Human life as been under treat in many parts of te world beause of ig arseni ontamination in ground water. Arseni is tasteless, olourless and odourless even if its onentration is very ig in water. Healt effets start to emerge after a long time use of arseni ontaminated water making te problem more serious. So many metods ave been tried to get rid of arseni problem. But tese metods ave been sowing omplexities and ineffiieny resulting from use of emials, areful ontrol of ph, sludge andling, bateriologial growt, requirement of expertise for proper operation et. Moreover, implementation of a tenique or metod only for removal of arseni is not feasible in most ases. Te HVR ouse-old water purifier, wi is based on membrane distillation proess, is an effetive tool for prodution of pure drinking water. It an effetively remove all nonvolatile impurities inluding arseni from water. In tis tesis work, teoretial studies on membrane distillation proess were done. Different arseni removal metods and tere omparison wit HVR water purifier were investigated. Experiments were done on te HVR water purifier to see its impurities removal apaity. Results were quite satisfatory wi justify te effetiveness of te membrane distillation proess and te purifier for safe drinking water prodution. iii
Index 1. Introdution...1 2. Te membrane distillation proess...2 3. Te air gap membrane distillation (AGMD) proess...4 4. Carateristis of te membrane.....5 5. Advantages of membrane distillation proess over oter membrane tenologies...6 6. Teoretial model for mass and eat transport...7 7. Grapial representation of eat and mass transfer in AGMD...9 8. Fators affeting eat and mass transfer in MD..12 8.1. Membrane wetting...12 8.2. Flux deay...13 8.3. Resistane to eat transfer...14 8.4. Heat loss aross te membranes..15 9. Extent of arseni problem 17 10. Arseni removal metods or teniques. 18 10.1. Oxidation 18 10.2. Co-preipitation and Adsorption proesses...18 10.3. Sorption teniques...18 10.4. Ion-exange...19 10.5. Membrane teniques 19 11. Aeptability of an arseni removal tenology 19 12. Te HVR water purifier...20 13. Experimental study on HVR purifier and te results.21 14. Advantages of HVR water purifier in removal of arseni...27 15. Disadvantages of HVR water purifier 27 16. Conlusions....28 iv
Figures Figure 1. Vapour-liquid interfae in MD. 2 Figure 2. Types of MD proess: (i) DCMD; (ii) AGMD; (iii) SGMD; (iv) VMD. 3 Figure 3. Semati representation of te membrane distillation module..4 Figure 4. Te mass flux as a temperature of te ot solution att = 20 C, b = 0. 2 mm, φ = 0.8, kptfe = 0. 22 W/m. K and L= 1 mm..9 Figure 5. Te flux as a funtion of te temperature of te ot solution at te onstant temperature differene, T = 10 ºC at b = 0. 2 mm, φ = 0.8, kptfe = 0.22 W/m. K and L=1 mm.10 Figure 6. Te mass flux as a funtion of te temperature differene, T = T T at T = 60ºC, b = 0. 2 mm, φ = 0. 8, k PTFE = 0. 22 W/m. K and L=1 mm 10 Figure 7. Te mass flux as a funtion of te lengt of te diffusion pat, L in te air gap at T = 60ºC, T = 20 C, b = 0. 2 mm, φ = 0.8, kptfe = 0.22 W/m. K 11 Figure 8. Te mass flux as a funtion of te membrane tikness, b att = 60 C, T = 20 ºC, b = 0. 2 mm, k = 0. 22 W/m. K 11 PTFE Figure 9. Te mass flux as a funtion of te effetive net pore area at T = 60 ºC, T = 20 ºC, b = 0. 2 mm, k PTFE = 0. 22 W/m. K 12 Figure 10. Carateristi grap of liquid flux versus pressure drop in porous ydropobi membranes...13 Figure 11. Profile diagram of air gap membrane distillation...14 Figure 12. Te eat loss as a funtion of te lengt of te diffusion pat in te air gap at T = 60 ºC, T = 20 ºC, b = 0. 2 mm, k PTFE = 0. 22 W/m. K 15 Figure 13. Te eat loss as a funtion of te lengt of te diffusion pat in te air gap at T = 60 ºC, T = 20 ºC, k PTFE = 0. 22 W/m. K...16 Figure 14. Te eat loss as a funtion of te temperature of te ot solution at T = 20 ºC, b = 0. 2 mm, k PTFE = 0. 22 W/m. K (ref. 2) and L = 1mm 16 Figure 15. Some models of HVR water purifier.20 Figure 16. Te lab unit of HVR water purifier (side view)..21 Figure 17. Semati diagram sowing te proedure of HVR water purifier (ross-setional side view).....22 Figure 18. Treated water vs. time grap for sample A.. 24 Figure 19. Treated water flow rate vs. time grap for sample A...24 Figure 20. Treated water temperature vs. time grap for sample A...25 Figure 21. Treated water vs. time diagrams for sample B..26 Figure 22. Treated water flow rate vs. time grap for sample B 26 Figure 23. Treated water temperature vs. time grap for sample B 27 v
1. Introdution Membrane distillation (MD) is a proess in wi a liquid (su as water) is separated from impurities by eating and evaporating te liquid and allowing te vapour to pass troug a miro-porous ydropobi membrane. Ten te vapours are allowed to ondense into liquid by ooling in te oter side of te membranes. Te membrane pores are suffiiently small tat apillary fores prevent diret mixing of te pases on eiter side of te membrane. Te vapour pressure differene aross te membrane aused by te orresponding temperature differene provides te driving fore tat auses diffusion of vapours troug te membrane pores. Membrane distillation proess an be used for effiient purifiation of drinking water, wi an remove all sorts of non-volatiles. Currently, tere is no ommerial produt available in te market, wi is based on te membrane distillation proess. HVR Water Purifiation AB is a Stokolm based ompany, wi as ommerialized a ouse-old level drinking water purifier. Te purifier is based on membrane distillation proess. Te purifier as been proved to be very effiient in removing all sorts of non-volatiles from drinking water inluding arseni. Tis purifier an be an effetive tool for effiient removal of arseni from arseni ontaminated ground water. Tis purifier offers a number of advantages over oter impurities and arseni removal proess. Te objetive of te projet were to make a teoretial study on te membrane distillation proess, to study arseni removal metods used in Banglades and to ompare tem wit te HVR water purifier, to ondut experiments on te HVR purifier, and to make a feasibility study of it. Projet ativities were in Sweden and Banglades. 1
2. Te membrane distillation proess In a membrane distillation proess, a eated, aqueous feed solution is brougt into ontat wit one side (feed side) of a ydropobi, miro porous membrane. Te ydropobi nature of te membrane prevents penetration of te aqueous solution into te pores, resulting in a vapour-liquid interfae at ea pore entrane. Fig. 1 sows a ross setional view of a ydropobi membrane wit straigt ylindrial pores in ontat wit an aqueous solution to illustrate ow te vapour-liquid interfaes are supported at te pore openings. Aqueous Solution Sweep Gas θ vapour r Vauum or Aqueous Solution Air Gap Figure 1. Vapour-liquid interfae in MD Various types of metods may be employed to impose a vapour pressure differene aross te membrane to drive te flux. Tere are mainly 4 onfigurations of te MD proess, wi are mainly dependent on ow te ondensing surfaes are separated from te membrane. Tey are: (i) DCMD (Diret Contat Membrane Distillation): Te permeate side of te membrane may onsist of a ondensing fluid in diret ontat wit te membrane (see Fig. 2(i)). (ii) AGMD (Air Gap Membrane Distillation): Te ondensing surfae is separated from membrane by an air gap (see Fig. 2(ii)). (iii) SGMD (Sweeping Gas Membrane Distillation): Te ondensing surfae is separated from membrane by a sweeping gas (see Fig. 2(iii)). (iv) VMD (Vauum Membrane Distillation): Te ondensing surfae is separated from membrane by a vauum (see Fig. 2(iv)). 2
(i) Aq. Sol n (ii) Air Gap Cold Surfae Aq. Sol n Aq. Sol n Sweep Gas vauum (iii) (iv) Aq. Sol n Aq. Sol n Figure 2. Types of MD proess: (i) DCMD; (ii) AGMD; (iii) SGMD; (iv) VMD. Te membrane distillation proess is based on te following onditions of parameters: (1) Te membrane sould be porous. (2) Te membrane sould not be wetted by te proess liquids. (3) No apillary ondensation sould take plae inside te pores of te membrane. (4) Te membrane must not alter te vapour-liquid equilibrium of te different omponents in te proess liquids. (5) At least one side of te liquid sould be in diret ontat wit te proess liquid (see artile 4). (6) For ea omponent te driving fore of tis membrane operation is a partial pressure gradient in te vapour pase. Te advantages of membrane distillation proess over oter prominent separation proess are: (1) 100% (teoretial) removal of ions, maromoleules, olloids, ells, and oter nonvolatiles. (2) Lower operating temperature tan te onventional distillation. (3) Lower operating pressure tan te onventional pressure-driven membrane separation proesses. (4) Redued emial interation between membrane membranes and proess solution. (5) Less membrane meanial requirement in terms of pressure and eat sustaining apabilities. (6) Redued vapour spaes ompared to onventional distillation proess. Te advantages of membrane distillation over some oter membrane separation proess are disussed in te artile 7. 3
3. Te air gap membrane distillation (AGMD) proess Among te different membrane distillation proesses te air gap membrane distillation (AGMD) is te main fous in tis tesis work. Membrane Condensing wall 0 1 2 3 4 5 X Coolant Pore Vapour Hot solution b L Figure 3. Semati representation of te membrane distillation module. Te figure sows te air gap membrane distillation system. 0-1 : a flow annel for te ot solution 1-2 : igly porous membrane 2-3 : an air gap 3-4 : a tin ondensate layer 4-5 : a annel for te oolant Te meanism is explained in te following: (1) Heat transfer from te bulk to te membrane surfae at 1. (2) Evaporation of water from te ot solution surfae 1. (3) Diffusion of vapour from te membrane, from 1 to 2. (4) Diffusion of water vapour troug te air gap, from membrane surfae to te ondensate film 3. (5) Condensation of water vapour on te ondensate film 3. (6) Heat transfer from troug te ondensate film to te oolant, from 3 to 4. Z 4
4. Carateristis of te membrane Te membranes used in MD modules are igly ydropobi polyolefine or fluoroplasti membranes. For MD, te typial average pore diameter is in te range 0.1-5 µm [2]. Seletion of pore size depends on two fators, te pore-size must be large enoug to allow te required flux and te pores must be small enoug to prevent liquid penetrating troug te pores under operating onditions. Te porosity of te membrane is te value determined by dividing te total volume of pores by te total volume of te membrane. Molar flux troug a pore, N is related to te average pore size <r α >, te membrane porosity φ, membrane tortuosityτ, te membrane tiknessb, α < r > * φ N (A) τb Equation (A) illustrates te importane (in terms of molar flux) of maximizing te membrane porosity and pore size wile minimizing te transport pat lengt troug te membrane, τ b, for MD systems, porosity an be te most influential fator affeting mass transfer rates aross te membrane [1]. For pure water, molar flux in DCMD aross a relatively tik membrane is proportional to 1 / b as predited by equation (A). However, flux beomes independent of membrane tikness at small tiknesses (tat is, wen b << km / U were k m is te termal ondutivity of te membrane and1 / U = 1/ f + 1/ p, te ombined feed and permeate boundary layer resistane). Tis penomenon is a result of te inreased ondutive eat loss assoiated wit tinner membranes ( Q m 1/ b ) [1]. For aqueous solutions wit signifiant osmoti pressures, te effet an beome so pronouned tat a negative flux( from permeate to feed) results if te membrane is too tin. Porosity an also beome important in terms of reduing te amount of eat lost by ondution. Q = T m m m mg m = φ + ( 1 φ) ms Were φ is te membrane porosity, and mg and ms represent te eat transfer oeffiients of te gas (vapour) witin te membrane pores and te solid membrane material, respetively. Sine mg is generally an order of magnitude smaller tan ms, te value of m an be minimized by maximizing te membrane porosity. 5
Table 1. Commerially available membrane ommonly used in MD[1]: Manufaturer Trade name Material Tikness (µm) Average Void Pore size (µm) fration 3M PP <100 a a Enka(Akzo) PP(Tube) 150 0.43 0.70 PP 140 0.10 0.75 PP 100 0.20 0.75 Gore Gore-Tex PTFE b <50 a a Gelman Inst. Co. TF 200 PTFE b 60 0.20 0.60 TF 450 PTFE b 60 0.45 0.60 TF 1000 PTFE b 60 1.00 0.60 Hoest-Celanese Celgard 2400 PP 25 0.02 0.38 Celgard X-20 PP (tube) 25 0.03 0.35 Millipore Durapore PVDF 110 0.45 0.75 Durapore PVDF 140 0.22 0.75 a = Membranes wit a wide range of parameters ave been used in MD b = Membrane supposed on polymer fabri. Reported values of tikness and porosity do not inlude support. 5. Advantages of membrane distillation proess over oter membrane tenologies Some of te advantages of te MD proess over oter membrane tenologies are as following: Less vapour veloity Conventional distillation olumn relies on ig vapour veloities to provide intimate vapourliquid ontat. MD employs a ydropobi miro porous membrane to support a vapourliquid interfae. Less vapour spae Te large vapour spae required by a onventional distillation olumn is replaed in MD by te pore volume of a miro porous membrane, wi is generally on te order of 100 µm tik [1]. Smaller equipment In membrane distillation membrane surfae per unit volume is ig, tus te equipment an tus be made very dense [2]. Lower temperature Te required operating temperatures are mu lower, beause it is not neessary to eat te proess liquids above teir boiling temperatures. Feed temperatures in MD typially range from 60 to 90 C, altoug temperatures as low as 30 C ave been used [1].Terefore low grade, waste and/or alternative energy soures su as solar and geotermal energy an be oupled wit MD systems for a ost effiient, energy effiient liquid separation system. 6
Less eat loss Lower operating temperature ombined wit redued equipment surfae area results in less eat lost to te environment troug te equipment surfaes. Lower pressure MD is safer, more effiient proess tan RO for removing ioni omponents and non-volatile organi ompounds from water. Sine MD is a termally driven proess, operating pressures are generally on te order of zero to a few undred kpa, relatively low ompared to pressure driven proesses su as RO [1]. Higer rejetion of impurities Sine MD operates on te priniples of vapour-liquid equilibrium, 100% (teoretial) of ions, maromoleules, olloids, ells, and oter non-volatile onstituents are rejeted; pressure driven proess su as reverse osmosis (RO), ultra filtration (UF), miro filtration (MF) ave not sown to aieve su ig levels of rejetion. Wen applied to desalination, a well designed MD system typially aieves water fluxes as ig as 75 kg/ m 2., wi is omparable to RO [1]. Less meanial demands Sine MD operates at pressures signifiantly lower tan tose enountered in te pressuredriven proesses, te meanial demands (tat is, resistane to ompation) on tese miro porous membranes is greatly redued [1]. More emially resistant membrane An advantage of MD over RO, UF, and MF arises from te minimal role tat te membrane plays in te atual separation. In MD te membrane ats merely as a support for a vapourliquid interfae and does not distinguis between solution omponents on a emial basis, nor does it at as a sieve. Terefore, MD membranes an be fabriated from emially resistant polymers su as polytetrafluoroetylene (PTFE), polypropylene (PP), and ployvinylidenedifluoride (PVDF) [1]. Less fouling Membrane fouling is less of a problem tan in te oter membrane separations beause te pores are relatively large ompared to te pores or diffusional patways in oter membrane separations beause te pores are relatively large ompared to te pores or diffusional patways in RO or UF [1]. 6. Teoretial model for mass and eat transport [2] Te assumption, tat membrane distillation an be desribed as a proess in wi a ot ondensable vapour is diffusing at steady state troug a stagnant film of nonondesable gas to a old surfae were te vapour ondenses, is te basis for te alulations. Te molar flux of a vapour diffusing at steady state troug a stagnant air film is given by (see fig. 3), N D dx = (1) 1 x dz Were, N is te molar flux, is te molar onentration, and D is te diffusion oeffiient for te water vapour-air mixture. Altoug te molar flux is only affeted to a minor degree by 7
te simultaneous mass transfer, te rate of eat transfer is diretly affeted by te simultaneous mass transfer. Te sensible eat, E, is made up of one term desribing te ondutive energy flux and one term desribing te energy flux aused by diffusion. E dt = k + NC ( ) p T T (2) dz Solving te differential equations (1) and (2), in te membrane region and in te air gap region, te mass and eat transport may be alulated for different membrane parameters and proesses. Te oeffiients for diffusion D, and te termal ondutivity k depend on bot temperature and onentration. In order to simplify te alulations approximate expressions for D and k: 5 D = 6.3*10 T (3) k air 3 = 1.5*10 T (4) Te effetive oeffiient for te membrane region an be written as, k k air * φ + k (1 φ) (5) = membrane If tese equations are substituted into equations (1) and (2) te molar and energy flux in te membrane region may be expressed as, N 5 6.3*10 T dx = φ * (6) 1 x dz 3 dt 3 E = {1.5*10 T * φ + 0.22(1 φ)}* + 1.86 *10 N( T T ) (7) dz and in te air gap region as, N 5 6.3*10 = 1 x T dx dz (8) E = {1.5*10 3 dt 3 T * φ + 0.22(1 φ) * + 1.86 *10 N( T T ) } (9) dz Te above equations may not yield te exat analytial solution. But N and E an be obtained from te following good approximations, 5 1 1 x N = 6.3*10 ln( ) (10) b /( φ * T ) + L / T 1 x 8
1.5*10 E = ( b / γ * φ * 3 ( T T ) T ) + L / T 1- x *{1+ 1.14*ln( 1- x b /( γ * φ * T ) ) * } b ( ) + ( L / T ) φ T (11) were, γ = kmembrane /( φ * kair ) and k sould be evaluated at T. Te mass flux equation an be obtained from te molar flux equation. It an be used to alulate te teoretial pure water prodution rate. Q = 4.1*10 3 * b /( φ * 1 T ) + L / T 1 x *ln( 1 x ) (12) Equation (12) is used in te teoretial alulations were, T and T is te average bulk temperature in ot and old sides, T, in + T, out T = 2 T, in + T, out T = 2 7. Grapial representation of eat and mass transfer in AGMD Te rate of evaporation or mass flux an be alulated from equation (12) for a number of different proesses and membrane parameters. In an air gap membrane distillation (AGMD) te diffusion pat is te summation of te tikness of te membrane, b, and te lengt of te diffusion pat in te air gap, L (see fig. 3). Te temperature of ot solution, T and te oolant, T an strongly influene te rate of evaporation. Figure 4. Te mass flux as a temperature of te ot solution att = 20 C, b = 0. 2 mm, φ = 0.8, k = 0. 22 W/m. K and L= 1 mm [2]. PTFE 9
Wit te derease in diffusion pat, te rate of evaporation inreases (see fig 5, 6). Te inrease is more pronouned as te temperature of ot solution, T inreases (fig 5). Te vapour pressure inreases as te temperature inreases, wi result in te inrease of te driving fore. Tat is wy, te rate of evaporation inreases if te temperature of te ot solution is inreased (see fig 4), even if, te temperature differene, T = T T, remains onstant (see fig. 5). Figure 5. Te flux as a funtion of te temperature of te ot solution at te onstant temperature differene, T = 10 ºC at b = 0. 2 mm, φ = 0.8, kptfe = 0.22 W/m. K and L=1 mm [2]. If te ot solution temperature remains onstant, evaporation inrease wit te inrease in temperature differene between ot and old side but te rate of te evaporation dereases (see fig. 6). Figure 6. Te mass flux as a funtion of te temperature differene, T = T T at T = 60ºC, b = 0. 2 mm, φ = 0. 8, k = 0. 22 W/m. K and L=1 mm [2]. PTFE 10
Wit te derease of air gap te flux inreases (see fig. 7, 8, and 9). Figure 7. Te mass flux as a funtion of te lengt of te diffusion pat, L in te air gap at T = 60ºC, T = 20 C, b = 0. 2 mm, φ = 0.8, k = 0.22 W/m. K [2]. Flux dereases wit te inrease of membrane tikness (see fig 8). As sown in te equation (12), for te alulation of mass flux in AGMD, te denominator onsists of te membrane term, b /( φ T ), and te air gap term, L / T. Te air gap term is mu larger tan te membrane term and ene an anges in L an ave mu more effets tan anges in b (see fig. 8). PTFE Figure 8. Te mass flux as a funtion of te membrane tikness,b att = 60 ºC, T = 20 ºC, b = 0.2 mm, k = 0. 22 W/m. K [2]. PTFE 11
Inrease of porosity inreases te flux (see fig. 9). Figure 9. Te mass flux as a funtion of te effetive net pore area at T = 60 ºC, T = 20 ºC, b = 0. 2 mm, k = 0. 22 W/m. K [2]. PTFE 8. Fators affeting eat and mass transfer in MD 8.1. Membrane wetting Te relationsip between a membrane s largest allowable pore size and operating onditions is given by te Laplae (Cantor) equation [1]: 2Bγ L osθ Pliquid Pvapor = Pint erfae < Pentry = (B) rmax were γ L is te liquid surfae tension, θ is te liquid-solid ontat angle, r max is te largest pore radius, and B is a geometri fator determined by pore struture. Even toug equation (B) is not impliit in operating temperature and proess solution omposition, tese parameters an ave signifiant effets on te liquid-solid ontat angle and te liquid surfae tension. Tese affets must be taken into onsideration during seletion of membrane. In general, te ontat angleθ must be greater tan 90º for te system to be used in MD. For a typial water-ydropobi membrane ontat angle of 130º, te penetration pressure of a ylindrial ( B =1) pore of diameter 1µm is only 185 kpa (27 psi) [1]. Te presene of strong surfatants an greatly redue te value of γ L osθ, terefore are must be taken to prevent ontamination of proess equipment and proess solutions wit detergents or oter surfating agents. 12
Liquid Flux, N (3) K (2) P entry Pressure Drop, P Figure 10. Carateristi grap of liquid flux versus pressure drop in porous ydropobi membranes [1]. If P int erfae exeeds Pentry (1) te liquid an penetrate into and troug te membrane pores wi means te membrane is wetted and needs to be ompletely dried and leaned before it an be used one again as a support for vapour-liquid interfae. As te pressure is inreased no liquid an flow troug te membrane until Pentry is exeeded. At tis point liquid begins to penetrate te largest pores and is able to pass troug te membrane. As te pressure is inreased furter, more and more pores and beome flooded and te liquid flux aross te membrane obeys Dary s law ( N = K P ), and dereasing te pressure results in a linear derease in flux. It is evident from te figure tat one tat one te membrane is wetted te derease in te ydrostati pressure on te membrane will not restore te membrane to its un-wetted state. Deterioration of permeate quality inreases wit te inrease in number of wetted pores. Terefore, te goal of any MD system design sould be to ompletely prevent pore wetting. 8.2. Flux deay One ypotesis made by Franken et al(1987) tat flux deay was aused by membrane wetting [1].Tey ypotesized tat as time progressed, more and more pores beome wetted, wi results in bak flow of permeate to te feed. Tey validated teir ypotesis by varying te ydrostati pressure of te permeate and noting tat as te permeate pressure inreased te flux dereased. But Franken s explanation is not suffiient for te ase were te ydrostati pressure of te feed is iger tan tat of te permeate, in wi ase pore wetting leads to enaned flux (wit redued seletivity or rejetion) as desribed in te membrane wetting setion. Anoter meanism for flux deay is membrane fouling, wi as not been torougly examined in te MD proess. Tere are several types of fouling su as (1) biologial fouling,(2) saling, (3) partiulate or olloidal speies. Biologial fouling or growt of bateria on te membrane surfae is easily avoided wit UV treatment or te addition of te appropriate emials to proess liquids. Saling an build up on te membrane surfae if te onentration of minerals or salts beomes too ig. Saling an lead to bot pore logging and pore wetting, but as only been observed wit saturated proess solutions. Partiulate or 13
olloidal speies in te proess liquids an lead to membrane fouling as well. Tese partiles or olloids tend to beome trapped at te membrane-liquid interfae by interfaial tension fores. Tere is a lot of teoretial and experimental work to be done in tis area of flux deay [2]. 8.3. Resistane to eat transfer Vapour flow Air gap Condensate wall Solute T b1 C b1 T m1 C m1 T m2 T b2 Vapour pressure P b1 P m1 P m2 P b2 Figure 11. Profile diagram of air gap membrane distillation As a large quantity of eat as be supplied to te surfae of te membrane to vaporize te liquid, eat transfer aross te boundary layers is often te rate limiting step for mass transfer in MD. A ommonly used measure of te magnitudes of te boundary layer resistanes relative to te total eat transfer resistane of te system is given by te temperature polarization oeffiient, Tm 1 Tm 2 Θ = Tb 1 Tb 2 were, T m1 is te interfaial feed temperature, T m2 is te interfaial permeate temperature, T b1 is te bulk feed temperature, and T b2 is te bulk permeate temperature. Te value of Θ approaes unity for well-designed systems tat are mass transfer limited, and it approaes zero for poorly designed systems tat are limited by eat transfer troug te boundary layers. Temperature polarization oeffiient varies between 0.2 and 0.9 [5]. 14
Temperature polarization an ave a signifiant affet on te flux. Low feed stirring rates or low feed veloities, wi result in low Reynolds numbers, an redue te flux by orders of magnitude relative to te ig Reynolds number ase [5]. 8.4. Heat loss aross te membranes Tere are two important meanisms of eat transfer aross te membrane: (1) ondution troug te membrane material and te vapour witin te membrane pores, and (2) transfer of te latent eat of vaporization assoiated wit te mass flux. Te rate of eat transfer aross te membrane is given by, Qv = N H v, were N is te molar flux and H v is te molar latent eat of vaporization. Condution of eat aross te membrane, Qm = m Tm, were m = φmg + ( 1 φ) ms ; φ = membrane porosity, mg = eat transfer oeffiient of gas witin membrane, ms = eat transfer oeffiient of membrane solid materials. T = temperature drop between te bot side of te membrane. m Figure 12. Te eat loss as a funtion of te lengt of te diffusion pat in te air gap at T = 60 ºC, T = 20 ºC, b = 0. 2 mm, k PTFE = 0. 22 W/m. K [2]. Bot investigators reommended using tiker membranes to derease te problems aused by eat loss [1]. But it is surprising tat te eat loss is diminised wen te tikness of te membrane is redued (see fig. 13). Tis is due to te large inrease of te mass flux wen te membrane tikness is dereased. 15
Figure 13. Te eat loss as a funtion of te lengt of te diffusion pat in te air gap at T = 60 ºC, T = 20 ºC, k = 0. 22 W/m. K [2]. PTFE Figure 14. Te eat loss as a funtion of te temperature of te ot solution at T = 20 ºC, b = 0.2 mm, k = 0. 22 W/m. K (ref. 2) and L = 1mm [2]. PTFE Fane et al.(1987) found tat 20 to 50% of te total eat transferred in te MD proess is lost by ondution. Gostoli et al (1987) and Findley (1969) sowed tat if eat ondution aross 16
te membrane is too great, flux an be diminised and even reversed(from permeate to feed)[1]. Heat transfer oeffiients for polymeri materials and vapours are readily available from te termal ondutivities [1]. T k T =, were k is te termal ondutivity, and δ is te membrane tikness. δ Ten termal ondutivities for water and air are respetively [1], T 3 5 k H O = 2.72 *10 + 5.71*10 T, 2 k T 3 Air = 2.72 *10 + 7.77 *10 5 T T were k is in W/m.K and T is in K. Te termal ondutivities of polymers are dependent upon bot temperature and te degree of rystallinity [ 1]. As a result, te reported values of an vary, su as for polypropylene 0.15-0.20 W/m.K, for PVDF and PTFE 0.22-0.45 W/m.K [1]. So it is easily understandable tat water eat ondution troug te pores is less ompared to te eat loss troug membrane materials. Terefore, eat lost by ondution troug te membrane an be redued by inreasing te membrane porosity [1]. 9. Extent of arseni problem Naturally ourring arseni in groundwater as beome a global problem. In te sedimentary aquifers of te Bengal Delta Plan(BDP) in Banglades and neigboring Indian state of West Bengal, As is mobilized in groundwater by natural proesses, wi is an issue of major environmental ealt onern[5]. Widespread exploitation of groundwater from te BDP aquifer system, wi began in te 1970 s, urrently supplies drinking water for a population of more tan 150 million. Elevated As onentration in groundwater is owever is not unique to te BDP, but as been reported in several oter ountries, inluding Argentina, Canada, Cile, Cina, Hungary, Japan, Mexio, New Zealand, Taiwan, Tailand and te United states [5]. Te reent deision by US Environmental Protetion Ageny tat te Maximum Contamination Level (MCL) for As in drinking water will be lowered from 50 µg/l to 10 µg/l reflets reevaluation of ealt risks assoiated wit ingestion of tis metalloid (NRC, 1999) [6]. Currently, Banglades employs a drinking water standard of 50 ppb (or µg/l) wi is based on some earlier guidelines establised by te World Healt Organization (WHO, 1993). Many ountries inluding Banglades ave eiter kept tis as te national standard or as an interim target, wit te realization tat signifiant impats may also exist at lower onentrations in te 10-50 ppb range. It sould also be noted tat based on sientifi information available on ealt impats of arseni ontaminated water, a value lower tan 10 ppb is advisable (WHO, 1999). However, su a value is still onsidered provisional in part due to te lak of widely aeptable analytial teniques. Existing sientifi evidene sould be weiged arefully and te drinking water standard for arseni sould be revised downwards [3]. Sientifi resear as to be empasized to redue unertainty, wit due onsideration to loal onditions. Drinking water standards ave to be modified in view of te sientifi resear [3]. 17
10. Arseni removal metods or teniques 10.1. Oxidation Air Oxidation Arseni is present in groundwater in As(III) and As(IV) forms in different proportions. Most treatment metods are effetive in removing arseni in pentavalent form and ene inlude an oxidation step as pretreatment to onvert arsenite to arsenate. Arsenite an be oxidized by oxygen, ozone, free lorine, ypolorite, permanganate, ydrogen peroxide but atmosperi oxygen, ypoloride and permanganate are ommonly used for oxidation in developing ountries. Cemial Oxidation Te proesses remove only a part of arseni. H 3 AsO 3 + 1 / 2 O 2 = H 2 AsO 4 - + 2H + H 3 AsO 3 + HClO = HAsO4 -- + Cl - + 3H + 3H 3 AsO 3 + 2KMnO4 = HAsO4 --- + 2MnO 2 + + 2K + + 4H + + H 2 O Passive Sedimentation Oxidation of water during olletion and subsequent storage in ouses may ause a redution in arseni onentration in stored water. Arseni redution by plain sedimentation appears to be dependent on water quality partiularly te presene of preipitating iron in water. Most studies sowed a redution of zero to 25% of te initial onentration of arseni in groundwater. In rapid assessment of tenologies passive sedimentation failed to desired level of 50% µg/l in any well [3]. 10.2. Co-preipitation and adsorption proesses Water treatment wit oagulants su as aluminum alum, Al 2 (SO 4 ) 3.18H 2 O, ferri loride, FeCl 3 and ferri sulfate, Fe 2 (SO 4 ) 3.7H 2 O are effetive in removing arseni from water. Ferri salts ave been found to be more effetive in removing arseni tan alum on a weigt basis and effetive over a wider range of ph. In alum oagulation, te removal is most effetive in te ph range 7.2-7.5 and iron oagulation, effiient removal is aieved in a wider ph range usually between 6.0 and 8.5 [3]. As trivalent arseni ours in non-ionized form, it is not subjet to signifiant removal. Oxidation of As(III) to As(IV) is tus required as a pretreatment for effiient removal. Alum dissolution: Al 2 (SO 4 ) 3.18H 2 O = 2Al ++ + 3SO 4 -- + 18H 2 O Alum preipitation (aidi): 2Al ++ + 6H 2 O = 2Al(OH) 3 + 6H + Co-preipitation (Non-stoiiometri, non-defined produt): H 2 AsO 4 - + Al(OH) 3 = Al-As(omplex) + Oter produts Arseni is absorbed on aluminum ydroxide floulates as Al-As omplex removed by sedimentation. 10.3. Sorption teniques Sorption media tat are mostly used to remove arseni are ativated alumina, ativated arbon, iron and manganese oated sand, kaolinite lay, ydrated ferri oxide, ativated 18
bauxite, titanium oxide, siliium oxide and many natural and synteti media. Te effiieny of all sorptive media depends on te use of oxidizing agent as aids to sorption of arseni. Saturation (te point wen te effiieny in removing te desired impurities beome zero) of media by different ontaminants and omponents of water takes plae at different times of operation depending on te speifi sorption affinity of te medium to te given omponent. 10.4. Ion exange Te proess is similar to tat of ativated alumina, just te medium is a synteti resin of more well defined ion exange apaity. Te proess is normally used for removal of speifi undesirable ation or anion from water. As te resin beomes exausted, it needs to be regenerated. Te arseni exange and regeneration equations wit ommon salt solution as regeneration agent are as follows: Arseni exange: 2R-Cl + HAsO 4 -- = R 2 HAsO 2 + 2Cl - Regeneration: R 2 HAsO 4 + 2N + + 2Cl - = 2R-Cl + HAsO 4 -- + 2Na + Here R stands for ion exange resin: Te arseni removal apaity is dependent on sulfate and nitrate ontents of raw water as sulfate and nitrate are exanged before arseni. Te ion exange proess in less dependent on ph of water. Te effiieny of ion exange proess is radially improved by preoxidation of As(III) to As(V) but te exess of oxidant often needs to be removed before te ion exange in order to avoid te damage of sensitive resins. 10.5. Membrane teniques A reverse osmosis and a ombined nanofiltration and reverse osmosis proess for te treatment of arseni were tested in Banglades. Tose are in te experimental pases, ave not it te market. MRT-1000 and Reid System Ltd. A ouseold reverse osmosis water dispenser MRT-1000 are manufatured by B & T Siene Co. Limited, Taiwan. Tis system was tested at BUET and sowed a arseni(iii) removal effiieny more tan 80% [3]. Low-pressure nanofiltration and reverse osmosis O et al.(2000) applied reverse osmosis and nanofiltration membrane proesses for te treatment of arseni ontaminated water applying low pressure by biyle pump. A nanofiltration membrane proess oupled wit a biyle pump ould be operated under ondition of low reovery and low pressure range from 0.2 to 0.7 MPa. Arsenite was found to ave lower rejetion tan arsenate in ionized forms and ene water ontaining iger arsenite requires pre-oxidation for redution of total arseni to an aeptable level [3]. In many arseni affeted areas, arseni removal may be te only option in te absene of an alternative safe soure of water supply [3]. 11. Aeptability of an arseni removal tenology It sowed tat arseni removal is just one small element in te prodution of a suessful and aeptable tenology. Many would argue tat, given tat te tenologies are mainly designed to be a sort/medium solution, bateriologial ontamination is potentially a far more serious and immediate azard to ealt tan drinking untreated water. Tere are reommendations made for ea of te tenologies were tere are problems wit bateriologial ontamination. Tese prinipally onern regular leaning wit ypolorite 19
to minimize ontamination and a ygiene eduation program as a part of te distribution of tenologies. Tis adds furter stages to te water treatment proess and potentially furter redue aeptability of te tenologies to users [3]. Tere is some rooms for a trade off between ost and performane. However, lower ost sould not be made te priority if it results in a tenology wi is unaeptable to users, does not produe suffiient water and wi may deliver water of a lower quality and ontaining faeal oliforms [3]. In Banglades arseni removal tenologies will be sored based upon te establised riteria as to teir suitability for use in Banglades. Te sreening will onsider te following aspets of te treatment system [3]: A. Treatment/proess inluding emial/pysial meanisms, expeted treatment performane, potential limitations on performane, proess emial requirements, power requirements, flow dynamis, ardware requirements, serviing requirements, media regeneration and waste disposal requirements. B. Soial/ultural ompatibility inluding feasibility of distributing te equipment and materials, ease of system use by women, and feasibility of loal system maintenane. C. Capital/operating osts inluding installation/startup osts, operating and maintenane osts, and osts related to disposal of spent units and/or emial wastes 12. Te HVR water purifier Te membrane material is PTFE (Polytetrafluoroetylene) wit a porosity of 80% and tikness of 0.2 mm. Te lengt of air gap is 1 mm. Tere are two membranes used wit an area of 42 m * 24 m. HVR developed different models of te ouseold water purifier. Figure 15. Some models of HVR water purifier 20
13. Experimental study on HVR purifier and te results Figure 16. Te lab unit of HVR water purifier (side view) Te proedure for operating te lab unit of te purifier was as follows: 1. To onnet to eletriity supply. 2. To onnet to ill water supply. 3. To open te ap of te orrosion-free tank. 4. To pour approx. 4.5 to 4.8 l test feed water troug fluid inlet. 5. To onnet test vessels to permeate/ treated water outlet. 6. To set temperature of te eater at a temperature between 60-99 C. 7. Start te proess by pressing te start button. Let te feed water eated for 15 min. 8. Start te ill water supply approx. 1.5-2 l/min. 9. If te proess is disontinued before approx. 1-1.3 l permeate is produed, ten press te stop button. 21
Cill water inlet Feed water inlet Feed water Heater Cill water Feed water Cill water Eletriity supply St Sp P Feed water outlet Cill water outlet Cill water outlet Treated water outlet Figure 17. Semati diagram sowing te proedure of HVR water purifier (ross-setional side view) Experiments were onduted at Royal Institute of Tenology (KTH), Sweden and Banglades University of Tenology (BUET), Banglades. Feed water used were KTH and BUET supply water, water from arseni ontaminated site in Sweden, well water in Banglades wit extremely ig naturally ourring arseni ontent et. Several experiments were also onduted by using tose supply water mixed wit NaCl salt up to 4% (wt/wt) onentration. Higly arseni ontaminated (0,24 mg/l) water from ontaminated surfae water in Högsby kommun in Sweden was treated in te lab unit.te onentration found in te treated water was less tan 0,0005 mg/l. For 4% NaCl (wt/wt) solution te ondutivity in te treated water was 0,94 µs/m. In all te ases TDS was zero. Te exeption ame only for a water sample (sample A) were TDS were found 30, 40 and 50 mg/l and te ondutivity 98 µs/m. Furter study needs to be done to find out te element ausing te unexpeted (for te lab unit) level of TDS and ondutivity. It is to be mentioned 22
ere tat tere is no perolator or evaporator in te storage tank in te lab unit unlike te ommerialized model. Tat migt ave aused some volatiles oter tan water to remain in te feed solution. Anoter ypotesis may be te presene of old membranes (about 4 years). But still te removal effiieny of te purifier is really exellent for te same sample. Te Te tests onduted in te Environmental Engineering Laboratory at BUET (Banglades University of Engineering & Tenology) for sample A in Table 2. All oter parameters exept Eletrial ondutivity sowed good results. Table 2. Laboratory experiment results for sample A Parameter Unit Conentration in Conentration in Raw Water Treated Water Arseni (As) µg/l 334 < 1.0 Manganese (Mn) mg/l 0,102 0.001 Pospate (PO4) mg/l 3,28 0.03 Eletrial Condutivity µs/m 845 98 Experimental results for two samples (sample A & B) are desribed ere. Sample A: Te feed water was olleted from a tube well loated at Village:Basail Bog Tana(subdistrit): Srinagar Distrit: Munsigonj Banglades Heater temperature = 85 C; Cill water flow rate = 1.5 l/min Cill water started 15 min after te starting time. Room/ill water temp. = 29 C Te feed water TDS = 270 mg/l ph of te raw water = 7,5 Table 3. Experimental results for sample A Treated water Treated water Treated water TDS ph volume (ml) flow rate (l/) temp. ( C) (mg/l) 0-12 70 0,35 34 30 6,5 13-18 75 0,75 39,5 30 6,5 19-22 85 1,275 41 30 6,5 23-27 105 1,26 41 40 6,5 28-32 123 1,476 41 40 6,5 33-36 98 1,425 46 40 6,5 37-39 72 1,44 47 50 6,5 40-42 78 1,56 47 50 6,5 23
Figure 18. Treated water vs. time grap for sample A Treated Water Flow Rate Vs. Time Treated Water Flow Rate(l/) 1,8 1,6 1,4 1,2 1 0,8 0,6 0,4 0,2 0 0-12 13-18 19-22 23-27 28-32 33-36 37-39 40-42 Time(min) Figure 19. Treated water flow rate vs. time grap for sample A 24
Figure 20. Treated water temperature vs. time grap for sample A Sample B: Te feed water was olleted from te BUET (Banglades University of Engineering & Tenology) supply water wi is extrated wit deep tube wells loated at BUET. Heater temperature = 85 C; Cill water flow rate = 1.5 l/min Cill water started 15 min after te starting time. Room/ill water temp. = 29 C Te feed water TDS = 280 mg/l ph of te raw water = 7,5 Table 4. Experimental results for sample B Time (min) Treated water volume (ml) Treated water flow rate(l/) Treated water temp. ( C) TDS (mg/l) ph 0-11 55 0,3 33 0 6 12-16 43 0,516 39 0 6 17-20 92 1,38 42 0 6 21-24 105 1,575 43 0 6 25-27 82 1,64 44 0 6,5 28-30 60 1,2 44,5 0 6,5 31-33 75 1,5 44,5 0 6,5 34-36 89 1,78 46 0 6,5 37-40 120 1,8 46 0 6,5 41-43 85 1,7 46 0 6,5 44-46 87 1,74 46,5 0 6,5 47-49 90 1,8 46,5 0 6,5 50-51 61 1,83 47 0 6,5 25
Treated Water Vs. Time Treated Water(ml) 140 120 100 80 60 40 20 0 0-11 12-16 17-20 21-24 25-27 28-30 31-33 34-36 37-40 41-43 44-46 47-49 50-51 Time(min) Figure 21. Treated water vs. time diagrams for sample B Treated Water Flow Rate Vs. Time 2 Treated Water Flow Rate(l/) 1,5 1 0,5 0 0-11 12-16 17-20 21-24 25-27 28-30 31-33 34-36 37-40 41-43 44-46 47-49 50-51 Time(min) Figure 22. Treated water flow rate vs. time grap for sample B 26
Treated Water Temperature Vs. Time Treated Water Temperature(deg. C) 50 45 40 35 30 25 20 15 10 5 0 0-11 12-16 17-20 21-24 25-27 28-30 31-33 34-36 37-40 41-43 44-46 47-49 50-51 Time(min) Figure 23. Treated water temperature vs. time grap for sample B It is reommended tat to ave better results or grap (wi an well represent te atual situation) te following tings an be followed: (1) Te feed water sould be run separately for ea parameters like flow rate, volume, temp et. (2) Treated water sample sould be olleted more frequently like in every 5 min(if possible 3 min) until te ill water starts (until 15 min), and ten in every 3 min until te maine stops. (3) It is quite neessary to ave more tan one person wile running te purifier, olleting samples and readings if samples are to be taken frequently. 14. Advantages of te HVR water purifier in removal of arseni 1. No emial needs to be used. 2. No need to ontrol te ph of te arseni ontaminated water. 3. No bateriologial growt in te membrane to ontaminate te water. 4. No expertise is required to know weter te purifier is working properly. 5. All oter impurities are removed along wit arseni. 6. Water purifiation system is ompat and portable, so easy to transport. 6. Low pressure is required, so low energy demand, less vulnerability of te membrane and solar energy or waste eat an be used. 7. Easy to andle wastes. 8. Less maintenane ost. 9. Easy to ange te membranes. 15. Disadvantages of HVR purifier 1. Higer initial osts tan te emial teniques. 2. Eletriity is required. 3. Limited storage tank. 4. Restart is required after 40 to 60 minutes from ea start for furter treated water. 27
16. Conlusions In tis tesis work, mu more time sould ave been given to ondut tests on HVR te purifier in Banglades, wi ould not be done for different onstraints. In almost all te ases, test results were quite satisfatory. Wide range of ost/benefit analysis an be done and soial aeptability an be assessed in future. So mu money and time ave been spent to get rid of arseni problem, but unfortunately no real solution as been found. So te membrane distillation proess and its appliation like te HVR purifier deserve mu attention from various international agenies, NGO s et. so tat furter study and assessment an be done for its wider implementation. Larger ommunity level purifier an be built and plaed in te arseni affeted areas in ountries like Banglades were ground water arseni problem exists. As most of te power is used for eating te feed water, solar eat, natural gas (wi is abundant in Banglades) an be utilized. Low-grade waste eat an be used, if possible. As, tere is no emial requirement, no big sludge andling problem, no ealt risk for bateriologial growt in te purifiation system, te membrane distillation proess as a big advantage in reduing osts for effiient purifiation of drinking water. In su ases, power supply an be properly managed. It sould be mentioned ere tat te proess is not only removing arseni, it is removing all oter impurities from water. It an be a very attrative solution to te arseni problem. If due attention is given for its proper development and implementation, tis new tenology an beome a real lifesaver. 28
Symbols b= membrane tikness, m = molar onentration, mole/m 3 C P = eat apaity, J/mole K D = diffusion oeffiient for te water vapour- air mixture, m 2 /s E = energy flux, J/m 2 s k = termal ondutivity, W/m K k = termal ondutivity, W/m K air k PTFE = termal ondutivity; W/m K L = lengt of te diffusion pat in te air gap, m N = molar flux, mole/m 2 Q = mass flux, kg/m 2 T = absolute temperature, K T = absolute temperature, K T = absolute temperature, K x = mole fration of water vapour x = mole fration of water at te ondensate surfae x = mole fration of water z = diffusion lengt, m φ = effetive net pore area Θ = temperature polarization o-effiient 29
Referenes [1] Kevin W. Lawson & Douglas R. Lloyd, Review membrane distillation, J. Membr. Si., 124(1997) 1-25. [2] A.-S. Jönsson, R. Wimmerstedt and A.-C. Harrysson, Membrane distillation- a teoretial study of evaporation troug miroporous membranes, Desalination, 56(1985) 237-249. [3] M. Feroze Amed, M. Asraf Ali and Zafar Adeel, Tenologies for Aresni Removal from Drinking Water (2001). [4] Caroliene M. Guijt, Imre G. Raz, Jan Williem van Heuven, Tom Reit, Andre B. de. Haan, Modelling of a transmembrane evaporation module for desalination of seawater, Desalination, 126(1999) 119-125. [5] Prosun Battaarya, Alan H. Wel, Kazi Matin Amed, Gunnar Jaks, Ravi Naidu, Arseni in groundwater of sedimentary aquifers, Applied Geoemistry 19 (2004) 163-167. [6] Y. Zeng, M. Stute, A. van Geen, I. Gavrieli, R. Dar, H.J. Simpson, P. Sosser, K. M. Amed, Redox ontrol of arseni mobilization in Banglades groundwater, Applied Geoemistry 19 (2004) 201-214. [7] Tzai Y. Cat, V. Dean Adams, Amy E. Cildress, Experimental study of desalination: a new approa to flux enanement, J. Membr. Si. 228 (2004) 5-16. 30