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Ind. Eng. Chem. Res. 2009, 48, 1270–1276
Simultaneous Oxygen and Carbon Variation within an RBC Biofilm as Function of Different Operating Conditions Sergio A. Martı´nez Delgadillo,† Denis Cantu´-Lozano,*,‡ Carlos Montalvo,‡ and Jesu´s Gonza´lez Herna`ndez§ Departamento de Energı´a, UniVersidad Auto´noma Metropolitana-Azcapotzalco, AVenida San Pablo 180, CP 02200, Me´xico D.F., Me´xico, DiVisio´n de Estudios de Posgrado e InVestigacio´n, Instituto Tecnolo´gico de Orizaba, Oriente 9, 852, Col. E. Zapata, Orizaba, 94320 Mexico, and CIMAV-Centro de InVestigacio´n en Materiales AVanzado, Miguel de CerVantes 120, Complejo Industrial 31109, Chihuahua, Me´xico
In this work, a model that considers the change in biofilm density at different biofilm depths was applied. Two correlations, describing the variations of the oxygen and substrate effective diffusivities as functions of the biofilm density and depth, were introduced. The model was validated with experimental data of oxygen concentrations measured with a microelectrode at different biofilm depths in a laboratory-scale rotating biological contactor (RBC) system at different rotational speeds (1.25, 2.75, and 5.25 rpm.), using municipal wastewater. The oxygen and substrate concentration profiles depend on the position on the biodisk. Comparisons between the COD and oxygen concentration profiles obtained with the model and those obtained considering the microorganism density and diffusivities to be constant were performed. The effects of the biokinetic parameters µmax, KS, and KO2 on the COD and oxygen concentration profiles inside the biofilm were tested. Introduction Rotating biological contactors (RBCs) have been used to eliminate contaminants from sewage waters and other specific organic compounds such as azo dye mixtures1 and 2,4dichlorophenol,2 among others, because such units are inexpensive and practically all liquid effluents contain large amounts of biodegradable material. RBCs have a very high interfacial area that is virtually independent of the rotational speed, which is an advantage over other biological wastewater treatments. In addition, they are easily adaptable to small and medium-sized wastewater treatment plants, with easy operation and maintenance.3 However, the aerobic biodegradation is frequently limited by oxygen diffusion into the biofilm.4 The depth at which oxygen penetrates into the biofilm depends on different factors such as the density and composition of the biofilm, the respiration rate, and the substrate consumption rate.5,6 Different studies on mass transport within biofilms have been conducted. These studies have showed that biofilms form cellular clusters separated by interstitial voids filled with water and biopolymers.7 Moreover, because of the irregular distribution of the biomass in biofilms, mass transfer is affected, and the effective diffusivity varies across the biofilm.8,9 It has been desmosntrated that the density of the biofilm depends on the depth and is higher at the bottom of the biofilm.10 This reduces the effective diffusivity of oxygen by 25-90% in comparison with the oxygen difussivity in water.11 It has also been reported that the tortuosity (κ) increases and the porosity (ε) decreases toward the bottom of a biofilm, thereby affecting the effective difussivity (D ) ε/κ). Several researchers have applied direct microelectrodes to measure the oxygen concentration and obtain concentration profiles in biofims. In another study, platinum microelectrodes were utilized to determine the oxygen concentrations in aerobic biofilms.12 It was concluded that the flow of the liquid into the biofilm is affected by the irregularities in the surface of the * To whom correspondence should be addressed. E-mail: dcantu@ itorizaba.edu.mx. † Universidad Auto´noma Metropolitana-Azcapotzalco. ‡ Instituto Tecnolo´gico de Orizaba. § CIMAV-Centro de Investigacio´n en Materiales Avanzado.
biofilm. Other researchers13 have utilized the method of finite differences to determine the effective diffusivity in a completely mixed bioreactor. They performed direct calculations of the oxygen concentrations inside the biofilm, at steady state, at an interval of 100 µm using a microelectrode, concluding that the effective diffusivity of oxygen is highly related to the density of the biofilm. Furthermore, different microelectrodes have been used to measure the concentrations of oxygen, ammonia nitrogen, and nitrates and the pH.10 In another study, a microelectrode with a platinum tip was utilized to determine oxygen flows inside the biofilm of a rotating biological contactor (RBC).14 When using a kinetic model at the steady state to compare the results with those obtained from the stoichometric quantity required to oxidize the nitrogen, a high correspondence was found. However, this test was performed at only one rotation speed, 1.0 rpm. Using a platinum microelectrode, handled through a micromanipulator, they calculated the local mass-transfer coefficient in the biofilm formed in an openchannel biofilm reactor, using synthetic water.15 In this study, a modified limiting current technique was applied, concluding that the mass-transfer coefficient is not constant; it varies both horizontally and vertically. Modeling to describe the profiles of oxygen in the biofilm has been performed in several studies.3,16-20 In those studies, it was assumed that the oxygen diffusivity and biofilm density were constant. However, it was mentioned before that the effective difussivity is affected by the density, tortuosity, and porosity in the biofilm, which change with the depth inside the biofilm. In this work, a model was applied that considers that the effective difussivity is affected by the biofilm density, which increases at increasing biofilm depth. Two correlations that describe the variations in the oxygen and substrate effective difussivities as functions of the biofilm density and depth (x) were introduced. The model was validated with the experimental data on oxygen concentrations measured with a microelectrode at different biofilm depths, in a laboratory-scale RBC system and at different rotational speeds (1.25, 2.75, and 5.25 rpm), using municipal wastewater. Moreover, the effects of 2-fold changes in the values of the biokinetic parameters of µmax, KS,
10.1021/ie8005885 CCC: $40.75 2009 American Chemical Society Published on Web 08/19/2008
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Figure 2. Sketch of the oxygen measurement points (P1-P5) in the RBC. Figure 1. Characteristics of RBC laboratory-scale reactor. Table 1. Experimental Design Parameters of the Rotating Biological Contactor Used in This Work parameter
value
number of stages number of disks per stage disk diameter (cm) disk thickness (cm) total surface area per stage (m2) volume of working water per stage (m3) area/volume ratio (m2/m3) percentage immersion of disks
5 4 24.1 0.35 0.3804 0.0022 176.122 45%
and KO2 on the COD and oxygen concentration profiles inside the biofilm were tested. Material and Methods The rotating biological contactor (RBC) used in this work was made up of 20 acrylic disks. The tub and the primary settler of the RBC were built out of fiberglass, and the form of the tub was a hollow semicylinder, placed in a metallic structure. The reactor tub was divided into five transverse compartments of equal size using fiberglass screens. The chambers of the reactor were interconnected through a lateral slot located on the upper part of each screen. These slots were positioned in a zigzag manner; they allowed the passage of both liquid and microbial flocculate toward the following chamber when the maximum capacity level has been reached (Figure 1). This particularity means that, in this type of system, the biological process of degradation is carried out sequentially. Table 1 lists the characteristics of the RBC system. Municipal wastewater was fed to the reactor with a variablespeed pump. It is important to note that the COD of the municipal wastewater was not constant but rather changed during the day. The same procedure as described in other studies13,14 was used to construct the microelectrode with only the following modification:21 The diameter of the tip was obtained with a Struers model Dap-7 file, using Buehler silicon carbide sandpaper (5000 and 1200 mesh). The tip of the microelectrode was reduced until a diameter of 67 µm was obtained. The microelectrode was installed in the last stage. To maintain a fixed distance between each disk, support axes were used to avoid perturbations during measurements. The microelectrode was attached to the micromanipulator, which was, in turn, attached to the disks. The micromanipulator had a displacement precision of 100 µm. A copper wire with has a special covering to avoid any induced voltage was attached to the microelectrode serving as a wire conductor, and it passed through the center of the
disks and the center of the RBC axis. The determinations of dissolved oxygen (DO) were carried out after 4 months of operation of the RBC. The DO measurements were carried out moving the microelectrode from the bulk liquid down through the biofilm in 300-µm increments for the rotational speeds of 1.25 and 5.25 rpm and in 100-µm increments for 2.75 rpm. In addition, measurements were taken at five points along the diameter of the disk, as shown in Figure 2. After the DO measurements were taken in one point, the biodisks were allowed to rotate once again at the fixed speed for 20 min; then, the biodisk was stopped, and the microelectrode was placed in the other point to perform the DO measurements. About 100 s elapsed between placing the microelectrode in the next position and carrying out the measurements in the biofilm. The results of the simulations with the model were obtained at 100 s. Because the experiments started at 2.75 rpm, the biofilm thickness was less than that reached at the other rotation speeds. For each point and at each distance of penetration, 20 data values were recorded, and their relative standard deviation and mean were determined. For deviations of less than 0.5, the data were accepted, the mean was taken as the value, and the next point was considered. A commercial Ag/AgCl reference electrode was used. A digital multimeter was utilized to measure the current on the microampere scale. The microelectrode calibration curve was obtained by plotting the oxygen concentration against current with a correlation coefficient of R2 ) 0.975. The oxygen concentrations were determined using eq 1 from the electric current measured with the microelectrode21 CO2 ) -0.00291812 + 0.00001429µAM -3
(1)
where CO2 is the concentration of oxygen (mg cm ) and µAM is the measured current (µA). The concentrations of oxygen at the top biofilm layer (biofilm depth ) 0.0 cm) and in the biofilm at different depths were measured with the microlectrode for each test carried out at the different rpm. Biofilm Model. Equations 2 and 4 with their boundary conditions were used to describe the variations of the substrate and oxygen effective diffusivities as functions of the biofilm depth (x). Because the biofilm density increased toward the substratum,9,10 the diffusion coefficients of the species had to be corrected. The proposed correlations, in eqs 3 and 5, describe the changes in substrate (COD) and oxygen effective diffusivities as functions of the biofilm density and the biofilm depth. Because of the difficulty in measuring the biofilm density at different biofilm depths, a linear relation was assumed as shown in eq 6. In addition, it is important to point out that, although nutrients can be laterally transferred by convection because water
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movement occurs inside the biofilm, it was demonstrated that nutrients are mainly transported by diffusion perpendicularly to the substratum.22,23 Therefore, the nutrients (oxygen and COD) are transferred by diffusion, and the following assumptions were made: (1) There is no substrate removal or oxygen consumption in the bulk liquid in the reactor. (2) Substrate removal in the liquid film occurs only by diffusion into the biofilm. (3) The diffusion of oxygen and COD obeys Fick’s law. (4) Biofilms grow on impermeable and inactive surfaces. (5) Only oxygen and COD are transferred, perpendicularly to the substratum. The following equations comprise the biofilm model Substrate (COD) utilization
) ( )(
(
CO2 µmaxXV ∂2S S ∂S ) DS(XV) 2 ∂t Ys KS + S KO2 + CO2 ∂x
)
Results and Discussion (2)
with the boundary conditions x)0
S ) Sb
x)L
∂S |)0 ∂t
COD effective diffusivity DS(XV) ) DSe-XVx Oxygen utilization ∂CO2 ∂t
) DO2(XV)
∂2CO2 2
-
∂x
( )( )(
(3)
)
CO2 µmaxXV S (4) YO2 KS + S KO2 + CO2
with the boundary conditions x)0 x)L
CO2 ) CO2b ∂CO2 ∂t
|x)L ) 0
Oxygen effective diffusivity DO2(XV) ) DO2e-XVx
(5)
Microorganism density as a function of biofilm depth XV ) mx + XVS
cm-3, (2) at 2.75 rpm, XV ) 18 mg cm-3, and (3) at 5.25 rpm, XV ) 23 mg cm-3. Model parameters adopted directly from the literature include the following:4,11 The effective diffusivity of the substrate at the top biofilm layer (at x ) 0.0 cm) is DS ) 0.0000084 cm2 s-1. The effective diffusivity of oxygen at the top biofilm layer (at x ) 0.0 cm) is DO2 ) 0.00005 cm2 s-1. The maximum specific growth rate is µmax ) 0.0000040 s-1. The saturation concentration of substrate is KS ) 0.015 mg cm-3. The saturation concentration of oxygen is KO2 ) 0.0005 mg cm-3. The yield growth coefficient is YS ) 0.4. The yield oxygen coefficient is YO2 ) 2.2.
(6)
In the model equations, S is the concentration of substrate (mg cm-3), Sb is the concentration of substrate at the top biofilm layer (at x ) 0.0 cm) (mg cm-3), CO2 is the concentration of oxygen in the biofilm (mg cm-3), CO2b is the concentration of oxygen at the top biofilm layer (at x ) 0.0 cm) (mg cm-3), XV is the microorganism density in the biofilm (mg cm-3), XVS is the microorganism density at the top biofilm layer (x ) 0.0 cm) (specifically, XVS ) 19.5 mg cm-3 at 1.25 rpm, 15 mg cm-3 at 2.75 rpm, and 20 mg cm-3 at 5.25 rpm), m is the variation in the microorganism density as a function of the biofilm depth (m ) 36.6 mg cm-3 cm-1), x is the biofilm depth (cm), L is the biofilm thickness (cm), DS(XV) is the effective diffusivity of substrate as a function of the microorganism density in the biofilm, and DO2(XV) is the effective diffusivity of oxygen as a function of the microorganism density in the biofilm. To compare the model of this work with conventional models, which consider that the biofilm density and the diffusivities are constant, oxygen and COD profiles (with D and XV constant) were obtained with constant values of COD diffusivity (DS) and oxygen diffusivity (DO2), both at the top biofilm layer. Moreover, the following constant biofilm mean densities for the three rotational speeds were used: (1) At 1.25 rpm, XV ) 22.5 mg
Based on the experimental results obtained at the different rotational speeds, biofilm depths, and sampling points (shown in Figure 2), the model was validated. Figures 3-5 show the experimental points and the model for the different sampling points at different biofilm depths at rotational speeds of 1.25, 2.75, and 5.25 rpm. As shown, the model fits satisfactorily the experimental results at the three rotational speeds. It has been reported that, if the densities of the bottom biofilm layers are higher than those at the top,10 then the oxygen and substrate diffusivities decrease as the biofilm depth increases. Therefore, the model is in accordance with this report. In Figure 3[point 2 (P2)], Figure 4 [point 5 (P5)], and Figure 5 (all points), the results obtained with the model and those obtained assuming that the diffusivities and biofilm density were constant are shown. As seen, the model (effective diffusivity changes as functions of the biofilm density and biofilm depth) fits the experimental data. On the other hand, considering that the microorganism density in the biofilm and the diffusivities are constant (XV and D constant) gives rise to significant deviations from the experimental behavior, mainly at biofilm depths greater than 0.05 cm for 1.25 and 5.25 rpm and greater than 0.02 cm at 2.75 rpm. As shown in the same figures, at the three rotational speeds, the highest oxygen concentration is reached at point 5, just before the disk enters the liquid. This highest concentration is because the oxygen has been transferred into the biofilm during the time that the biofilm remains in contact with the air phase. Afterward, the disk enters the liquid, and the liquid film is removed from the biofilm and mixed with the wastewater in the tank. Then, the oxygen concentration is reduced as shown by the oxygen profile at P1, and the microorganisms continually deplete the dissolved oxygen as it diffuses through the biofilm. The lowest oxygen concentrations at the different biofilm depths were measured at P3 for the three rotational speeds, just before the biofilm enters the air-exposure cycle. In P4, the oxygen increases again as a result of the transfer during the air-exposure cycle, to reach the maximum values at P5, as shown in Figures 3-5. The results show that the lowest biofilm oxygen concentrations (e0.32 × 10-3 mg cm-3) for the three rotational speeds were measured at P3 and at 1.25 rpm. At this rotational speed, the highest oxygen concentration reached was about 0.5 × 10-3 mg cm-3 at P5. These low oxygen concentrations reached inside the biofilm are because the oxygen transfer rate during the airexposure cycle is lower than those at higher rotational speeds. As reported, the mass-transfer coefficient is proportional to the rotational speed, thereby affecting the renewal number proportionally.24,25 Under the conditions of low rotational speed, the oxygen transfer rate during the air-exposure cycle is not sufficient to increase the oxygen concentration (CO2b) at the top
Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1273
Figure 3. Oxygen concentration profiles as a function of the biofilm depth at different sampling points at 1.25 rpm.
Figure 4. Oxygen concentration profiles as a function of the biofilm depth at different sampling points at 2.75 rpm.
biofilm layer (biofilm depth ) 0.0 cm). Moreover, the uptake of oxygen by the microorganisms reduces the oxygen penetration into the biofilm; hence, this concentration is low, and the oxygen concentrations in the biofilm are the lowest of the three rotational speeds. Therefore, at 1.25 rpm, the oxygen concentra-
tions in the deeper layers of the biofilm near the substratum are too low, producing anoxic conditions. The low oxygen concentrations at biofilm depths greater than 0.045 cm limit the oxidation of carbonaceous compounds, so the COD removal is carried out mainly in the upper biofilm layers. In addition, it is
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Figure 5. Oxygen concentration profiles as a function of the biofilm depth at different sampling points at 5.25 rpm.
Figure 6. COD and oxygen profiles obtained with the model, considering the biofilm density and diffusivities (D and XV) to be constant, in P5 and at 5.25 rpm.
important to note that there is an aerobic zone in the upper layers of the biofilm and other anoxic zone in the deeper layers, where denitrification reactions occur.26 It has been reported that the critical biofilm thickness at which internal mass transfer becomes limiting is approximately 0.015 cm.27 In our work, the experimental biofilm thicknesses were greater in all three cases. Consequently, under our conditions, the mass transfer is limiting. Comparing the COD and oxygen concentration profiles obtained with the model at point 5 (P5) and at 5.25 rpm with those obtained considering the microorganism density in the biofilm and the diffusivities are constant (XV and D constant), as shown in Figure 6, it can be seen that the COD model and constant D and XV profiles are similar at biofilm depths lower than 0.09 cm. Nevertheless, at biofilm depths higher than 0.09
cm, the COD concentrations obtained with the model are lower than the COD concentrations obtained with constant microorganism density in the biofilm and constant diffusivities. In addition, the oxygen concentration profiles, which fit the experimental data well, are lower than the oxygen concentrations obtained with D and XV constant at biofilm depths greater than 0.05 cm. These results can be explained by the microbial growth having an autocatalytic effect on the substrate and oxygen dynamics. Moreover, higher microbial growth affects the diffusivities. When the profiles are obtained with the model, the biofilm density increases toward the substratum, in this case from 20 mg cm-3 at the top biofilm layer to 25.8 mg cm-3 at x ) 0.16 cm. On one hand, this causes higher oxygen and COD uptake rates at deeper layers, but on the other, it causes lower COD and oxygen diffusivities at deeper layers of the biofilm, in agreement with the experimental results. When the diffusivities and biofilm density (23 mg cm-3) are constant, the oxygen and COD concentrations in deeper layers are higher than those obtained with the model, and the oxygen concentrations deviate from the experimental results at the three rotational speeds. These higher concentrations are because, on one hand, the diffusivities remain high at deeper biofilm layers, allowing the oxygen and COD to penetrate deeper inside the biofilm, and on the other hand, the biofilm density is constant and lower at deeper layers than in the model. Based on the model, the effects of the biokinetic parameters µmax, KS, and KO2 on the COD and oxygen concentration profiles inside the biofilm were tested. Figures 7 and 8 show the results for 2-fold changes in the parameter values. The solid lines in both figures show the profiles with the experimental parameters. Simulations at the conditions of P5 and at 5.25 rpm were performed. The figures show that the COD and oxygen profiles are more sensitive to the 2-fold changes in µmax than the other parameters, which is in agreement with other studies,17 and they are less sensitive to the to the 2-fold changes in KO2. It is
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biodisk rotational speeds. In addition, it was demonstrated that, for a model that considers the effective diffusivities constant and independent of the biofilm depth and biofilm density, the oxygen and COD concentration profiles deviate from the experimental results. Based on the validated model, the effects of the biokinetic parameters µmax, KS, and KO2 on the oxygen and COD concentration profiles inside the biofilm were evaluated. The changes in the values of the biokinetic parameters affect the oxygen and COD concentration profiles inside the biofilm, which are more sensitive to changes in µmax than the other biokinetic parameters. Acknowledgment This work was supported by COSNET, Mexico (Project E-390.98). The authors thank Dr. Juan Francisco Pe´rez Robles of CINVESTAV, Unidad Quere´taro, Mexico, for his contribution toward this research. The authors also express thanks to Ayax Martinez, who helped to check the text.
Figure 7. Effects of the biokinetic parameters µmax, KS, and KO2 on the COD concentration profiles inside the biofilm (P5 and 5.25 rpm).
Figure 8. Effects of the biokinetic parameters µmax, KS, and KO2 on the oxygen concentration profiles inside the biofilm (P5 and 5.25 rpm).
important to note that both profiles are more sensitive to increases than decreases of KS and KO2, whereas the reduction of µmax affects the profiles more than the increase of this biokinetic parameter. Conclusions In this work, based on experimental data, it was demonstrated that the oxygen and substrate effective diffusivities depend on the biofilm depth and biofilm density in an RBC system. The applied model, which considers the variations of the oxygen and substrate effective difussivities as functions of the biofilm depth due to the change in biofilm density, was validated using experimental data obtained with a microolectrode at diferent rotational biodisk speeds of the RBC system. The model adequately describes the oxygen and COD concentration profiles in the biofilm at diferent positions in the biodisk and different
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ReceiVed for reView April 12, 2008 ReVised manuscript receiVed June 30, 2008 Accepted July 8, 2008 IE8005885
