efecto de la razón sustrato/inóculo sobre el rendimiento celular en

Revista Mexicana de Ingeniería Química

CONTENIDO

Volumen 8, número 3, 2009 / Volume 8, number 3, 2009

213 Derivation and application of the Stefan-Maxwell equations

(Desarrollo y aplicación de las ecuaciones de Stefan-Maxwell)

Stephen Whitaker

Biotecnología / Biotechnology

245 Modelado de la biodegradación en biorreactores de lodos de hidrocarburos totales del petróleo

intemperizados en suelos y sedimentos

(Biodegradation modeling of sludge bioreactors of total petroleum hydrocarbons weathering in soil

and sediments)

S.A. Medina-Moreno, S. Huerta-Ochoa, C.A. Lucho-Constantino, L. Aguilera-Vázquez, A. Jiménez-

González y M. Gutiérrez-Rojas

259 Crecimiento, sobrevivencia y adaptación de Bifidobacterium infantis a condiciones ácidas

(Growth, survival and adaptation of Bifidobacterium infantis to acidic conditions)

L. Mayorga-Reyes, P. Bustamante-Camilo, A. Gutiérrez-Nava, E. Barranco-Florido y A. Azaola-

Espinosa

265 Statistical approach to optimization of ethanol fermentation by Saccharomyces cerevisiae in the

presence of Valfor® zeolite NaA

(Optimización estadística de la fermentación etanólica de Saccharomyces cerevisiae en presencia de

zeolita Valfor® zeolite NaA)

G. Inei-Shizukawa, H. A. Velasco-Bedrán, G. F. Gutiérrez-López and H. Hernández-Sánchez

Ingeniería de procesos / Process engineering

271 Localización de una planta industrial: Revisión crítica y adecuación de los criterios empleados en

esta decisión

(Plant site selection: Critical review and adequation criteria used in this decision)

J.R. Medina, R.L. Romero y G.A. Pérez

Vol. 13, No. 3 (2014) 749-755

EFFECT OF INITIAL SUBSTRATE/INOCULUM RATIO ON CELL YIELD IN THEREMOVAL OF HYDROPHOBIC VOCS IN FUNGAL BIOFILTERS

EFECTO DE LA RAZON SUSTRATO/INOCULO SOBRE EL RENDIMIENTOCELULAR EN LA REMOCION DE COVS HIDROFOBICOS EN BIOFILTROS

FUNGICOSA. Vergara-Fernandez1∗, J. San Martın-Davison3, L.A. Dıaz-Robles2, O. Soto-Sanchez3

1Facultad de Ingenierıa y Ciencias Aplicadas, Universidad de los Andes, Mons. Alvaro del Portillo 12455, LasCondes. Santiago, 7620001 Chile.

2Departamento de Ingenierıa Quımica, Facultad de Ingenierıa, Universidad de Santiago de Chile.3Escuela de Ingenierıa Ambiental, Facultad de Ingenierıa, Universidad Catolica de Temuco.

Received September 30, 2013; Accepted June 19, 2014

AbstractDifferent kinetic models have been proposed to describe the elimination of hydrophobic volatile organic compounds(VOCs) by fungal biofiltration. In this process the ratio of the initial substrate concentration (Cp

b0) to the initial biomass(X0) has been shown to influence the cell yield. This paper presents a study of the effect of the Cp

b0/X0 ratio on observed cellyield (Yobs) in a fixed bed batch system (microcosm) using a gaseous carbon source, as an approximation to its applicationin the fungal biofiltration of hydrophobic VOCs. Essays were carried out in fixed-bed microcosms using the filamentousfungus Fusarium solani as a biological agent and n-pentane as a carbon and energy source. The results indicated that Yobsin the gas phase is inversely proportional to the Cp

b0/X0 ratio, with values of 0.9 to 0.35 gbiomass g−1pentane being obtained

when the Cpb0/X0 ratio is changed from 0.1 to 1.0 gpentane g−1

biomass. The results indicate that more than 60% of n-pentanewas consumed due to energy spilling, and that strong dissociation of catabolism from anabolism occurred at higher Cp

b0/X0ratios.

Keywords: hydrophobic VOCs, fungal biofiltration, substrate/inoculum ratio, growth yield.

ResumenDiferentes modelos cineticos han sido propuestos para describir la eliminacion de compuestos organicos volatileshidrofobicos (COVs) en biofiltros fungicos. En este proceso la razon de la concentracion inicial de sustrato (Cp

b0) a labiomasa inicial (X0) ha mostrado influir en el rendimiento celular. Este artıculo presenta el estudio del efecto de la razonCp

b0/X0 en el rendimiento celular observado (Yobs) en un sistema por lote de lecho fijo (microcosmo) utilizando una fuentede carbono gaseosa, como una aproximacion a su aplicacion en la biofiltracion fungica de COVs hidrofobicos. Los ensayosfueron realizados en microcosmos de lecho fijo utilizando el hongo filamentoso Fusarium solani como agente biologico y n-pentano como fuente de carbono y energıa. Los resultados indican que Yobs en la fase gaseosa es inversamente proporcionala la razon Cp

b0/X0, con valores de 0.9 a 0.35 gbiomasa g−1pentano siendo obtenido cuando la razon Cp

b0/X0 es cambiada desde

0.1 a 1.0 gpentano g−1biomasa. Los resultados indican que mas del 60% del n-pentano fue consumido debido a perdida de

energıa, y que una fuerte disociacion del catabolismo y anabolismo ocurre para altas razones Cpb0/X0.

Palabras clave: COVs hidrofobicos, biofiltracion fungica, razon sustrato/inoculo, rendimiento de crecimiento.

∗Corresponding author. E-mail: [email protected]/fax: 56-02-2618 1000

Publicado por la Academia Mexicana de Investigacion y Docencia en Ingenierıa Quımica A.C. 749

Vergara-Fernandez et al./ Revista Mexicana de Ingenierıa Quımica Vol. 13, No. 3 (2014) 749-755

1 Introduction

Fungal biofiltration is a biological technology used forthe elimination of hydrophobic VOCs. It offers severalcomparative advantages over other systems, such asthe high hydrophobicity of the fungus surface, a largetransport area due to the presence of aerial hyphae, andfeasibility for use with high gas flows at low VOCsconcentrations (Vergara-Fernandez et al. 2006 and2011a; Vigueras et al. 2009; Rene et al. 2010).

The effect of the VOCs concentration and start-up on the elimination capacity of biofiltration systemshas been extensively studied in different mathematicalmodels representing the operation of the fungalbiofilter (Maestre et al. 2007; Devinny and Ramesh,2005; Spigno et al. 2003; Vergara-Fernandez et al.2008; Gonzalez-Vazquez et al. 2011). However,the relationship between them, and the effect of thisratio on cell yield, have not been sufficiently studiedfor the removal of hydrophobic VOCs in fungalbiofilters. The different mathematical models usedin biofiltration systems inoculated with bacteria andfilamentous fungi have used cell yield as a constantparameter (Vergara-Fernandez et al. 2008; Arriagaand Revah, 2009; Spigno and De Faveri, 2005; Doradoet al. 2008). This is because the models developedfor the biofiltration of gases only consider the cellyield for the substrate (VOCs) consumed in biomassgrowth, and do not include substrate consumptiondue to cell maintenance, product formation, heat loss,etc. (Liu and Guang-Hao, 1997). Moreover, thepresence of fungal biomass in biofiltration systemscauses an increase in the solubility of hydrophobicVOCs (Vergara-Fernandez et al. 2006), generatingexcess substrate with respect to the other nutrientspresents, which must cause strong uncoupling betweenanabolism and catabolism leading to energy spilling.This has not previously been considered, sincetraditionally the cell yield obtained in the liquidmedium has been used, being one of the main reasonsit is difficulty measuring the presence of the othernutrients in the medium.

For many years it was assumed that livingorganisms always utilized ATP in a highly efficientmanner, but simple growth studies with bacteriaindicated that the efficiency of biomass production wasoften at least 3-fold lower than the amount that wouldbe predicted from standard biosynthetic pathways.These ideas and thermodynamic arguments indicatedthat cells might have another avenue of energyutilization. This phenomenon has also been called‘uncoupling’, ‘spillage’ and ‘overflow metabolism’,

but ‘energy spilling’ is probably the most descriptiveterm (Russell, 2007).

Although cell yield has been considered as aconstant value, some studies in the liquid phase usingbatch and continuous cultures have suggested thatcell yield varies according to the ratio between theinitial substrate and the initial biomass (Liu, 1996;Liu et al. 1998; Liu et al. 2005; Wang et al.2007). Vergara-Fernandez et al. (2008) developed aphenomenological model with experimental validationfor n-hexane fungal biofiltration; they noted adeviation between the results in the simulation andthe experiment which shows a change in the cell yieldfrom 0.1 to 0.8 for model validation. This indicatesthe need for a model to establish cell yield variationsfor biofiltration systems under different conditions ofsubstrate and biomass concentrations. On the otherhand, it is important to consider that in biofiltrationsystems is undesirable to have high yields of biomassand accumulation reservation molecules, as this couldclog the system; however it is desirable to have highvalues of energy dissipation.

The influence of the substrate/inoculum ratio alsohas been studied to determine its effect on methaneyield in the anaerobic digestion of different rawmaterials such as bean curd refuse-okara (Zhou etal. 2011) and Microcystis spp. (Zeng et al. 2010).Furthermore, Aktas (2012) determined the effect ofthe substrate/inoculum ratio on the biodegradation ofphenolic compounds in an activated sludge, indicatingthat the kinetic parameters obtained in a batch reactorcan reflect the biomass activity in continuous-flowreactors. Wang et al. (2007) studied the effect of theratio on the biomass growth of Ralstonia eutropha forthe production of polyhydrobutyrate (PHB).

The aim of this study is to show the effectof the ratio between the initial concentrations ofhydrophobic VOCs/fungal biomass (Cp

b0/X0) on theobserved growth yield (Yobs) in a fixed bed batchsystem. For this, tests were conducted on microcosmsin solid media inoculated with Fusarium solani andfed with n-pentane as the carbon source. Cell yieldsobserved at different values of substrate-inoculumratio were adjusted to energy uncoupling model thataccounts for this phenomenon.

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2 Materials and methods

2.1 Microorganisms and microcosmpreparation

Fusarium solani B1 was grown in solid media(perlite imbibed with liquid mineral medium) in closedenvironments (microcosms). The experiments wereperformed in triplicate in 125 mL serum bottlessealed with Mininert Teflon Valves (VICI; PrecisionSampling, Baton Rouge, LA).

Series of batch experiments were carried outat different Cp

b0/X0 ratios with initial n-pentaneconcentrations (Cp

b0) in wet biomass between 22and 435 g m−3. The range of values obtainedfor Cp

b0 correspond to initial n-pentane headspaceconcentrations (Cp

h0) between 0.5 and 10 g m−3 (rangeof concentrations commonly used in biofiltrationsystems for removing hydrophobic VOCs) (Doradoet al. 2008; Arriaga and Revah, 2009; Vergara-Fernandez et al. 2012). The Cp

h0 was determinedby direct gas chromatography injection. In additionCp

b0 was obtained by n-pentane/biomass partitioncoefficient (Kw

b ) as:

Kwb =

CPh0

CPb0

(1)

The partition coefficient for the biomass on a wet basisat 25 ºC (Kw

b = 0.023) was obtained from Vergara-Fernandez et al. (2011b). Control experimentsshowed negligible sorption of n-pentane on perlite.

To evaluate the observed cell yield (Yobs), 1.5 gsupport (perlite) was mixed with 10 mL of mineralmedium containing an initial active fungal mycelium(initial biomass concentration, X0) at 0.35 gbiomass L−1

(equivalent to 2.33 mgbiomass g−1dry perlite). This was

placed in 125 mL serum bottles for all batch testsand a desired Cp

b0 /X0 ratio was obtained by varyingthe initial substrate concentration. Batch tests werecarried out at 25 ºC.

2.2 Carbon source and mineral medium

The model compound used as the carbon source was n-pentane (Merck, 99%). Mineral medium was preparedin a buffered phosphate solution (pH 4) and contained(g L−1): 18 NaNO3; 1.3 KH2PO4; 0.38 MgSO4·7H2O;0.25 CaSO4·2H2O; 0.055 CaCl2; 0.015 FeSO4·7H2O;0.012 MnSO4·H2O; 0.013 ZnSO4·7H2O; 0.0023CuSO4·7H2O; 0.0015 CoCl2·6H2O; 0.0015 H3BO3.

2.3 Energy uncoupling model

The Cpb0 /X0 ratio represents the availability of carbon

and energy source for microbial growth in a batchculture. To determine the effect of the Cp

b0 /X0 ratioon the yield in a batch culture, the Yobs were adjustedaccording to the model developed by Liu (2000) (Eq.2), considering the substrate consumed due to growth,cell maintenance and energy spilling.

1Yobs

=1

(Yobs)max+

1(Yw)min

CPb0

/X0

CPb0

/X0 + KS /X

(2)

where Yobs is the observed cell yield, (Yobs)maxis the observed growth yield of substrate-limitedculture, (Yw)min is the minimum energy spilling-related cell yield and KS/X is the Cp

b0 /X0 ratio-related saturation constant. Eq. (2) shows that theratio between anabolism and catabolism is taken tobe dependent on the Cp

b0 /X0 ratio (Liu et al. 1998).Following Liu et al. (1998), the concept of theenergy uncoupling coefficient is used in this work todescribe the uncoupling observed between anabolismand catabolism under substrate-sufficient conditions(Eq. 3).

Eu =(Yobs)max −Yobs

(Yobs)max(3)

where Eu is the energy uncoupling coefficient.This parameter features reduction in the efficiencyof converting energy into cell biosynthesis undersubstrate-sufficient conditions. Substituting Eq. (2)into Eq. (3) produces the Cp

b0 /X0 -dependentexpression for the energy uncoupling coefficient:

Eu = Eu,maxCP

b0

/X0

CPb0

/X0 + K∗S/X

(4)

whereEu,max =

(Yobs)max(Yobs)max + (Yw)min

(5)

K∗S/X =(Yw)min

(Yobs)max + (Yw)minKS/X (6)

where Eu,max is the maximum energy uncouplingcoefficient and K∗S/X is the yield-related saturationconstant.

2.4 Analytical methods

The gaseous n-pentane concentration was measuredin triplicate by FID-GC, Shimadzu 2014 (detectiontemperature 220ºC, injection temperature 80ºC andcolumn temperature 200ºC), equipped with a capillary

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column, model Rtx-5 Restex UE (30 m × 0.32 mm ×0.25 µm), using nitrogen as a carrier gas.

The biomass was measured as volatile solids witha thermogravimetric analyzer according to Arriaga andRevah (2005). This analysis allowed the mass lossesto be quantified and associated with the processes ofwater and carbon combustion. Measurements weredone in duplicate.

3 Results and discussionFig. 1 shows the experimental values of Yobs obtainedfor different values of the CP

b0/X0 ratio using F. solanigrown in n-pentane gas. The experimental valuesof Yobs were adjusted to the mathematical modelproposed by Liu (2000) based on the CP

b0/X0 ratio (Eq.2). Thus, the parameters of the model were (Yobs)max =

0.96 gbiomass g−1pentane; (Yw)min = 0.12 gbiomass g−1

pentaneand KS/X = 2.71 gpentane g−1

biomass.As shown in Fig. 1, Yobs decreased by a factor

of three when the CPb0/X0 ratio was increased by

a factor of ten, becoming approximately constantfor a CP

b0/X0 ratio higher than 0.5 (Yobs ≈ 0.35),while for ratios between 0.1 and 0.5 a sharpdecline in Yobs is observed, from 0.9 to 0.35,respectively. Reduced cell yields show dissociation ofanabolism from catabolism. In fact, much researchshows that under substrate-sufficient conditions,Yobs decreases significantly with increasing residualsubstrate concentration for continuous and batchculture (Yamame et al. 1992; Liu, 1996; Liu andGuang-Hao, 1997).

The range of Yobs obtained (between 0.9 to 0.35gbiomass g−1

pentane) in this work was similar to valuesreported by Vergara-Fernandez et al. (2008) (0.1 -0.8 gbiomass g−1

hexane) in the simulation of n-hexanefungal biofiltration using the fungus Fusarium solani.In comparable works, Macris and Kokke (1978)reported values of 0.71 gbiomass g−1

carob sugar using thefungus Fusarium moniliforme, while Shellart (1975)and Lareo et al. (2006) reported average values of0.54 gbiomass g−1

substrate using the fungi Trichodermaviride with corn waste effluent substrate, and Mucorbacilliformis with glucose, respectively.

On the other hand, Garnier et al. (1999), using n-pentane as the carbon source, reported values of 0.9gbiomass g−1

pentane with Pseudomonas aeruginosa; whileTakahashni (1970), using bacteria from a soil sample,reported values of 0.85 gbiomass g−1

pentane. Miller andJohnson (1966), using yeast, reported values of 0.8gbiomass g−1

pentane.

Figure 1

Fig. 1 Relationship between Yobs and /X0 ratiofor Fusarium solani grown on n-pentane gas. (•)Experimental data; (—) model prediction (Eq. 2).(Yobs)max = 0.96 gbiomass g−1

pentane; (Yw)min = 0.12gbiomass g−1

pentane and KS/X = 2.71 gpentane g−1biomass.

Figure 2

Fig. 2 Relationship between the Cp

b0/X0 ratio andthe energy uncoupling coefficient for Fusatium solanigrown on n-pentane gas. (•) Experimental data; (—)model prediction (Eq. 4). Eu,max = 0.89; K∗S/X= 0.30gpentane g−1

biomass.

Fig. 1 shows that the mathematical model fittedthe experimental data, proving that the effect ofthe CP

b0/X0 ratio reported by Liu (2000) in liquidmedium bioreactors can be extrapolated to fixed bedbioreactors with a gaseous carbon source.

Fig. 2 shows a comparison between the observedEu and the values obtained using Eq. (4) indicatingthat the model proposed by Liu et al. (1998)for substrate-sufficient batch culture of activatedsludge microorganisms can be adequately applied to

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a gaseous carbon source in a fixed bed bioreactorbatch culture inoculated with filamentous fungus. Theparameters of the model (Eq. 4) were Eu,max = 0.89and K∗S/X = 0.30 gpentane g−1

biomass.Fig. 2 shows that the energy uncoupling coefficient

reaches 0.6 when the CPb0/X0 ratio is greater than

0.5 gpentane g−1biomass. A similar energy uncoupling

coefficient of 0.65 was observed by Chang et al.(1993) at CP

b0/X0 ratios greater than 10 mgCOD

mg−1MLS S , while Liu et al. (1998) reported an energy

uncoupling coefficient value of 0.5 using a batchculture of activated sludge.

The results obtained in this study indicate thatmore than 60% of n-pentane was consumed due toenergy spilling, and that a strong dissociation ofcatabolism from anabolism occurred at higher CP

b0/X0

ratios. These results show that at higher CPb0/X0

ratios, bioenergy may be produced faster than itwould be used by microorganisms, and this result isfavoured by increasing the solubility of n-pentane onthe hydrophobic surface of the fungus.

The results of Figs. 1 and 2 fit the changing Yobsobtained by Vergara-Fernandez et al. (2008) for a n-hexane biofiltration system. In this system, low Yobsvalues (0.1 gbiomass g−1

hexane) are found in the latencystage due to low biomass concentrations (high CP

b0/X0ratio), while after the steady state was reached thehigher Yobs (0.8 gbiomass g−1

hexane) represents a highintake of biomass for the same amount of carbonsource (low CP

b0/X0 ratio). This effect is consistentwith the findings of Wang et al. (2007), Liu et al.(2005) and Liu (2000), and demonstrates that thephysiological uncoupling of catabolism and anabolismextends to biofiltration processes with a gaseouscarbon source.

Carbon balance results of about 50% to 70%,considering cell maintenance, biomass growth andmineralization of the carbon source, have beenobtained in different works on fungal biofiltration ofVOCs, in which the rest of the carbon and energysource has not been clearly quantified (Arriaga andRevah, 2009; Vergara-Fernandez et al. 2012; Vergara-Fernandez et al. 2011b; Hernandez-Melendez et al.2008). These results in continuous culture biofiltrationsystems indicate that the energy loss due to uncouplingmay be up to 50%, similar to the 60% maximumobtained in this work in a fixed bed batch system usinga gaseous carbon source. These results indicate thatit is feasible to use microcosm results to determine

the effect of the CPb0/X0 ratio on fungal biofiltration of

hydrophobic VOCs.These results indicate the feasibility of

determining CPb0/X0 ratios that allow decreasing

the amount of biomass generated and possibleaccumulation of material in long periods of a biofilteroperation, so as to prevent possible clogging andmaximizing the energy dissipation source of carbon.

ConclusionsThe results indicate that it is reasonable to consider theinitial substrate concentration normalized to the initialbiomass concentration (CP

b0/X0 ratio) to describe theavailability of carbon and energy for fungal growthin the modeling of a biofilter hydrophobic VOCs.These results indicate that the CP

b0/X0 ratio has aneffect on Yobs in fixed bed systems using a gaseouscarbon source. In this case, when F. solani is fed witha gaseous carbon source (n-pentane) in a fixed bedsystem, the effect observed is similar to that reportedin the liquid phase, show maximum energy uncouplingof 60%.

AcknowledgementsThe present research was financed by theCONICYT (National Commission for Scientificand Technological Research) (FONDECYT ProjectNº11080036).

NomenclatureCP

b0 initial n-pentane concentration in wetbiomass gpentane m−3

wet biomassCP

h0 initial n-pentane headspaceconcentration gpentane m−3

headspaceEu energy uncoupling coefficientEu,max maximum energy uncoupling

coefficientKw

b n-pentane/biomass partitioncoefficient gpentane m−3

wet biomass /

gpentane m−3headspace

KS/X CPb0/X0 ratio-related saturation

constant gpentane g−1biomass

K∗S/X yield-related saturation constantgpentane g−1

biomassYobs observed growth yield gbiomass

g−1pentane

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(Yobs)max observed growth yield of substrate-limited culture gbiomass g−1

pentane(Yw)min minimal energy spilling-related

growth yield gbiomass g−1pentane

X0 initial biomass concentration gbiomassm−3

dry perlite

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