Biofiltration of hydrogen sulphide using schist as packing material: biofilters performances and tortuosity assessment of the packed bed

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(1)BIOFILTRATION OF HYDROGEN SULPHIDE USING SCHIST AS PACKING MATERIAL: BIOFILTERS PERFORMANCES AND TORTUOSITY ASSESSMENT OF THE PACKED BED. LAURA MARÍA AYALA GUZMÁN UNIVERSIDAD DE LOS ANDES DEPARTAMENTO DE INGENIERÍA CIVIL Y AMBIENTAL. JULIO DE 2010.

(2) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. BIOFILTRATION OF HYDROGEN SULPHIDE USING SCHIST AS PACKING MATERIAL: BIOFILTERS PERFORMANCES AND TORTUOSITY ASSESSMENT OF THE PACKED BED. Laura María Ayala Guzmán Tesis para optar por el título de Magister en Ingeniería Civil Director Ph.D Manuel Rodríguez Susa. Universidad de los Andes Departamento de Ingeniería Civil y Ambiental. Julio de 2010. Universidad de los Andes – 2010. 2.

(3) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. Al ser pocos, son indispensables… Gracias familia. A mi mama, la compañera constante de todas mis luchas. A mi papa, por su apoyo y espera. A Nana y Dani, mis mejores amigas. A Oscar, paciente confidente y ante todo, amigo.. Universidad de los Andes – 2010. 3.

(4) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. AGRADECIMIENTOS. Expreso mis agradecimientos al Ingeniero Manuel Rodríguez Susa, por su constante apoyo y confianza. Así mismo, al equipo de l’Ecole des Mines de Nantes, quienes guiaron con sus conocimientos todo el proceso experimental de la tesis. A todas las personas que intervinieron con su oportuna colaboración, muchas gracias.. Universidad de los Andes – 2010. 4.

(5) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. Table of contents Nomenclature ...................................................................................................................................7 1. INTRODUCTION ........................................................................................................................9 1.1 Objectives.......................................................................................................................... 10 2. PROBLEMATIC OF HYDROGEN SULPHIDE .................................................................... 11 3. BIOFILTRATION PROCESS.................................................................................................. 14 3.1 Definition .......................................................................................................................... 14 3.2 Packing materials........................................................................................................... 14 3.2.1 Natural packing materials....................................................................................................15 3.2.2 Synthetic materials..................................................................................................................16 3.3 Factors affecting the biofiltration process............................................................. 17 3.3.1 pH....................................................................................................................................................17 3.3.2 Temperature ..............................................................................................................................18 3.3.3 Humidity ......................................................................................................................................19 3.4 Operational parameters in biofiltration ................................................................ 20 3.5 Biofitration modelling .................................................................................................. 21 3.5.1 Michaelis‐Menten model ......................................................................................................22 3.5.2 Ottengraf model........................................................................................................................22 3.5.3 Andrews model.........................................................................................................................24 3.6 Biomass growth .............................................................................................................. 25 3.7 Mass transfer in biofilters ........................................................................................... 26 3.7.1 Pollutants adsorption in the bed.......................................................................................27 3.7.2 Gas‐liquid mass transfer .......................................................................................................28 3.7.3 Liquid‐solid mass transfer ...................................................................................................28 3.8 Bed tortuosity and pressure drop ............................................................................ 30 3.8.1 Bed tortuosity in fixed beds packed with identical particles................................30 3.8.2 Tortuosity model for porous beds....................................................................................31 3.8.3 Tortuosity in binary beds packed of low density.......................................................31 3.8.4 Tortuosity calculation based on the pressure drop..................................................32 3.8.5 Pressure drop ............................................................................................................................33 4. HYDROGEN SULPHIDE BIOFILTRATION WITH SCHIST ............................................ 36 4.1 Background ...................................................................................................................... 36 4.2 Biofitration process....................................................................................................... 39 4.2.1 Packing materials.....................................................................................................................39 4.3 Experimental setup ....................................................................................................... 41 4.4 Results and discussion ................................................................................................. 42 4.4.1 Biofilters performance...........................................................................................................42 4.4.2 Pressure drop ............................................................................................................................45 4.4.3 Porosity, tortuosity and superficial area of the bed .................................................46 4.4.4 Tortuosity and superficial area determination...........................................................47. Universidad de los Andes – 2010. 5.

(6) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ 4.5 Conclusions ......................................................................................................................... 48 5. REFERENCES .............................................................................................................................. 49. Tables Table 2.1 Main sources of H2S ....................................................................................................................11 Table 2.2. Effects of hydrogen sulphide exposure on human health.........................................12 Table 3.1. pH values in H2S biofiltration with different support materials............................18 Table 3.2. Temperature values in H2S biofiltration with different support materials ......19 Table 3.3. Humidity values in H2S biofiltration with different support materials ..............19 Table 3.4. Values of operational parameters in biofiltration........................................................21 Table 3.5. Values of dynamic tortuosity and surface area obtained from the Comiti and Renaud model..........................................................................................................................................33 Table 3.6. Values of pressure drop in H2S biofiltration with different packing materials34 Table 4.1. Physical properties of the packing materials .................................................................41 Table 4.2. Bioflters configuration .............................................................................................................41 Table 4.3. Porosity values for biofilters .................................................................................................46 Table 4.4. Tortuosity and superficial values for biofilters.............................................................47. Figures Figure 4.1. UP20 material.............................................................................................................................36 Figure 4.2. Materials tested in the first study .....................................................................................37 Figure 4.3. Experimental setup of the third study ............................................................................38 Figure 4.4. Packing materials, left: schist, right: UP20 ....................................................................41 Figure 4.5. Biofilters configuration ..........................................................................................................42 Figure 4.6. Removal efficiency for biofilters ........................................................................................44 Figure 4.7. Elimination capacity versus loading rate for biofilters............................................45 Figure 4.8. Superficial velocity versus pressure drops for biofilers..........................................46. Universidad de los Andes – 2010. 6.

(7) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ Nomenclature A: Biofilter area (m2) avd: Superficial dynamic area of material (m2•m‐3) avs: Superficial area of material (m2•m‐3) C: Pollutant concentration (g•m‐3) CGi: Pollutant concentration in the gas phase (mg•l‐1) CGl: Pollutant concentration in the liquid phase (mg•l‐1) Cin: Inlet concentration (g•m‐3) Cln: Mean logarithmic concentration (mgH2S•m‐3) Cout: Outlet concentration (g•m‐3) D: Fick’s diffusion coefficient (m2•s‐1) dc: Column diameter (m) Dif, max, Dif, min, Dif, mean: Minimum, maximum and average diameter of the pore channel between i and f (m) dp: Particle diameter (m) dpore: Tortuous canal diameter or pore diameter (m) H: Henry’s constant (atm•l•mol‐1) k: Total mass transfer coefficient (m•s‐1) kG: Gas‐liquid mass transfer coefficient (h‐1) Ki: Monod’s inhibition constant (mgH2S•m‐3) KiP: Monod’s inhibition constant (mgH2S•m‐3) kL: Liquid‐solid mass transfer coefficient (m•s‐1) Ks: Monod's apparent half‐saturation constant (mgH2S•m‐3) KSC, KSO: Pollutant and oxigen Monod’s constants, respectively (mg•m‐3) L: Column length (m) l: Tortuous channels length (m) le: Pore length (m) LR: Pollutant’s loading rate (g•m‐3•h‐1) M*: Experimental coefficient (Pa•s2•m‐3) N*: Experimental coefficient (Pa•s•m‐2) ∆P: Pressure drop (Pa) Q: Inlet flow (m3•s‐1) R: Universal gas constant (atm•l•K‐1•mol‐1) R0: Biomass growth rate (g•m3 •s.1) RC: Apparent removal rate or removal capacity (g•m‐3•h‐1) RE: Removal efficiency (%) Δrif: Tortuous distance between the pores i and f (m) RS: Biological reaction rate (g•m‐3•s‐1) S: Substrate concentration (g•m‐3) Sb, Sg: Substrate concentration in the biofilm and in the gas phase, respectively (g•m‐3) SC, SO: Pollutant and oxigen concentrations, respectively (g•m‐3) Sp: External particle surface (m2) SL: Substrate concentration in the gas‐biofilm interface (g•m‐3) t: Time (s) T: Temperature, K U0: Gas velocity (m•s‐1) Universidad de los Andes – 2010. 7.

(8) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ Upore: Flow velocity in tortuous channel or in pore (m•s‐1) V: Biofilter volume (m3) Vm: Maximum apparent removal rate (mgH2S•kgmaterial‐1•s‐1) Vp: Particle volume (m3) x: Biofilm thickness (m) Δzif: Distance between the pores i and f (m) Greek Letters δ: Biofilm effective thickness (m) ε: Bed porosity (adimensional) µ: Gas viscosity (kg•m‐1•s‐1) µ, µmax: Maximum velocity in Monod’s equation (m‐1) ρg: Gas density (kg•m‐3) τ: Bed tortuosity (adimensional) τif: Local bed tortuosity between the sites i and f (adimensional) ϕ: Particle sphericity (adimensional) φ1: Thiele number (adimensional) Dimensionless numbers K’H: Henry's proportionality constant Lif: Local length factor between pores i and f mi: Henry's distribution coefficient Rep: Particle Reynolds number Repore: Pore Reynolds number Sc: Schmidt number Sif: Local form factor of the channel between the pores i and f Sh: Sherwood number Shp: Particle Sherwood number Shpore: Pore Sherwood number X: Particle fraction exposed to the gas flow XeW: Energy criterion of biofilter walls. Universidad de los Andes – 2010. 8.

(9) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. 1. INTRODUCTION. The Department of Energy Systems and Environment of the Ecole des Mines de Nantes has been developing biofiltration projects of gaseous pollutants, including hydrogen sulphide and ammonia. In the context of these investigations, different support materials have been investigated in order to determine the major factors affecting the biofiltration process. In this sense a material called UP20 has been fabricated and tested since 2008. The innovative characteristics of this material are its buffering effect and its nutrients content, making unnecessary the addition of extra buffer and nutrients solutions. It also offers good removal at high pollutant concentrations and adequate conditions for biomass growth. However, it was found that is better to combine UP20 with other support materials, such as fibrous peat, pine bark and pozzolan. This combination enables the presence of required nutrients inside the bed and avoids high pressure drops. In the present work it was tested the performance of schist, a volvanic rock, and the possible improving on mixing it with UP20. The experimental setup was the same used in previous works and the analysis included the biofilter removal capacity and physical parameters such as porosity, pressure drop, tortuosity and superficial area. The second chapter of this document describes the problematic asociated to the hydrogen sulphide, that is focused on public health issues. The third chapter gives an approximation to the biofiltration process in terms of packing materials, operational parameters and models of biomass growth and mass transfer. Finally, the fourth chapter compiles the hydrogen sulphide biofiltration with schist and includes the description of previous works, the results, discussion and general conclusions.. Universidad de los Andes – 2010. 9.

(10) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ 1.1 Objectives The main objective of this work is the evaluation of schist as a carrier material for hydrogen sulphide biofiltration. The other objectives are: •. The evaluation of hydrogen sulphide biofiltration in terms of efficiency and mechanic parameters, such as porosity and tortuosity.. •. The study of material UP20 in the improving of hydrogen sulphide biofiltration in terms of nutrient requirements of the microorganisms.. Universidad de los Andes – 2010. 10.

(11) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. 2. PROBLEMATIC OF HYDROGEN SULPHIDE Hydrogen sulphide is a toxic, colourless, flammable and offensive gas produced in several industries and activities (Oyarzún et al., 2003). It is highly corrosive and produces damage to several devices (Bouzaza et al., 2004). Nowadays one of the major problems of H2S is its presence in the biogas that causes corrosion in power engines (Redondo et al., 2008). Table 2.1 shows some of the most common sources of H2S, which are oil refining, landfills operation, livestock facilities, food processing, paper manufacturing and sewage systems. Table 2.1 Main sources of Hydrogen Sulphide Activity o industry Oil refining and hydrocarbons production. References. Sewage systems and wastewater treatment plants. Vaiopoulou et al., 2005; Robinson, 2006; Lins & Guimarães, 2007; Zhu et al., 2010 Kim et al., 2005; Truong & Abatzoglou, 2005; Eun et al., 2007; Song et al., 2007; Tyndall & Colleti, 2007; Blunden et al., 2008; Qu et al., 2008 Delgado et al., 1999; Hvitved‐Jacobsen, 2002; Lahav et al., 2006. Textile industry. Vanhoorne et al., 1995; Kraakman, 2001. Compost production. Cohen, 2001; Canovai et al., 2004; Li et al., 2008. Landfills operation Lifestock facilities. Food processing Paper and mill manufacturing Leather manufacturing. Oyarzún et al., 2003; Cudmore & Gostomski, 2005; Kim et al., 2008; Landaud et al., 2008; Catalan et al., 2009 Bordado & Gomes, 1997; Iliuta & Larachi, 2003; Oyarzún et al., 2003 van Groenestijn et al., 2002; Nazer et al., 2006; Dutta et al., 2010. The health effects of H2S are shown in Table 2.2. The odor threshold of the gas on humans ranges between 0.01 and 0.3 ppm. Slight effects on human health occur at concentrations within the range of 20‐50 ppm of H2S, while that from 100 ppm. Universidad de los Andes – 2010. 11.

(12) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ irreversible after‐effects are presented such as acute conjunctivitis and pulmonary edema (Agency for Toxic Substances and Disease Registry, 2008). Table 2.2. Effects of hydrogen sulphide exposure on human health (Agency for Toxic Substances and Disease Registry, 2008). H2S concentration (ppm) 0,01 – 0,30 20‐50. Health effects Odor treshold on humans Offensive odors, nausea, irritated eyes and headaches at prolonged exposure. 100‐200. Irritation of nose, throat and lungs. Fatigue of the smell sense and loss of appetite. There may be episodes of acute conjunctivitis. 250‐500. Severe irritation of nose, throat and lungs. Loss of the ability to smell the gas. 500. Pulmonary edema (Caused by the lungs fluids when they interact with the gas). 500‐1000. Severe irritation of the lungs, headache, dizziness, sudden collapse, unconsciousness and death within hours. >1000. Immediate unconsciousness, lost the ability to breathe and death. Due to the various sources of hydrogen sulphide emission and its high toxicity, it is crucial to investigate effective methods for its removal, especially when these sources are located near human settlements such as sewages and industries. The removal techniques can be biological and non‐biological, depending on the pollutant concentration and the loading rate. Biological techniques are recommended for streams with low pollutants concentrations and high volumetric loading rates (Kennes & Veiga, 2001; Schwarz et al., 2001). Among the non‐biological techniques there are gravity chambers, cyclones, filters, scrubbers, absorption and adsorption columns, condensers and membranes. The biological techniques include biofilters, bioscrubbers, biotrickling towers and. Universidad de los Andes – 2010. 12.

(13) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ membrane bioreactors, among others (Kennes & Veiga, Bioreactors for Waste Gas Treatment, 2001).. Universidad de los Andes – 2010. 13.

(14) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. 3. BIOFILTRATION PROCESS. 3.1 Definition In biofiltration the pollutant stream is passed across the filter bed where the microorganims are immobilized (Andrès et al., 2006). In the case of gaseous streams, the pollutant is transfered to the aqueous phase where it can be absorbed by the microorganisms (Duan et al., 2007). Biofiltration offers advantages in instalation and operation costs, for this reason it is a widely used technique in the treatment of liquid and gaseous pollutants (van Groenestijn & Hesselink, 1993; Ramírez, 2001; Cudmore & Gostomski, 2005). Previously biofiltration studies were focussed in the removal of wastewater treatment odours (Revah et al., 1996). Nowadays its application includes the treatment of highly degradable compounds such as volatile substances and ethanol, as recalcitrant compounds such as toluene and halogenated substances (Elías et al., 2002). It is a technology that requires low energy consumption, it is easy to operate, has long‐ term life (Soroushian et al., 2006) and occupies little space (Cho et al, 2000). In the case of H2S is an excellent option due to its high degradability (Devinny et al., 1999). The efluents of the process are water, carbon dioxide and sulfates, which have easy disposal (Ma et al., 2006).. 3.2 Packing materials At present day there is a great variety of natural and synthetic packing materials. These materials are the biofilm support since the extracellular polymers of the microorganisms need a solid surface for their formation (Rittmann & McCarty, 2001). The packing material must provide high water retention and buffer capacity, besides low density and high values of surface area and porosity. These factors promote the biofilm formation, the nutrients transport and the pass of pollutant stream across the bed (Gaudin et al., 2008; Park et al., 2009). Another authors emphasize that a good packing material must have low cost and be chemically stable (Elías et al., 2002).. Universidad de los Andes – 2010. 14.

(15) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ The bed materials can be inoculated in order to favour the microorganisms growth in the medium. In the case of high concentrations of gaseous pollutants, it is necessary to inoculate the medium with specific microorganisms or waste sludges from the same industry that produces the pollutant (Chung et al., 1996; Shojaosadati & Elyasi, 1999; Oyarzún et al., 2003). However, some studies have obtained good removal results in biofilters inoculated with wastewaters and activated slugdes (Chung et al., 2001; Xi, Hu, & Qian, 2006; Chung, 2007; Filho et al., 2010).. 3.2.1 Natural packing materials The most common natural materials used in biofiltration are compost, soil and peat (Gaudin, et al., 2008). These materials include the nutrients required by the microorganisms but they present long‐term problems due to clogging and high pressure drops (Cho et al., 2000; Galera et al., 2008). Soil was the first material used in biofiltration. It is composed by organic matter, minerals and microorganisms. Sands and clays are widely used in the process, which have an organic matter content of 95% and a density between 0.5 and 2.7 g•m‐3 (Kennes & Veiga, 2001). The use of clays such as bentonite and zeolite is a good option in the treatment of variable H2S loads (Park & Han, 2007). However some of the problems asociated with these materials are their low values of water retention and permeability (Kennes & Veiga, 2001). In the case of compost it has been found good removal in biofiltration of volatile substances, thanks to the high microbial growth that promotes and its content of nitrogen, phosphorus and other nutrients (Sercu et al., 2006; Znad et al., 2007). Other study showed a good performance of this material with high loading rates and long starvation periods (Kim, Chung, & Oh, 2004). The phisical characteristics of compost vary according to the manufacturer, but usually its porosity ranges between 60 and 90%, its pH is neutral and its density is between 0.1 and 0.3 g•m‐3 (Kennes & Veiga, 2001). These are optimal characteristics for a packing material, nonetheless the excess of biomass growth causes high pressure drops.. Universidad de los Andes – 2010. 15.

(16) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ Recently other natural materials have been studied, such as sugarcane bagasse, coconut fibre and peat, which have reached a removal of 100% of high concentrations of H2S (Hartikainen et al., 2002; Filho et al., 2010). On the other hand cork biofilters have proved to be efficient in the treatment of mixed currents of H2S and other gases (Park et al., 2009). In the same way biofilters with volcanic rocks have good hydrogen sulphide removal efficiencies and prevent the increase of pressure drop (Dumont et al., 2008; Jeong et al., 2008; Dumont & Andrès, 2010).. 3.2.2 Synthetic materials The advantages of synthetic materials are their long‐term mechanical stability because they do not compact and their buffer capacity that prevents drastic changes in pH inside the biofilter (Webster et al., 2000). However these materials require periodic nutrient addition which complicates their operation and increases costs (Maestre et al., 2007). The most used synthetic materials are pieces made of plastic and ceramic (Jin‐Ying et al., 2005). The investigation of the efficiency of other materials, such as molecular sieve, porous carbon and ferric oxide desulfurizer has been quite successful in terms of material regeneration, nonetheless the interaction with undesirable substances (such as Al+3 in the case of molecular sieve) could drastically alter the biofiltration process (Li et al., 2008). Activated carbon has been widely used in biofiltration due to its resistance at fluctuating pollutant concentrations and its regeneration capacity (Duan et al., 2006; Jiang et al., 2001). This material adsorbs a large amount of pollutant at the beginning of biofiltration and releases it gradually so it can be degrated by microorganisms. In the same way it acts as a catalyst for the oxidation of H2S into sulfurs and sulfates (Smet et al., 1998). These facts make activated carbon an excelent packing material for biofiltration in comparision with other synthetic materials (Ng, et al., 2004). The desadvantages of this material are the operational costs and its disposal after used (Tian et al., 2007). Recently other synthetic materials have been used such as polypropylene fibers (Jeong et al., 2008) and polyuretane foams (González‐Sánchez et al., 2008; Ryu et al., 2009), which have obtained 100% of H2S removal. The biofiltration of other gaseous pollutants. Universidad de los Andes – 2010. 16.

(17) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ has been investigated with plastic and metallic rings of several forms and perlites (Wu et al., 2001; Prado et al., 2002; Kim & Deshusses, 2008). The influence of the packing material in the biofilter efficiency has led to the manufacture of new support materials, as in the study by Dumont and Andrès (2008) in which the synthetic material UP20 has been proposed. This material provides for itself the nutrients required by microorganisms. The biofilm formation in this material helps to attain a good removal efficiency, being chemically stable and avoiding large pressure drops.. 3.3 Factors affecting the biofiltration process In the biofiltration the pollutant gas is transferred to the liquid phase of the system in order to be adsorbed by the filter bed and be available for the microorganisms (Duan et al., 2007). In the process the hydrogen sulphide is biologically degraded to sulfate (van Groenestijn & Kraakman, 2005). The hydrogen sulphide provides the energy required for the microbial growth, which passes from equivalence ‐2 to 0:. H 2 S + OH − → HS − + H 2O 1 HS − + O2 → S 0 + OH − 2. (1). The major influence parameters of this reaction are pH, humidity and temperature of the filter media (Kennes et al., 2009). € 3.3.1 pH The optimum pH value of biofiltration is close to neutral because in this range there is no inhibition of sulphide‐oxidizing bacterias. The pH value could fall drastically during the operation due to sulfuric acid formation (Kennes et al., 2009), so the packing material must act as a buffer agent in order to support these pH drops. Besides of affecting the microbial activity, the pH falls are asociated to a decrease in the water solubility of H2S and mass transfer in the filter bed (Xiaobing et al., 2003). In the H2S biofiltration it is recommended to keep pH values between 3 and 8 (McNevin & Barford, 2000), however a biofitration study with peat shows how it was neccesary to. Universidad de los Andes – 2010. 17.

(18) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ increase the pH value until 6.0 in order to maintain the removal eficiency (Oyarzún et al., 2003). Regarding organic and inorganic materials (ceramic pieces, peat, wood chips and activated carbon), Ma et al. (2006) showed that there was no outstanding differences in the pH values, which ranged between 6.8 an 7.4. Other pH values and the removal efficiencies of H2S obtained with different materials are shown in Table 3.1, where it can be observed that the biofiltration is possible in both acidic and alkaline conditions. Table 3.1. pH values in H2S biofiltration with different support materials pH (Units). Removal efficiency (%). Compost. 3.8 a 7.3. >99. Kim et al., 2004. Activated carbon. 3.8 a 7.0. >92. Ng et al., 2004. Compost. 3.5. 100. Compost. 5.0 a 8.0. >93. Morgan‐Sagastume & Noyola, 2006 Sercu et al, 2006. 10.0. >98. González‐Sánchez et al., 2008. Material. Polyurethane foam. References. In order to prevent high pH drops it is recommended the use of buffer solutions (Lu et al., 2002). Ethanol has a good performance as buffer solution in the biofiltration of volatile sulphur compounds (Zhang et al., 2007). 3.3.2 Temperature Most of sulphide‐oxidizing bacterias are mesophilic (Rittmann & McCarty, 2001) that is why for H2S biofiltration the temperature must be between 20 and 50ºC (McNevin & Barford, 2000). Table 3.2 shows some of the temperature values reported in the literature where it can be observed that it is possible to obtain good H2S removal in this temperature range. However some of the industrial processes reach temperature values of 70ºC, such as paper mill fabrication, where the removal of H2S falls drastically (Datta et al., 2007). Regarding this aspect it is important to maintain the pollutant gas temperature between 20 y 22ºC (Dumont & Andrès, 2008).. Universidad de los Andes – 2010. 18.

(19) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ Table 3.2. Temperature values in H2S biofiltration with different support materials Temperature (ºC). Removal efficiency (%). Stainless steel. 70. 35. Datta et al., 2007. Active carbon Polyurethane foams. 27 a 32 60. 100 95. Rattanapan et al., 2009 Ryu et al., 2009. Material. References. 3.3.3 Humidity The proper humidity of the bed provides the ideal medium for microbial activity (Lu, Lin, & Chu, 2002). It is also neccesary to assure the separation of the sulfur ions contained in H2S to form HS‐ (Bandosz, 1999). Low humidity leads to bacterial inhibition and low removal efficiencies, while high humidity decreases the porosity of medium, forms channels and anaerobic zones in the bed and increases the pressure drops (Morales et al., 2003). For these reasons in biofiltration it is recomended to maintain the humidity values between 20 and 60% (McNevin & Barford, 2000). However, as can be seen in Table 3.3 it is possible to reach excelent removal rates with humidity values close to 90%. Another fact that affects the bed humidity is the medium drying. It could be caused by the increase of microbial activity (Morales et al., 1998), the high temperatures that cause evaporation (Morales et al., 2003) and low humidity of pollutant gases. Xiaobing et al. (2003) recommend a value of gas humidity higher than 95% in order to prevent this problem. Packing materials should have a high water retention value, which have been recommended to be between 40 and 70% (Dehghanzadeh et al., 2005). Higher values are asociated with clogging problems (Ramírez‐López et al., 2003). Table 3.3. Humidity values in H2S biofiltration with different support materials Material Compost. Humidity (%) 65‐90. Universidad de los Andes – 2010. Removal efficiency (%) >99. References Shojaosadati 1999. &. Elyasi,. 19.

(20) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. Material Activated carbon Compost. Humidity (%) 40. Removal efficiency (%) >94. Duan et al., 2006. 37 a 48. >93%. Sercu et al, 2006. References. 3.4 Operational parameters in biofiltration The operational parameters in biofiltration are defined in the design stage and help to evaluate the process performance: •. The loading rate is defined as the quantity of pollutant gas that enters the biofilter (Equation 2).. •. The superficial gas velocity is the relation between the pollutant gas flow and the superficial area of the support material (Equation 3).. •. The removal efficiency is the parameter that assesses the biofiltration yield and it is the percentage of pollutant gas eliminated (Equation 4).. •. The removal capacity is the mass of pollutant that are removed during a time interval (Equation 5).. •. Finally, the empty bed residence time is the time that a pollutant lasts inside the filter bed (Equation 6). LR =. Q Cin V. U0 = €. RE = 100 * € RC =. €. Q A. (3). Cin − Cout Cin. Q (Cin − Cout ) V. EBRT =. €. (2). V Q. (4) (5). (6). Generally, the superficial gas velocity is between 50 y 150 m•h.1 (Andrès et al., 2006).. €. The treatment of gases with concentrations higher than 5 g•m‐3 requires a high EBRT. Universidad de los Andes – 2010. 20.

(21) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ value. This fact increases the investment due to the need of biofilters with bigger length and expensive support materials with good adsorption capacity (Kennes & Veiga, 2001). Also as EBRT value increases the removal efficiency declines (Devinny & Hodge, 1995; Park & Jung, 2006). Recommended EBRT values are between 20 and 40 s (Wani et al., 1998; Cox & Deshusses, 2002; Ramírez et. al., 2009) but good H2S removal efficiencies have been found out of this range (Table 6). Table 3.4. Values of operational parameters in biofiltration Máximum LR (g•m3•h1 ). RE (%). U0 (m•h1). EBRT (s). 6.25. 100. 20‐163. 32. Chung et al., 2005. 6.5. 99. 16‐100. <10. Lee et al., 2005. 27.4. 99. 4. 0‐120. 99. 31‐81. 0‐25.5. 50‐100. 65. 57. References. Jiang et al., 2008 Ramírez et al., 2009 Dumont 2010. &. Andrès,. 3.5 Biofitration modelling Models are usefull to predict the microbial behavior in pilot and large‐scale biofilters (Kennes & Veiga, 2001). Cinetic models of Michaelis‐Menten and Ottengraf have been used in order to determine the contaminants removal and the transport across the biofilm (Oyarzún et al., 2003; Pré & Le Cloirec, 2007). However these models should not be used in the study of mixtures of pollutants and changeable loads; in these cases the Andrews model should be considered (Veiga & Kennes, 2001).. Universidad de los Andes – 2010. 21.

(22) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ 3.5.1 Michaelis‐Menten model Some authors have modified the Michaelis‐Menten model in order to know the saturation level and the maximal removal rate of the biofiltration system (Chung et al., 2001):. 1 K 1 1 = S + RC Vm Cin Vm. (7). By doing linear regresion of the variables 1/Cln and 1/RC the values of KS and Vm can be. €. obtained, being the intercept and the slope of the line. The KS parameter can be related to the microorganisms afinity to the H2S (Chung et al., 2001). To determine the concentration at which the system will fail by inhibition of microorganisms, the parameter Ki can be used:. 1 Cln 1 = + RC Vm * K i Vm. €. Cln =. Cin − Cout C  ln in   Cout . (8). (9). € 3.5.2 Ottengraf model This model is based on the balance of reaction equations inside the biofilter and in the diffusive transport of nutrients. It assumes that the Monod model describes the pollutant biodegradation, that the resistance to the mass transport can be negligible (Pré & Le Cloirec, 2007) and that the biofilm can be studied as a liquid phase (Jani & Madvar, 2010). Based on the mass balance equation, three cinetic scenarios are obtained depending on whether the degradation is limited by the substrate concentration, substrate diffusion or biodegradation rate. This equation must consider both the biofilm and the gas phase. Can be zero order (when is limited by substrate concentration and diffusion) or first orden (when is limited by biodegradation rate):. Universidad de los Andes – 2010. 22.

(23) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________.  Dd 2 S  − R0 Zero order equation: 0 =  2   dx . (10).  Dd 2 S  S − R0 L 2  KS  dx . First order equation: 0 = . €. (11). To solve these equations it is assumed that the substrate concentrations in the gas and. €. liquid phase (biofilm) may be calculated with Henry's law and that the substrate concentration along the biofilm is constant. Sg H. (12). dSb =0 dx. (13). Sb =. €. Based on the assumption that, in initial conditions, the substrate concentrations along € the column is constant (Sg,0=Sg,L), the mass balance is:  dS   dS  0 = −U 0  g  + avsD b   dx  x= 0  dL . (14). According to Henry’s law and the mass balance equation, the three scenarios are € described by: •. Zero order kinetics: Biodegradation is limited by substrate diffusion 0.5 2  Sg,L   avsL  R0 D   = 1−    Sg,0   U 0  2HSg,0    . •. (15). Zero order kinetics: Biodegradation is limited by reaction rate.  a δLR  Sg,L 0 = 1−  vs  Sg,0 U S  0 g,0 . €.  2dSg  0.5 δ =   R0 H . €. (16). (17). € Universidad de los Andes – 2010. 23.

(24) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ •. First order kinetics: Biodegradation is limited by substrate concentration.  −( LavsDφ1 tan φ1)  Sg,L = exp  Sg,0 δHU 0  . (18). δ is the effective biofilm thickness that is exposed to the substrate passage. φ1 is the. €. Thiele number which measures if biodegradation is limited by reaction rate or substrate diffusion. If φ1 is lower than. 2 , biodegradation is governed by limitation of reaction. rate and viceversa.. !.  R0  0.5 φ1= δ  (19)  KS D. 3.5.3 Andrews model € This model describes the behavior of the biofilter when the current is composed by a mixture of gases and the pollution load is not constant. In the case of pollutants mixtures includes the Michaelis‐Menten inhibition parameter caused by simultaneous biodegradation.. RS = R0. S S2 KS + S + Ki. (20). RS is the reaction rate (g•m‐3•s‐1). Some authors have found significant decreases in the € biofilters efficiency treating gas mixtures (Galera et al., 2008; Chan & Lai, 2010). However, in some cases H2S removal can occur simultaneously with the presence of ammonia and toluene, reaching RE values higher than 90% (Chung et al., 2001; Park et al., 2009). These gas mixtures are common in the treatment of biogas. Wani et al. (1999) also observed no inhibition in H2S biofiltration with presence of other sulfur compounds.. Universidad de los Andes – 2010. 24.

(25) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ Some biofiltration systems require certain oxygen concentration to be success, therefore Equation 20 is modified by the concentration and the degradation rate of oxygen.. RS = R0.  K SP . SP SO (21) S p2  + SP + (K SO + SO ) K iP . 3.6 Biomass€growth The biomass growth in the bed is a sensitive variable of the process whose behavior has been modeled with the Monod equations (Alonso et al, 2000; Devinny & Ramesh, 2005). This growth is not uniform along the biofilter due to differences in the substrate availability along the biofilter (Veiga y Kennes, 2001). Several authors have reported higher biomass growth at biofilter entrance because of the higher pollutant concentration at this point (Moe & Irvine, 2001; Chen, 2006; Kim & Sorial, 2007). Jani and Madvar (2010) modeled the relation between porosity changes and biomass growth, finding that at the biofilter entrance biofilms are thicker and there are greater bed settlement, while close to the exit biofilm thickness decreases. Monod model takes into account the pollutant concentration (or substrate) and microorganisms content in order to model the biomass growth in the biofilter. Assuming that biomass growth depends only on pollutant concentration, Monod equation must be modified (Veiga & Kennes, 2001):.  S  µ = µmax   (22)  KS + S  Another approach to the biomass growth is to measure the pressure drop which raises. €. with the increase of the biofilm thickness, as will be discussed later (Mendoza et al., 2004; Hand et al., 2008; Yang et al., 2008). An important factor to consider in biofiltration studies is the acclimation period of microorganisms to the biofilter medium. Although since the beginning of the biofiltration there is some pollutant removal, it does not reach its peak until the. Universidad de los Andes – 2010. 25.

(26) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ microorganisms are fully adapted to the medium. It has been reported in literature acclimation periods of five and six days (Duan et al., 2006), even in simultaneous biofiltration of H2S and other compounds (Cox & Deshusses, 2002). However other studies showed that depending on the support material it is no necessary take into account an acclimation period in order to obtain good removal efficiencies (Duan et al., 2005; Jiang et al., 2008; Ramírez et al., 2009). An excessive biomass growth causes clogging problems inside the biofilter which has been demostrated to affect the removal efficiency (Iliuta et al., 2005; Jani & Madvar, 2010). Besides it induces the formation of channels inside the bed that block the passage of air and drastically reduce the porosity (Schwarz et al., 2001). Several authors have proposed models to determine the clogging level inside the biofilter which is commonly called “bioclogging”. Thullner et al. (2002), Engesgaard et al. (2006) y Seifert & Engesgaard (2007) developed biomass growth models based on the porosity changes and hydraulic conductivity. Soleimani et al. (2009) determined a biofilter saturation rate taking into account the changes in pressure drops, permeability and conductivity.. 3.7 Mass transfer in biofilters Mass transfer in biofilters has two stages, namely the transfer between the gas phase (pollutants) and liquid phase (bed humidity), and between liquid and solid phases (support material). Flow inside the bioreactors is laminar because it has a low Reynolds number. In the case of biofilters packed with spheres the particle Reynolds number is between 1 and 10 (Comiti & Renaud, 1989; Seguin et al., 1996). It is assumed that this transport is diffusive because the amount of water between the liquid phases and the filter material is negligible (Ramírez, 2001; Devinny & Ramesh, 2005) and depends on the pressure drop and the pores size (Kärger & Vasenkov, 1992; Schwarz et al., 2001). The diffusive transport of the contaminant inside the bed is also limited by the resistance exerted by the biofilm and high rates of biodegradation (Devinny & Ramesh, 2005). In the same way it is important to consider the bed tortuosity and the material geometry (Lanfrey et al., 2010). In order to know the mass transfer rate it is necessary to determine the pollutant concentrations in both liquid and gaseous phases. Assuming an state of equilibrium. Universidad de los Andes – 2010. 26.

(27) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ inside the biofilter and that the pollutant is an ideal gas, the pollutant concentrations in both liquid and gaseous phase can be calculated by Henry’s law:. CGi = K H CLi (23). €. CGi H = = mi CLi RT. (24). Several models for the mass transfer through the filter bed have been proposed based. €. on experimental tests. The particle Reynolds number (Rep) and the Schmidt number are used in these models:. Re p =. Sc = €. ρ gU 0 d p µ. µ Dρ g. (25) (26). € 3.7.1 Pollutants adsorption in the bed The adsorption of pollutants in the filter bed depends on the macro‐structural characteristics of the bed, such as external porosity, particle density, superficial area and bed tortuosity. External porosity is proportional to the contact area between liquid and solid phases, thus affects directly the adsorption of contaminants. Particle density is a measure of the internal configuration of the material and affects the porosity. The specific area and tortuosity of the bed affect both compaction and contact area between phases (Mauguet et al., 2005). The particle superficial area can be dynamic (avd) or static (avs). The difference between these two parameters is that avd refers to the fraction of the particle that is in contact with the fluid, i.e. is an area where mass transfer occurs while avs is obtained from geometry particle (Mauguet et al., 2003). The dynamic superficial area and bed tortuosity change during the operation with the biofilm development. Several studies have obtained these values from models that take into account pressure drop, as. Universidad de los Andes – 2010. 27.

(28) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ discussed below. In the case of spheres avd can be assumed equal to avs because the entire surface area of the particle is in contact with the fluid (X=1):. avd = X ≤1 avs. (27). Some authors have also remarked the influence of other parameters such as EBRT,. €. operation time and changes in pollutant loads (Ma et al., 2006). Others mention that the density of filter material is directly proportional to the mass transfer of liquid to solid phase, since higher densities imply larger contact area between the phases (Kim & Deshusses, 2008). 3.7.2 Gas‐liquid mass transfer The mass transfer between these two phases has been hardly investigated for the case of biofilters since most models have been proposed to describe processes in all bioreactors where the liquis phase is predominant. Kim and Deshusses (2006) developed an equation to determine a gaseous mass transfer coefficient based on studies of CO2 transfer in compost and wood biofilters. This equation includes the effect of superficial gas velocity and the pollutant concentrations.. kG =. U 0  Cin  ln  V  Cout . (28). € 3.7.3 Liquid‐solid mass transfer. 3.7.3.1 RanzMarshall diffusion theory This theory has been used in studies of hydrogen sulphide biofiltration (Barona et al., 2005) and calculates a mass transfer coefficient (kf) based on the Sherwood number, the diffusion coefficient from Fick's law and the particle Reynolds number. The Sherwood number is the ratio between convection and diffusion, tending to 0 when the transport is mainly diffusive. This number is a measure of the mass transfer inside the bed.. Universidad de los Andes – 2010. 28.

(29) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. Sh =. dp k f = 2.0 + 0.6Re 0.53 Sc 0.33 p D. (29). 3.7.3.2 Mass flow in highly porous fixed beds This model is € based on Darcy’s flow theory and assumes that the mass transfer is primarily a function of porosity (Seguin et al., 1996). It describes the mass transfer in fluids with low Reynolds numbers and proposes two Sherwood numbers, one for the entire column and one for the particle:.  d  Sh = 1.615 Re p Sc c   L. €. 1. 3.   4ε Sh p = 1.615   avd (1− ε) le . (30) (31). 3.7.3.3 Influence€of biofilter walls in mass transfer The biofilter walls can cause kinetic energy losses which decreases the available energy for mass transfer between phases. Comiti et al. (2000) proposed a mass transfer model that takes into account this phenomenon and modifies the Sherwood number equations including the tortuosity parameter which will be explain next. The kinetic energy losses are represented by Xew and depend on the gas velocity, the superficial area and the porosity. This model was proved by Comiti et al. giving good results in fixed beds packed with different particles shapes..  π 1− ε 1 1  1   ε  1 1− ε Sh = 3.66 + 0.1011+ τΥXew8  * Xew0.229 Sc 3 (τ −1) * 1+ 2 Sc 3 exp−   τ  3 ε   4 ε (32). Xew =. €. U 0ρg. µ. *. 4ε avs (1− ε). (33). Y is the aspect ratio of the particles, defined as the ratio of the particle dimensions in € two directions. These directions are placed in the normal axis of the plane respect to the. Universidad de los Andes – 2010. 29.

(30) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ main flow direction. Y is equal to 1 in the case of spheres and square plates and equal to 5.5 in the case of cylindrical particles.. 3.8 Bed tortuosity and pressure drop The tortuosity on the filter medium refers to the flow sinuosity along the biofilter and mainly depends on the porosity of the bed and the Reynolds number (Cicerón et al., 2002; Zacarias et al., 2005). It is affected by the configuration and density of the filler material and the presence of channels within the bed, among other factors (Dias et al., 2006; Lanfrey et al., 2010). Usually it is calculated from the length of the tortuosity channels and the total length of the biofilter:. τ=. l L. (34). Due to the uncertainty of the parameter l, some models have been developed that € depend on the porosity of the bed (Dias et al., 2006; Lanfrey et al., 2010). Additionally the tortuosity can be measured following the trajectory of a particle through a tracer as in the studies of Plessis & Masliyah (1988) and Habisreuther et al. (2009).. 3.8.1 Bed tortuosity in fixed beds packed with identical particles This model developed by Lanfrey et al. (2010) proposes an equation to describe the tortuosity of different types of filler materials, which is directly proportional to the sphericity and void ratio:. (1− ε) τ = 1.23 εϕ 2. 4. 3. (35). The sphericity value varies from one material to another, taking the value of 1 for round € particles and less than 1 for other forms.. Universidad de los Andes – 2010. 30.

(31) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. (36πV ) ϕ= 2 p. 1. Sp. 3. (36). 3.8.2 Tortuosity model € for porous beds This model assumes that the total bed tortuosity is the average of the tortuosity of all channels between the pores i and f (Armatas, 2006). In this case, computer modeling was performed with N repetitions of the local tortuosity:. τ=. ∑τ i. if. N. τ if = Lif * Sif €. Lif =. € Sif = 1+ €. Δrif Δz if. (37) (38) (39). Dif ,max − Dif ,min Dif ,mean. (40). 3.8.3 Tortuosity€in binary beds packed of low density Binary beds are formed by materials of different sizes and shapes. They have been utilized in biofiltration because these characteristics are not uniform in the materials found in the market (Mauguet et al., 2003). In the case of beds with low density it can establish an inverse relationship between the porosity and tortuosity:. τ=. 1 εn. (41). Dias et al. (2006) worked on determining the value of the parameter n, finding that it € mainly depends on the difference of particles size. For materials in which this difference is smaller the value of n is 0.5, otherwise it is recommend a value of 0.4.. Universidad de los Andes – 2010. 31.

(32) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ 3.8.4 Tortuosity calculation based on the pressure drop The tortuosity value increases with the biofilm development because it makes the flow length greater. Thus, it can be concluded that the pressure drop is proportional to the tortuosity, since the increase of the biofilm causes the compactation of the filler material (Habisreuther et al., 2009). Comiti and Renaud (1989) developed a model to determine both tortuosity and dynamic specific area, which has been applied in several studies (Sabiri & Comiti, 1994; Seguin et al., 1998; Ramírez‐López, 2001; Cicéron & Comiti, 2002; Mauguet et al., 2003; Mauguet et al., 2005). This model was proposed for beds composed by identical spheres, where the pressure drop is given by:. ΔP = M *U 0 + N * L. (42).    d  2   d p 2 3 (1− ε) p  M = 0.04131− 1−   + 0.09681−  τ ρavd dc   ε3   dc   €   *. (43). 2.   (1− ε) 2 4 N = 2πτ a 1+  3  avd dc (1− ε)  ε *. €. 2. 2 vd. (44). Comiti and Renaud also assumed that the diameter and flow velocity along the tortuous € channels are dpore and Upore, respectively:. d pore =. 4ε avd (1− ε). U pore =. U 0τ ε. (45) (46). € Based on these parameters it is possible to calculate the Reynolds number and € Sherwood number of the pores:. Re pore =. € – 2010 Universidad de los Andes. Ud pore 4U 0τ = ν νavd (1− ε). (47). 32.

(33) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. Sh pore =. 4kε Davd (1− ε). (48). Values of dynamic tortuosity and surface area obtained from the Comiti and Renaud € model are shown in Table 3.5. These parameters are proportional to the pressure drop, so it is concluded that this value is higher in materials such as polystyrene, polyvinyl chloride and activated carbon. Table 3.5. Values of dynamic tortuosity and surface area obtained from the Comiti and Renaud model Packing material. τ. avd (m1). References. Polystyrene. 3.49. 2315. Polyvinyl chloride. 2.77. 1438. Stainless steel. 1.90. 3490. Glass. 1.44. 2055. Glass. 1.35‐1.53. 1217‐3077. Cicéron & Comiti, 2002. Activated carbon. 1.41‐1.63. 4100‐4871. Mauguet et al., 2005. Sabiri & Comiti, 1994. 3.8.5 Pressure drop The pressure drop along the column is a measure of the bed compaction. Large pressure drops are the most common cause of biofiltration failures(Cudmore & Gostomski, 2005). The increase of this parameter is due to the biofilm growth (Xi, Hu, & Qian, 2006) and the formation of channels inside the bed (Morgan‐Sagastume & Noyola, 2003). High pressure drop values result in low biofiltration yields since the compaction creates channels that do not allow the flow of water and gas in the bed (Ryu, Cho, & Chung, 2010). The pressure drop is equivalent to a higher energy consumption and an increase of operational costs (Morgan‐Sagastume et al., 2001; Cudmore & Gostomski, 2005; Govind & Narayan, 2005). Characteristics such as high density and low porosity of the packing material could increase the pressure drops (Kim & Deshusses, 2008), for this reason it is recommended choosing packing materials with homogeneous shapes and sizes in order to avoid this problem (Kennes & Veiga, 2001). Universidad de los Andes – 2010. 33.

(34) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ Table 3.6. Values of pressure drop in H2S biofiltration with different packing materials Pressure drop. Packing materials. (Pa•m1) <3500. References. Compost. Yang & Allen, 1994. Calcium alginate beads. Chung et al., 2001. 8 ‐ 156. Compost. Kim, Chung, & Oh, 2004. 65 ‐ 520. UP20 (synthetic material). Dumont & Andrès, 2008. 510 ‐ 550. Sugarcane. 226 ‐ 706. bagasse. and Filho et al., 2010. coconut fibre In the study by Yang and Allen (1994) it was noted that the beds with higher density have greater values of pressure drop. It has been shown that natural materials have a higher pressure drop than synthetic in biofilters operating under the same conditions (Morgan‐Sagastume et al., 2001). Several authors have proposed physical, chemical and biological methods to prevent the increase in pressure drop, which are focused on removing biomass and avoiding the medium compaction. The physical methods include washing with water and air, periodic mixture, change of air flow direction and interruption of the feed (Iliuta & Larachí, 2004; Znad, Katoh, & Kawase, 2007). Among these, water washing and periodic agitation do not affect the removal efficiency of biofilters, giving at the same time good results in reducing the pressure drop (Delhoménie et al., 2003). Ryu et al. (2010) observed a decrease from 873 to 29 Pa•m‐1 in the pressure drop by injecting water and air flows in opposite directions in order to remove excess biofilm in the bed. Ryu et al., 2008, Jani and Madvad (2010) and Hassan and Sorial (2009) also established the efficiency of this technique. On the other hand, Mendoza et al. (2004) compared various technologies such as water washing, air injection and manual mixing of the medium, the latter being the one that threw the best results which is consistent with the findings of Yang et al. (2008). Prado et al. (2002) found that periodic injection of air helps to maintain the removal efficiency of biofilters close to 100%. Additionally, the operation of rotated drum biofilters has. Universidad de los Andes – 2010. 34.

(35) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ been successful in forming homogeneous biofilm along the bed and in the prevention of excessive biomass growth (Yang et al., 2008). Chemical methods of biomass removal consist on injecting solutions of sodium hydroxide and peroxide, reaching biomass removals of 50 to 70% (Cox & Deshusses, 1999). Likewise, nutrient limitation is probably the method of easiest application, being carbon and nitrogen the nutrients that are commonly restricted. The restriction values of a given nutrient vary from one study to another, depending on the needs of microorganisms and the type of pollutant (Kennes & Veiga, 2002). The acclimation period for microorganisms after the nutrientes limitation can take hours or days, depending on the biofilter conditions (Cox & Deshusses, 2002). Despite the ease of application of these methods, they have not been found successful in reducing the pressure drop and it has been considerable the decrease in removal efficiency (Delhoménie et al., 2003). The use of biological methods for biomass control is starting and requires further investigation. These techniques are based on the ecology of the species in the food chain. To date good results have been reported in the pressure drop decrease with the use of protozoa and nematodes as agents of biological predation (Yang et al., 2010). Before the application of these methods it is important to know the long‐term consequences that they can generate, such as the increase of operating costs in the case of mechanical methods or inhibition of microorganisms, in the case of chemical and biological methods (Kennes & Veiga, 2002; Xi, Hu, & Qian, 2006).. Universidad de los Andes – 2010. 35.

(36) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. 4. HYDROGEN SULPHIDE BIOFILTRATION WITH SCHIST. 4.1 Background Previous works of the École des Mines de Nantes have studied in depth in producing an ideal support material for biofiltration of gaseous pollutants. In this regard, in 2007 eight support materials were made with the purpose of provide the nutrients for biofiltration and exclude the addition of air or water currents. These materials are made of salts and an organic agent to provide cohesion (Dumont, Andrès, Le Cloirec, & Gaudin, 2008). The salts consist of phosphoric acid, calcium carbonate and urea, which are responsible for nutrient release to the biofilter medium, while organic cohesion agent contains mainly ethylene and vinyl acetate monomer. The main differences of the materials were due to the amount of salt and cohesion agent added resulting in different values of bulk density, porosity and water retention capacity of the materials. The removal efficiency of the biofiltration of hydrogen sulphide and ammonia was tested with each of these materials, resulting that material called UP20 (manufactured from urea and phosphate, Figure 4.1) allows greater microbial growth after being inoculated and better buffer capacity, resulting in a removal efficiency of about 90%.. Figure 4.1. UP20 material (Dumont, Andrès, Le Cloirec, & Gaudin, 2008).. Once determined the material with the best performance in H2S biofiltration, other investigations were conducted to compare it with other common support materials. In. Universidad de los Andes – 2010. 36.

(37) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ all the studies the support materials were inoculated with sludge from the Nantes wastewater plant in order to promote biofilm formation at its surface area. In one of the studies it was made a comparison between the removal efficiencies achieved with sapwood (soft wood fiber from trees), pine bark with pozzolan (siliceous volcanic rock) and fibrous peat (Figure 4.2). Additionally, the sapwood, peat and pozzolan were mixed with UP20 material to determine if there was an improve in the removal efficiency.. Figure 4.2. Materials tested in the first study (Dumont & Andrès, 2008).. This study had an operating time of 95 days. Among the main results it was found that by mixing each UP20 material the efficiency of removal of H2S was improved. This fact is due to the amount of nutrients provided by the material that promote the metabolism of microorganisms. The material that showed a better performance was fibrous peat whose removal efficiency was increased by adding UP20. Another study was aimed to evaluate the removal efficiency of pine bark and pozzolan with UP20 in three biofilters. H2S concentration ranged up to 100 ppmv and load values were increased from 0.45 to 9.24 g• m‐3•h‐1. The operating time was 95 days. The acclimation period of microorganisms took a month which were fed with sucrose. The three biofilters had removal efficiency values close to 95% when the loads did not exceed 5 g•m‐3•h‐1. However as the load increases to 10 g•m‐3•h‐1, the removal efficiency of the biofilter with pine bark decreased to 69% while the mixture of UP20 and pozzolan showed a removal of 74%. Again, the biofilter with UP20 alone was the best performing who kept its removal efficiency above 93% (Dumont et al., 2008).. Universidad de los Andes – 2010. 37.

(38) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ Finally it was made a deeper study of biofiltration with peat and UP20 as carrier materials. The configuration of the pilots is shown in Figure 4.3, which shows that the left column was completely filled with peat, the next with a mixture of peat and UP20 and the last with two separate layers of the two materials. The initial concentration of H2S was varied to 100 ppmv, and the load to 25.5 g•m‐3•h‐1. After 111 days of operation the obtained results allowed to conclude that by increasing the loading rate of polluted gas to maximum, the biofilter filled with a mixture of UP20 and peat showed better damping and a removal efficiency of 80%. The biofilter filled with peat alone presented good performance at loads lower than 6 g•m‐3•h‐1. However by increasing the loading rate, the removal efficiency was significantly decreased. In biofilters containing the two materials the removal efficiency remained to 100% at loads lower than 10 g•m‐3•h‐1. Regarding the configuration of the materials it was demostrated that the biofilter with two separate layers was the best performing (Dumont et al. 2009). In addition, pressure drops were higher for biofilters containing UP20 because this material promotes the biofilm growth which increases compaction.. Figure 4.3. Experimental setup of the third study (Dumont et al. 2009).. The last two studies had three biofiltration stages. The first corresponds to the acclimationperiod of microorganisms in which the removal efficiency is minimal. The second stage corresponds to maximum removal efficiencies where loading rates are. Universidad de los Andes – 2010. 38.

(39) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________ held constant. Finally the biofilters presented a decrease in removal efficiencies as a result of an increment in loading rates and pressure drop.. 4.2 Biofitration process 4.2.1 Packing materials The packing materials used in this study are schist and UP20. The first material is a volcanic rock naturally expanded and the second material is made in the laboratories of l’École des Mines de Nantes. 4.2.1.1 Schist characteristics These material was fabricated at Mayenne (France). The schist pieces are round with an average diameter of 10 mm. This material is composed by SiO2 (63%), Al2O3 (21%), Fe2O3 (8,5%), K2O (3,6%), Na2O (1,5%), MgO (1%), CaO (0,5%) and C (0,02%).. 4.2.1.2 UP20 fabrication For the present study 1000 g of UP20 were fabricated. In the study of Gaudin (2008) it was found that the organic cohesion agent must represent 20% of the total weight. The other components, which are calcium carbonate (CaCO3) and urea phosphate (CH4N2OH3PO4), must conform a nutrients relation of 100:10:1 and represent 80% of the total weight. The molecular weight of calcium carbonate and urea phosphate are 100.9 g and 158.05 g, respectively. Each mol of calcium carbonate contains one mol of carbon, while one mol of urea phosphate contains two moles of nitrogen. So, to obtain a nutrient relation of 100:10 between carbon and nitrogen, the stoichiometric analysis is:. 10molCaCO3 →10molC 1molCaCO3 →100.09g € € €. 10molCaCO3 →1000.9g 0.5molCH 4 N 2OH 3 PO4 →1molN 1molCH 4 N 2OH 3 PO4 →158.05g. € € – 2010 Universidad de los Andes. 39.

(40) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. 0.5molCH 4 N 2OH 3 PO4 → 79.03g In this way, 1079.92 g is the total weight to obtain a nutrient relation of 100:10. The € percentage of each substance is: 1079.92g(CaCO3 + CH 4 N 2OH 3PO4 ) →100%. 1000gCaCO3 → 92.68% 79.02gCH 4 N 2OH 3 PO4 → 7.32%. €. € In order to make 1000 g of UP20, it is necessary to add 200 g of the organic cohesion € agent and 800 g of calcium carbonate and urea phosphate. The proportion of these last components is: 100% → 800g(CaCO3 + CH 4 N 2OH 3PO4 ). 92.68% → 741.41gCaCO3 €. 7.32% → 58.58gCH 4 N 2OH 3 PO4. € After the determination of each component weight, the fabrication of UP20 material was € made as follows: 1. First, the dry salt powders and the binder were mixed in a container for 20 min, 2. Then, water was added in a proportion approximated of 2/3 of the mixture (600 ml) and all the ingredients were blended, 3. Next, the mix was extruded in a cilindrical shape with a meat mincer, 4. Last, the pieces were dried at 50°C for 20 h and cut in small pieces. The pieces measure 7 mm in diameter and 15 mm in length. The main physical properties of the two packing materials are summarized in Table 4.1. Universidad de los Andes – 2010. 40.

(41) Biofiltration of Hydrogen Sulphide using schist as packing material:. MIC 2010‐II‐54. Biofilters performances and tortuosity assessment of the packed bed. _____________________________________________________________________________________________________. Figure 4.4. Packing materials, left: schist, right: UP20. Table 4.1. Physical properties of the packing materials Parameter. Schist. UP20. Bulk density (g•cm‐3). 0.67. 0.94. Apparent density (g•m‐3). 1.25. 1.89. Median pore diameter (mm). 10. 7. Water retention capacity (%). 6. 48. Specific surface area (m3•m‐2). 600. 705. 4.3 Experimental setup The experimental setup is shown in Figures 4.5 and 4.6. The biofiltration study was based on the different configuration of biofilters, which are shown in Table 4.2. Table 4.2. Bioflters configuration Biofilter. Configuration. 1. Schist and microorganisms. 2. Schist alone. 3. Schist, UP20 and microorganisms. The biofilters were 0,1 m diameter and 1 m height. The first and second biofilters were filled with 3,97 kg of schiste (0,87 m height each, dark material). The third biofilter was filled with 3,57 kg of schiste (0,77 m height) and 0,48 kg of UP20 (0,1 m height, white. Universidad de los Andes – 2010. 41.